Protein Termini and Their Modifications Revealed by Positional Proteomics

One-Step Purification and N-Terminal Functionalization of Bioactive Proteins via Atypically Split Inteins

Site-specific installation of non-natural functionality onto proteins has enabled countless applications in biotechnology, chemical biology, and biomaterials science. Though the N-terminus is an attractive derivatization location, prior methodologies targeting this site have suffered from low selectivity, a limited selection of potential chemical modifications, and/or challenges associated with divergent protein purification/modification steps. In this work, we harness the atypically split VidaL intein to simultaneously N-functionalize and purify homogeneous protein populations in a single step. Our method─referred to as VidaL-tagged expression and protein ligation (VEPL)─enables modular and scalable production of N-terminally modified proteins with native bioactivity. Demonstrating its flexibility and ease of use, we employ VEPL to combinatorially install 4 distinct (multi)functional handles (e.g., biotin, alkyne, fluorophores) to the N-terminus of 4 proteins that span three different classes: fluorescent (Enhanced Green Fluorescent Protein, mCherry), enzymatic (β-lactamase), and growth factor (epidermal growth factor). Moving forward, we anticipate that VEPL's ability to rapidly generate and isolate N-modified proteins will prove useful across the growing fields of applied chemical biology.

C-Terminal Arginine-Selective Cleavage of Peptides as a Method for Mimicking Carboxypeptidase B

C-Terminal residues play a pivotal role in dictating the structure and functions of proteins. Herein, we report a mild, efficient, chemoselective, and site-selective chemical method that allows for precise chemical proteolysis at C-terminal arginine dictated by 9,10-phenanthrenequinone independent of the remaining sequence. This biomimetic approach also exhibits the potential to synthesize C-terminal methyl ester (-CO2Me) peptides.

Impact of Protein Nα-Modifications on Cellular Functions and Human Health

Most human proteins are modified by enzymes that act on the α-amino group of a newly synthesized polypeptide. Methionine aminopeptidases can remove the initiator methionine and expose the second amino acid for further modification by enzymes responsible for myristoylation, acetylation, methylation, or other chemical reactions. Specific acetyltransferases can also modify the initiator methionine and sometimes the acetylated methionine can be removed, followed by further modifications. These modifications at the protein N-termini play critical roles in cellular protein localization, protein-protein interaction, protein-DNA interaction, and protein stability. Consequently, the dysregulation of these modifications could significantly change the development and progression status of certain human diseases. The focus of this review is to highlight recent progress in our understanding of the roles of these modifications in regulating protein functions and how these enzymes have been used as potential novel therapeutic targets for various human diseases.

In-Depth Characterization of Apoptosis N-terminome Reveals a Link Between Caspase-3 Cleavage and Post-Translational N-terminal Acetylation

The N-termini of proteins contain information about their biochemical properties and functions. These N-termini can be processed by proteases, and can undergo other co- or post-translational modifications. We have developed LATE (LysN Amino Terminal Enrichment), a method that uses selective chemical derivatization of α-amines to isolate the N-terminal peptides, in order to improve N-terminome identification in conjunction with other enrichment strategies. We applied LATE alongside another N-terminomic method to study caspase-3 mediated proteolysis both in vitro and during apoptosis in cells. This has enabled us to identify many unreported caspase-3 cleavages, some of which cannot be identified by other methods. Moreover, we have found direct evidence that neo-N-termini generated by caspase-3 cleavage can be further modified by Nt-acetylation. Some of these neo-Nt-acetylation events occur in the early phase of the apoptotic process and may have a role in translation inhibition. This has provided a comprehensive overview of the caspase-3 degradome and has uncovered previously unrecognized crosstalk between post-translational Nt-acetylation and caspase proteolytic pathways.

Increasing the coverage of the N-terminome with LysN amino terminal enrichment (LATE)

The field of N-terminomics has been advancing with the development of novel methods that provide a comprehensive and unbiased view of the N-terminome. Negative selection N-terminomics enables the identification of free and naturally modified protein N-termini. Here, we present a streamlined protocol that combines two negative selection N-terminomics methods, LATE and HYTANE, to increase N-terminome coverage by 1.5-fold compared to using a single methodology. Our protocol includes sample preparation and data analysis of both methods and can be applied to studying the N-terminome of diverse samples. The suggested approach enables researchers to achieve a more detailed and accurate understanding of the N-terminome.

Shifting the balance: soluble ADAM10 as a potential treatment for Alzheimer's disease

Introduction

Accumulation of amyloid β in the brain is regarded as a key initiator of Alzheimer's disease pathology. Processing of the amyloid precursor protein (APP) in the amyloidogenic pathway yields neurotoxic amyloid β species. In the non-amyloidogenic pathway, APP is processed by membrane-bound ADAM10, the main α-secretase in the nervous system. Here we present a new enzymatic approach for the potential treatment of Alzheimer's disease using a soluble form of ADAM10.

Methods

The ability of the soluble ADAM10 to shed overexpressed and endogenous APP was determined with an ADAM10 knockout cell line and a human neuroblastoma cell line, respectively. We further examined its effect on amyloid β aggregation by thioflavin T fluorescence, HPLC, and confocal microscopy. Using N-terminal and C-terminal enrichment proteomic approaches, we identified soluble ADAM10 substrates. Finally, a truncated soluble ADAM10, based on the catalytic domain, was expressed in Escherichia coli for the first time, and its activity was evaluated.

Results

The soluble enzyme hydrolyzes APP and releases the neuroprotective soluble APPα when exogenously added to cell cultures. The soluble ADAM10 inhibits the formation and aggregation of characteristic amyloid β extracellular neuronal aggregates. The proteomic investigation identified new and verified known substrates, such as VGF and N-cadherin, respectively. The truncated variant also exhibited α-secretase capacity as shown with a specific ADAM10 fluorescent substrate in addition to shedding overexpressed and endogenous APP.

Discussion

Our in vitro study demonstrates that exogenous treatment with a soluble variant of ADAM10 would shift the balance toward the non-amyloidogenic pathway, thus utilizing its natural neuroprotective effect and inhibiting the main neurotoxic amyloid β species. The potential of such a treatment for Alzheimer's disease needs to be further evaluated in vivo.

No Substrate Left behind-Mining of Shotgun Proteomics Datasets Rescues Evidence of Proteolysis by SARS-CoV-2 3CLpro Main Protease

Proteolytic processing is the most ubiquitous post-translational modification and regulator of protein function. To identify protease substrates, and hence the function of proteases, terminomics workflows have been developed to enrich and detect proteolytically generated protein termini from mass spectrometry data. The mining of shotgun proteomics datasets for such 'neo'-termini, to increase the understanding of proteolytic processing, is an underutilized opportunity. However, to date, this approach has been hindered by the lack of software with sufficient speed to make searching for the relatively low numbers of protease-generated semi-tryptic peptides present in non-enriched samples viable. We reanalyzed published shotgun proteomics datasets for evidence of proteolytic processing in COVID-19 using the recently upgraded MSFragger/FragPipe software, which searches data with a speed that is an order of magnitude greater than many equivalent tools. The number of protein termini identified was higher than expected and constituted around half the number of termini detected by two different N-terminomics methods. We identified neo-N- and C-termini generated during SARS-CoV-2 infection that were indicative of proteolysis and were mediated by both viral and host proteases-a number of which had been recently validated by in vitro assays. Thus, re-analyzing existing shotgun proteomics data is a valuable adjunct for terminomics research that can be readily tapped (for example, in the next pandemic where data would be scarce) to increase the understanding of protease function and virus-host interactions, or other diverse biological processes.

Degradomics technologies in matrisome exploration

Consisting of a defined set of extracellular proteins secreted from resident cells and with minor contributions from serum proteins, the extracellular matrix (ECM) is an essential component of all tissues. Maintaining tissue homeostasis, structural support and cellular control through cell-ECM communication, the ECM has come to be viewed as not just a passive structural entity but rather as a dynamic signaling conduit between cells and the extracellular compartment. Proteins and their cleavage products mediate this communication, and aberrant signaling, either directly or indirectly distorting the ECM, results in pathological conditions including cancer, inflammation, fibrosis, and neurodegenerative diseases. Characterization of ECM components, the matrisome, the extracellular environment and their changes in disease is therefore of importance to understand and mitigate by developing novel therapeutics. Liquid chromatography-mass spectrometry (LC-MS) proteomics has been integral to protein and proteome research for decades and long superseded the obsolescent gel-based approaches. A continuous effort has ensured progress with increased sensitivity and throughput as more advanced equipment has been developed hand in hand with specialized enrichment, detection, and identification methods. Part of this effort lies in the field of degradomics, a branch of proteomics focused on discovering novel protease substrates by identification of protease-generated neo-N termini, the N-terminome, and characterizing the responsible protease networks. Various methods to do so have been developed, some specialized for specific tissue types, others for particular proteases, throughput, or ease of use. This review aims to provide an overview of the state-of-the-art proteomics techniques that have successfully been recently utilized to characterize proteolytic cleavages in the ECM and thereby guided new research and understanding of the ECM and matrisome biology.

Regulation of alphaherpesvirus protein via post-translational phosphorylation

An alphaherpesvirus carries dozens of viral proteins in the envelope, tegument and capsid structure, and each protein plays an indispensable role in virus adsorption, invasion, uncoating and release. After infecting the host, a virus eliminates unfavourable factors via multiple mechanisms to escape or suppress the attack of the host immune system. Post-translational modification of proteins, especially phosphorylation, regulates changes in protein conformation and biological activity through a series of complex mechanisms. Many viruses have evolved mechanisms to leverage host phosphorylation systems to regulate viral protein activity and establish a suitable cellular environment for efficient viral replication and virulence. In this paper, viral protein kinases and the regulation of viral protein function mediated via the phosphorylation of alphaherpesvirus proteins are described. In addition, this paper provides new ideas for further research into the role played by the post-translational modification of viral proteins in the virus life cycle, which will be helpful for understanding the mechanisms of viral infection of a host and may lead to new directions of antiviral treatment.

PBC, an easy and efficient strategy for high-throughput protein C-terminome profiling

High-throughput profiling of protein C-termini is still a challenging task. Proteomics provides a powerful technology for systematic and high-throughput study of protein C-termini. Various C-terminal peptide enrichment strategies based on chemical derivatization and chromatography separation have been reported. However, they are still costly and time-consuming, with low enrichment efficiency for C-terminal peptides. In this study, by taking advantage of the high reaction selectivity of 2-pyridinecarboxaldehyde (2-PCA) with an α-amino group on peptide N-terminus and high affinity between biotin and streptavidin, we developed a 2-PCA- and biotin labeling–based C-terminomic (PBC) strategy for a high-efficiency and high-throughput analysis of protein C-terminome. Triplicates of PBC experiments identified a total of 1,975 C-terminal peptides corresponding to 1,190 proteins from 293 T cell line, which is 180% higher than the highest reported number of C-terminal peptides identified from mammalian cells by chemical derivatization–based C-terminomics study. The enrichment efficiency (68%) is the highest among the C-terminomics methods currently reported. In addition, we not only uncovered 50 proteins with truncated C-termini which were significantly enriched in extracellular exosome, vesicle, and ribosome by a bioinformatic analysis but also systematically characterized the whole PTMs on C-terminal in 293 T cells, suggesting PBC as a powerful tool for protein C-terminal degradomics and PTMs investigation. In conclusion, the PBC strategy would benefit high-efficiency and high-throughput profiling of protein C-terminome.

Escherichia coli recombinant expression of SARS-CoV-2 protein fragments

We have developed a method for the inexpensive, high-level expression of antigenic protein fragments of SARS-CoV-2 proteins in Escherichia coli. Our approach uses the thermophilic family 9 carbohydrate-binding module (CBM9) as an N-terminal carrier protein and affinity tag. The CBM9 module was joined to SARS-CoV-2 protein fragments via a flexible proline–threonine linker, which proved to be resistant to E. coli proteases. Two CBM9-spike protein fragment fusion proteins and one CBM9-nucleocapsid fragment fusion protein largely resisted protease degradation, while most of the CBM9 fusion proteins were degraded at some site in the SARS-CoV-2 protein fragment. All of the fusion proteins were highly expressed in E. coli and the CBM9-ID-H1 fusion protein was shown to yield 122 mg/L of purified product. Three purified CBM9-SARS-CoV-2 fusion proteins were tested and found to bind antibodies directed to the appropriate SARS-CoV-2 antigenic regions. The largest intact CBM9 fusion protein, CBM9-ID-H1, incorporates spike protein amino acids 540–588, which is a conserved region overlapping and C-terminal to the receptor binding domain that is widely recognized by human convalescent sera and contains a putative protective epitope.

基于精氨酸酶切的蛋白质C端肽段富集方法的优化及评估

基于聚合物的蛋白质C端反向富集策略是用于研究蛋白质C端最为广泛的策略之一。目前,基于胰蛋白酶(trypsin)切割精氨酸残基C端(ArgC型酶切)的蛋白C端组学方法对蛋白质C端的鉴定深度仍有待提高。为解决这一问题,该研究对此方法进行了优化和评估:建立了基于“V型”过滤装置的“一锅法”富集流程,避免了副反应的干扰,缩短了样本的制备时间;优化了蛋白水平乙酰化反应条件,最大限度地降低了丝氨酸、苏氨酸、酪氨酸残基上的副反应,提高了肽段鉴定的可信性;优化了基于固相萃取枪头膜片过滤柱(StageTip柱)的样品分离过程,使C端肽段的鉴定深度增加至原来的4倍。通过以上优化,按照肽段水平错误发现率(FDR)<0.01、离子分数(ion score)≥20,且C端带有乙醇胺修饰的数据筛选标准,从人HEK 293T细胞中共鉴定出696个蛋白质C端。若仅要求肽段水平FDR<0.01,鉴定数目进一步增加到933个,这是基于聚合物富集策略的蛋白质C端组学方法所得的最大数据集之一。探索了胰蛋白酶镜像酶(LysargiNase)切割精氨酸残基N端(ArgN型酶切)与不同肽段N端衍生化修饰组合对蛋白质C端鉴定数目和种类的影响,“LysargiNase酶切+肽段N端乙酰化”新策略在原有“胰蛋白酶酶切+肽段N端二甲基化”策略的基础上将鉴定蛋白质C端的种类提升了47%。综上,该研究通过对基于Arg型酶切的蛋白C端组学方法的优化,提升了C端肽段的鉴定深度,扩大了C端肽段鉴定的覆盖范围。该方法将有望成为系统性表征蛋白质C端的有力工具。

One-Step Isolation of Protein C-Terminal Peptides from V8 Protease-Digested Proteins by Metal Oxide-Based Ligand-Exchange Chromatography

We have developed a one-step method to isolate protein C-terminal peptides from V8 protease-digested proteins by metal oxide-based ligand-exchange (MOLEX) chromatography. V8 protease cleaves the C-terminal side of Asp and Glu, affording a digested peptide with two carboxy groups at the C-terminus, whereas the protein C-terminal peptide has only one α-carboxy group. In MOLEX chromatography, a stable chelate is formed between dicarboxylates and metal atoms, so that the nonterminal (i.e., internal) peptide is retained, whereas the protein C-terminal peptide flows through the MOLEX column. After the optimization of the MOLEX chromatographic conditions, 1619 protein C-termini were identified from 30 μg of peptides (10 μg each, in triplicate) derived from human HeLa cells by means of nanoLC/MS/MS. When the MOLEX-isolated sample from 200 μg of HeLa peptides was further divided into six fractions by high-pH reversed-phase liquid chromatography (LC) prior to nanoLC/MS/MS, 2203 protein C-termini were identified with less than 3% contamination with internal peptides. We believe that this is the largest coverage with the highest purity reported to date in human protein C-terminomics. This fast, simple, sensitive, and selective method to isolate protein C-terminal peptides should be useful for profiling protein C-termini on a proteome-wide scale.

End-Binding E3 Ubiquitin Ligases Enable Protease Signaling

Post-translational modifications (PTMs) direct the assembly of protein complexes. In this context, proteolysis is a unique PTM because it is irreversible; the hydrolysis of the peptide backbone generates separate fragments bearing a new N and C terminus. Proteolysis can "re-wire" protein-protein interactions (PPIs) via the recruitment of end-binding proteins to new termini. In this review, we focus on the role of proteolysis in specifically creating complexes by recruiting E3 ubiquitin ligases to new N and C termini. These complexes potentiate proteolytic signaling by "erasing" proteolytic modifications. This activity tunes the duration and magnitude of protease signaling events. Recent work has shown that the stepwise process of proteolysis, end-binding by E3 ubiquitin ligases, and fragment turnover is associated with both the nascent N terminus (i.e., N-degron pathways) and the nascent C terminus (i.e., the C-degron pathways). Here, we discuss how these pathways might harmonize protease signaling with protein homeostasis (i.e., proteostasis).

The Arabidopsis PeptideAtlas: Harnessing worldwide proteomics data to create a comprehensive community proteomics resource

We developed a resource, the Arabidopsis PeptideAtlas (www.peptideatlas.org/builds/arabidopsis/), to solve central questions about the Arabidopsis thaliana proteome, such as the significance of protein splice forms and post-translational modifications (PTMs), or simply to obtain reliable information about specific proteins. PeptideAtlas is based on published mass spectrometry (MS) data collected through ProteomeXchange and reanalyzed through a uniform processing and metadata annotation pipeline. All matched MS-derived peptide data are linked to spectral, technical, and biological metadata. Nearly 40 million out of ∼143 million MS/MS (tandem MS) spectra were matched to the reference genome Araport11, identifying ∼0.5 million unique peptides and 17,858 uniquely identified proteins (only isoform per gene) at the highest confidence level (false discovery rate 0.0004; 2 non-nested peptides ≥9 amino acid each), assigned canonical proteins, and 3,543 lower-confidence proteins. Physicochemical protein properties were evaluated for targeted identification of unobserved proteins. Additional proteins and isoforms currently not in Araport11 were identified that were generated from pseudogenes, alternative start, stops, and/or splice variants, and small Open Reading Frames; these features should be considered when updating the Arabidopsis genome. Phosphorylation can be inspected through a sophisticated PTM viewer. PeptideAtlas is integrated with community resources including TAIR, tracks in JBrowse, PPDB, and UniProtKB. Subsequent PeptideAtlas builds will incorporate millions more MS/MS data.

tipNrich: A Tip-Based N-Terminal Proteome Enrichment Method

The mass spectrometry-based analysis of protein post-translational modifications requires large amounts of sample, complicating the analysis of samples with limited amounts of proteins such as clinical biopsies. Here, we present a tip-based N-terminal analysis method, tipNrich. The entire procedure is processed in a single pipette tip to minimize sample loss, which is so highly optimized to analyze small amounts of proteins, even femtomole-scale of a single protein. With tipNrich, we investigated various single proteins purified from different organisms using a low-resolution mass spectrometer and identified several N-terminal peptides with different Nt-modifications such as ragged N-termini. Furthermore, we applied matrix-assisted laser desorption ionization time-of-flight mass spectrometry to our method for shortening the analysis time. Moreover, we showed that our method could be utilized in disease diagnosis as exemplified by the characterization of wild-type transthyretin amyloidosis patients compared to the healthy individuals based on N-terminome profiling. In summary, tipNrich will satisfy the need of identifying N-terminal peptides even with highly scarce amounts of proteins and of having faster processing time to check the quality of protein products or to characterize N-terminal proteoform-related diseases.

Protein Characterization by MALDI In-Source Decay Mass Spectrometry in Support of Safety Assessments of Genetically Modified Crops

The advancement of mass spectrometry provides advantages for transgenic protein characterization in support of safety assessments of genetically modified crops. Here, we describe how matrix-assisted laser desorption ionization in-source decay (ISD) mass spectrometry (MS) in combination with intact mass and bottom-up analyses can be applied to achieve high confidence in the sequences of transgenic proteins expressed in plants and establish the biochemical equivalence of microbially produced protein surrogates. ISD confirmed 40-60 near terminal residues regardless of the protein size, including the improvement of the coverage of cysteine-rich proteins by the reduction/alkylation of disulfide bonds. Negative ISD significantly improved spectral quality and sequence coverage of acidic proteins. Various post-translational modifications, such as terminal truncations and N-terminal methionine excision and acetylation, were identified in plant-produced proteins by top-down MS. Finally, we demonstrated that a combination of top-down and bottom-up analyses provides high confidence in sequence equivalence of plant and microbially produced proteins.

Post-translational Modifications of the Protein Termini

Post-translational modifications (PTM) involve enzyme-mediated covalent addition of functional groups to proteins during or after synthesis. These modifications greatly increase biological complexity and are responsible for orders of magnitude change between the variety of proteins encoded in the genome and the variety of their biological functions. Many of these modifications occur at the protein termini, which contain reactive amino- and carboxy-groups of the polypeptide chain and often are pre-primed through the actions of cellular machinery to expose highly reactive residues. Such modifications have been known for decades, but only a few of them have been functionally characterized. The vast majority of eukaryotic proteins are N- and C-terminally modified by acetylation, arginylation, tyrosination, lipidation, and many others. Post-translational modifications of the protein termini have been linked to different normal and disease-related processes and constitute a rapidly emerging area of biological regulation. Here we highlight recent progress in our understanding of post-translational modifications of the protein termini and outline the role that these modifications play in vivo.

SAPT, a Fast and Efficient Approach for Simultaneous Profiling of Protein N- and C-Terminome

Protein termini play pivotal roles in various biological processes. Although several terminomic strategies have been proposed for the analysis of protein N-termini or protein C-termini separately, few can analyze both ends of proteins at the same time. Herein, we developed a workflow, termed Simultaneous Analysis of Protein N- and C-Terminome (SAPT) based on strong cation exchange chromatography (SCX) fractionation. Taking advantage of terminal peptides' reduced charge states in low pH SCX for their selective separation, we identified 3724 canonical human protein N-termini and 3129 canonical human protein C-termini, as well as 1463 neo-N-termini from the HeLa cell line, representing the largest human protein C-termini data set and the second largest human protein N-termini data set so far. The whole fractionation procedure was simple and rapid with considerable selectivity by utilizing a commercially available SCX-SPE column. In addition, we report for the first time the comprehensive screening of protein N-terminal and C-terminal modifications, leading to an identification of 8 kinds of protein N-terminal PTMs other than acetylation and 1 kind of protein C-terminal PTM other than amidation. Our results demonstrate the excellent performance and great potential of SAPT in terminomic studies. Data are available via ProteomeXchange with identifier PXD024573.

Substrate-biased activity-based probes identify proteases that cleave receptor CDCP1

CUB domain-containing protein 1 (CDCP1) is an oncogenic orphan transmembrane receptor and a promising target for the detection and treatment of cancer. Extracellular proteolysis of CDCP1 by poorly defined mechanisms induces pro-metastatic signaling. We describe a new approach for the rapid identification of proteases responsible for key proteolytic events using a substrate-biased activity-based probe (sbABP) that incorporates a substrate cleavage motif grafted onto a peptidyl diphenyl phosphonate warhead for specific target protease capture, isolation and identification. Using a CDCP1-biased probe, we identify urokinase (uPA) as the master regulator of CDCP1 proteolysis, which acts both by directly cleaving CDCP1 and by activating CDCP1-cleaving plasmin. We show that coexpression of uPA and CDCP1 is strongly predictive of poor disease outcome across multiple cancers and demonstrate that uPA-mediated CDCP1 proteolysis promotes metastasis in disease-relevant preclinical in vivo models. These results highlight CDCP1 cleavage as a potential target to disrupt cancer and establish sbABP technology as a new approach to identify disease-relevant proteases.

Proteomics approaches for the identification of protease substrates during virus infection

Proteases precisely and irreversibly catalyze the hydrolysis of peptide bonds, regulating the fate, localization, and activity of many proteins. Consequently, proteolytic activity plays an important role in fundamental cellular processes such as differentiation and migration, immunological and inflammatory reactions, apoptosis and survival. During virus infection, host proteases are involved in several processes, from cell entry to initiation, progression and resolution of inflammation. On the other hand, many viruses encode their own highly specific proteases, responsible for the proteolytic processing of viral proteins, but, at the same time, to cleave host proteins to corrupt antiviral host responses and adjust protein activity to favor viral replication. Traditionally, protease substrate identification has been addressed by means of hypothesis-driven approaches, but recent advances in proteomics have made a toolkit available to uncover the extensive repertoire of host proteins cleaved during infection, either by viral or host proteases. Here, we review the currently available proteomics-based methods that can and have contributed to the systematic and unbiased identification of new protease substrates in the context of virus-host interactions. The role of specific proteases during the course of virus infections will also be highlighted.

Insight into the human pathodegradome of the V8 protease from Staphylococcus aureus

Staphylococcus aureus possesses ten extracellular proteases with mostly unknown targets in the human proteome. To assist with bacterial protease target discovery, we have applied and compared two N-terminomics methods to investigate cleavage of human serum proteins by S. aureus V8 protease, discovering 85 host-protein targets. Among these are virulence-relevant complement, iron sequestration, clotting cascade, and host protease inhibitor proteins. Protein cleavage sites have been identified, providing insight into the disruption of host protein function by V8. Complement proteins are cleaved within peptidase and sushi domains, and host protease inhibitors are cleaved outside their protease-trapping motifs. Our data highlight the potential for further application of N-terminomics in discovery of bacterial protease substrates in other host niches and provide omics-scale insight into the role of the V8 protease in S. aureus pathogenesis.

S. aureus-secreted proteases are central to disease causation, but the discovery of their host substrates has been limited. Frey et al. use N-terminomic approaches to uncover human serum targets of the V8 protease that are from virulence-relevant processes such as the host inflammatory network and nutrient sequestration.

Bacterial riboproteogenomics: the era of N-terminal proteoform existence revealed

With the rapid increase in the number of sequenced prokaryotic genomes, relying on automated gene annotation became a necessity. Multiple lines of evidence, however, suggest that current bacterial genome annotations may contain inconsistencies and are incomplete, even for so-called well-annotated genomes. We here discuss underexplored sources of protein diversity and new methodologies for high-throughput genome reannotation. The expression of multiple molecular forms of proteins (proteoforms) from a single gene, particularly driven by alternative translation initiation, is gaining interest as a prominent contributor to bacterial protein diversity. In consequence, riboproteogenomic pipelines were proposed to comprehensively capture proteoform expression in prokaryotes by the complementary use of (positional) proteomics and the direct readout of translated genomic regions using ribosome profiling. To complement these discoveries, tailored strategies are required for the functional characterization of newly discovered bacterial proteoforms.

Site-selective modification of peptide backbones

Exciting developments in the site-selective modification of peptide backbones are allowing an outstanding fine-tuning of peptide conformation, folding ability, and physico-chemical and biological properties.

N-terminomics - its past and recent advancements

N-terminomics is a rapidly evolving branch of proteomics that encompasses the study of protein N-terminal sequence. A proteome-wide collection of such sequences has been widely used to understand the proteolytic cascades and in annotating the genome. Over the last two decades, various N-terminomic strategies have been developed for achieving high sensitivity, greater depth of coverage, and high-throughputness. We, in this review, cover how the field of N-terminomics has evolved to date, including discussion on various sample preparation and N-terminal peptide enrichment strategies. We also compare different N-terminomic methods and highlight their relative benefits and shortcomings in their implementation. In addition, an overview of the currently available bioinformatics tools and data analysis pipelines for the annotation of N-terminomic datasets is also included. SIGNIFICANCE: It has been recognized that proteins undergo several post-translational modifications (PTM), and a number of perturbed biological pathways are directly associated with modifications at the terminal sites of a protein. In this regard, N-terminomics can be applied to generate a proteome-wide landscape of mature N-terminal sequences, annotate their source of generation, and recognize their significance in the biological pathways. Besides, a system-wide study can be used to study complicated proteolytic machinery and protease cleavage patterns for potential therapeutic targets. Moreover, due to unprecedented improvements in the analytical methods and mass spectrometry instrumentation in recent times, the N-terminomic methodologies now offers an unparalleled ability to study proteoforms and their implications in clinical conditions. Such approaches can further be applied for the detection of low abundant proteoforms, annotation of non-canonical protein coding sites, identification of candidate disease biomarkers, and, last but not least, the discovery of novel drug targets.

Exploring Potential Signals of Selection for Disordered Residues in Prokaryotic and Eukaryotic Proteins

Intrinsically disordered proteins (IDPs) are an important class of proteins in all domains of life for their functional importance. However, how nature has shaped the disorder potential of prokaryotic and eukaryotic proteins is still not clearly known. Randomly generated sequences are free of any selective constraints, thus these sequences are commonly used as null models. Considering different types of random protein models, here we seek to understand how the disorder potential of natural eukaryotic and prokaryotic proteins differs from random sequences. Comparing proteome-wide disorder content between real and random sequences of 12 model organisms, we noticed that eukaryotic proteins are enriched in disordered regions compared to random sequences, but in prokaryotes such regions are depleted. By analyzing the position-wise disorder profile, we show that there is a generally higher disorder near the N- and C-terminal regions of eukaryotic proteins as compared to the random models; however, either no or a weak such trend was found in prokaryotic proteins. Moreover, here we show that this preference is not caused by the amino acid or nucleotide composition at the respective sites. Instead, these regions were found to be endowed with a higher fraction of protein–protein binding sites, suggesting their functional importance. We discuss several possible explanations for this pattern, such as improving the efficiency of protein–protein interaction, ribosome movement during translation, and post-translational modification. However, further studies are needed to clearly understand the biophysical mechanisms causing the trend.

Peptidomics-Driven Strategy Reveals Peptides and Predicted Proteases Associated With Oral Cancer Prognosis

Protease activity has been associated with pathological processes that can lead to cancer development and progression. However, understanding the pathological unbalance in proteolysis is challenging because changes can occur simultaneously at protease, their inhibitor, and substrate levels. Here, we present a pipeline that combines peptidomics, proteomics, and peptidase predictions for studying proteolytic events in the saliva of 79 patients and their association with oral squamous cell carcinoma (OSCC) prognosis. Our findings revealed differences in the saliva peptidome of patients with (pN+) or without (pN0) lymph-node metastasis and delivered a panel of ten endogenous peptides correlated with poor prognostic factors plus five molecules able to classify pN0 and pN+ patients (area under the receiver operating characteristic curve > 0.85). In addition, endopeptidases and exopeptidases putatively implicated in the processing of differential peptides were investigated using cancer tissue gene expression data from public repositories, reinforcing their association with poorer survival rates and prognosis in oral cancer. The dynamics of the OSCC-related proteolysis were further explored via the proteomic profiling of saliva. This revealed that peptidase/endopeptidase inhibitors exhibited reduced levels in the saliva of pN+ patients, as confirmed by selected reaction monitoring-mass spectrometry, while minor changes were detected in the level of saliva proteases. Taken together, our results indicated that proteolytic activity is accentuated in the saliva of patients with OSCC and lymph-node metastasis and, at least in part, is modulated by reduced levels of salivary peptidase inhibitors. Therefore, this integrated pipeline provided better comprehension and discovery of molecular features with implications in the oral cancer metastasis prognosis.

Targeted Metagenomics of Retting in Flax: The Beginning of the Quest to Harness the Secret Powers of the Microbiota

The mechanical and chemical properties of natural plant fibers are determined by many different factors, both intrinsic and extrinsic to the plant, during growth but also after harvest. A better understanding of how all these factors exert their effect and how they interact is necessary to be able to optimize fiber quality for use in different industries. One important factor is the post-harvest process known as retting, representing the first step in the extraction of bast fibers from the stem of species such as flax and hemp. During this process microorganisms colonize the stem and produce hydrolytic enzymes that target cell wall polymers thereby facilitating the progressive destruction of the stem and fiber bundles. Recent advances in sequencing technology have allowed researchers to implement targeted metagenomics leading to a much better characterization of the microbial communities involved in retting, as well as an improved understanding of microbial dynamics. In this paper we review how our current knowledge of the microbiology of retting has been improved by targeted metagenomics and discuss how related '-omics' approaches might be used to fully characterize the functional capability of the retting microbiome.

Recent advances in the synthesis of C-terminally modified peptides

C-Terminally modified peptides are important for the development and delivery of peptide-based pharmaceuticals because they impact peptide activity, stability, hydrophobicity, and membrane permeability. Additionally, the vulnerability of C-terminal esters to cleavage by endogenous esterases makes them excellent pro-drugs. Methods for post-SPPS C-terminal functionalization potentially enable access to libraries of modified peptides, facilitating tailoring of their solubility, potency, toxicity, and uptake pathway. Apparently minor structural changes can significantly impact the binding, folding, and pharmacokinetics of the peptide. This review summarizes developments in chemical methods for C-terminal modification of peptides published since the last review on this topic in 2003.

What Room for Two-Dimensional Gel-Based Proteomics in a Shotgun Proteomics World?

Two-dimensional gel electrophoresis was instrumental in the birth of proteomics in the late 1980s. However, it is now often considered as an outdated technique for proteomics—a thing of the past. Although this opinion may be true for some biological questions, e.g., when analysis depth is of critical importance, for many others, two-dimensional gel electrophoresis-based proteomics still has a lot to offer. This is because of its robustness, its ability to separate proteoforms, and its easy interface with many powerful biochemistry techniques (including western blotting). This paper reviews where and why two-dimensional gel electrophoresis-based proteomics can still be profitably used. It emerges that, rather than being a thing of the past, two-dimensional gel electrophoresis-based proteomics is still highly valuable for many studies. Thus, its use cannot be dismissed on simple fashion arguments and, as usual, in science, the tree is to be judged by the fruit.

Histone N-alpha terminal modifications: genome regulation at the tip of the tail

Histone proteins are decorated with numerous post-(PTMs) or co-(CTMs) translational modifications mainly on their unstructured tails, but also on their globular domain. For many decades research on histone modifications has been focused almost solely on the biological role of modifications occurring at the side-chain of internal amino acid residues. In contrast, modifications on the terminal N-alpha amino group of histones-despite being highly abundant and evolutionarily conserved-have been largely overlooked. This oversight has been due to the fact that these marks were being considered inert until recently, serving no regulatory functions. However, during the past few years accumulating evidence has drawn attention towards the importance of chemical marks added at the very N-terminal tip of histones and unveiled their role in key biological processes including aging and carcinogenesis. Further elucidation of the molecular mechanisms through which these modifications are regulated and by which they act to influence chromatin dynamics and DNA-based processes like transcription is expected to enlighten our understanding of their emerging role in controlling cellular physiology and contribution to human disease. In this review, we clarify the difference between N-alpha terminal (Nt) and internal (In) histone modifications; provide an overview of the different types of known histone Nt-marks and the associated histone N-terminal transferases (NTTs); and explore how they function to shape gene expression, chromatin architecture and cellular phenotypes.

MinProtMaxVP: Generating a minimized number of protein variant sequences containing all possible variant peptides for proteogenomic analysis

Identifying single-amino-acid variants (SAVs) from mass spectrometry-based experiments is critical for validating single-nucleotide variants (SNVs) at the protein level to facilitate biomedical research. Currently, two approaches are usually applied to convert SNV annotations into SAV-harboring protein sequences. One approach generates one sequence containing exactly one SAV, and the other all SAVs. However, they may neglect the possibility of SAV combinations, e.g., haplotypes, existing in bio-samples. Therefore, it is necessary to consider all SAV combinations of a protein when generating SAV-harboring protein sequences. In this paper, we propose MinProtMaxVP, a novel approach which selects a minimized number of SAV-harboring protein sequences generated from the exhaustive approach, while still accommodating all possible variant peptides, by solving a classic set covering problem. Our study on known haplotype variations of TAS2R38 justifies the necessity for MinProtMaxVP to consider all combinations of SAVs. The performance of MinProtMaxVP is demonstrated by an in silico study on OR2T27 with five SAVs and real experimental data of the HEK293 cell line. Furthermore, assuming simulated somatic and germline variants of OR2T27 in tumor and normal tissues demonstrates that when adopting the appropriate somatic and germline SAV integration strategy, MinProtMaxVP is adaptable to labeling and label-free mass spectrometry-based experiments. SIGNIFICANCE: We present MinProtMaxVP, a novel approach to generate SAV-harboring protein sequences for constructing a customized protein sequence database, which is used in database searching for variant peptide identification. This approach outperforms the existing approaches in generating all possible variant peptides to be included in protein sequences and possibly leading to identification of more variant peptides in proteogenomic analysis.

Mapping of polyglutamylation in tubulins using nanoLC-ESI-MS/MS

Tubulin polyglutamylation is a polymeric modification that extends from the carboxyl-terminus of tubulins. Molecular description of amino acids and their branching polyglutamyls is a hallmark of tubulin in microtubules. There are different chemical approaches for detecting these polymeric structures, mostly reported prior to development of nESI peptide analysis. Here we demonstrate a novel and simple approach to detect shared regions of amino acid ions from tubulin polyglutamylated peptides in nanoLC-MS/MS. This involves two parallel in gel digestions with trypsin and subtilisin followed by mapping of di- and triglutamyl modifications of α- and β-tubulins using a routine proteomics assay. We present three levels of information: i) identification of proteomics MS/MS data, ii) description of internal fragment ion series common across digests, and iii) extracted ion chromatograms mapped relative to retention time standards for confirmation of relative hydrophobicity values. Our nanoLC assay positive ion ESI detects up to 3 conjugated glutamates in tubulins. We implemented an analytical column only bottom up approach that characterizes molecular features of polyglutamylated tubulins.

Tryp-N: A Thermostable Protease for the Production of N-terminal Argininyl and Lysinyl Peptides

Bottom-up proteomics is a mainstay in protein identification and analysis. These studies typically employ proteolytic treatment of biological samples to generate suitably sized peptides for tandem mass spectrometric (MS) analysis. In MS, fragmentation of peptides is largely driven by charge localization. Consequently, peptides with basic centers exclusively on their N-termini produce mainly b-ions. Thus, it was long ago realized that proteases that yield such peptides would be valuable proteomic tools for achieving simplified peptide fragmentation patterns and peptide assignment. Work by several groups has identified such proteases, however, structural analysis of these suggested that enzymatic optimization was possible. We therefore endeavored to find enzymes that could provide enhanced activity and versatility while maintaining specificity. Using these previously described proteases as informatic search templates, we discovered and then characterized a thermophilic metalloprotease with N-terminal specificity for arginine and lysine. This enzyme, dubbed Tryp-N, affords many advantages including improved thermostability, solvent and detergent tolerance, and rapid digestion time.

Discovery of Cellular Roles of Intramembrane Proteases

Intramembrane proteases (IMPs) are localized within lipid bilayers of membranes-either the cell membrane or membranes of various organelles. Cleavage of substrates often results in release from the membrane, leading to a downstream biological effect. This mechanism allows different signaling events to happen through intramembrane proteolysis. Over the years, various mechanistically distinct families of IMPs have been discovered, but the research progress has generally been slower than for soluble proteases due to the challenges associated with membrane proteins. In this review we summarize how each mechanistic family of IMPs was discovered, which chemical tools are available for the study of IMPs, and which techniques have been developed for the discovery of IMP substrates. Finally, we discuss the various roles in cellular physiology of some of these IMPs.

Enrichments of post-translational modifications in proteomic studies

More than 300 different protein post-translational modifications are currently known, but only a few have been extensively investigated because modified proteoforms are commonly present in sub-stoichiometry amount. For this reason, improvement of specific enrichment techniques is particularly useful for the proteomic characterization of post-translationally modified proteins. Enrichment proteomic strategies could help the researcher in the challenging issue to decipher the complex molecular cross-talk existing between the different factors influencing the cellular pathways. In this review the state of art of the platforms applied for the enrichment of specific and most common post-translational modifications, such as glycosylation and glycation, phosphorylation, sulfation, redox modifications (i.e. sulfydration and nitrosylation), methylation, acetylation, and ubiquitinylation, are described. Enrichments strategies applied to characterize less studied post-translational modifications are also briefly discussed.

LysargiNase and Chemical Derivatization Based Strategy for Facilitating In-Depth Profiling of C-Terminome

Global identification of protein C-termini is highly challenging due to their low abundance in conventional shotgun proteomics. Several enrichment strategies have been developed to facilitate the detection of C-terminal peptides. One major issue of previous approaches is the limited C-terminome coverage. Herein, we integrated LysargiNase digestion, chemical acetylation on neo-N-terminus, and a-ion-aided peptide matching into poly(allylamine)-based C-terminomics (termed as LAACTer). In this strategy, we leveraged LysargiNase, a protease with cleavage specificity N-terminal to Lys and Arg residues, to cover previously unidentifiable C-terminome and employed chemical acetylation and a-ion-aided peptide matching to efficiently boost peptide identifications. Triplicates of LAACTer identified a total of 834 C-termini from proteome of 293T cell, which expanded the coverage by 164% (643 more unique C-termini) compared with the parallel experiments using the original workflow. Compared with the largest human C-terminome data sets (containing 800-900 C-termini), LAACTer not only achieved comparable profiling depth but also yielded 465 previously unidentified C-termini. In a SILAC (stable isotope labeling with amino acids in cell culture)-based quantitative study for identification of GluC-cleaved products, LAACTer quantified 300% more C-terminal peptides than the original workflow. Using LAACTer and the original workflow, we performed global analysis for the C-terminal sequences of 293T cell. The original and processed C-termini displayed distinct sequence patterns, implying the "C-end rules" that regulates protein stability could be more complex than just amino acid motifs. In conclusion, we reason LAACTer could be a powerful proteomic tool for in-depth C-terminomics and would benefit better functional understanding of protein C-termini.

Top-Down Proteomic Characterization of Truncated Proteoforms

A top-down proteomic strategy with semiautomated analysis of data sets has proven successful for the global identification of truncated proteins without the use of chemical derivatization, enzymatic manipulation, immunoprecipitation, or other enrichment. This approach provides the reliable identification of internal polypeptides formed from precursor gene products by proteolytic cleavage of both the N- and C-termini, as well as truncated proteoforms that retain one or the other termini. The strategy has been evaluated by application to the immunosuppressive extracellular vesicles released by myeloid-derived suppressor cells. More than 1000 truncated proteoforms have been identified, from which binding motifs are derived to allow characterization of the putative proteases responsible for truncation.

Protamines from liverwort are produced by post-translational cleavage and C-terminal di-aminopropanelation of several male germ-specific H1 histones

Protamines are small, highly-specialized, arginine-rich, and intrinsically-disordered chromosomal proteins that replace histones during spermiogenesis in many organisms. Previous evidence supports the notion that, in the animal kingdom, these proteins have evolved from a primitive replication-independent histone H1 involved in terminal cell differentiation. Nevertheless, a direct connection between the two families of chromatin proteins is missing. Here, we primarily used electron transfer dissociation MS-based analyses, revealing that the protamines in the sperm of the liverwort Marchantia polymorpha result from post-translational cleavage of three precursor H1 histones. Moreover, we show that the mature protamines are further post-translationally modified by di-aminopropanelation, and previous studies have reported that they condense spermatid chromatin through a process consisting of liquid-phase assembly likely involving spinodal decomposition. Taken together, our results reveal that the interesting evolutionary ancestry of protamines begins with histone H1 in both the animal and plant kingdoms.

The carboxy-terminus, a key regulator of protein function

All proteins end with a carboxyl terminus that has unique biophysical properties and is often disordered. Although there are examples of important C-termini functions, a more global role for the C-terminus is not yet established. In this review, we summarize research on C-termini, a unique region in proteins that cells exploit. Alternative splicing and proteolysis increase the diversity of proteins and peptides in cells with unique C-termini. The C-termini of proteins contain minimotifs, short peptides with an encoded function generally characterized as binding, posttranslational modifications, and trafficking. Many of these activities are specific to minimotifs on the C-terminus. Approximately 13% of C-termini in the human proteome have a known minimotif, and the majority, if not all of the remaining termini have conserved motifs inferring a function that remains to be discovered. C-termini, their predictions, and their functions are collated in the C-terminome, Proteus, and Terminus Oriented Protein Function INferred Database (TopFIND) database/web systems. Many C-termini are well conserved, and some have a known role in health and disease. We envision that this summary of C-termini will guide future investigation of their biochemical and physiological significance.

Exploring Extracellular Matrix Degradomes by TMT-TAILS N-Terminomics

Global characterization of protein N termini provides valuable information on proteome dynamics and diversity in health and disease. Driven by the progress in mass spectrometry-based proteomics, novel approaches for the dedicated investigation of protein N termini and protease substrates have been recently developed. Terminal amine isotopic labeling of substrates (TAILS) is a quantitative proteomics approach suitable for high-throughput and system-wide profiling of protein N termini in complex biological matrices. TAILS employs isotopic labeling of primary amines of intact proteins in combination with an amine-reactive high molecular weight polymer (HPG-ALD) for depletion of internal tryptic peptides and high enrichment of protein N termini by negative selection. Thereby, TAILS allows simultaneous identification of the natural N termini, protease-generated neo-N termini, and endogenously modified (e.g., acetylated) N termini. In this chapter, we provide a protocol for tandem mass tag (TMT)-TAILS analysis and further discuss specific considerations regarding N-terminome data interpretation using Proteome Discoverer™ software.

The unraveling of substrate specificity of histone deacetylase 6 domains using acetylome peptide microarrays and peptide libraries

Histone deacetylase 6 (HDAC6) is a multidomain cytosolic hydrolase acting mostly on nonhistone protein substrates. Investigations of the substrate specificity of HDAC6 are confounded by the presence of 2 catalytically active deacetylase domains (DD1 and DD2). In this study, acetylome peptide microarrays and peptide libraries were used to map the substrate specificity of DD1 and DD2 of human HDAC6. The results show that DD1 is solely responsible for the deacetylation of substrates harboring the acetyllysine at their C terminus, whereas DD2 exclusively deacetylates peptides with an internal acetyllysine residue. Also, statistical analysis of the deacetylation data revealed amino acid preferences at individual positions flanking the acetyllysine, where glycine and arginine residues are favored at positions N-terminal to the central acetyllysine; negatively charged glutamate is strongly disfavored throughout the sequence. Finally, the deacylation activity of HDAC6 was profiled by using a panel of acyl derivatives of the optimized peptide substrate and showed that HDAC6 acts as a proficient deformylase. Our data thus offer a detailed insight into the substrate preferences of the individual HDAC6 domains at the peptide level, and these findings can in turn help in elucidating the biologic roles of the enzyme and facilitate the development of new domain-specific inhibitors as research tools or therapeutic agents.-Kutil, Z., Skultetyova, L., Rauh, D., Meleshin, M., Snajdr, I., Novakova, Z., Mikesova, J., Pavlicek, J., Hadzima, M., Baranova, P., Havlinova, B., Majer, P., Schutkowski, M., Barinka, C. The unraveling of substrate specificity of histone deacetylase 6 domains using acetylome peptide microarrays and peptide libraries.

Untargeted Metabolite Profiling of Cerebrospinal Fluid Uncovers Biomarkers for Severity of Late Infantile Neuronal Ceroid Lipofuscinosis (CLN2, Batten Disease)

Late infantile neuronal ceroid lipofuscinosis (CLN2 disease) is a rare lysosomal storage disorder caused by a monogenetic deficiency of tripeptidyl peptidase-1 (TPP1). Despite knowledge that lipofuscin is the hallmark disease product, the relevant TPP1 substrate and its role in neuronal physiology/pathology is unknown. We hypothesized that untargeted metabolite profiling of cerebrospinal fluid (CSF) could be used as an effective tool to identify disease-associated metabolic disruptions in CLN2 disease, offering the potential to identify biomarkers that inform on disease severity and progression. Accordingly, a mass spectrometry-based untargeted metabolite profiling approach was employed to differentiate CSF from normal vs. CLN2 deficient individuals. Of 1,433 metabolite features surveyed, 29 linearly correlated with currently employed disease severity scores. With tandem mass spectrometry 8 distinct metabolite identities were structurally confirmed based on retention time and fragmentation pattern matches, vs. standards. These putative CLN2 biomarkers include 7 acetylated species - all attenuated in CLN2 compared to controls. Because acetate is the major bioenergetic fuel for support of mitochondrial respiration, deficient acetylated species in CSF suggests a brain energy defect that may drive neurodegeneration. Targeted analysis of these metabolites in CSF of CLN2 patients offers a powerful new approach for monitoring CLN2 disease progression and response to therapy.

'Head-to-Head' mRNA display for the translation of multi-copied proteins with a free C-terminus

With the development of various methods for affinity-based selection of proteins such as phage display, ribosomal display, and mRNA display, the progress in this field has been gradually shifting to function-based selection, such as through single-molecule observation, genetic selection, and compartmentalization technologies. In this vein, we present an opposite link mode of mRNA display termed as a 'Head-to-Head' (H2H) link. The key technique in H2H, formation of a covalent bond between O⁶-benzylguanine (BG) and O⁶-alkylguanine-DNA alkyltransferase (AGT), was demonstrated to be workable in H2H ligation, where mRNA is linked to a nascent AGT via a BG-DNA linker, resulting in a "(C-terminus) protein-BG-DNA linker-mRNA (5'-terminus)" conjugate. Thus, a head (N-terminus) to head (5'-terminus) linkage is formed. Among the advantages of H2H, the generation of multi-copied proteins is the most promising and was proven to be possible owing to the restored stop codon, which had been intentionally removed in the conventional mRNA display. Another advantage is obviously having a free C-terminus of the protein, which can be used for modifications such as C-terminal methylation, α-amidation, and others, which occur in nature. A superior merit of H2H is that it makes it possible to use a single construct commonly in mRNA display (affinity-based) and compartmentalization technologies (function-based) without requiring complicated construct changes.