Johnson, A. ; Stadlmeier, M. ; Wühr, M. TMTPro Complementary Ion Quantification Increases Plexing and Sensitivity for Accurate Multiplexed Proteomics at the MS2 Level. bioRxiv 2020.Abstract
Multiplexed proteomics is a powerful tool to assay cell states in health and disease, but accurate quantification of relative protein changes is impaired by interference from co-isolated peptides. Interference can be reduced by using MS3-based quantification, but this reduces sensitivity and requires specialized instrumentation. An alternative approach is quantification by complementary ions, which allows accurate and precise multiplexed quantification at the MS2 level and is compatible with the most widely distributed instruments. However, complementary ions of the popular TMT tag form inefficiently and multiplexing is limited to five channels. Here, we evaluate and optimize complementary ion quantification for the recently released TMTPro tag, which increases plexing capacity to eight channels (TMTProC). We find that the beneficial fragmentation properties of TMTPro increase quantification signal five-fold compared to TMT. This increased sensitivity results in ~65% more proteins quantified compared to TMTPro-MS3 and even slightly outperforms TMTPro-MS2. Furthermore, TMTProC quantification is more accurate than TMTPro-MS2 and even superior to TMTPro-MS3. To demonstrate the power of TMTProC, we analyzed a human and yeast interference sample and were able to quantify 13,290 proteins in 24 fractions. Thus, TMTProC advances multiplexed proteomics data quality and widens access to accurate multiplexed proteomics beyond laboratories with MS3-capable instrumentation.Competing Interest StatementThe authors have declared no competing interest.
Crapse, J. ; Pappireddi, N. ; Gupta, M. ; Shvartsman, S. Y. ; Wieschaus, E. ; Wühr, M. Evaluating the Simple Arrhenius Equation for the Temperature Dependence of Complex Developmental Processes. bioRxiv 2020. Publisher's VersionAbstract
The famous Arrhenius equation is well motivated to describe the temperature dependence of chemical reactions but has also been used for complicated biological processes. Here, we evaluate how well the simple Arrhenius equation predicts complex multistep biological processes, using frog and fruit fly embryogenesis as two canonical models. We find the Arrhenius equation provides a good approximation for the temperature dependence of embryogenesis, even though individual developmental stages scale differently with temperature. At low and high temperatures, however, we observed significant departures from idealized Arrhenius Law behavior. When we model multistep reactions of idealized chemical networks we are unable to generate comparable deviations from linearity. In contrast, we find the single enzyme GAPDH shows non-linearity in the Arrhenius plot similar to our observations of embryonic development. Thus, we find that complex embryonic development can be well approximated by the simple Arrhenius Law and propose that the observed departure from this law results primarily from non-idealized individual steps rather than the complexity of the system.Competing Interest StatementThe authors have declared no competing interest.
Hart, E. M. ; Gupta, M. ; Wühr, M. ; Silhavy, T. J. The gain-of-function allele bamAE470K bypasses the essential requirement for BamD in β-barrel outer membrane protein assembly. Proceedings of the National Academy of Sciences 2020. Publisher's VersionAbstract
The assembly of β-barrel outer membrane proteins (OMPs) is broadly conserved in diderm bacteria as well as in mitochondria and chloroplasts. The β-barrel assembly machine (BAM), which assembles OMPs into the outer membrane of gram-negative microbes, contains two essential proteins, BamA and BamD. Here we identify a genetic background in which BamD is nonessential, indicating that BamD does not function in OMP catalysis, but rather plays a regulatory role in OMP assembly. BamD is not conserved in the complexes that assemble OMPs in chloroplasts and mitochondria, likely because these organelles, unlike bacteria, inhabit a carefully controlled cytoplasmic environment.The outer membrane (OM) of gram-negative bacteria confers innate resistance to toxins and antibiotics. Integral β-barrel outer membrane proteins (OMPs) function to establish and maintain the selective permeability of the OM. OMPs are assembled into the OM by the β-barrel assembly machine (BAM), which is composed of one OMP—BamA—and four lipoproteins—BamB, C, D, and E. BamB, C, and E can be removed individually with only minor effects on barrier function; however, depletion of either BamA or BamD causes a global defect in OMP assembly and results in cell death. We have identified a gain-of-function mutation, bamAE470K, that bypasses the requirement for BamD. Although bamD::kan bamAE470K cells exhibit growth and OM barrier defects, they assemble OMPs with surprising robustness. Our results demonstrate that BamD does not play a catalytic role in OMP assembly, but rather functions to regulate the activity of BamA.
Cao, W. X. ; Kabelitz, S. ; Gupta, M. ; Yeung, E. ; Lin, S. ; Rammelt, C. ; Ihling, C. ; Pekovic, F. ; Low, T. C. H. ; Siddiqui, N. U. ; et al. Precise Temporal Regulation of Post-transcriptional Repressors Is Required for an Orderly Drosophila Maternal-to-Zygotic Transition. Cell Reports 2020. Publisher's VersionAbstract
In animal embryos, the maternal-to-zygotic transition (MZT) hands developmental control from maternal to zygotic gene products. We show that the maternal proteome represents more than half of the protein-coding capacity of Drosophila melanogaster’s genome, and that 2% of this proteome is rapidly degraded during the MZT. Cleared proteins include the post-transcriptional repressors Cup, Trailer hitch (TRAL), Maternal expression at 31B (ME31B), and Smaug (SMG). Although the ubiquitin-proteasome system is necessary for clearance of these repressors, distinct E3 ligase complexes target them: the C-terminal to Lis1 Homology (CTLH) complex targets Cup, TRAL, and ME31B for degradation early in the MZT and the Skp/Cullin/F-box-containing (SCF) complex targets SMG at the end of the MZT. Deleting the C-terminal 233 amino acids of SMG abrogates F-box protein interaction and confers immunity to degradation. Persistent SMG downregulates zygotic re-expression of mRNAs whose maternal contribution is degraded by SMG. Thus, clearance of SMG permits an orderly MZT.
Li, A. ; Mao, D. ; Yoshimura, A. ; Rosen, P. C. ; Martin, W. L. ; Gallant, É. ; Wühr, M. ; Seyedsayamdost, M. R. Multi-Omic Analyses Provide Links between Low-Dose Antibiotic Treatment and Induction of Secondary Metabolism in Burkholderia thailandensis. mBio 2020, 11, e03210-19. Publisher's VersionAbstract
Low doses of antibiotics can trigger secondary metabolite biosynthesis in bacteria, but the underlying mechanisms are generally unknown. We sought to better understand this phenomenon by studying how the antibiotic trimethoprim activates the synthesis of the virulence factor malleilactone in Burkholderia thailandensis. Using transcriptomics, quantitative multiplexed proteomics, and primary metabolomics, we systematically mapped the changes induced by trimethoprim. Surprisingly, even subinhibitory doses of the antibiotic resulted in broad transcriptional and translational alterations, with ∼8.5% of the transcriptome and ∼5% of the proteome up- or downregulated >4-fold. Follow-up studies with genetic-biochemical experiments showed that the induction of malleilactone synthesis can be sufficiently explained by the accumulation of methionine biosynthetic precursors, notably homoserine, as a result of inhibition of the folate pathway. Homoserine activated the malleilactone gene cluster via the transcriptional regulator MalR and gave rise to a secondary metabolome which was very similar to that generated by trimethoprim. Our work highlights the expansive changes that low-dose trimethoprim induces on bacterial physiology and provides insights into its stimulatory effect on secondary metabolism.IMPORTANCE The discovery of antibiotics ranks among the most significant accomplishments of the last century. Although the targets of nearly all clinical antibiotics are known, our understanding regarding their natural functions and the effects of subinhibitory concentrations is in its infancy. Stimulatory rather than inhibitory functions have been attributed to low-dose antibiotics. Among these, we previously found that antibiotics activate silent biosynthetic genes and thereby enhance the metabolic output of bacteria. The regulatory circuits underlying this phenomenon are unknown. We take a first step toward elucidating these circuits and show that low doses of trimethoprim (Tmp) have cell-wide effects on the saprophyte Burkholderia thailandensis. Most importantly, inhibition of one-carbon metabolic processes by Tmp leads to an accumulation of homoserine, which induces the production of an otherwise silent cytotoxin via a LuxR-type transcriptional regulator. These results provide a starting point for uncovering the molecular basis of the hormetic effects of antibiotics.
Yeung, E. ; McFann, S. ; Marsh, L. ; Dufresne, E. ; Filippi, S. ; Harrington, H. A. ; Shvartsman, S. Y. ; Wühr, M. Inference of Multisite Phosphorylation Rate Constants and Their Modulation by Pathogenic Mutations. Current Biology 2020. Publisher's VersionAbstract
Summary Multisite protein phosphorylation plays a critical role in cell regulation [1, 2, 3]. It is widely appreciated that the functional capabilities of multisite phosphorylation depend on the order and kinetics of phosphorylation steps, but kinetic aspects of multisite phosphorylation remain poorly understood [4, 5, 6]. Here, we focus on what appears to be the simplest scenario, when a protein is phosphorylated on only two sites in a strict, well-defined order. This scenario describes the activation of ERK, a highly conserved cell-signaling enzyme. We use Bayesian parameter inference in a structurally identifiable kinetic model to dissect dual phosphorylation of ERK by MEK, a kinase that is mutated in a large number of human diseases [7, 8, 9, 10, 11, 12]. Our results reveal how enzyme processivity and efficiencies of individual phosphorylation steps are altered by pathogenic mutations. The presented approach, which connects specific mutations to kinetic parameters of multisite phosphorylation mechanisms, provides a systematic framework for closing the gap between studies with purified enzymes and their effects in the living organism.
Stadlmeier, M. ; Runtsch, L. S. ; Streshnev, F. ; Wühr, M. ; Carell, T. A Click-Chemistry-Based Enrichable Crosslinker for Structural and Protein Interaction Analysis by Mass Spectrometry. ChemBioChem 2020, 21, 103-107. Publisher's VersionAbstract
Abstract Mass spectrometry is the method of choice for the characterisation of proteomes. Most proteins operate in protein complexes, in which their close association modulates their function. However, with standard MS analysis, information on protein–protein interactions is lost and no structural information is retained. To gain structural and interactome data, new crosslinking reagents are needed that freeze inter- and intramolecular interactions. Herein, the development of a new reagent, which has several features that enable highly sensitive crosslinking MS, is reported. The reagent enables enrichment of crosslinked peptides from the majority of background peptides to facilitate efficient detection of low-abundant crosslinked peptides. Due to the special cleavable properties, the reagent can be used for MS2 and potentially for MS3 experiments. Thus, the new crosslinking reagent, in combination with high-end MS, should enable sensitive analysis of interactomes, which will help researchers to obtain important insights into cellular states in health and diseases.
Cao, W. X. ; Kabelitz, S. ; Gupta, M. ; Yeung, E. ; Lin, S. ; Rammelt, C. ; Ihling, C. ; Pekovic, F. ; Low, T. C. H. ; Siddiqui, N. U. ; et al. Precise temporal regulation of post-transcriptional repressors is required for an orderly Drosophila maternal-to-zygotic transition. bioRxiv 2019, 862490. Publisher's VersionAbstract
In animal embryos the maternal-to-zygotic transition (MZT) hands developmental control from maternal to zygotic gene products. We show that the maternal proteome represents over half of the protein coding capacity of the Drosophila melanogaster genome and that 2% of this proteome is rapidly degraded during the MZT. Cleared proteins include the post-transcriptional repressors Cup, Trailer hitch (TRAL), Maternal expression at 31B (ME31B), and Smaug (SMG). While the ubiquitin-proteasome system is necessary for clearance of all four repressors, distinct E3 ligase complexes target them: the C-terminal to Lis1 Homology (CTLH) complex targets Cup, TRAL and ME31B for degradation early in the MZT; the Skp/Cullin/F-box-containing (SCF) complex targets SMG at the end of the MZT. Deleting the C-terminal 233 amino acids of SMG makes the protein immune to degradation. We show that artificially persistent SMG downregulates the zygotic re-expression of mRNAs whose maternal contribution is cleared by SMG. Thus, clearance of SMG permits an orderly MZT.
Peshkin, L. *; Gupta, M. *; Ryazanova, L. ; Wühr, M. Bayesian Confidence Intervals for Multiplexed Proteomics Integrate Ion-Statistics with Peptide Quantification Concordance. Molecular & Cellular Proteomics 2019. Publisher's VersionAbstract
Multiplexed proteomics has emerged as a powerful tool to measure relative protein expression levels across multiple conditions. The relative protein abundances are inferred by comparing the signals generated by isobaric tags, which encode the samples’ origins. Intuitively, the trust associated with a protein measurement depends on the similarity of ratios from the protein’s peptides and the signal-strength of these measurements. However, typically the average peptide ratio is reported as the estimate of relative protein abundance, which is only the most likely ratio with a very naive model. Moreover, there is no sense on the confidence in these measurements. Here, we present a mathematically rigorous approach that integrates peptide signal strengths and peptidemeasurement agreement into an estimation of the true protein ratio and the associated confidence (BACIQ). The main advantages of BACIQ are: 1) It removes the need to threshold reported peptide signal based on an arbitrary cut-off, thereby reporting more measurements from a given experiment; 2) Confidence can be assigned without replicates; 3) For repeated experiments BACIQ provides confidence intervals for the union, not the intersection, of quantified proteins; 4) For repeated experiments, BACIQ confidence intervals are more predictive than confidence intervals based on protein measurement agreement. To demonstrate the power of BACIQ we reanalyzed previously published data on subcellular protein movement upon treatment with an Exportin-1 inhibiting drug. We detect ~2x more highly significant movers, down to subcellular localization changes of ~1% . Thus, our method drastically increases the value obtainable from quantitative proteomics experiments helping researchers to interpret their data and prioritize resources. To make our approach easily accessible we distribute it via a Python/Stan package
Hart, E. M. ; Gupta, M. ; Wühr, M. ; Silhavy, T. J. The Synthetic Phenotype of ΔbamB ΔbamE Double Mutants Results from a Lethal Jamming of the Bam Complex by the Lipoprotein RcsF. mBio 2019, 10, e00662-19. Publisher's VersionAbstract
The selective permeability of the Gram-negative outer membrane (OM)
is maintained by integral -barrel outer membrane proteins (OMPs). The heteropentomeric
-barrel assembly machine (Bam) folds and inserts OMPs into the OM. Coordination
of the essential proteins BamA and BamD is critical for OMP assembly and
therefore the viability of the cell. The role of the nonessential lipoproteins BamBCE
has yet to be characterized; however, genetic evidence suggests that they have nonoverlapping
roles in OMP assembly. In this work, we quantify changes of the proteome
in the conditional lethal ΔbamB ΔbamE double mutant. We show that cells
lacking BamB and BamE have a global OMP defect that is a result of a lethal obstruction
of an assembly-competent Bam complex by the lipoprotein RcsF. RcsF is a
stress-sensing lipoprotein that is threaded through the lumen of abundant -barrel
OMPs by the Bam complex to expose the amino terminus on the cell surface. We
demonstrate that simply removing this lipoprotein corrects the severe OMP assembly
defect of the double mutant nearly as efficiently as a previously isolated suppressor
mutation in bamA. We propose that BamB and BamE play crucial, nonoverlapping
roles to coordinate the activities of BamA and BamD during OMP
Peshkin, L. ; Lukyanov, A. ; Kalocsay, M. ; Gage, R. M. ; Wang, D. Z. ; Pells, T. J. ; Karimi, K. ; Vize, P. D. ; Wühr, M. ; Kirschner, M. W. The protein repertoire in early vertebrate embryogenesis. bioRxiv 2019. Publisher's VersionAbstract
We present an unprecedentedly comprehensive characterization of protein
dynamics across early development in Xenopus laevis, available immediately via a convenient
Web portal. This resource allows interrogation of the protein expression data in conjunction with
other data modalities such as genome wide mRNA expression. This study provides detailed
data for absolute levels of ~14K unique Xenopus proteins representing homologues of ~9K
unique human genes -- a rich resource for developmental biologists. The purpose of this
manuscript is limited to presenting and releasing the data browser.
Tye, B. W. ; Commins, N. ; Ryazanova, L. V. ; Wühr, M. ; Springer, M. ; Pincus, D. ; Churchman, S. L. Proteotoxicity from aberrant ribosome biogenesis compromises cell fitness. eLife 2019, 8:e43002. Publisher's VersionAbstract
To achieve maximal growth, cells must manage a massive economy of ribosomal proteins (r-proteins) and RNAs (rRNAs) to produce thousands of ribosomes every minute. Although ribosomes are essential in all cells, natural disruptions to ribosome biogenesis lead to heterogeneous phenotypes. Here, we model these perturbations in Saccharomyces cerevisiae and show that challenges to ribosome biogenesis result in acute loss of proteostasis. Imbalances in the synthesis of r-proteins and rRNAs lead to the rapid aggregation of newly synthesized orphan r-proteins and compromise essential cellular processes, which cells alleviate by activating proteostasis genes. Exogenously bolstering the proteostasis network increases cellular fitness in the face of challenges to ribosome assembly, demonstrating the direct contribution of orphan r-proteins to cellular phenotypes. We propose that ribosome assembly is a key vulnerability of proteostasis maintenance in proliferating cells that may be compromised by diverse genetic, environmental, and xenobiotic perturbations that generate orphan r-proteins.
Nguyen, T. ; Mitchison, T. J. ; Wühr, M. Immunofluorescence of Microtubule Assemblies in Amphibian Oocytes and Early Embryos. Methods Mol Biol. 2019, 1920, 17-32. Publisher's VersionAbstract
Amphibian oocytes and embryos are classical models to study cellular and developmental processes. For these studies, it is often advantageous to visualize protein organization. However, the large size and yolk distribution make imaging of deep structures in amphibian zygotes challenging. Here we describe in detail immunofluorescence (IF) protocols for imaging microtubule assemblies in early amphibian development. We developed these protocols to elucidate how the cell division machinery adapts to drastic changes in embryonic cell sizes. We describe how to image mitotic spindles, microtubule asters, chromosomes, and nuclei in whole-mount embryos, even when they are hundreds of micrometers removed from the embryo’s surface. Though the described methods were optimized for microtubule assemblies, they have also proven useful for the visualization of other proteins.
Pappireddi, N. ; Martin, L. ; Wühr, M. A Review on Quantitative Multiplexed Proteomics. ChemBioChem 2019. Publisher's VersionAbstract
Over the last few decades, mass spectrometry‐based proteomics has become an increasingly powerful tool that is now able to routinely detect and quantify thousands of proteins. A major advance for global protein quantification was the introduction of isobaric tags, which in a single experiment enable the global quantification of proteins across multiple samples. In this review, we refer to these methods as multiplexed proteomics. We discuss the principles, advantages, and drawbacks of various multiplexed proteomics techniques, and compare them to alternative approaches. We discuss how the emerging combination of multiplexing with targeted proteomics might enable the reliable and high‐quality quantification of very low‐abundance proteins across multiple conditions. Lastly, we suggest that fusing multiplexed proteomics with data‐independent acquisition approaches might enable the comparison of hundreds of different samples without missing values while maintaining the superb measurement precision and accuracy obtainable with isobaric tag quantification.
Nguyen, T. ; Pappireddi, N. ; Wühr, M. Proteomics of nucleocytoplasmic partitioning. Current Opinion in Chemical Biology 2019, 2019, 55–63. Publisher's VersionAbstract

The partitioning of the proteome between nucleus and cytoplasm affects nearly every aspect of eukaryotic biology. Despite this central role, we still have a poor understanding of which proteins localize in the nucleus and how this varies in different cell types and conditions. Recent advances in quantitative proteomics and high-throughput imaging are starting to close this knowledge gap. Studies on protein interaction are beginning to reveal the spectrum of cargos of nuclear import and export receptors.
We anticipate that it will soon be possible to predict each protein’s nucleocytoplasmic localization based on its importin/exportin interactions and its estimated diffusion rate through the nuclear pore. This insight is likely to provide us with a fundamental understanding of how cells use nucleocytoplasmic partitioning to encode and relay information.

Gupta, M. ; Sonnett, M. ; Ryazanova, L. ; Presler, M. ; Wühr, M. Quantitative Proteomics of Xenopus Embryos I, Sample Preparation. Methods in molecular biology 2018, 175–194. Publisher's VersionAbstract
Xenopus oocytes and embryos are model systems optimally suited for quantitative proteomics. This is due to the availability of large amount of protein material and the ease of physical manipulation. Furthermore, facile in vitro fertilization provides superbly synchronized embryos for cell cycle and developmental stages. Here, we detail protocols developed over the last few years for sample preparation of multiplexed proteomics with TMT-tags followed by quantitative mass spectrometry analysis using the MultiNotch MS3 approach. In this approach, each condition is barcoded with an isobaric tag at the peptide level. After barcoding, samples are combined and the relative abundance of \textasciitilde100,000 peptides is quantified on a mass spectrometer. High reproducibility of the sample preparation process prior to peptides being tagged and combined is of upmost importance for obtaining unbiased data. Otherwise, differences in sample handling can inadvertently appear as biological changes. We detail and exemplify the application of our sample workflow on an embryonic time-series of ten developmental stages of Xenopus laevis embryos ranging from the egg to stage 35 (just before hatching). Our accompanying paper (Chapter 14 ) details a bioinformatics pipeline to analyze the quality of the given sample preparation and strategies to convert spectra of X. laevis peptides into biologically interpretable data.
Sonnett, M. ; Gupta, M. ; Nguyen, T. ; Wühr, M. Quantitative Proteomics for Xenopus Embryos II, Data Analysis. Methods in molecular biology 2018, 1865, 195-215. Publisher's VersionAbstract
The oocytes, embryos, and cell-free lysates of the frog Xenopus laevis have emerged as powerful models for quantitative proteomic experiments. In the accompanying paper (Chapter 13) we describe how to prepare samples and acquire multiplexed proteomics spectra from those. As an illustrative example we use a 10-stage developmental time series from the egg to stage 35 (just before hatching). Here, we outline how to convert the ~700,000 acquired mass spectra from this time series into protein expression dynamics for ~9000 proteins. We first outline a preliminary quality-control analysis to discover any errors that occurred during sample preparation. We discuss how peptide and protein identification error rates are controlled, and how peptide and protein species are quantified. Our analysis relies on the freely available MaxQuant proteomics pipeline. Finally, we demonstrate how to start interpreting this large dataset by clustering and gene-set enrichment analysis.
Sonnett, M. ; Yeung, E. ; Wühr, M. Accurate, Sensitive, and Precise Multiplexed Proteomics Using the Complement Reporter Ion Cluster. Analytical Chemistry 2018. Publisher's VersionAbstract
Quantitative analysis of proteomes across multiple time points, organelles, and
perturbations is essential for understanding both fundamental biology and disease
states. The development of isobaric tags (e.g. TMT) have enabled the simultaneous
measurement of peptide abundances across several different conditions. These
multiplexed approaches are promising in principle because of advantages in throughput
and measurement quality. However, in practice existing multiplexing approaches suffer
from key limitations. In its simple implementation (TMT-MS2), measurements are
distorted by chemical noise leading to poor measurement accuracy. The current state-ofthe-
art (TMT-MS3) addresses this, but requires specialized quadrupole-iontrap-Orbitrap
instrumentation. The complement reporter ion approach (TMTc) produces high accuracy
measurements and is compatible with many more instruments, like quadrupole-
Orbitraps. However, the required deconvolution of the TMTc cluster leads to poor
measurement precision. Here, we introduce TMTc+, which adds the modeling of the MS2-
isolation step into the deconvolution algorithm. The resulting measurements are
comparable in precision to TMT-MS3/MS2. The improved duty cycle, and lower filtering
requirements make TMTc+ more sensitive than TMT-MS3 and comparable with TMT-MS2.
At the same time, unlike TMT-MS2, TMTc+ is exquisitely able to distinguish signal from
chemical noise even outperforming TMT-MS3. Lastly, we compare TMTc+ to quantitative
label-free proteomics of total HeLa lysate and find that TMTc+ quantifies 7.8k versus 3.9k
proteins in a 5-plex sample. At the same time the median coefficient of variation
improves from 13% to 4%. Thus, TMTc+ advances quantitative proteomics by enabling
accurate, sensitive, and precise multiplexed experiments on more commonly used
Stadlmeier, M. ; Bogena, J. ; Wallner, M. ; Wühr, M. ; Carell, T. A sulfoxide-based isobaric labelling reagent for accurate quantitative mass spectrometry. Angewandte Chemie 2018.Abstract
Modern proteomics requires reagents for exact quantification of peptides in complex mixtures. Peptide labelling is most typically achieved with isobaric tags that consist of a balancer and a reporter part that separate in the gas phase. An ingenious distribution of stable isotopes provides multiple reagents with identical molecular weight but a different mass of the reporter groups, allowing relative quantification of multiple samples in one measurement. Current generation reagents require a high fragmentation energy for cleavage, leading to incomplete fragmentation and hence loss of signal intensity. Here we report a new isobaric labelling reagent, where the balancer and the reporter are linked by a sulfoxide group, which, based on the sulfoxide pyrolysis, leads to easy and asymmetric cleavage at low fragmentation energy. The fragmentation of our new design is significantly improved, yielding more intense complementary ion signals, allowing complementary ion cluster analysis as well.
Presler, M. ; Van Itallie, E. ; Klein, A. M. ; Kunz, R. ; Coughlin, M. L. ; Peshkin, L. ; Gygi, S. P. ; Wühr, M. *; Kirschner, M. W. *. Proteomics of phosphorylation and protein dynamics during fertilization and meiotic exit in the Xenopus egg. PNAS 2017.Abstract
Fertilization releases the meiotic arrest and initiates the events that prepare the egg for the ensuing developmental program. Protein degradation and phosphorylation are known to regulate protein activity during this process. However, the full extent of protein loss and phosphoregulation is still unknown. We examined absolute protein and phosphosite dynamics of the fertilization response by mass spectrometry-based proteomics in electroactivated eggs. To do this, we developed an approach for calculating the stoichiometry of phosphosites from multiplexed proteomics that is compatible with dynamic, stable, and multisite phosphorylation. Overall, the data suggest that degradation is limited to a few low-abundance proteins. However, this degradation promotes extensive dephosphorylation that occurs over a wide range of abundances during meiotic exit. We also show that eggs release a large amount of protein into the medium just after fertilization, most likely related to the blocks to polyspermy. Concomitantly, there is a substantial increase in phosphorylation likely tied to calcium-activated kinases. We identify putative degradation targets and components of the slow block to polyspermy. The analytical approaches demonstrated here are broadly applicable to studies of dynamic biological systems.
Sonnett, M. ; Yeung, E. ; Wühr, M. Accurate, Sensitive, and Precise Multiplexed Proteomics using the Complement Reporter Ion Cluster. bioRxiv 2017. Publisher's VersionAbstract

Quantitative analysis of proteomes across multiple time points, organelles, and perturbations is essential for understanding both fundamental biology and disease states. The development of isobaric tags (e.g. TMT) have enabled the simultaneous measurement of peptide abundances across several different conditions. These multiplexed approaches are promising in principle because of advantages in throughput and measurement quality. However, in practice existing multiplexing approaches suffer from key limitations. In its simple implementation (TMT-MS2), measurements are distorted by chemical noise leading to poor measurement accuracy. The current state-ofthe-art (TMT-MS3) addresses this, but requires specialized quadrupole-iontrap-Orbitrap instrumentation. The complement reporter ion approach (TMTc) produces high accuracy measurements and is compatible with many more instruments, like quadrupoleOrbitraps. However, the required deconvolution of the TMTc cluster leads to poor measurement precision. Here, we introduce TMTc+, which adds the modeling of the MS2- isolation step into the deconvolution algorithm. The resulting measurements are comparable in precision to TMT-MS3/MS2. The improved duty cycle, and lower filtering requirements make TMTc+ more sensitive than TMT-MS3 and comparable with TMT-MS2. At the same time, unlike TMT-MS2, TMTc+ is exquisitely able to distinguish signal from chemical noise even outperforming TMT-MS3. Lastly, we compare TMTc+ to quantitative label-free proteomics of total HeLa lysate and find that TMTc+ quantifies 7.8k versus 3.9k proteins in a 5-plex sample. At the same time the median coefficient of variation improves from 13% to 4%. Thus, TMTc+ advances quantitative proteomics by enabling accurate, sensitive, and precise multiplexed experiments on more commonly used instruments.

Presler, M. S. ; Van Itallie, E. ; Klein, A. M. ; Kunz, R. ; Coughlin, P. ; Peshkin, L. ; Gygi, S. ; Wühr, M. *; Kirschner, M. *. Proteomics of phosphorylation and protein dynamics during fertilization and meiotic exit in the Xenopus egg. bioRxiv 2017, 145086.Abstract

Fertilization triggers release from meiotic arrest and initiates events that prepare for the ensuing developmental program. Protein degradation and phosphorylation are known to regulate protein activity during this process. However, the full extent of protein loss and phospho-regulation is still unknown. We examined absolute protein and phospho-site dynamics after fertilization by mass spectrometry-based proteomics. To do this, we developed a new approach for calculating the stoichiometry of phospho-sites from multiplexed proteomics that is compatible with dynamic, stable and multi-site phosphorylation. Overall, the data suggest that degradation is limited to a few low abundance proteins. However, this degradation promotes extensive dephosphorylation that occurs over a wide range of abundances during meiotic exit. We also show that eggs release a large amount of protein into the medium just after fertilization, most likely related to the blocks to polyspermy. Concomitantly, there is a substantial increase in phosphorylation likely tied to calcium activated kinases. We identify putative degradation targets as well as new components of the block to polyspermy. The analytical approaches demonstrated here are broadly applicable to studies of dynamic biological systems.

Remillard, D. ; Buckley, D. L. ; Paulk, J. ; Brien, G. L. ; Sonnett, M. ; Seo, H. S. ; Dastjerdi, S. ; Wühr, M. ; Dhe-Paganon, S. ; Armstrong, S. A. ; et al. Degradation of the BAF Complex Factor BRD9 by Heterobifunctional Ligands. Angewandte Chemie 2017, 56, 5738-5743.Abstract

The bromodomain-containing protein BRD9, a subunit of the human BAF (SWI/SNF) nucleosome remodeling complex, has emerged as an attractive therapeutic target in cancer. Despite the development of chemical probes targeting the BRD9 bromodomain, there is a limited understanding of BRD9 function beyond acetyl-lysine recognition. We have therefore created the first BRD9-directed chemical degraders, through iterative design and testing of heterobifunctional ligands that bridge the BRD9 bromodomain and the cereblon E3 ubiquitin ligase complex. Degraders of BRD9 exhibit markedly enhanced potency compared to parental ligands (10- to 100-fold). Parallel study of degraders with divergent BRD9-binding chemotypes in models of acute myeloid leukemia resolves bromodomain polypharmacology in this emerging drug class. Together, these findings reveal the tractability of non-BET bromodomain containing proteins to chemical degradation, and highlight lead compound dBRD9 as a tool for the study of BRD9.

Tassan, J. P. ; Wühr, M. ; Hatte, G. ; Kubiak, J. Asymmetries in Cell Division, Cell Size, and Furrowing in the Xenopus laevis Embryo. Results and problems in cell differentiation 2017, 61, 243-260.Abstract

Asymmetric cell divisions produce two daughter cells with distinct fate. During embryogenesis, this mechanism is fundamental to build tissues and organs because it generates cell diversity. In adults, it remains crucial to maintain stem cells. The enthusiasm for asymmetric cell division is not only motivated by the beauty of the mechanism and the fundamental questions it raises, but has also very pragmatic reasons. Indeed, misregulation of asymmetric cell divisions is believed to have dramatic consequences potentially leading to pathogenesis such as cancers. In diverse model organisms, asymmetric cell divisions result in two daughter cells, which differ not only by their fate but also in size. This is the case for the early Xenopus laevis embryo, in which the two first embryonic divisions are perpendicular to each other and generate two pairs of blastomeres, which usually differ in size: one pair of blastomeres is smaller than the other. Small blastomeres will produce embryonic dorsal structures, whereas the larger pair will evolve into ventral structures. Here, we present a speculative model on the origin of the asymmetry of this cell division in the Xenopus embryo. We also discuss the apparently coincident asymmetric distribution of cell fate determinants and cell-size asymmetry of the 4-cell stage embryo. Finally, we discuss the asymmetric furrowing during epithelial cell cytokinesis occurring later during Xenopus laevis embryo development.

Erickson, B. K. ; Rose, C. M. ; Braun, C. R. ; Erickson, A. R. ; Knott, J. ; McAlister, G. C. ; Wühr, M. ; Paulo, J. A. ; Everley, R. A. ; Gygi, S. P. A Strategy to Combine Sample Multiplexing with Targeted Proteomics Assays for High-Throughput Protein Signature Characterization. Molecular Cell 2017.Abstract

Targeted mass spectrometry assays for protein quantitation monitor peptide surrogates, which are easily multiplexed to target many peptides in a single assay. However, these assays have generally not taken advantage of sample multiplexing, which allows up to ten analyses to occur in parallel. We present a two-dimensional multiplexing workflow that utilizes synthetic peptides for each protein to prompt the simultaneous quantification of >100 peptides from up to ten mixed sample conditions. We demonstrate that targeted analysis of unfractionated lysates (2 hr) accurately reproduces the quantification of fractionated lysates (72 hr analysis) while obviating the need for peptide detection prior to quantification. We targeted 131 peptides corresponding to 69 proteins across all 60 National Cancer Institute cell lines in biological triplicate, analyzing 180 samples in only 48 hr (the equivalent of 16 min/sample). These data further elucidated a correlation between the expression of key proteins and their cellular response to drug treatment.

Hasley, A. ; Chavez, S. ; Danilchik, M. ; Wühr, M. ; Pelegri, F. Vertebrate Embryonic Cleavage Pattern Determination. Advances in experimental medicine and biology 2017, 953, 117-171.Abstract

The pattern of the earliest cell divisions in a vertebrate embryo lays the groundwork for later developmental events such as gastrulation, organogenesis, and overall body plan establishment. Understanding these early cleavage patterns and the mechanisms that create them is thus crucial for the study of vertebrate development. This chapter describes the early cleavage stages for species representing ray-finned fish, amphibians, birds, reptiles, mammals, and proto-vertebrate ascidians and summarizes current understanding of the mechanisms that govern these patterns. The nearly universal influence of cell shape on orientation and positioning of spindles and cleavage furrows and the mechanisms that mediate this influence are discussed. We discuss in particular models of aster and spindle centering and orientation in large embryonic blastomeres that rely on asymmetric internal pulling forces generated by the cleavage furrow for the previous cell cycle. Also explored are mechanisms that integrate cell division given the limited supply of cellular building blocks in the egg and several-fold changes of cell size during early development, as well as cytoskeletal specializations specific to early blastomeres including processes leading to blastomere cohesion. Finally, we discuss evolutionary conclusions beginning to emerge from the contemporary analysis of the phylogenetic distributions of cleavage patterns. In sum, this chapter seeks to summarize our current understanding of vertebrate early embryonic cleavage patterns and their control and evolution.

Pierre, A. ; Salle, J. ; Wühr, M. ; Minc, N. Generic Theoretical Models to Predict Division Patterns of Cleaving Embryos. Developmental Cell 2016, 39, 667-682.Abstract

Life for all animals starts with a precise 3D choreography of reductive divisions of the fertilized egg, known as cleavage patterns. These patterns exhibit conserved geometrical features and striking interspecies invariance within certain animal classes. To identify the generic rules that may govern these morphogenetic events, we developed a 3D-modeling framework that iteratively infers blastomere division positions and orientations, and consequent multicellular arrangements. From a minimal set of parameters, our model predicts detailed features of cleavage patterns in the embryos of fishes, amphibians, echinoderms, and ascidians, as well as the genetic and physical perturbations that alter these patterns. This framework demonstrates that a geometrical system based on length-dependent microtubule forces that probe blastomere shape and yolk gradients, biased by cortical polarity domains, may dictate division patterns and overall embryo morphogenesis. These studies thus unravel the default self-organization rules governing early embryogenesis and how they are altered by deterministic regulatory layers.

Boke, E. ; Ruer, M. ; Wühr, M. ; Coughlin, M. ; Lemaitre, R. ; Gygi, S. P. ; Alberti, S. ; Drechsel, D. ; Hyman, A. A. ; Mitchison, T. J. Amyloid-like Self-Assembly of a Cellular Compartment. Cell 2016, 166, 637–650.Abstract

Most vertebrate oocytes contain a Balbiani body, a
large, non-membrane-bound compartment packed
with RNA, mitochondria, and other organelles. Little
is known about this compartment, though it specifies
germline identity in many non-mammalian vertebrates.
We show Xvelo, a disordered protein with
an N-terminal prion-like domain, is an abundant constituent
of Xenopus Balbiani bodies. Disruption of the
prion-like domain of Xvelo, or substitution with a
prion-like domain from an unrelated protein, interferes
with its incorporation into Balbiani bodies
in vivo. Recombinant Xvelo forms amyloid-like networks
in vitro. Amyloid-like assemblies of Xvelo recruit
both RNA and mitochondria in binding assays.
We propose that Xenopus Balbiani bodies form by
amyloid-like assembly of Xvelo, accompanied by
co-recruitment of mitochondria and RNA. Prion-like
domains are found in germ plasm organizing proteins
in other species, suggesting that Balbiani
body formation by amyloid-like assembly could be
a conserved mechanism that helps oocytes function
as long-lived germ cells.

Huttlin, E. L. ; Ting, L. ; Bruckner, R. J. ; Gebreab, F. ; Gygi, M. P. ; Szpyt, J. ; Tam, S. ; Zarraga, G. ; Colby, G. ; Baltier, K. ; et al. The BioPlex Network: A Systematic Exploration of the Human Interactome. Cell 2015, 162, 425-40.Abstract

Protein interactions form a network whose structure drives cellular function and whose organization informs biological inquiry. Using high-throughput affinity-purification mass spectrometry, we identify interacting partners for 2,594 human proteins in HEK293T cells. The resulting network (BioPlex) contains 23,744 interactions among 7,668 proteins with 86% previously undocumented. BioPlex accurately depicts known complexes, attaining 80%-100% coverage for most CORUM complexes. The network readily subdivides into communities that correspond to complexes or clusters of functionally related proteins. More generally, network architecture reflects cellular localization, biological process, and molecular function, enabling functional characterization of thousands of proteins. Network structure also reveals associations among thousands of protein domains, suggesting a basis for examining structurally related proteins. Finally, BioPlex, in combination with other approaches, can be used to reveal interactions of biological or clinical significance. For example, mutations in the membrane protein VAPB implicated in familial amyotrophic lateral sclerosis perturb a defined community of interactors.

Braun, C. R. ; Bird, G. H. ; Wühr, M. ; Erickson, B. K. ; Rad, R. ; Walensky, L. D. ; Gygi, S. P. ; Haas, W. Generation of multiple reporter ions from a single isobaric reagent increases multiplexing capacity for quantitative proteomics. Anal Chem 2015, 87, 9855-63.Abstract

Isobaric labeling strategies for mass spectrometry-based proteomics enable multiplexed simultaneous quantification of samples and therefore substantially increase the sample throughput in proteomics. However, despite these benefits, current limits to multiplexing capacity are prohibitive for large sample sizes and impose limitations on experimental design. Here, we introduce a novel mechanism for increasing the multiplexing density of isobaric reagents. We present Combinatorial Isobaric Mass Tags (CMTs), an isobaric labeling architecture with the unique ability to generate multiple series of reporter ions simultaneously. We demonstrate that utilization of multiple reporter ion series improves multiplexing capacity of CMT with respect to a commercially available isobaric labeling reagent with preserved quantitative accuracy and depth of coverage in complex mixtures. We provide a blueprint for the realization of 16-plex reagents with 1 Da spacing between reporter ions and up to 28-plex at 6 mDa spacing using only 5 heavy isotopes per reagent. We anticipate that this improvement in multiplexing capacity will further advance the application of quantitative proteomics, particularly in high-throughput screening assays.

Wühr, M. ; Güttler, T. ; Peshkin, L. ; McAlister, G. C. ; Sonnett, M. ; Ishihara, K. ; Groen, A. C. ; Presler, M. ; Erickson, B. K. ; Mitchison, T. J. ; et al. The Nuclear Proteome of a Vertebrate. Curr Biol 2015, 25, 2663-71.Abstract

The composition of the nucleoplasm determines the behavior of key processes such as transcription, yet there is still no reliable and quantitative resource of nuclear proteins. Furthermore, it is still unclear how the distinct nuclear and cytoplasmic compositions are maintained. To describe the nuclear proteome quantitatively, we isolated the large nuclei of frog oocytes via microdissection and measured the nucleocytoplasmic partitioning of ∼9,000 proteins by mass spectrometry. Most proteins localize entirely to either nucleus or cytoplasm; only ∼17% partition equally. A protein's native size in a complex, but not polypeptide molecular weight, is predictive of localization: partitioned proteins exhibit native sizes larger than ∼100 kDa, whereas natively smaller proteins are equidistributed. To evaluate the role of nuclear export in maintaining localization, we inhibited Exportin 1. This resulted in the expected re-localization of proteins toward the nucleus, but only 3% of the proteome was affected. Thus, complex assembly and passive retention, rather than continuous active transport, is the dominant mechanism for the maintenance of nuclear and cytoplasmic proteomes.

Peshkin, L. *; Wühr, M. *; Pearl, E. ; Haas, W. ; Freeman, R. M. ; Gerhart, J. C. ; Klein, A. M. ; Horb, M. ; Gygi, S. P. ; Kirschner, M. W. On the Relationship of Protein and mRNA Dynamics in Vertebrate Embryonic Development. Dev Cell 2015, 35, 383-94.Abstract

A biochemical explanation of development from the fertilized egg to the adult requires an understanding of the proteins and RNAs expressed over time during embryogenesis. We present a comprehensive characterization of protein and mRNA dynamics across early development in Xenopus. Surprisingly, we find that most protein levels change little and duplicated genes are expressed similarly. While the correlation between protein and mRNA levels is poor, a mass action kinetics model parameterized using protein synthesis and degradation rates regresses protein dynamics to RNA dynamics, corrected for initial protein concentration. This study provides detailed data for absolute levels of ∼10,000 proteins and ∼28,000 transcripts via a convenient web portal, a rich resource for developmental biologists. It underscores the lasting impact of maternal dowry, finds surprisingly few cases where degradation alone drives a change in protein level, and highlights the importance of transcription in shaping the dynamics of the embryonic proteome.

Mitchison, T. J. ; Ishihara, K. ; Nguyen, P. ; Wühr, M. Size Scaling of Microtubule Assemblies in Early Xenopus Embryos. Cold Spring Harb Perspect Biol 2015, 7 a019182.Abstract

The first 12 cleavage divisions in Xenopus embryos provide a natural experiment in size scaling, as cell radius decreases ∼16-fold with little change in biochemistry. Analyzing both natural cleavage and egg extract partitioned into droplets revealed that mitotic spindle size scales with cell size, with an upper limit in very large cells. We discuss spindle-size scaling in the small- and large-cell regimes with a focus on the "limiting-component" hypotheses. Zygotes and early blastomeres show a scaling mismatch between spindle and cell size. This problem is solved, we argue, by interphase asters that act to position the spindle and transport chromosomes to the center of daughter cells. These tasks are executed by the spindle in smaller cells. We end by discussing possible mechanisms that limit mitotic aster size and promote interphase aster growth to cell-spanning dimensions.

Wühr, M. *; Freeman, R. M. *; Presler, M. ; Horb, M. E. ; Peshkin, L. ; Gygi, S. P. ; Kirschner, M. W. Deep proteomics of the Xenopus laevis egg using an mRNA-derived reference database. Curr Biol 2014, 24, 1467-75.Abstract

BACKGROUND: Mass spectrometry-based proteomics enables the global identification and quantification of proteins and their posttranslational modifications in complex biological samples. However, proteomic analysis requires a complete and accurate reference set of proteins and is therefore largely restricted to model organisms with sequenced genomes. RESULTS: Here, we demonstrate the feasibility of deep genome-free proteomics by using a reference proteome derived from heterogeneous mRNA data. We identify more than 11,000 proteins with 99% confidence from the unfertilized Xenopus laevis egg and estimate protein abundance with approximately 2-fold precision. Our reference database outperforms the provisional gene models based on genomic DNA sequencing and references generated by other methods. Surprisingly, we find that many proteins in the egg lack mRNA support and that many of these proteins are found in blood or liver, suggesting that they are taken up from the blood plasma, together with yolk, during oocyte growth and maturation, potentially contributing to early embryogenesis. CONCLUSION: To facilitate proteomics in nonmodel organisms, we make our platform available as an online resource that converts heterogeneous mRNA data into a protein reference set. Thus, we demonstrate the feasibility and power of genome-free proteomics while shedding new light on embryogenesis in vertebrates.

McAlister, G. C. ; Nusinow, D. P. ; Jedrychowski, M. P. ; Wühr, M. ; Huttlin, E. L. ; Erickson, B. K. ; Rad, R. ; Haas, W. ; Gygi, S. P. MultiNotch MS3 enables accurate, sensitive, and multiplexed detection of differential expression across cancer cell line proteomes. Anal Chem 2014, 86, 7150-8.Abstract

Multiplexed quantitation via isobaric chemical tags (e.g., tandem mass tags (TMT) and isobaric tags for relative and absolute quantitation (iTRAQ)) has the potential to revolutionize quantitative proteomics. However, until recently the utility of these tags was questionable due to reporter ion ratio distortion resulting from fragmentation of coisolated interfering species. These interfering signals can be negated through additional gas-phase manipulations (e.g., MS/MS/MS (MS3) and proton-transfer reactions (PTR)). These methods, however, have a significant sensitivity penalty. Using isolation waveforms with multiple frequency notches (i.e., synchronous precursor selection, SPS), we coisolated and cofragmented multiple MS2 fragment ions, thereby increasing the number of reporter ions in the MS3 spectrum 10-fold over the standard MS3 method (i.e., MultiNotch MS3). By increasing the reporter ion signals, this method improves the dynamic range of reporter ion quantitation, reduces reporter ion signal variance, and ultimately produces more high-quality quantitative measurements. To demonstrate utility, we analyzed biological triplicates of eight colon cancer cell lines using the MultiNotch MS3 method. Across all the replicates we quantified 8,378 proteins in union and 6,168 proteins in common. Taking into account that each of these quantified proteins contains eight distinct cell-line measurements, this data set encompasses 174,704 quantitative ratios each measured in triplicate across the biological replicates. Herein, we demonstrate that the MultiNotch MS3 method uniquely combines multiplexing capacity with quantitative sensitivity and accuracy, drastically increasing the informational value obtainable from proteomic experiments.

Ishihara, K. ; Nguyen, P. A. ; Wühr, M. ; Groen, A. C. ; Field, C. M. ; Mitchison, T. J. Organization of early frog embryos by chemical waves emanating from centrosomes. Philos Trans R Soc Lond B Biol Sci 2014, 369.Abstract

The large cells in early vertebrate development face an extreme physical challenge in organizing their cytoplasm. For example, amphibian embryos have to divide cytoplasm that spans hundreds of micrometres every 30 min according to a precise geometry, a remarkable accomplishment given the extreme difference between molecular and cellular scales in this system. How do the biochemical reactions occurring at the molecular scale lead to this emergent behaviour of the cell as a whole? Based on recent findings, we propose that the centrosome plays a crucial role by initiating two autocatalytic reactions that travel across the large cytoplasm as chemical waves. Waves of mitotic entry and exit propagate out from centrosomes using the Cdk1 oscillator to coordinate the timing of cell division. Waves of microtubule-stimulated microtubule nucleation propagate out to assemble large asters that position spindles for the following mitosis and establish cleavage plane geometry. By initiating these chemical waves, the centrosome rapidly organizes the large cytoplasm during the short embryonic cell cycle, which would be impossible using more conventional mechanisms such as diffusion or nucleation by structural templating. Large embryo cells provide valuable insights to how cells control chemical waves, which may be a general principle for cytoplasmic organization.

Nguyen, P. A. ; Groen, A. C. ; Loose, M. ; Ishihara, K. ; Wühr, M. ; Field, C. M. ; Mitchison, T. J. Spatial organization of cytokinesis signaling reconstituted in a cell-free system. Science 2014, 346, 244-7.Abstract

During animal cell division, the cleavage furrow is positioned by microtubules that signal to the actin cortex at the cell midplane. We developed a cell-free system to recapitulate cytokinesis signaling using cytoplasmic extract from Xenopus eggs. Microtubules grew out as asters from artificial centrosomes and met to organize antiparallel overlap zones. These zones blocked the interpenetration of neighboring asters and recruited cytokinesis midzone proteins, including the chromosomal passenger complex (CPC) and centralspindlin. The CPC was transported to overlap zones, which required two motor proteins, Kif4A and a Kif20A paralog. Using supported lipid bilayers to mimic the plasma membrane, we observed the recruitment of cleavage furrow markers, including an active RhoA reporter, at microtubule overlaps. This system opens further approaches to understanding the biophysics of cytokinesis signaling.

Wühr, M. ; Haas, W. ; McAlister, G. C. ; Peshkin, L. ; Rad, R. ; Kirschner, M. W. ; Gygi, S. P. Accurate multiplexed proteomics at the MS2 level using the complement reporter ion cluster. Anal Chem 2012, 84, 9214-21.Abstract

Isobaric labeling strategies, such as isobaric tags for relative and absolute quantitation (iTRAQ) or tandem mass tags (TMT), have promised to dramatically increase the power of quantitative proteomics. However, when applied to complex mixtures, both the accuracy and precision are undermined by interfering peptide ions that coisolate and cofragment with the target peptide. Additional gas-phase isolation steps, such as proton-transfer ion-ion reactions (PTR) or higher-order MS3 scans, can almost completely eliminate this problem. Unfortunately, these methods come at the expense of decreased acquisition speed and sensitivity. Here we present a method that allows accurate quantification of TMT-labeled peptides at the MS2 level without additional ion purification. Quantification is based on the fragment ion cluster that carries most of the TMT mass balance. In contrast to the use of low m/z reporter ions, the localization of these complement TMT (TMT(C)) ions in the spectrum is precursor-specific; coeluting peptides do not generally affect the measurement of the TMT(C) ion cluster of interest. Unlike the PTR or MS3 strategies, this method can be implemented on a wide range of high-resolution mass spectrometers like the quadrupole Orbitrap instruments (QExactive). A current limitation of the method is that the efficiency of TMT(C) ion formation is affected by both peptide sequence and peptide ion charge state; we discuss potential routes to overcome this problem. Finally, we show that the complement reporter ion approach allows parallelization of multiplexed quantification and therefore holds the potential to multiply the number of distinct peptides that can be quantified in a given time frame.

Mitchison, T. ; Wühr, M. ; Nguyen, P. ; Ishihara, K. ; Groen, A. ; Field, C. M. Growth, interaction, and positioning of microtubule asters in extremely large vertebrate embryo cells. Cytoskeleton (Hoboken) 2012, 69, 738-50.Abstract

Ray Rappaport spent many years studying microtubule asters, and how they induce cleavage furrows. Here, we review recent progress on aster structure and dynamics in zygotes and early blastomeres of Xenopus laevis and Zebrafish, where cells are extremely large. Mitotic and interphase asters differ markedly in size, and only interphase asters span the cell. Growth of interphase asters occurs by a mechanism that allows microtubule density at the aster periphery to remain approximately constant as radius increases. We discuss models for aster growth, and favor a branching nucleation process. Neighboring asters that grow into each other interact to block further growth at the shared boundary. We compare the morphology of interaction zones formed between pairs of asters that grow out from the poles of the same mitotic spindle (sister asters) and between pairs not related by mitosis (non-sister asters) that meet following polyspermic fertilization. We argue growing asters recognize each other by interaction between antiparallel microtubules at the mutual boundary, and discuss models for molecular organization of interaction zones. Finally, we discuss models for how asters, and the centrosomes within them, are positioned by dynein-mediated pulling forces so as to generate stereotyped cleavage patterns. Studying these problems in extremely large cells is starting to reveal how general principles of cell organization scale with cell size.

Nguyen, P. A. ; Ishihara, K. ; Wühr, M. ; Mitchison, T. J. Pronuclear migration: no attachment? No union, but a futile cycle!. Curr Biol 2012, 22, R409-11.Abstract

How do pronuclei migrate towards each other? The zebrafish futile cycle gene is shown to encode a maternally expressed membrane protein required for nuclear attachment and migration along the sperm aster.

Field, C. M. ; Wühr, M. ; Anderson, G. A. ; Kueh, H. Y. ; Strickland, D. ; Mitchison, T. J. Actin behavior in bulk cytoplasm is cell cycle regulated in early vertebrate embryos. J Cell Sci 2011, 124, 2086-95.Abstract

The mechanical properties of cells change as they proceed through the cell cycle, primarily owing to regulation of actin and myosin II. Most models for cell mechanics focus on actomyosin in the cortex and ignore possible roles in bulk cytoplasm. We explored cell cycle regulation of bulk cytoplasmic actomyosin in Xenopus egg extracts, which is almost undiluted cytoplasm from unfertilized eggs. We observed dramatic gelation-contraction of actomyosin in mitotic (M phase) extract where Cdk1 activity is high, but not in interphase (I-phase) extract. In spread droplets, M-phase extract exhibited regular, periodic pulses of gelation-contraction a few minutes apart that continued for many minutes. Comparing actin nucleation, disassembly and myosin II activity between M-phase and I-phase extracts, we conclude that regulation of nucleation is likely to be the most important for cell cycle regulation. We then imaged F-actin in early zebrafish blastomeres using a GFP-Utrophin probe. Polymerization in bulk cytoplasm around vesicles increased dramatically during mitosis, consistent with enhanced nucleation. We conclude that F-actin polymerization in bulk cytoplasm is cell cycle regulated in early vertebrate embryos and discuss possible biological functions of this regulation.

Wühr, M. ; Obholzer, N. D. ; Megason, S. G. ; Detrich, H. W. ; Mitchison, T. J. Live imaging of the cytoskeleton in early cleavage-stage zebrafish embryos. Methods Cell Biol 2011, 101, 1-18.Abstract

The large and transparent cells of cleavage-stage zebrafish embryos provide unique opportunities to study cell division and cytoskeletal dynamics in very large animal cells. Here, we summarize recent progress, from our laboratories and others, on live imaging of the microtubule and actin cytoskeletons during zebrafish embryonic cleavage. First, we present simple protocols for extending the breeding competence of zebrafish mating ensembles throughout the day, which ensures a steady supply of embryos in early cleavage, and for mounting these embryos for imaging. Second, we describe a transgenic zebrafish line [Tg(bactin2:HsENSCONSIN17-282-3xEGFP)hm1] that expresses the green fluorescent protein (GFP)-labeled microtubule-binding part of ensconsin (EMTB-3GFP). We demonstrate that the microtubule-based structures of the early cell cycles can be imaged live, with single microtubule resolution and with high contrast, in this line. Microtubules are much more easily visualized using this tagged binding protein rather than directly labeled tubulin (injected Alexa-647-labeled tubulin), presumably due to lower background from probe molecules not attached to microtubules. Third, we illustrate live imaging of the actin cytoskeleton by injection of the actin-binding fragment of utrophin fused to GFP. Fourth, we compare epifluorescence-, spinning-disc-, laser-scanning-, and two-photon-microscopic modalities for live imaging of the microtubule cytoskeleton in early embryos of our EMTB-3GFP-expressing transgenic line. Finally, we discuss future applications and extensions of our methods.

Wühr, M. Spatial Organization of Large Cells, Harvard University: Cambridge, 2010.Abstract

The rationale for my research was to investigate unusually large cells, fertilized frog and fish eggs, to obtain a unique perspective on a cell’s spatial organization with a focus on cell division.
First, we investigated how spindle size changes during cleavage stages while cell size changes by orders of magnitude. To do so we improved techniques for immunofluorescence in amphibian embryos and generated a transgenic fish line with fluorescently labeled microtubules. We show that in smaller cells spindle length scales with cell length, but in very large cells spindle
length approaches an upper limit and seems uncoupled from cell size. Furthermore, we were able to assemble mitotic spindles in embryonic extract that had similar size as in vivo spindles. This indicates that spindle size is set by a mechanism that is intrinsic to the spindle and not downstream of cell size.
Second we investigated how relatively small spindles in large cells are positioned, and oriented for symmetric cell division. We show that the localization and orientation of these spindles are determined by location and orientation of sister centrosomes set by the anaphase-telophase aster of the previous cycle.
Third we researched the mechanism by which asters center and align centrosomes relative to the longest axis. With a novel light-activated drug we could asymmetrically perturb asters. The asters moved away from the site of perturbation indicating that asters are positioned with pulling and not pushing forces. We show that the pulling forces are dynein dependent and act before astral microtubules contact the cortex they are moving toward, so dynein must pull on bulk cytoplasm, not the cell cortex as usually proposed. The cortex acts to limit microtubule length, which indirectly controls the strength of dyneindependent pulling forces, and thus centers the aster in the cell. Where two sister asters overlap a microtubules sparse region we call “exclusion zone” emerges.
This zone limits microtubule length, and thus promotes outward movement of sister asters
We present a model in which dynein pulling on bulk cytoplasm, operating in conjunction with microtubule-length limiting mechanisms, centers nascent spindles, and orient them along the long axis of the cell. This model explains cleavage plane geometry in early vertebrate embryos, and may apply more generally.

Wühr, M. ; Tan, E. S. ; Parker, S. K. ; Detrich, W. H. ; Mitchison, T. J. A model for cleavage plane determination in early amphibian and fish embryos. Curr Biol 2010, 20, 2040-5.Abstract

Current models for cleavage plane determination propose that metaphase spindles are positioned and oriented by interactions of their astral microtubules with the cellular cortex, followed by cleavage in the plane of the metaphase plate [1, 2]. We show that in early frog and fish embryos, where cells are unusually large, astral microtubules in metaphase are too short to position and orient the spindle. Rather, the preceding interphase aster centers and orients a pair of centrosomes prior to nuclear envelope breakdown, and the spindle assembles between these prepositioned centrosomes. Interphase asters center and orient centrosomes with dynein-mediated pulling forces. These forces act before astral microtubules contact the cortex; thus, dynein must pull from sites in the cytoplasm, not the cell cortex as is usually proposed for smaller cells. Aster shape is determined by interactions of the expanding periphery with the cell cortex or with an interaction zone that forms between sister-asters in telophase. We propose a model to explain cleavage plane geometry in which the length of astral microtubules is limited by interaction with these boundaries, causing length asymmetries. Dynein anchored in the cytoplasm then generates length-dependent pulling forces, which move and orient centrosomes.

Wühr, M. ; Dumont, S. ; Groen, A. C. ; Needleman, D. J. ; Mitchison, T. J. How does a millimeter-sized cell find its center?. Cell Cycle 2009, 8 1115-21.Abstract

Microtubules play a central role in centering the nucleus or mitotic spindle in eukaryotic cells. However, despite common use of microtubules for centering, physical mechanisms can vary greatly, and depend on cell size and cell type. In the small fission yeast cells, the nucleus can be centered by pushing forces that are generated when growing microtubules hit the cell boundary. This mechanism may not be possible in larger cells, because the compressive force that microtubules can sustain are limited by buckling, so maximal force decreases with microtubule length. In a well-studied intermediate sized cell, the C. elegans fertilized egg, centrosomes are centered by cortex-attached motors that pull on microtubules. This mechanism is widely assumed to be general for larger cells. However, re-evaluation of classic experiments in a very large cell, the fertilized amphibian egg, argues against such generality. In these large eggs, movement of asters away from a part of the cell boundary that they are touching cannot be mediated by cortical pulling, because the astral microtubules are too short to reach the opposite cell boundary. Additionally, Herlant and Brachet discovered a century ago that multiple asters within a single egg center relative to the cell boundary, but also relative to each other. Here, we summarize current understanding of microtubule organization during the first cell cycle in a fertilized Xenopus egg, discuss how microtubule asters move towards the center of this very large cell, and how multiple asters shape and position themselves relative to each other.

Wühr, M. ; Mitchison, T. J. ; Field, C. M. Size and speed go hand in hand in cytokinesis. Cell 2009, 137, 798-800.Abstract

In animal cells, cytokinesis is mediated by the constriction of a cortical ring. In this issue, Carvalho et al. (2009) show in embryos of the worm Caenorhabditis elegans that the rate of ring constriction during cytokinesis is proportional to the initial cell perimeter, ensuring that the duration of cytokinesis is cell-size independent.

Wühr, M. ; Chen, Y. ; Dumont, S. ; Groen, A. C. ; Needleman, D. J. ; Salic, A. ; Mitchison, T. J. Evidence for an upper limit to mitotic spindle length. Curr Biol 2008, 18, 1256-61.Abstract

Size specification of macromolecular assemblies in the cytoplasm is poorly understood [1]. In principle, assemblies could scale with cell size or use intrinsic mechanisms. For the mitotic spindle, scaling with cell size is expected, because the function of this assembly is to physically move sister chromatids into the center of nascent daughter cells. Eggs of Xenopus laevis are among the largest cells known that cleave completely during cell division. Cell length in this organism changes by two orders of magnitude ( approximately 1200 microm to approximately 12 microm) while it develops from a fertilized egg into a tadpole [2]. We wondered whether, and how, mitotic spindle length and morphology adapt to function at these different length scales. Here, we show that spindle length increases with cell length in small cells, but in very large cells spindle length approaches an upper limit of approximately 60 microm. Further evidence for an upper limit to spindle length comes from an embryonic extract system that recapitulates mitotic spindle assembly in a test tube. We conclude that early mitotic spindle length in Xenopus laevis is uncoupled from cell length, reaching an upper bound determined by mechanisms that are intrinsic to the spindle.

Wühr, M. ; Mitchison, T. J. ; Field, C. M. Mitosis: new roles for myosin-X and actin at the spindle. Curr Biol 2008, 18, R912-4.Abstract

Roles for actin and myosin in positioning mitotic spindles in the cell are well established. A recent study of myosin-X function in early Xenopus embryo mitosis now reports that this unconventional myosin is required for pole integrity and normal spindle length by localizing to poles and exerting pulling forces on actin filaments within the spindle.

Gorr, I. H. ; Reis, A. ; Boos, D. ; Wühr, M. ; Madgwick, S. ; Jones, K. T. ; Stemmann, O. Essential CDK1-inhibitory role for separase during meiosis I in vertebrate oocytes. Nat Cell Biol 2006, 8 1035-7.Abstract

Separase not only triggers anaphase of meiosis I by proteolytic cleavage of cohesin on chromosome arms, but in vitro vertebrate separase also acts as a direct inhibitor of cyclin-dependent kinase 1 (Cdk1) on liberation from the inhibitory protein, securin. Blocking separase-Cdk1 complex formation by microinjection of anti-separase antibodies prevents polar-body extrusion in vertebrate oocytes. Importantly, proper meiotic maturation is rescued by chemical inhibition of Cdk1 or expression of Cdk1-binding separase fragments lacking cohesin-cleaving activity.

Franzmann, T. M. ; Wühr, M. ; Richter, K. ; Walter, S. ; Buchner, J. The activation mechanism of Hsp26 does not require dissociation of the oligomer. J Mol Biol 2005, 350, 1083-93.Abstract

Small heat shock proteins (sHsps) are molecular chaperones that specifically bind non-native proteins and prevent them from irreversible aggregation. A key trait of sHsps is their existence as dynamic oligomers. Hsp26 from Saccharomyces cerevisiae assembles into a 24mer, which becomes activated under heat shock conditions and forms large, stable substrate complexes. This activation coincides with the destabilization of the oligomer and the appearance of dimers. This and results from other groups led to the generally accepted notion that dissociation might be a requirement for the chaperone mechanism of sHsps. To understand the chaperone mechanism of sHsps it is crucial to analyze the relationship between chaperone activity and stability of the oligomer. We generated an Hsp26 variant, in which a serine residue of the N-terminal domain was replaced by cysteine. This allowed us to covalently crosslink neighboring subunits by disulfide bonds. We show that under reducing conditions the structure and function of this variant are indistinguishable from that of the wild-type protein. However, when the cysteine residues are oxidized, the dissociation into dimers at higher temperatures is no longer observed, yet the chaperone activity remains unaffected. Furthermore, we show that the exchange of subunits between Hsp26 oligomers is significantly slower than substrate aggregation and even inhibited in the presence of disulfide bonds. This demonstrates that the rearrangements necessary for shifting Hsp26 from a low to a high affinity state for binding non-native proteins occur without dissolving the oligomer.