Profiling of Protein N-Termini and Their Modifications in Complex Samples

Ion Mobility-Based Enrichment-Free N-Terminomics Analysis Reveals Novel Legumain Substrates in Murine Spleen

Aberrant levels of the asparaginyl endopeptidase legumain have been linked to inflammation, neurodegeneration, and cancer, yet our understanding of this protease is incomplete. Systematic attempts to identify legumain substrates have been previously confined to in vitro studies, which fail to mirror physiological conditions and obscure biologically relevant cleavage events. Using high-field asymmetric waveform ion mobility spectrometry (FAIMS), we developed a streamlined approach for proteome and N-terminome analyses without the need for N-termini enrichment. Compared to unfractionated proteomic analysis, we demonstrate FAIMS fractionation improves N-termini identification by >2.5 fold, resulting in the identification of >2882 unique N-termini from limited sample amounts. In murine spleens, this approach identifies 6366 proteins and 2528 unique N-termini, with 235 cleavage events enriched in WT compared to legumain-deficient spleens. Among these, 119 neo-N-termini arose from asparaginyl endopeptidase activities, representing novel putative physiological legumain substrates. The direct cleavage of selected substrates by legumain was confirmed using in vitro assays, providing support for the existence of physiologically relevant extra-lysosomal legumain activity. Combined, these data shed critical light on the functions of legumain and demonstrate the utility of FAIMS as an accessible method to improve depth and quality of N-terminomics studies.

Hallmarks of the metabolic secretome

The identification of novel secreted factors is advancing at an unprecedented pace. However, there is a critical need to consolidate and integrate this knowledge to provide a framework of their diverse mechanisms, functional significance, and inter-relationships. Complicating this effort are challenges related to nonstandardized methods, discrepancies in sample handling, and inconsistencies in the annotation of unknown molecules. This Review aims to synthesize the rapidly expanding field of the metabolic secretome, encompassing the five major types of secreted factors: proteins, peptides, metabolites, lipids, and extracellular vesicles. By systematically defining the functions and detection of the components within the metabolic secretome, this Review provides a primer into the advances of the field, and how integration of the techniques discussed can provide a deeper understanding of the mechanisms underlying metabolic homeostasis and its disorders.

Limited evidence for protein products of non-coding transcripts in the HEK293T cellular cytosol

Ribosome profiling has revealed translation outside of canonical coding sequences (CDSs) including translation of short upstream ORFs, long non-coding RNAs, overlapping ORFs, ORFs in UTRs or ORFs in alternative reading frames. Studies combining mass spectrometry, ribosome profiling and CRISPR-based screens showed that hundreds of ORFs derived from non-coding transcripts produce (micro)proteins, while other studies failed to find evidence for such types of non-canonical translation products. Here, we attempted to discover translation products from non-coding regions by strongly reducing the complexity of the sample prior to mass spectrometric analysis. We used an extended database as the search space and applied stringent filtering of the identified peptides to find evidence for novel translation events. We show that, theoretically our strategy facilitates the detection of translation events of transcripts from non-coding regions, but experimentally only find 19 peptides that might originate from such translation events. Finally, Virotrap based interactome analysis of two N-terminal proteoforms originating from non-coding regions finally showed the functional potential of these novel proteins.

Sensitive Plant N-Terminome Profiling with HUNTER

Protein N-termini provide unique and distinguishing information on proteolytically processed or N-terminally modified proteoforms. Also splicing, use of alternative translation initiation sites, and a variety of co- and post-translational N-terminal modifications generate distinct proteoforms that are unambiguously identified by their N-termini. However, N-terminal peptides are only a small fraction among all peptides generated in a shotgun proteome digest, are often of low stoichiometric abundance, and therefore require enrichment. Various protocols for enrichment of N-terminal peptides have been established and successfully been used for protease substrate discovery and profiling of N-terminal modification, but often require large amounts of proteome. We have recently established the High-efficiency Undecanal-based N-Termini EnRichment (HUNTER) as a fast and sensitive method to enable enrichment of protein N-termini from limited sample sources with as little as a few microgram proteome. Here we present our current HUNTER protocol for sensitive plant N-terminome profiling, including sample preparation, enrichment of N-terminal peptides, and mass spectrometry data analysis.

A User Guide to Validation, Annotation, and Evaluation of N-Terminome Datasets with MANTI

A large variety of enrichment procedures for protein N-termini have been developed to trace protease activity and determine precise cleavage sites, as well as other N-terminal protein modifications. Typically, enriched N-terminal peptides are identified by tandem mass spectrometry using standard database search engines, in many cases the popular MaxQuant software package. MaxQuant Advanced N-termini Interpreter (MANTI) is a software package that helps to validate, annotate, and visualize peptide identifications in N-termini datasets in a rapid and straightforward manner. Usage of MANTI and especially its graphical interface Yoğurtlu MANTI in detail are described to enable users to take full advantage of the software package and the multitude of options it has to offer.

Protein Processing in Plant Mitochondria Compared to Yeast and Mammals

Limited proteolysis, called protein processing, is an essential post-translational mechanism that controls protein localization, activity, and in consequence, function. This process is prevalent for mitochondrial proteins, mainly synthesized as precursor proteins with N-terminal sequences (presequences) that act as targeting signals and are removed upon import into the organelle. Mitochondria have a distinct and highly conserved proteolytic system that includes proteases with sole function in presequence processing and proteases, which show diverse mitochondrial functions with limited proteolysis as an additional one. In virtually all mitochondria, the primary processing of N-terminal signals is catalyzed by the well-characterized mitochondrial processing peptidase (MPP). Subsequently, a second proteolytic cleavage occurs, leading to more stabilized residues at the newly formed N-terminus. Lately, mitochondrial proteases, intermediate cleavage peptidase 55 (ICP55) and octapeptidyl protease 1 (OCT1), involved in proteolytic cleavage after MPP and their substrates have been described in the plant, yeast, and mammalian mitochondria. Mitochondrial proteins can also be processed by removing a peptide from their N- or C-terminus as a maturation step during insertion into the membrane or as a regulatory mechanism in maintaining their function. This type of limited proteolysis is characteristic for processing proteases, such as IMP and rhomboid proteases, or the general mitochondrial quality control proteases ATP23, m-AAA, i-AAA, and OMA1. Identification of processing protease substrates and defining their consensus cleavage motifs is now possible with the help of large-scale quantitative mass spectrometry-based N-terminomics, such as combined fractional diagonal chromatography (COFRADIC), charge-based fractional diagonal chromatography (ChaFRADIC), or terminal amine isotopic labeling of substrates (TAILS). This review summarizes the current knowledge on the characterization of mitochondrial processing peptidases and selected N-terminomics techniques used to uncover protease substrates in the plant, yeast, and mammalian mitochondria.

Deep N-terminomics of Mycobacterium tuberculosis H37Rv extensively correct annotated encoding genes

Mycobacterium tuberculosis (MTB) is a severe causing agent of tuberculosis (TB). Although H37Rv, the type strain of M. tuberculosis was sequenced in 1998, annotation errors of encoding genes have been frequently reported in hundreds of papers. This phenomenon is particularly severe at the 5' end of the genes. Here, we applied a TMPP [(N-Succinimidyloxycarbonylmethyl) tris (2,4,6-trimethoxyphenyl) phosphonium bromide] labeling combined with StageTip separating strategy on M. tuberculosis H37Rv to characterize the N-terminal start sites of its annotated encoding genes. Totally, 1047 proteins were identified with 2058 TMPP labeled N-terminal peptides from all the 2625 mass spectrometer (MS) sequenced proteins. Comparative genomics analysis allowed the re-annotation of 43 proteins' N-termini in H37Rv and 762 proteins in Mycobacteriaceae. All revised N-termini start sites were distributed in 5'-UTR of annotated genes due to over-annotation of previous N-terminal initiation codon, especially the ATG. In addition, we identified and verified a novel gene Rv1078A in +3 frame different from the annotated gene Rv1078 in +2 frame. Altogether, our findings contribute to the better understanding of N-terminal of H37Rv and other species from Mycobacteriaceae that can assist future studies on biological study.

Proteolysis and inflammation of the kidney glomerulus

Proteases play a central role in regulating renal pathophysiology and are increasingly evaluated as actionable drug targets. Here, we review the role of proteolytic systems in inflammatory kidney disease. Inflammatory kidney diseases are associated with broad dysregulations of extracellular and intracellular proteolysis. As an example of a proteolytic system, the complement system plays a significant role in glomerular inflammatory kidney disease and is currently under clinical investigation. Based on two glomerular kidney diseases, lupus nephritis, and membranous nephropathy, we portrait two proteolytic pathomechanisms and the role of the complement system. We discuss how profiling proteolytic activity in patient samples could be used to stratify patients for more targeted interventions in inflammatory kidney diseases. We also describe novel comprehensive, quantitative tools to investigate the entirety of proteolytic processes in a tissue sample. Emphasis is placed on mass spectrometric approaches that enable the comprehensive analysis of the complement system, as well as protease activities and regulation in general.

Abundance of metalloprotease FtsH12 modulates chloroplast development in Arabidopsis thaliana

The ATP-dependent metalloprotease FtsH12 (filamentation temperature sensitive protein H 12) has been suggested to participate in a heteromeric motor complex, driving protein translocation into the chloroplast. FtsH12 was immuno-detected in proplastids, seedlings, leaves, and roots. Expression of Myc-tagged FtsH12 under its native promotor allowed identification of FtsHi1, 2, 4, and 5, and plastidic NAD-malate dehydrogenase, five of the six interaction partners in the suggested import motor complex. Arabidopsis thaliana mutant seedlings with reduced FTSH12 abundance exhibited pale cotyledons and small, deformed chloroplasts with altered thylakoid structure. Mature plants retained these chloroplast defects, resulting in slightly variegated leaves and lower chlorophyll content. Label-free proteomics revealed strong changes in the proteome composition of FTSH12 knock-down seedlings, reflecting impaired plastid development. The composition of the translocon on the inner chloroplast membrane (TIC) protein import complex was altered, with coordinated reduction of the FtsH12-FtsHi complex subunits and accumulation of the 1 MDa TIC complex subunits TIC56, TIC214 and TIC22-III. FTSH12 overexpressor lines showed no obvious phenotype, but still displayed distinct differences in their proteome. N-terminome analyses further demonstrated normal proteolytic maturation of plastid-imported proteins irrespective of FTSH12 abundance. Together, our data suggest that FtsH12 has highest impact during seedling development; its abundance alters the plastid import machinery and impairs chloroplast development.

Identification of Putative Mitochondrial Protease Substrates

Mitochondrial proteases constitute a fundamental part of the organellar protein quality control system to ensure the timely removal of damaged or obsolete proteins. The analysis of proteases is often limited to the identification of bona fide substrates that are degraded in the presence and become more abundant in the absence of the respective protease. However, proteases in numerous organisms from bacteria to humans can process specific substrates to release shortened proteins with potentially altered activities. Here, we describe an adaptation of the substrate-trapping approach, as well as the N-terminal profiling protocol Terminal Amine Isotope Labeling of Substrates (TAILS) for the identification of bona fide substrates and mitochondrial proteins that undergo complete or partial proteolysis.

The Mouse Heart Mitochondria N Terminome Provides Insights into ClpXP-Mediated Proteolysis

The mammalian mitochondrial proteome consists of more than 1100 annotated proteins and their proteostasis is regulated by only a few ATP-dependent protease complexes. Technical advances in protein mass spectrometry allowed for detailed description of the mitoproteome from different species and tissues and their changes under specific conditions. However, protease-substrate relations within mitochondria are still poorly understood. Here, we combined Terminal Amine Isotope Labeling of Substrates (TAILS) N termini profiling of heart mitochondria proteomes isolated from wild type and Clpp-/- mice with a classical substrate-trapping screen using FLAG-tagged proteolytically active and inactive CLPP variants to identify new ClpXP substrates in mammalian mitochondria. Using TAILS, we identified N termini of more than 200 mitochondrial proteins. Expected N termini confirmed sequence determinants for mitochondrial targeting signal (MTS) cleavage and subsequent N-terminal processing after import, but the majority were protease-generated neo-N termini mapping to positions within the proteins. Quantitative comparison revealed widespread changes in protein processing patterns, including both strong increases or decreases in the abundance of specific neo-N termini, as well as an overall increase in the abundance of protease-generated neo-N termini in CLPP-deficient mitochondria that indicated altered mitochondrial proteostasis. Based on the combination of altered processing patterns, protein accumulation and stabilization in CLPP-deficient mice and interaction with CLPP, we identified OAT, HSPA9 and POLDIP2 and as novel bona fide ClpXP substrates. Finally, we propose that ClpXP participates in the cooperative degradation of UQCRC1. Together, our data provide the first landscape of the heart mitochondria N terminome and give further insights into regulatory and assisted proteolysis mediated by ClpXP.

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.

Photoprotective Acclimation of the Arabidopsis thaliana Leaf Proteome to Fluctuating Light

Plants are subjected to strong fluctuations in light intensity in their natural growth environment, caused both by unpredictable changes due to weather conditions and movement of clouds and upper canopy leaves and predictable changes during day-night cycle. The mechanisms of long-term acclimation to fluctuating light (FL) are still not well understood. Here, we used quantitative mass spectrometry to investigate long-term acclimation of low light-grown Arabidopsis thaliana to a FL condition that induces mild photooxidative stress. On the third day of exposure to FL, young and mature leaves were harvested in the morning and at the end of day for proteome analysis using a stable isotope labeling approach. We identified 2,313 proteins, out of which 559 proteins exhibited significant changes in abundance in at least one of the four experimental groups (morning-young, morning-mature, end-of-day-young, end-of-day-mature). A core set of 49 proteins showed significant responses to FL in three or four experimental groups, which included enhanced accumulation of proteins involved in photoprotection, cyclic electron flow around photosystem I, photorespiration, and glycolysis, while specific glutathione transferases and proteins involved in translation and chlorophyll biosynthesis were reduced in abundance. In addition, we observed pathway- and protein-specific changes predominantly at the end of day, whereas few changes were observed exclusively in the morning. Comparison of the proteome data with the matching transcript data revealed gene- and protein-specific responses, with several chloroplast-localized proteins decreasing in abundance despite increased gene expression under FL. Together, our data shows moderate but widespread alterations of protein abundance during acclimation to FL and suggests an important role of post-transcriptional regulation of protein abundance.

Sensitive Determination of Proteolytic Proteoforms in Limited Microscale Proteome Samples

Protein N termini unambiguously identify truncated, alternatively translated or modified proteoforms with distinct functions and reveal perturbations in disease. Selective enrichment of N-terminal peptides is necessary to achieve proteome-wide coverage for unbiased identification of site-specific regulatory proteolytic processing and protease substrates. However, many proteolytic processes are strictly confined in time and space and therefore can only be analyzed in minute samples that provide insufficient starting material for current enrichment protocols. Here we present High-efficiency Undecanal-based N Termini EnRichment (HUNTER), a robust, sensitive and scalable method for the analysis of previously inaccessible microscale samples. HUNTER achieved identification of >1000 N termini from as little as 2 μg raw HeLa cell lysate. Broad applicability is demonstrated by the first N-terminome analysis of sorted human primary immune cells and enriched mitochondrial fractions from pediatric cancer patients, as well as protease substrate identification from individual Arabidopsis thaliana wild type and Vacuolar Processing Enzyme-deficient mutant seedlings. We further implemented the workflow on a liquid handling system and demonstrate the feasibility of clinical degradomics by automated processing of liquid biopsies from pediatric cancer patients.

The cellular prion protein and its derived fragments in human prion diseases and their role as potential biomarkers

Introduction: Human prion diseases are a heterogeneous group of incurable and debilitating conditions characterized by a progressive degeneration of the central nervous system. The conformational changes of the cellular prion protein and its formation into an abnormal isoform, spongiform degeneration, neuronal loss, and neuroinflammation are central to prion disease pathogenesis. It has been postulated that truncated variants of aggregation-prone proteins are implicated in neurodegenerative mechanisms. An increasing body of evidence indicates that proteolytic fragments and truncated variants of the prion protein are formed and accumulated in the brain of prion disease patients. These prion protein variants provide a high degree of relevance to disease pathology and diagnosis. Areas covered: In the present review, we summarize the current knowledge on the occurrence of truncated prion protein species and their potential roles in pathophysiological states during prion diseases progression. In addition, we discuss their usability as a diagnostic biomarker in prion diseases. Expert opinion: Either as a primary factor in the formation of prion diseases or as a consequence from neuropathological affection, abnormal prion protein variants and fragments may provide independent information about mechanisms of prion conversion, pathological states, or disease progression.

The Scope, Functions, and Dynamics of Posttranslational Protein Modifications

Assessing posttranslational modification (PTM) patterns within protein molecules and reading their functional implications present grand challenges for plant biology. We combine four perspectives on PTMs and their roles by considering five classes of PTMs as examples of the broader context of PTMs. These include modifications of the N terminus, glycosylation, phosphorylation, oxidation, and N-terminal and protein modifiers linked to protein degradation. We consider the spatial distribution of PTMs, the subcellular distribution of modifying enzymes, and their targets throughout the cell, and we outline the complexity of compartmentation in understanding of PTM function. We also consider PTMs temporally in the context of the lifetime of a protein molecule and the need for different PTMs for assembly, localization, function, and degradation. Finally, we consider the combined action of PTMs on the same proteins, their interactions, and the challenge ahead of integrating PTMs into an understanding of protein function in plants.

New beginnings and new ends: methods for large-scale characterization of protein termini and their use in plant biology

Dynamic regulation of protein function and abundance plays an important role in virtually every aspect of plant life. Diversifying mechanisms at the RNA and protein level result in many protein molecules with distinct sequence and modification, termed proteoforms, arising from a single gene. Distinct protein termini define proteoforms arising from translation of alternative transcripts, use of alternative translation initiation sites, and different co- and post-translational modifications of the protein termini. Also site-specific proteolytic processing by endo- and exoproteases generates truncated proteoforms, defined by distinct protease-generated neo-N- and neo-C-termini, that may exhibit altered activity, function, and localization compared with their precursor proteins. In eukaryotes, the N-degron pathway targets cytosolic proteins, exposing destabilizing N-terminal amino acids and/or destabilizing N-terminal modifications for proteasomal degradation. This enables rapid and selective removal not only of unfolded proteins, but also of substrate proteoforms generated by proteolytic processing or changes in N-terminal modifications. Here we summarize current protocols enabling proteome-wide analysis of protein termini, which have provided important new insights into N-terminal modifications and protein stability determinants, protein maturation pathways, and protease-substrate relationships in plants.

Quantitative proteomics in plant protease substrate identification

Contents Summary 936 I. Introduction 936 II. The quest for plant protease substrates - proteomics to the rescue? 937 III. Quantitative proteome comparison reveals candidate substrates 938 IV. Dynamic metabolic stable isotope labeling to measure protein turnover in vivo 938 V. Terminomics - large-scale identification of protease cleavage sites 939 VI. Substrate or not substrate, that is the question 940 VII. Concluding remarks 941 Acknowledgements 941 References 941 SUMMARY: Proteolysis is a central regulatory mechanism of protein homeostasis and protein function that affects all aspects of plant life. Higher plants encode for hundreds of proteases, but their physiological substrates and hence their molecular functions remain mostly unknown. Current quantitative mass spectrometry-based proteomics enables unbiased large-scale interrogation of the proteome and its modifications. Here we provide an overview of proteomics techniques that allow profiling of changes in protein abundance, measurement of proteome turnover rates, identification of protease cleavage sites in vivo and in vitro and determination of protease sequence specificity. We discuss how these techniques can help to reveal protease substrates and determine plant protease function, illustrated by recent studies on selected plant proteases.