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Data independent acquisition (DIA) mass spectrometry is a powerful technique that is improving the reproducibility and throughput of proteomics studies. Here, we introduce an experimental workflow that uses this technique to construct chromatogram libraries that capture fragment ion chromatographic peak shape and retention time for every detectable peptide in a proteomics experiment. These coordinates calibrate protein databases or spectrum libraries to a specific mass spectrometer and chromatography setup, facilitating DIA-only pipelines and the reuse of global resource libraries. We also present EncyclopeDIA, a software tool for generating and searching chromatogram libraries, and demonstrate the performance of our workflow by quantifying proteins in human and yeast cells. We find that by exploiting calibrated retention time and fragmentation specificity in chromatogram libraries, EncyclopeDIA can detect 20–25% more peptides from DIA experiments than with data dependent acquisition-based spectrum libraries alone. Data-independent acquisition (DIA)-based proteomics often relies on mass spectrum libraries from data-dependent acquisition experiments. Here, the authors present a method to generate DIA-based chromatogram libraries, enabling DIA-only workflows and detecting more peptides than with spectrum libraries alone.

Over the last three decades, human genetics has gone from dissecting high-penetrance Mendelian diseases to discovering the vast and complex genetic etiology of common human diseases. In tackling this complexity, scientists have discovered the importance of numerous genetic processes – most notably functional regulatory elements – in the development and progression of these diseases. Simultaneously, scientists have increasingly used multiplex assays of variant effect to systematically phenotype the cellular consequences of millions of genetic variants. In this article, we argue that the context of genetic variants – at all scales, from other genetic variants and gene regulation to cell biology to organismal environment – are critical components of how we can employ genomics to interpret these variants, and ultimately treat these diseases. We describe approaches to extend existing experimental assays and computational approaches to examine and quantify the importance of this context, including through causal analytic approaches. Having a unified understanding of the molecular, physiological, and environmental processes governing the interpretation of genetic variants is sorely needed for the field, and this perspective argues for feasible approaches by which the combined interpretation of cellular, animal, and epidemiological data can yield that knowledge.

Proteins can be phosphorylated at neighboring sites resulting in different functional states, and studying the regulation of these sites has been challenging. Here we present Thesaurus, a search engine that detects and quantifies phosphopeptide positional isomers from parallel reaction monitoring and data-independent acquisition mass spectrometry experiments. We apply Thesaurus to analyze phosphorylation events in the PI3K/AKT signaling pathway and show neighboring sites with distinct regulation.

Recent developments in proteomics have enabled signaling studies where > 10,000 phosphosites can be routinely identified and quantified. Yet, current analyses are limited in throughput, reproducibility, and robustness, hampering experiments that involve multiple perturbations, such as those needed to map kinase–substrate relationships, capture pathway crosstalks, and network inference analysis. To address these challenges, we introduce rapid‐robotic phosphoproteomics (R2‐P2), an end‐to‐end automated method that uses magnetic particles to process protein extracts to deliver mass spectrometry‐ready phosphopeptides. R2‐P2 is rapid, robust, versatile, and high‐throughput. To showcase the method, we applied it, in combination with data‐independent acquisition mass spectrometry, to study signaling dynamics in the mitogen‐activated protein kinase (MAPK) pathway in yeast. Our results reveal broad and specific signaling events along the mating, the high‐osmolarity glycerol, and the invasive growth branches of the MAPK pathway, with robust phosphorylation of downstream regulatory proteins and transcription factors. Our method facilitates large‐scale signaling studies involving hundreds of perturbations opening the door to systems‐level studies aiming to capture signaling complexity. Synopsis The study presents R2‐P2, an automated end‐to‐end phosphoproteomic sample preparation method. Application of R2‐P2 to study phosphorylation temporal dynamics in yeast stimulated to perturb MAPK signaling reveals treatment‐specific responses and pathway crosstalks. R2‐P2 is a novel workflow for proteomic and phosphoproteomic sample preparation that compares favorably to common methods. R2‐P2 is automated, high‐throughput, rapid, robust, reproducible and yields high phosphopeptide enrichment efficiency. R2‐P2 facilitates large‐scale cell signaling studies. R2‐P2 in combination with DIA‐MS reveals broad and specific signaling events along the different branches of the MAPK pathway, with robust phosphorylation of downstream regulatory proteins and transcription factors. Graphical Abstract The study presents R2‐P2, an automated end‐to‐end phosphoproteomic sample preparation method. Application of R2‐P2 to study phosphorylation temporal dynamics in yeast stimulated to perturb MAPK signaling reveals treatment‐specific responses and pathway crosstalks.

The success in profiling the phosphoproteome by mass spectrometry-based proteomics has been intimately related to the availability of methods that selectively enrich for phosphopeptides. To this end, we describe a protocol that combines two sequential enrichment steps. First, strong cation exchange (SCX) chromatography separates peptides by solution charge. Phosphate groups contribute to solution charge by adding a negative charge at pH 2.7. Therefore, at that pH, phosphopeptides are expected to elute earlier than their nonphosphorylated homologs. Second, immobilized metal affinity chromatography (IMAC) takes advantage of phosphate's affinity for metal ions such as Fe 3+ to uniformly enrich for phosphopeptides from the previously collected SCX fractions. We have successfully employed the SCX/IMAC enrichment strategy in the exploration of phosphoproteomes from several systems including mouse liver and Drosophila embryos characterizing over 5,500 and 13,000 phosphorylation events, respectively. The SCX/IMAC enrichment protocol requires 2 days, and the entire procedure from cells to a phosphorylation data set can be completed in less than 10 days.

Two methods for identifying protein isoforms that are concurrently phosphorylated and ubiquitylated are applied in yeast to identify phosphorylation sites that regulate ubiquitin proteasome–mediated proteome degradation. Cross-talk between different types of post-translational modifications on the same protein molecule adds specificity and combinatorial logic to signal processing, but it has not been characterized on a large-scale basis. We developed two methods to identify protein isoforms that are both phosphorylated and ubiquitylated in the yeast Saccharomyces cerevisiae , identifying 466 proteins with 2,100 phosphorylation sites co-occurring with 2,189 ubiquitylation sites. We applied these methods quantitatively to identify phosphorylation sites that regulate protein degradation via the ubiquitin-proteasome system. Our results demonstrate that distinct phosphorylation sites are often used in conjunction with ubiquitylation and that these sites are more highly conserved than the entire set of phosphorylation sites. Finally, we investigated how the phosphorylation machinery can be regulated by ubiquitylation. We found evidence for novel regulatory mechanisms of kinases and 14-3-3 scaffold proteins via proteasome-independent ubiquitylation.

MicroRNAs are endogenous ∼23-nucleotide RNAs that can pair to sites in the messenger RNAs of protein-coding genes to downregulate the expression from these messages. MicroRNAs are known to influence the evolution and stability of many mRNAs, but their global impact on protein output had not been examined. Here we use quantitative mass spectrometry to measure the response of thousands of proteins after introducing microRNAs into cultured cells and after deleting mir-223 in mouse neutrophils. The identities of the responsive proteins indicate that targeting is primarily through seed-matched sites located within favourable predicted contexts in 3′ untranslated regions. Hundreds of genes were directly repressed, albeit each to a modest degree, by individual microRNAs. Although some targets were repressed without detectable changes in mRNA levels, those translationally repressed by more than a third also displayed detectable mRNA destabilization, and, for the more highly repressed targets, mRNA destabilization usually comprised the major component of repression. The impact of microRNAs on the proteome indicated that for most interactions microRNAs act as rheostats to make fine-scale adjustments to protein output. MiRNA tunes proteins MicroRNAs can regulate gene expression by either inhibiting translation of a messenger RNA, or inducing its degradation. While previous studies have measured regulation at the mRNA level, it was unknown how much regulation occurred at the protein level. Now two groups led by David Bartel and Nikolaus Rajewsky have used variants of the technique known as SILAC (stable isotope labelling with amino acids in cell culture) to measure proteome-wide changes in protein level as a function of expression of endogenous and exogenous microRNAs. They find that while microRNAs can directly repress the translation of hundreds of genes, additional indirect effects result in changes in expression of thousands of genes. Many of the changes observed are less than twofold in magnitude, however, indicating either directly or indirectly, microRNAs can act as rheostats to fine-tune protein synthesis to match the needs of the cell at any given time. In one of two studies, a technique known as SILAC is used to measure, on a large scale, changes in protein level as a function of expression of endogenous and exogenous miRNAs. It is found that although miRNAs directly repress the translation of hundreds of genes, additional indirect effects result in changes in expression of thousands of genes.

A method (and resource) demonstrating the mining of information from large-scale phosphoproteomics data sets is presented, allowing users to build targeted parallel reaction monitoring mass spectrometry assays to study phosphosites of interest. Systematic approaches to studying cellular signaling require phosphoproteomic techniques that reproducibly measure the same phosphopeptides across multiple replicates, conditions, and time points. Here we present a method to mine information from large-scale, heterogeneous phosphoproteomics data sets to rapidly generate robust targeted mass spectrometry (MS) assays. We demonstrate the performance of our method by interrogating the IGF-1/AKT signaling pathway, showing that even rarely observed phosphorylation events can be consistently detected and precisely quantified.

The eukaryotic cell division cycle is characterized by a sequence of orderly and highly regulated events resulting in the duplication and separation of all cellular material into two newly formed daughter cells. Protein phosphorylation by cyclin-dependent kinases (CDKs) drives this cycle. To gain further insight into how phosphorylation regulates the cell cycle, we sought to identify proteins whose phosphorylation is cell cycle regulated. Using stable isotope labeling along with a two-step strategy for phosphopeptide enrichment and high mass accuracy mass spectrometry, we examined protein phosphorylation in a human cell line arrested in the G₁ and mitotic phases of the cell cycle. We report the identification of >14,000 different phosphorylation events, more than half of which, to our knowledge, have not been described in the literature, along with relative quantitative data for the majority of these sites. We observed >1,000 proteins with increased phosphorylation in mitosis including many known cell cycle regulators. The majority of sites on regulated phosphopeptides lie in [S/T]P motifs, the minimum required sequence for CDKs, suggesting that many of the proteins may be CDK substrates. Analysis of non-proline site-containing phosphopeptides identified two unique motifs that suggest there are at least two undiscovered mitotic kinases.

Data analysis and interpretation remain major logistical challenges when attempting to identify large numbers of protein phosphorylation sites by nanoscale reverse-phase liquid chromatography/tandem mass spectrometry (LC-MS/MS) ( Supplementary Figure 1 online). In this report we address challenges that are often only addressable by laborious manual validation, including data set error, data set sensitivity and phosphorylation site localization. We provide a large-scale phosphorylation data set with a measured error rate as determined by the target-decoy approach, we demonstrate an approach to maximize data set sensitivity by efficiently distracting incorrect peptide spectral matches (PSMs), and we present a probability-based score, the Ascore, that measures the probability of correct phosphorylation site localization based on the presence and intensity of site-determining ions in MS/MS spectra. We applied our methods in a fully automated fashion to nocodazole-arrested HeLa cell lysate where we identified 1,761 nonredundant phosphorylation sites from 491 proteins with a peptide false-positive rate of 1.3%.