Reviews & Perspectives
Deciphering Spatial Protein–Protein Interactions in Brain Using Proximity Labeling
In Brief PL has emerged as a powerful tool to identify proteomes in distinct cell types in brain as well as proteomes and protein–protein interaction networks in structures difficult to isolate, such as the synaptic cleft, axonal projections, or astrocyte–neuron junctions. Here, we review recent advances in PL methods and their application to neurobiology.New Views of Old Proteins: Clarifying the Enigmatic Proteome
In Brief Capturing the biology of proteins will require improved technologies to readout their composition in space and time. Developing these improved technologies presents a major opportunity for biomedical research. How might we proceed in the decades ahead?MHC Class I Immunopeptidome: Past, Present, and Future
In Brief A pioneer in studying CD8+ T-cell immunosurveillance of viruses and tumors reviews the critical contributions of MS-based studies to early, current, and future understanding of how cells generate the MHC class I immunopeptidome: the repertoire of foreign and self-peptides presented by MHC class I molecules for T-cell recognition.An Expanding Repertoire of Protein Acylations
In Brief In this work, we give a general overview of the 12 main protein acylations, also including novel acylations, such as benzoylation and 2-hydroxyisobutyrylation. We summarize the recent advances in protein acylation, mainly focus on their substrates, enzymes, biological functions, and novel detecting methods and related diseases, especially in cancer. We believe that the review will provide an unprecedented and comprehensive view of protein acylations and bring important reference significance for future research.Subcellular Transcriptomics and Proteomics: A Comparative Methods Review
In Brief The interior of the cell is molecularly crowded. Its compartmentalization within organelles enables the regulation of biochemical processes and allows multifunctionality of proteins and RNAs. Subcellular information can thus give insights into the function of these biomolecules. Multiple techniques to measure such information have been established, with ever-increasing throughput and sensitivity. These techniques are covered in this review, and demonstrating their application is providing valuable insights into cellular biology, such as aiding our understanding of single-cell heterogeneity and posttranslational modifications.Scaling Up Single-Cell Proteomics
In Brief Single-cell proteomics will drive the next wave of single-cell biology. This requires broad adoption of existing methods, the application of rigorous quality control standards, and the continuous advancement of the technology. The advancement will be driven by numerous innovations, including highly parallelized analysis, and will increase the throughput, quantitative accuracy, and the accessibility of the single-cell proteomics.Ethical Principles, Constraints, and Opportunities in Clinical Proteomics
In Brief We introduce bioethical principles and use these as operational definitions to carry out a systematic review of the literature on ethical issues in clinical proteomics. We identify 10 ethical themes across 16 studies, many of which are familiar from other fields. We therefore survey how genomics has dealt with ethical issues and regulation. We also add our own perspectives on the ethical aspects of study design and sample treatment as well as the ethical potential of preventive proteomics profiling.Reflections on the HUPO Human Proteome Project, the Flagship Project of the Human Proteome Organization, at 10 Years
In Brief Starting from several organ-oriented projects, HUPO in 2010 launched the Human Proteome Project to identify and characterize the protein parts list and integrate proteomics into multiomics research. Key steps were partnerships with neXtProt, PRIDE, PeptideAtlas, Human Protein Atlas, and instrument makers; global engagement of researchers; creation of ProteomeXchange; adoption of HPP Guidelines for Interpretation of MS Data and SRMAtlas for proteotypic peptides; annual metrics of finding “missing proteins” and functionally annotating proteins; and initiatives for early career scientists.Data Management of Sensitive Human Proteomics Data: Current Practices, Recommendations, and Perspectives for the Future
In Brief Availability of proteomics data in the public domain has become the norm, as it has been the case in genomics and transcriptomics for many years. Analogously to sequencing data, there are increasing ethical issues and legal requirements related to sensitive human clinical proteomics data. We review the current state of the art and make concrete recommendations to address these issues in the proteomics field, which are summarized in four different areas.Neuroproteomics of the Synapse: Subcellular Quantification of Protein Networks and Signaling Dynamics
In Brief Advancements in MS-based proteomics have increased the study of synaptic proteins using neuroproteomics. The development of proximity, genetic labeling and bio-orthogonal amino acid labeling approaches now allow for the study of synaptic protein–protein interactions and protein signaling dynamics. In this review, we highlight studies from the last 5 years, with a focus on synapse structure, composition, functioning, or signaling and finally discuss some recent developments that could further advance the field of neuroproteomics.Are There Indeed Spliced Peptides in the Immunopeptidome?
In Brief Peptide splicing was suggested to significantly contribute ligands to the immunopeptidome. This article argues that peptide splicing is at most very rare, even if it happens at all. Considerations against peptide splicing are based on bioinformatics calculations related to the analysis of the LC-MS/MS data, and on the abundance of water in the cells, which should compete effectively with the transpeptidation reaction, needed for peptide splicing.Proteomics-Based Insights Into the SARS-CoV-2–Mediated COVID-19 Pandemic: A Review of the First Year of Research
In Brief SARS-CoV-2, the betacoronavirus that caused the COVID-19 pandemic, became a major source of human disease and death in 2020. The fundamental constituents of a virus being its genome and proteome, characterizing the proteome is essential to understanding its biology. In this review article, we survey the proteomics literature from the first year of the COVID-19 pandemic, including protein–protein interaction studies, post-translational modification studies, and work using proteomics technologies to probe host response, which collectively inform efforts to ameliorate the pandemic.MS-Based HLA-II Peptidomics Combined With Multiomics Will Aid the Development of Future Immunotherapies
In Brief Although challenges remain in leveraging MS-based HLA-II peptidomics, investigations into the interplay between disease pathologies and the presentation of HLA-II peptides to CD4+ T cells will enable the development of future immunotherapies. In this Review article, we discuss our current understanding of HLA-II peptidomics and outstanding questions in the field and how MS-based innovations will enable us to fill knowledge gaps and help improve our ability to select HLA-II-presented antigens as targets for personalized immunotherapies.Decoding Post-Translational Modification Crosstalk With Proteomics
In Brief We provide an overview of current experimental and computational proteomic methods, as well as a perspective on emerging technologies to study PTM crosstalk.Uncovering the Depths of the Human Proteome: Antibody-based Technologies for Ultrasensitive Multiplexed Protein Detection and Quantification
In Brief Probing the human plasma proteome is attractive for biomarker and drug target discovery. Recent breakthroughs in multiplex proteomics technologies enable the simultaneous and sensitive quantification of thousands of proteins in biofluids. We provide a comprehensive guide to the methodologies, performance, advantages, and disadvantages of established and emerging technologies for the multiplexed ultrasensitive measurement of proteins. Gaining knowledge on these innovations is crucial for choosing the right multiplexed proteomics tool to critically complement traditional proteomics methods.An Introduction to Advanced Targeted Acquisition Methods
In Brief The analytical power of targeted proteomics depends on how efficiently the mass spectrometer detects target peptides. A number of “smart” acquisition approaches have been developed that enable more targets per run and improve analytical performance such as sensitivity, specificity, and quantitative accuracy. This review provides an introduction to these methods and highlights their inherent strengths and weaknesses.Recent Advances in Software Tools for More Generic and Precise Intact Glycopeptide Analysis
In Brief This article provides a systematic review of the most recent MS-based strategies and corresponding software tools for the analysis of intact glycopeptides, particularly intact N-glycopeptides, reported in the last decade, including the process of identifying N-glycopeptides from MS data, the existing methods of MS data acquisition and interpretation, the quality control methods, the display of results, and the software applications.Recent Advances in Analytical Approaches for Glycan and Glycopeptide Quantitation
In Brief Recent years have seen an explosion in novel strategies for quantitative glycomics and glycoproteomics. Whether through metabolic incorporation of stable isotopes, deposition of custom isotopic labels, or high-throughput isobaric chemical tags, these numerous novel strategies provide ease of access to glycomic and glycoproteomic investigation. This review highlights the recent innovations in labeling methods, label-free strategies, acquisition modes, and bioinformatic tools for glycan and glycopeptide quantitation, while providing critical evaluations and technical considerations to enable effective analysis.The Role of Data-Independent Acquisition for Glycoproteomics
In Brief As a highly abundant and diverse post-translational modification, protein glycosylation is challenging to characterize in various approaches including MS. In MS-based proteomics, data-independent acquisition (DIA) has been advanced rapidly and showed outstanding analytical performances. DIA now started to be applied in different facets of glycoproteomics, including deglycosylated and intact N-linked and O-linked glycopeptides, and screening of oxonium ions. We summarized current applications of DIA in glycoproteomics and discussed its limitations and perspectives.Chromatin Proteomics to Study Epigenetics — Challenges and Opportunities
In Brief MS-based analysis of chromatin has emerged as a powerful tool to identify proteins associated with gene regulation. Total chromatin isolated from cells can be directly analyzed using MS, further fractionated into transcriptionally active and inactive chromatin, enriched for specific compartment or regions, and potentially used for single-locus isolation. This review highlights recent advances and discusses current challenges that should be addressed to further advance the field of chromatin proteomics.Calculating Glycoprotein Similarities From Mass Spectrometric Data
In Brief To understand the roles of glycoproteins in biological processes, it is necessary to quantify the changes that occur to glycosylation at individual sites and to the whole molecule. That glycoprotein glycosylation is inherently heterogeneous means that the distribution of glycoforms at each glycosite must be quantified in order to inform calculation of molecular similarities. We review analytical and statistical methods for determining glycoprotein molecular similarities from glycoproteomics data.Glycomics, Glycoproteomics, and Glycogenomics: An Inter-Taxa Evolutionary Perspective
In Brief This review article i) assesses the utility of current glycomic, glycoproteomic, and glycogenomic methods to characterize protein glycosylation in less-well-studied eukaryotes; ii) assembles a plausible evolutionary lineage of eukaryotic glycan–protein linkages from the last eukaryotic common ancestor through protists to multicellular plants, invertebrates, and vertebrates; and iii) highlights the diversity of peripheral glycan specializations and modifications with an emphasis on available information from diverse protist kingdoms and invertebrate animals.Developments in Mass Spectrometry for Glycosaminoglycan Analysis: A Review
In Brief Glycosaminoglycans (GAGs) participate in a variety of biological functions and have a multitude of medicinal properties. Due to their non template driven biosynthesis, GAGs are produced as nonuniform complex mixtures. Mass spectrometry paired with on-line separation techniques has been utilized to determine the composition of these complex mixtures. Advances in tandem mass spectrometry have also made determining sequence information such as sulfation location and C-5 epimerization possible. This review covers recent developments in the analysis of GAGs using mass spectrometry.The Peptide Vaccine of the Future
In Brief Therapeutic peptide-based vaccination approaches for the treatment of cancer patients have shown first glimmers of success. However, to achieve broad clinical efficacy and implement peptide vaccinations in the standard treatment of cancer patients future peptide vaccines need further optimization in terms of target antigen selection, adjuvant choice, vaccination schedules, delivery routes, biomarkers, and combinatorial drugs.Methods for Enrichment and Assignment of N-Acetylglucosamine Modification Sites
In Brief This review article summarizes methods for O-GlcNAc enrichment and different mass spectrometric approaches for acquiring data on modified peptides and describes software strategies for analyzing data, including the challenges of reliably identifying modification sites and differentiating between other potential HexNAc modifications. It then presents a new dataset to exemplify what is currently achievable.A Pragmatic Guide to Enrichment Strategies for Mass Spectrometry–Based Glycoproteomics
In Brief Interest in mass spectrometry–based glycoproteomics analysis is increasing because of recent advances in instrumentation and data analysis tools. Such studies can provide a wealth of information across a wide spectrum of glycan classes and biological systems. However, many studies require the choice of an enrichment strategy for glycosylated species prior to analysis to obtain the maximum amount of analytical information. Here, common enrichment strategies are reviewed with strengths and weaknesses, and the practical considerations for various methods are discussed.Meta-heterogeneity: Evaluating and Describing the Diversity in Glycosylation Between Sites on the Same Glycoprotein
In Brief Diversity in protein glycosylation can be described in terms of micro-heterogeneity and macro-heterogeneity, respectively, referring to the variation and occupancy of glycans at a given glycosylation site. However, these terms are not sufficient to describe a higher level of regulation when proteins are multiply glycosylated. For this, we propose the term meta-heterogeneity: variation in glycosylation across multiple sites of a given glycoprotein. In this review, we describe several remarkable examples of glycoprotein meta-heterogeneity and underline the need for its investigation.Accelerating the Field of Epigenetic Histone Modification Through Mass Spectrometry–Based Approaches
In Brief Histone post-translational modifications play essential roles in the epigenetic regulation of chromatin-related functions. Because of its high throughput, accuracy, and flexibility, mass spectrometry has emerged as a powerful tool in the epigenetic field. In this review, we describe the contributions of mass spectrometry–based proteomics in combination with distinct labeling strategies and various biological techniques to understand the roles of histone post-translational modifications and how they regulate chromatin function.Proteome Turnover in the Spotlight: Approaches, Applications, and Perspectives
In Brief In this review, we outline historical and current approaches to measure the kinetics of protein turnover on a proteome-wide scale in both steady-state and dynamic systems, with an emphasis on metabolic tracing using stable isotope–labeled amino acids. In addition, we highlight important considerations for designing proteome turnover experiments, key biological findings regarding the conserved principles of proteome turnover regulation, and future perspectives for both technological and biological investigations.Single-cell Proteomics: Progress and Prospects
Single-cell proteomics capabilities have the potential to transform biomedical research and enable understanding of biological systems with a new level of granularity. Recent advances in sample processing, separations and MS instrumentation now make it possible to quantify >1000 proteins from individual mammalian cells.Proteomics and Metaproteomics Add Functional, Taxonomic and Biomass Dimensions to Modeling the Ecosystem at the Mucosal-luminal Interface
Proteomics and metaproteomics are important tools for studying the spatiotemporal heterogeneous ecosystem in our gut. We review strategies and their applications to gut ecology studies, such as building a dynamical model of the MLI.Organellar Maps Through Proteomic Profiling – A Conceptual Guide
Protein subcellular localization is highly regulated and critical for protein function. Spatial proteomics aims at capturing the localization dynamics of all proteins expressed in a given cell type. Among different approaches, organellar mapping through proteomic profiling stands out as the only method capable of determining the subcellular localizations of thousands of proteins in a single experiment. Importantly, it can also detect movements of proteins between subcellular compartments, providing an unbiased systems analysis tool for investigating physiological and pathological cellular processes.Peptide-based Interaction Proteomics
Protein-protein interactions that are mediated by short linear motifs (SLiMs) in intrinsically disordered regions (IDRs) of proteins are notoriously difficult to study. Recently, pull-downs with synthetic peptides in combination with quantitative mass spectrometry emerged as a powerful screening approach. Here, we briefly highlight the relevance of SLiMs for protein-protein interactions, outline existing screening technologies, discuss unique advantages of peptide-based interaction screens, and provide practical suggestions for setting up such peptide-based screens.Developments and Applications of Functional Protein Microarrays
Functional protein microarray is a crucial tool in the study of proteins in native, unbiased, and high-throughput manner. There is a wide variety of applications, including the study of proteome-wide molecular interactions, analysis of post-translational modifications, identification of novel drug targets, and examination of pathogen-host interactions. Functional protein microarray is also useful in profiling antibody specificity, as well as in the discovery of novel biomarkers, especially for autoimmune diseases, infectious diseases, and cancers. Recently, the virion display method has been applied to produce functional GPCR array for various research and pharmaceutical applications.Profiling Cell Signaling Networks at Single-cell Resolution
Signaling network responses can be highly heterogeneous across cells in a tissue because of many sources of genetic and non-genetic variance. The emergence of multiplexed single-cell technologies has made it possible to evaluate this heterogeneity. In this review, we categorize currently established single-cell signaling network profiling approaches by their methodology, coverage, and application, and we discuss the advantages and limitations of these technologies. We describe the computational tools for network characterization using single-cell data and discuss potential confounding factors that may affect single-cell analyses.Proximity Dependent Biotinylation: Key Enzymes and Adaptation to Proteomics Approaches
Proximity-dependent biotinylation approaches such as BioID and APEX overcome classical limitations of biochemical purification and have gained widespread use in recent years for revealing cellular neighborhoods. Here we focus on the structural diversity and mechanisms of the two classes of enzymes, biotin protein ligases and peroxidases, and discuss current and emerging applications of these enzymes for proximity dependent biotinylation. We provide guidelines for enzyme selection and experimental design for performing and interpreting proximity-dependent biotinylation experiments.Phosphoproteomic Approaches to Discover Novel Substrates of Mycobacterial Ser/Thr Protein Kinases
Mycobacterial STPKs are responsible for orchestrating phosphorylation-dependent signaling cascades that mediate bacterial growth and environmental adaptation. Recent advances in MS-based phosphoproteomics have significantly expanded the candidate substrate lists for individual mycobacterial STPKs. Integration of the available phosphoproteomic datasets provide new insights into the functional roles of specific STPKs in cell physiology. Future research should focus on in vivo phosphorylation network reconstruction to expose the fundamental signaling pathways in mycobacteria. Linking STPKs with their physiological substrates may reveal novel antimycobacterial agents.Advances in Tools to Determine the Glycan-Binding Specificities of Lectins and Antibodies
Lectins and glycan-binding antibodies are powerful tools in biological research, provided detailed information is available about their glycan-binding specificities. Glycan-arrays, in combination with bioinformatics tools to mine the data, offer the ability to obtain such information. This review focuses on the bioinformatics tools and resources that are available for the analysis of glycan-array data. The tools are enabling new insights into protein-glycan interactions and enhancing the value of glycan-binding proteins in research.Next-generation Interactomics: Considerations for the Use of Co-elution to Measure Protein Interaction Networks
Interactome studies are necessary to understand cellular processes and co-elution methods are well suited for the simultaneous and global exploration of the interactome, as well as the assessment of biological perturbations of the network. These methods rely on the fundamental idea that proteins from the same complex migrate together during fractionation. We review the different separation techniques along with the quantification and bioinformatic approaches used for co-elution methods and provide design considerations to choose between them.Why Glycosylation Matters in Building a Better Flu Vaccine
Immunodominant influenza A virus (IAV) antigens mutate rapidly, allowing the virus to escape host antibodies. The question remains how to design vaccines that recognize conserved but subdominant IAV antigens for broader immune protection. Glycosylation is a mechanism whereby IAV evades the innate and adaptive immune systems. However, its influence on immunodominance remains poorly understood. Although mass spectrometry methods for identifying glycopeptides are maturing, quantifying glycosylation variation among sets of IAV mutants remains a technical challenge.Proteomics, Glycomics, and Glycoproteomics of Matrisome Molecules
Extracellular networks of matrisome proteins and their binding partners give rise to dynamic cell and tissue-specific microenvironments. The extreme complexity of matrisome molecule glycosylation and other post-translational modifications belies the need for specialized omics methods. It is necessary to map the modifications of matrisome molecules in detail in order to understand their roles in normal and pathological physiology. We review proteomics, glycomics and glycoproteomics methods for matrisome molecules toward the goal of achieving detailed matrisome maps.Exploiting Interdata Relationships in Next-generation Proteomics Analysis
Mass spectrometry-based proteomics and other technologies have matured to enable routine acquisition of system-wide data sets that describe concentrations, modifications, and interactions of proteins, mRNAs, and other molecules. Productive integrative studies differ from parallel data analysis by quantitative modeling of the relationships between data. We outline steps and considerations towards integromic studies to exploit the synergy between data sets.E3 Ubiquitin Ligases: Key Regulators of Hormone Signaling in Plants
Ubiquitin-mediated control of protein stability is central to most aspects of plant hormone signaling. Attachment of ubiquitin to target proteins occurs via an enzymatic cascade with the final step being catalyzed by a family of enzymes known as E3 ubiquitin ligases, which have been classified based on their protein domains and structures. Although E3 ubiquitin ligases are conserved among eukaryotes, in plants they are well-known to fulfill unique roles as central regulators of phytohormone signaling, including hormone perception and regulation of hormone biosynthesis.Analysis of Mammalian O-Glycopeptides—We Have Made a Good Start, but There is a Long Way to Go
Glycosylation is perhaps the most common post-translational modification. Recently there has been growing interest in cataloging the glycan structures, glycoproteins, and specific sites modified and deciphering the biological functions of glycosylation. Although the results are piling up for N-glycosylation, O-glycosylation is seriously trailing behind. In our review we reiterate the difficulties researchers have to overcome in order to characterize O-glycosylation. We describe how an ingenious cell engineering method delivered exciting results, and what could we gain from “wild-type” samples.Methods, Tools and Current Perspectives in Proteogenomics
With combined technological advancements in high-throughput next-generation sequencing and deep mass spectrometry-based proteomics, proteogenomics, i.e. the integrative analysis of proteomic and genomic data, has emerged as a new research field. Early efforts in the field were focused on improving protein identification using sample-specific genomic and transcriptomic sequencing data. More recently, integrative analysis of quantitative measurements from genomic and proteomic studies have identified novel insights into gene expression regulation, cell signaling, and disease.Protein Footprinting Comes of Age: Mass Spectrometry for Biophysical Structure Assessment
Protein footprinting mediated by mass spectrometry has evolved over the last 30 years from proof of concept to commonplace biophysics tool, with unique capabilities for assessing structure and dynamics of purified proteins in physiological states in solution. This review outlines the history and current capabilities of two major methods of protein footprinting: reversible hydrogen-deuterium exchange (HDX) and hydroxyl radical footprinting (HRF), an irreversible covalent labeling approach. Technological advances in both approaches now permit high-resolution assessments of protein structure including secondary and tertiary structure stability mediated by backbone interactions (measured via HDX) and solvent accessibility of side chains (measured via HRF).The Promise of Proteomics in the Study of Oncogenic Viruses
Oncogenic viruses are responsible for about 15% human cancers. This article explores the promise and challenges of viral proteomics in the study of the oncogenic human DNA viruses, HPV, McPyV, EBV and KSHV. These viruses have coevolved with their hosts and cause persistent infections. Each virus encodes oncoproteins that manipulate key cellular pathways to promote viral replication and evade the host immune response. Viral proteomics can identify cellular pathways perturbed by viral infection, identify cellular proteins that are crucial for viral persistence and oncogenesis, and identify important diagnostic and therapeutic targets.Proteomics Tracing the Footsteps of Infectious Disease
Every year, a major cause of human disease and death worldwide is infection with the various pathogens—viruses, bacteria, fungi, and protozoa—that are intrinsic to our ecosystem. In efforts to control the prevalence of infectious disease and develop improved therapies, the scientific community has focused on building a molecular picture of pathogen infection and spread. These studies have been aimed at defining the cellular mechanisms that allow pathogen entry into hosts cells, their replication and transmission, as well as the core mechanisms of host defense against pathogens.Sharpening Host Defenses during Infection: Proteases Cut to the Chase
The human immune system consists of an intricate network of tightly controlled pathways, where proteases are essential instigators and executioners at multiple levels. Invading microbial pathogens also encode proteases that have evolved to manipulate and dysregulate host proteins, including host proteases during the course of disease. The identification of pathogen proteases as well as their substrates and mechanisms of action have empowered significant developments in therapeutics for infectious diseases.Protein Interactions during the Flavivirus and Hepacivirus Life Cycle
Protein–protein interactions govern biological functions in cells, in the extracellular milieu, and at the border between cells and extracellular space. Viruses are small intracellular parasites and thus rely on protein interactions to produce progeny inside host cells and to spread from cell to cell. Usage of host proteins by viruses can have severe consequences e.g. apoptosis, metabolic disequilibria, or altered cell proliferation and mobility. Understanding protein interactions during virus infection can thus educate us on viral infection and pathogenesis mechanisms.Extracellular Protein Phosphorylation, the Neglected Side of the Modification
The very existence of extracellular phosphorylation has been questioned for a long time, although casein phosphorylation was discovered a century ago. In addition, several modification sites localized on secreted proteins or on extracellular or lumenal domains of transmembrane proteins have been catalogued in large scale phosphorylation analyses, though in most such studies this aspect of cellular localization was not considered. Our review presents examples when additional analyses were performed on already public data sets that revealed a wealth of information about this “neglected side” of the modification.Omics Profiling in Precision Oncology
Cancer causes significant morbidity and mortality worldwide, and is the area most targeted in precision medicine. Recent development of high-throughput methods enables detailed omics analysis of the molecular mechanisms underpinning tumor biology. These studies have identified clinically actionable mutations, gene and protein expression patterns associated with prognosis, and provided further insights into the molecular mechanisms indicative of cancer biology and new therapeutics strategies such as immunotherapy.Recent Advances in Clinical Glycoproteomics of Immunoglobulins (Igs)
Antibody glycosylation analysis has seen methodological progress resulting in new findings with regard to antibody glycan structure and function in recent years. For example, antigen-specific IgG glycosylation analysis is now applicable for clinical samples because of the increased sensitivity of measurements, and this has led to new insights in the relationship between IgG glycosylation and various diseases. Furthermore, many new methods have been developed for the purification and analysis of IgG Fc glycopeptides, notably multiple reaction monitoring for high-throughput quantitative glycosylation analysis.Maturing Glycoproteomics Technologies Provide Unique Structural Insights into the N-glycoproteome and Its Regulation in Health and Disease
The glycoproteome remains severely understudied because of significant analytical challenges associated with glycoproteomics, the system-wide analysis of intact glycopeptides. This review introduces important structural aspects of protein N-glycosylation and summarizes the latest technological developments and applications in LC-MS/MS-based qualitative and quantitative N-glycoproteomics. These maturing technologies provide unique structural insights into the N-glycoproteome and its synthesis and regulation by complementing existing methods in glycoscience.A Biologist's Field Guide to Multiplexed Quantitative Proteomics
High-throughput genomic and proteomic studies have generated near-comprehensive catalogs of biological constituents within many model systems. Nevertheless, static catalogs are often insufficient to fully describe the dynamic processes that drive biology. Quantitative proteomic techniques address this need by providing insight into closely related biological states such as the stages of a therapeutic response or cellular differentiation. The maturation of quantitative proteomics in recent years has brought about a variety of technologies, each with their own strengths and weaknesses.Histone H4 Lysine 20 (H4K20) Methylation, Expanding the Signaling Potential of the Proteome One Methyl Moiety at a Time
Covalent post-translational modifications (PTMs) of proteins can regulate the structural and functional state of a protein in the absence of primary changes in the underlying sequence. Common PTMs include phosphorylation, acetylation, and methylation. Histone proteins are critical regulators of the genome and are subject to a highly abundant and diverse array of PTMs. To highlight the functional complexity added to the proteome by lysine methylation signaling, here we will focus on lysine methylation of histone proteins, an important modification in the regulation of chromatin and epigenetic processes.Neural Stem Cells (NSCs) and Proteomics
Neural stem cells (NSCs) can self-renew and give rise to the major cell types of the CNS. Studies of NSCs include the investigation of primary, CNS-derived cells as well as animal and human embryonic stem cell (ESC)-derived and induced pluripotent stem cell (iPSC)-derived sources. NSCs provide a means with which to study normal neural development, neurodegeneration, and neurological disease and are clinically relevant sources for cellular repair to the damaged and diseased CNS. Proteomics studies of NSCs have the potential to delineate molecules and pathways critical for NSC biology and the means by which NSCs can participate in neural repair.Proteomics of the Synapse – A Quantitative Approach to Neuronal Plasticity
The advances in mass spectrometry based proteomics in the past 15 years have contributed to a deeper appreciation of protein networks and the composition of functional synaptic protein complexes. However, research on protein dynamics underlying core mechanisms of synaptic plasticity in brain lag far behind. In this review, we provide a synopsis on proteomic research addressing various aspects of synaptic function. We discuss the major topics in the study of protein dynamics of the chemical synapse and the limitations of current methodology.Signaling Over Distances
Neurons are extremely polarized cells. Axon lengths often exceed the dimension of the neuronal cell body by several orders of magnitude. These extreme axonal lengths imply that neurons have mastered efficient mechanisms for long distance signaling between soma and synaptic terminal. These elaborate mechanisms are required for neuronal development and maintenance of the nervous system. Neurons can fine-tune long distance signaling through calcium wave propagation and bidirectional transport of proteins, vesicles, and mRNAs along microtubules.Neurodegeneration and Alzheimer's disease (AD). What Can Proteomics Tell Us About the Alzheimer's Brain?
Neurodegenerative diseases, such as Alzheimer′s diseases (AD), are becoming more prevalent as the population ages. However, the mechanisms that lead to synapse destabilization and neuron death remain elusive. The advent of proteomics has allowed for high-throughput screening methods to search for biomarkers that could lead to early diagnosis and treatment and to identify alterations in the cellular proteome that could provide insight into disease etiology and possible treatment avenues. In this review, we have concentrated mainly on the findings that are related to how and whether proteomics studies have contributed to two aspects of AD research, the development of biomarkers for clinical diagnostics, and the recognition of proteins that can help elucidate the pathways leading to AD brain pathology.Glial Contributions to Neural Function and Disease
The nervous system consists of neurons and glial cells. Neurons generate and propagate electrical and chemical signals, whereas glia function mainly to modulate neuron function and signaling. Just as there are many different kinds of neurons with different roles, there are also many types of glia that perform diverse functions. For example, glia make myelin; modulate synapse formation, function, and elimination; regulate blood flow and metabolism; and maintain ionic and water homeostasis to name only a few.Molecular and Cellular Mechanisms of Axonal Regeneration After Spinal Cord Injury
Following axotomy, a complex temporal and spatial coordination of molecular events enables regeneration of the peripheral nerve. In contrast, multiple intrinsic and extrinsic factors contribute to the general failure of axonal regeneration in the central nervous system. In this review, we examine the current understanding of differences in protein expression and post-translational modifications, activation of signaling networks, and environmental cues that may underlie the divergent regenerative capacity of central and peripheral axons.Emerging Proteomic Technologies Provide Enormous and Underutilized Potential for Brain Cancer Research
High-throughput technologies present immense opportunities to characterize brain cancer biology at a systems level. However, proteomic studies of brain cancers are still relatively scarce. Here we discuss the latest proteomic technologies, their application to profiling and quantitation of brain proteomes and how we expect these technologies will be applied to study brain cancer proteomes in the future. Mass spectrometry based proteomics with increased specificity, coverage and throughput will be pervasive in proteomics investigations of brain.Low Mass Blood Peptides Discriminative of Inflammatory Bowel Disease (IBD) Severity: A Quantitative Proteomic Perspective
Breakdown of the protective gut barrier releases effector molecules and degradation products into the blood stream making serum and plasma ideal as a diagnostic medium. The enriched low mass proteome is unexplored as a source of differentiators for diagnosing and monitoring inflammatory bowel disease (IBD) activity, that is less invasive than colonoscopy. Differences in the enriched low mass plasma proteome (<25 kDa) were assessed by label-free quantitative mass-spectrometry. A panel of marker candidates were progressed to validation phase and “Tier-2” FDA-level validated quantitative assay.The Expanding Landscape of the Thiol Redox Proteome
Cysteine occupies a unique place in protein chemistry. The nucleophilic thiol group allows cysteine to undergo a broad range of redox modifications beyond classical thiol-disulfide redox equilibria, including S-sulfenylation (-SOH), S-sulfinylation (-SO2H), S-sulfonylation (-SO3H), S-nitrosylation (-SNO), S-sulfhydration (-SSH), S-glutathionylation (-SSG), and others. Emerging evidence suggests that these post-translational modifications (PTM) are important in cellular redox regulation and protection against oxidative damage.Protein Neighbors and Proximity Proteomics
Within cells, proteins can co-assemble into functionally integrated and spatially restricted multicomponent complexes. Often, the affinities between individual proteins are relatively weak, and proteins within such clusters may interact only indirectly with many of their other protein neighbors. This makes proteomic characterization difficult using methods such as immunoprecipitation or cross-linking. Recently, several groups have described the use of enzyme-catalyzed proximity labeling reagents that covalently tag the neighbors of a targeted protein with a small molecule such as fluorescein or biotin.Less is More: Membrane Protein Digestion Beyond Urea–Trypsin Solution for Next-level Proteomics
The goal of next-level bottom-up membrane proteomics is protein function investigation, via high-coverage high-throughput peptide-centric quantitation of expression, modifications and dynamic structures at systems scale. Yet efficient digestion of mammalian membrane proteins presents a daunting barrier, and prevalent day-long urea–trypsin in-solution digestion proved insufficient to reach this goal. Many efforts contributed incremental advances over past years, but involved protein denaturation that disconnected measurement from functional states.The Proteomics of Networks and Pathways: A Movie Is Worth A Thousand Pictures
This special issue of Molecular and Cellular Proteomics highlights some of the science that has come out of the National Technology Centers for Networks and Pathways (TCNP)1 program, supported by the National Institutes of Health Common Fund. That program, which has drawn to a close after a planned 10-year run, was a response to the perceived need for greater emphasis on developing and using proteomics to understand the spatial and temporal dynamics of protein interactions. Proteomics was not adding as much as it could to our understanding of how systems change.A Proteome-wide Domain-centric Perspective on Protein Phosphorylation
Phosphorylation is a widespread post-translational modification that modulates the function of a large number of proteins. Here we show that a significant proportion of all the domains in the human proteome is significantly enriched or depleted in phosphorylation events. A substantial improvement in phosphosites prediction is achieved by leveraging this observation, which has not been tapped by existing methods. Phosphorylation sites are often not shared between multiple occurrences of the same domain in the proteome, even when the phosphoacceptor residue is conserved.Methods for the Detection of Peptidylarginine Deiminase (PAD) Activity and Protein Citrullination
The post-translational conversion of peptidylarginine to peptidylcitrulline, a process also known as citrullination, is catalyzed by the enzyme family of peptidylarginine deiminases (PADs) and has been demonstrated to be involved in many physiological processes, including the regulation of gene expression. In addition, citrullination has been shown to be associated with several diseases, such as cancer, multiple sclerosis, rheumatoid arthritis, and Alzheimer's disease. To get more insight into the role of PAD enzymes and citrullination in both health and disease, experimental strategies to study PAD activity and to characterize citrullinated proteins in complex biological samples are crucial.A Proteomic Perspective of Sirtuin 6 (SIRT6) Phosphorylation and Interactions and Their Dependence on Its Catalytic Activity
Sirtuin 6 (SIRT6), a member of the mammalian sirtuin family, is a nuclear deacetylase with substrate-specific NAD+-dependent activity. SIRT6 has emerged as a critical regulator of diverse processes, including DNA repair, gene expression, telomere maintenance, and metabolism. However, our knowledge regarding its interactions and regulation remains limited. Here, we present a comprehensive proteomics-based analysis of SIRT6 protein interactions and their dependence on SIRT6 catalytic activity. We also identify evolutionarily conserved SIRT6 phosphorylations, including four within a proline-rich disordered region, and show that the conserved S338 phosphorylation can modulate selected SIRT6 interactions.Using the Ubiquitin-modified Proteome to Monitor Protein Homeostasis Function
The ubiquitin system is essential for the maintenance of proper protein homeostasis function across eukaryotic species. Although the general enzymatic architecture for adding and removing ubiquitin from substrates is well defined, methods for the comprehensive investigation of cellular ubiquitylation targets have just started to emerge. Recent advances in ubiquitin-modified peptide enrichment have greatly increased the number of identified endogenous ubiquitylation targets, as well as the number of sites of ubiquitin attachment within these substrates.Regulation of Protein Degradation by O-GlcNAcylation: Crosstalk with Ubiquitination
The post-translational modification of intracellular proteins by O-linked N-acetylglucosamine (O-GlcNAc) regulates essential cellular processes such as signal transduction, transcription, translation, and protein degradation. Misfolded, damaged, and unwanted proteins are tagged with a chain of ubiquitin moieties for degradation by the proteasome, which is critical for cellular homeostasis. In this review, we summarize the current knowledge of the interplay between O-GlcNAcylation and ubiquitination in the control of protein degradation.Spatiotemporal Dynamics of Phosphorylation in Lipid Second Messenger Signaling
The plasma membrane serves as a dynamic interface that relays information received at the cell surface into the cell. Lipid second messengers coordinate signaling on this platform by recruiting and activating kinases and phosphatases. Specifically, diacylglycerol and phosphatidylinositol 3,4,5-trisphosphate activate protein kinase C and Akt, respectively, which then phosphorylate target proteins to transduce downstream signaling. This review addresses how the spatiotemporal dynamics of protein kinase C and Akt signaling can be monitored using genetically encoded reporters and provides information on how the coordination of signaling at protein scaffolds or membrane microdomains affords fidelity and specificity in phosphorylation events.Alpha-synuclein Post-translational Modifications as Potential Biomarkers for Parkinson Disease and Other Synucleinopathies
The development of novel therapies against neurodegenerative disorders requires the ability to detect their early, presymptomatic manifestations in order to enable treatment before irreversible cellular damage occurs. Precocious signs indicative of neurodegeneration include characteristic changes in certain protein levels, which can be used as diagnostic biomarkers when they can be detected in fluids such as blood plasma or cerebrospinal fluid. In the case of synucleinopathies, cerebrospinal alpha-synuclein (α-syn) has attracted great interest as a potential biomarker; however, there is ongoing debate regarding the association between cerebrospinal α-syn levels and neurodegeneration in Parkinson disease and synucleinopathies.The Coming of Age of Phosphoproteomics—from Large Data Sets to Inference of Protein Functions
Protein phosphorylation is one of the most common post-translational modifications used in signal transduction to control cell growth, proliferation, and survival in response to both intracellular and extracellular stimuli. This modification is finely coordinated by a network of kinases and phosphatases that recognize unique sequence motifs and/or mediate their functions through scaffold and adaptor proteins. Detailed information on the nature of kinase substrates and site-specific phosphoregulation is required in order for one to better understand their pathophysiological roles.A Proteomic Perspective of Inbuilt Viral Protein Regulation: pUL46 Tegument Protein is Targeted for Degradation by ICP0 during Herpes Simplex Virus Type 1 Infection
Much like the host cells they infect, viruses must also regulate their life cycles. Herpes simples virus type 1 (HSV-1), a prominent human pathogen, uses a promoter-rich genome in conjunction with multiple viral trans-activating factors. Following entry into host cells, the virion-associated outer tegument proteins pUL46 and pUL47 act to increase expression of viral immediate–early (α) genes, thereby helping initiate the infection life cycle. Because pUL46 has gone largely unstudied, we employed a hybrid mass spectrometry-based approach to determine how pUL46 exerts its functions during early stages of infection.Combining Results of Multiple Search Engines in Proteomics
A crucial component of the analysis of shotgun proteomics datasets is the search engine, an algorithm that attempts to identify the peptide sequence from the parent molecular ion that produced each fragment ion spectrum in the dataset. There are many different search engines, both commercial and open source, each employing a somewhat different technique for spectrum identification. The set of high-scoring peptide-spectrum matches for a defined set of input spectra differs markedly among the various search engine results; individual engines each provide unique correct identifications among a core set of correlative identifications.Identification of Protein Interactions Involved in Cellular Signaling
Protein-protein interactions drive biological processes. They are critical for all intra- and extracellular functions, and the technologies to analyze them are widely applied throughout the various fields of biological sciences. This study takes an in-depth view of some common principles of cellular regulation and provides a detailed account of approaches required to comprehensively map signaling protein-protein interactions in any particular cellular system or condition. We provide a critical review of the benefits and disadvantages of the yeast two-hybrid method and affinity purification coupled with mass spectrometric procedures for identification of signaling protein-protein interactions.Glycoproteomic Analysis of Antibodies
Antibody glycosylation has been shown to change with various processes. This review presents mass spectrometric approaches for antibody glycosylation analysis at the level of released glycans, glycopeptides, and intact protein. With regard to IgG fragment crystallizable glycosylation, mass spectrometry has shown its potential for subclass-specific, high-throughput analysis. In contrast, because of the vast heterogeneity of peptide moieties, fragment antigen binding glycosylation analysis of polyclonal IgG relies entirely on glycan release.Density-dependent Lectin–Glycan Interactions as a Paradigm for Conditional Regulation by Posttranslational Modifications
Mice with null mutations in specific Golgi glycosyltransferases show evidence of glycan compensation where missing carbohydrate epitopes are found on biosynthetically related structures. Repetitive saccharide sequences within the larger glycan structures are functional epitopes recognized by animal lectins. These studies provide the first in vivo support for the existence of a feedback system that maintains and regulates glycan epitope density in cells. Receptor regulation by lectin–glycan interactions and the Golgi provides a mechanism for the adaptation of cell surface receptors and solute transporters in response to environmental cues and intracellular signaling.Putting the Pieces Together: High-performance LC-MS/MS Provides Network-, Pathway-, and Protein-level Perspectives in Populus
High-performance mass spectrometry (MS)-based proteomics enabled the construction of a detailed proteome atlas for Populus, a woody perennial plant model organism. Optimization of experimental procedures and implementation of current state-of-the-art instrumentation afforded the most detailed look into the predicted proteome space of Populus, offering varying proteome perspectives: (1) network-wide, (2) pathway-specific, and (3) protein-level viewpoints. Together, enhanced protein retrieval through a detergent-based lysis approach and maximized peptide sampling via the dual-pressure linear ion trap mass spectrometer (LTQ Velos), have resulted in the identification of 63,056 tryptic peptides.Proteome Dynamics: Revisiting Turnover with a Global Perspective
Although bulk protein turnover has been measured with the use of stable isotope labeled tracers for over half a century, it is only recently that the same approach has become applicable to the level of the proteome, permitting analysis of the turnover of many proteins instead of single proteins or an aggregated protein pool. The optimal experimental design for turnover studies is dependent on the nature of the biological system under study, which dictates the choice of precursor label, protein pool sampling strategy, and treatment of data.Mass Spectrometry Based Glycoproteomics—From a Proteomics Perspective
Glycosylation is one of the most important and common forms of protein post-translational modification that is involved in many physiological functions and biological pathways. Altered glycosylation has been associated with a variety of diseases, including cancer, inflammatory and degenerative diseases. Glycoproteins are becoming important targets for the development of biomarkers for disease diagnosis, prognosis, and therapeutic response to drugs. The emerging technology of glycoproteomics, which focuses on glycoproteome analysis, is increasingly becoming an important tool for biomarker discovery.Dehydration-responsive Nuclear Proteome of Rice (Oryza sativa L.) Illustrates Protein Network, Novel Regulators of Cellular Adaptation, and Evolutionary Perspective
Water deficit or dehydration is the most crucial environmental constraint on plant growth and development and crop productivity. It has been postulated that plants respond and adapt to dehydration by altering their cellular metabolism and by activating various defense machineries. The nucleus, the regulatory hub of the eukaryotic cell, is a dynamic system and a repository of various macromolecules that serve as modulators of cell signaling dictating the cell fate decision. To better understand the molecular mechanisms of dehydration-responsive adaptation in plants, we developed a comprehensive nuclear proteome of rice.
