High-Throughput Characterization of Histidine Phosphorylation Sites Using UPAX and Tandem Mass Spectrometry

Not your Mother's MAPKs: Apicomplexan MAPK function in daughter cell budding

Reversible phosphorylation by protein kinases is one of the core mechanisms by which biological signals are propagated and processed. Mitogen-activated protein kinases, or MAPKs, are conserved throughout eukaryotes where they regulate cell cycle, development, and stress response. Here, we review advances in our understanding of the function and biochemistry of MAPK signaling in apicomplexan parasites. As expected for well-conserved signaling modules, MAPKs have been found to have multiple essential roles regulating both Toxoplasma tachyzoite replication and sexual differentiation in Plasmodium. However, apicomplexan MAPK signaling is notable for the lack of the canonical kinase cascade that normally regulates the networks, and therefore must be regulated by a distinct mechanism. We highlight what few regulatory relationships have been established to date, and discuss the challenges to the field in elucidating the complete MAPK signaling networks in these parasites.

Histidine phosphorylation in human cells; a needle or phantom in the haystack?

It has been suggested that in mammalian cells histidine residues in proteins may become as frequently phosphorylated as serine, threonine and tyrosine, and may play a key role in mammalian signaling. Here we applied a robust workflow that earlier allowed us to detect histidine phosphorylation in bacteria unambiguously, to probe for histidine phosphorylation in four human cell lines. Initially, seemingly hundreds of protein histidine phosphorylations were picked up in all studied human cell lines. However, careful examination of the data, and several control experiments, led us to the conclusion that >99% of these initially assigned pHis sites were not genuine, and should be site localized to neighboring Ser/Thr residues. Nevertheless, our methods are selective enough to detect just a handful of genuine pHis sites in mammalian cells, representing well-known enzymatic intermediates. Consequently, we do not find any evidence in our data supporting that protein histidine phosphorylation plays a role in mammalian signaling.

ProMetheusDB: An In-Depth Analysis of the High-Quality Human Methyl-proteome

Protein arginine (R) methylation is a post-translational modification involved in various biological processes, such as RNA splicing, DNA repair, immune response, signal transduction, and tumor development. Although several advancements were made in the study of this modification by mass spectrometry, researchers still face the problem of a high false discovery rate. We present a dataset of high-quality methylations obtained from several different heavy methyl stable isotope labeling with amino acids in cell culture experiments analyzed with a machine learning–based tool and show that this model allows for improved high-confidence identification of real methyl-peptides. Overall, our results are consistent with the notion that protein R methylation modulates protein–RNA interactions and suggest a role in rewiring protein–protein interactions, for which we provide experimental evidence for a representative case (i.e., NONO [non-POU domain–containing octamer-binding protein]–paraspeckle component 1 [PSPC1]). Upon intersecting our R-methyl-sites dataset with the PhosphoSitePlus phosphorylation dataset, we observed that R methylation correlates differently with S/T-Y phosphorylation in response to various stimuli. Finally, we explored the application of heavy methyl stable isotope labeling with amino acids in cell culture to identify unconventional methylated residues and successfully identified novel histone methylation marks on serine 28 and threonine 32 of H3. The database generated, named ProMetheusDB, is freely accessible at https://bioserver.ieo.it/shiny/app/prometheusdb.

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hmSEEKER 2.0 identifies methyl-peptides from hmSILAC data through machine learning.

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Arginine methylation plays a role in modulating protein–protein interactions.

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Arginine methylations occur more frequently in proximity of phosphorylation sites.

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hmSEEKER 2.0 was used to identify methylations occurring on nonstandard amino acids.

IreK-Mediated, Cell Wall-Protective Phosphorylation in Enterococcus faecalis

Enterococcus faecalis is a Gram-positive bacterium that is a major cause of hospital-acquired infections due, in part, to its intrinsic resistance to cell wall-active antimicrobials. One critical determinant of this resistance is the transmembrane kinase IreK, which belongs to the penicillin-binding protein and serine/threonine kinase-associated kinase family of bacterial signaling proteins involved with the regulation of cell wall homeostasis. The activity of IreK is enhanced in response to cell wall stress, but direct substrates of IreK phosphorylation, leading to antimicrobial resistance, are largely unknown. To better understand stress-modulated phosphorylation events contributing to antimicrobial resistance, wild type E. faecalis cells treated with cell wall-active antimicrobials, chlorhexidine or ceftriaxone, were examined via phosphoproteomics. Among the most prominent changes was increased phosphorylation of divisome components after both treatments, suggesting that E. faecalis modulates cell division in response to cell wall stress. Phosphorylation mediated by IreK was then determined via a similar analysis with a E. faecalis ΔireK mutant strain, revealing potential IreK substrates involved with the regulation of peptidoglycan biosynthesis and within the E. faecalis CroS/R two-component system, another signal transduction pathway that promotes antimicrobial resistance. These results reveal critical insights into the biological functions of IreK.

Strategies for mass spectrometry-based phosphoproteomics using isobaric tagging

INTRODUCTION

Protein phosphorylation is a primary mechanism of signal transduction in cellular systems. Isobaric tagging can be used to investigate alterations in phosphorylation events in sample multiplexing experiments where quantification extends across all conditions. As such, innovations in tandem mass tag methods can facilitate the expansion of the depth and breadth of phosphoproteomic analyses.

AREAS COVERED

This review discusses the current state of tandem mass tag-centric phosphoproteomics and highlights advances in reagent chemistry, instrumentation, data acquisition, and data analysis. We stress that approaches for phosphoproteomic investigations require high-specificity enrichment, sensitive detection, and accurate phosphorylation site localization.

EXPERT OPINION

Tandem mass tag-centric phosphoproteomics will continue to be an important conduit for our understanding of signal transduction in living organisms. We anticipate that progress in phosphopeptide enrichment methodologies, enhancements in instrumentation and data acquisition technologies, and further refinements in analytical strategies will be key to the discovery of biologically relevant findings from phosphoproteomics studies.

WIDENING THE BOTTLENECK OF PHOSPHOPROTEOMICS: EVOLVING STRATEGIES FOR PHOSPHOPEPTIDE ENRICHMENT

Phosphorylation is a form of protein posttranslational modification (PTM) that regulates many biological processes. Whereas phosphoproteomics is a scientific discipline that identifies and quantifies the phosphorylated proteome using mass spectrometry (MS). This task is extremely challenging as ~30% of the human proteome is phosphorylated; and each phosphoprotein may exist as multiple phospho-isoforms that are present in low abundance and stoichiometry. Hence, phosphopeptide enrichment techniques are indispensable to (phospho)proteomics laboratories. These enrichment methods encompass widely-adopted techniques such as (i) affinity-based chromatography; (ii) ion exchange and mixed-mode chromatography (iii) enrichment with phospho-specific antibodies and protein domains, and (iv) functionalized polymers and other less common but emerging technologies such as hydroxyapatite chromatography and precipitation with inorganic ions. Here, we review these techniques, their history, continuous development and evaluation. Besides, we outline associating challenges of phosphoproteomics that are linked to experimental design, sample preparation, and proteolytic digestion. In addition, we also discuss about the future outlooks in phosphoproteomics, focusing on elucidating the noncanonical phosphoproteome and deciphering the "dark phosphoproteome". © 2020 John Wiley & Sons Ltd.

Advances in quantitative high-throughput phosphoproteomics with sample multiplexing

Eukaryotic protein phosphorylation modulates nearly every major biological process. Phosphorylation regulates protein activity, mediates cellular signal transduction, and manipulates cellular structure. Consequently, the dysregulation of kinase and phosphatase pathways has been linked to a multitude of diseases. Mass spectrometry-based proteomic techniques are increasingly used for the global interrogation of perturbations in phosphorylation-based cellular signaling. Strategies for studying phosphoproteomes require high-specificity enrichment, sensitive detection, and accurate localization of phosphorylation sites with advanced LC-MS/MS techniques and downstream informatics. Sample multiplexing with isobaric tags has also been integral to recent advancements in throughput and sensitivity for phosphoproteomic studies. Each of these facets of phosphoproteomics analysis present distinct challenges and thus opportunities for improvement and innovation. Here, we review current methodologies, explore persistent challenges, and discuss the outlook for isobaric tag-based quantitative phosphoproteomic analysis.

Bis(zinc(II)-dipicolylamine)-functionalized sub-2 μm core-shell microspheres for the analysis of N-phosphoproteome

Protein N-phosphorylation plays a critical role in central metabolism and two/multicomponent signaling of prokaryotes. However, the current enrichment methods for O-phosphopeptides are not preferred for N-phosphopeptides due to the intrinsic lability of P-N bond under acidic conditions. Therefore, the effective N-phosphoproteome analysis remains challenging. Herein, bis(zinc(II)-dipicolylamine)-functionalized sub-2 μm core-shell silica microspheres (SiO₂@DpaZn) are tailored for rapid and effective N-phosphopeptides enrichment. Due to the coordination of phosphate groups to Zn(II), N-phosphopeptides can be effectively captured under neutral conditions. Moreover, the method is successfully applied to an E.coli and HeLa N-phosphoproteome study. These results further broaden the range of methods for the discovery of N-phosphoproteins with significant biological functions.

N-phosphorylation plays a critical role in central metabolism and signaling processes, however, enrichment methods for N-phosphopeptides are limited by the P-N bond lability. Here, the authors report the synthesis and use of silica microspheres functionalized with bis(zinc(II)-dipicolylamine) in N-phosphopeptides effective enrichment.

NME/NM23/NDPK and Histidine Phosphorylation

The NME (Non-metastatic) family members, also known as NDPKs (nucleoside diphosphate kinases), were originally identified and studied for their nucleoside diphosphate kinase activities. This family of kinases is extremely well conserved through evolution, being found in prokaryotes and eukaryotes, but also diverges enough to create a range of complexity, with homologous members having distinct functions in cells. In addition to nucleoside diphosphate kinase activity, some family members are reported to possess protein-histidine kinase activity, which, because of the lability of phosphohistidine, has been difficult to study due to the experimental challenges and lack of molecular tools. However, over the past few years, new methods to investigate this unstable modification and histidine kinase activity have been reported and scientific interest in this area is growing rapidly. This review presents a global overview of our current knowledge of the NME family and histidine phosphorylation, highlighting the underappreciated protein-histidine kinase activity of NME family members, specifically in human cells. In parallel, information about the structural and functional aspects of the NME family, and the knowns and unknowns of histidine kinase involvement in cell signaling are summarized.