Extracellular Ser/Thr/Tyr phosphorylated proteins ofPseudomonas aeruginosaPA14 strain

Insights into peculiar fungal LPMO family members holding a short C-terminal sequence reminiscent of phosphate binding motifs

Lytic polysaccharide monooxygenases (LPMOs) are taxonomically widespread copper-enzymes boosting biopolymers conversion (e.g. cellulose, chitin) in Nature. White-rot Polyporales, which are major fungal wood decayers, may possess up to 60 LPMO-encoding genes belonging to the auxiliary activities family 9 (AA9). Yet, the functional relevance of such multiplicity remains to be uncovered. Previous comparative transcriptomic studies of six Polyporales fungi grown on cellulosic substrates had shown the overexpression of numerous AA9-encoding genes, including some holding a C-terminal domain of unknown function ("X282"). Here, after carrying out structural predictions and phylogenetic analyses, we selected and characterized six AA9-X282s with different C-term modularities and atypical features hitherto unreported. Unexpectedly, after screening a large array of conditions, these AA9-X282s showed only weak binding properties to cellulose, and low to no cellulolytic oxidative activity. Strikingly, proteomic analysis revealed the presence of multiple phosphorylated residues at the surface of these AA9-X282s, including a conserved residue next to the copper site. Further analyses focusing on a 9 residues glycine-rich C-term extension suggested that it could hold phosphate-binding properties. Our results question the involvement of these AA9 proteins in the degradation of plant cell wall and open new avenues as to the divergence of function of some AA9 members.

Bacterial Chitinases and Their Role in Human Infection

It has been widely appreciated that numerous bacterial species express chitinases for the purpose of degrading environmental chitin. However, chitinases and chitin-binding proteins are also expressed by pathogenic bacterial species during infection even though mammals do not produce chitin. Alternative molecular targets are therefore likely present within the host. Here, we will describe our current understanding of chitinase/chitin-binding proteins as virulence factors that promote bacterial colonization and infection. The targets of these chitinases in the host have been shown to include immune system components, mucins, and surface glycans. Bacterial chitinases have also been shown to interact with other microorganisms, targeting the peptidoglycan or chitin in the bacterial and fungal cell wall, respectively. This review highlights that even though the name "chitinase" implies activity toward chitin, chitinases can have a wide diversity of targets, including ones relevant to host infection. Chitinases may therefore be useful as a target of future anti-infective therapeutics.

A Review of the Bacterial Phosphoproteomes of Beneficial Microbes

The number and variety of protein post-translational modifications (PTMs) found and characterized in bacteria over the past ten years have increased dramatically. Compared to eukaryotic proteins, most post-translational protein changes in bacteria affect relatively few proteins because the majority of modified proteins exhibit substoichiometric modification levels, which makes structural and functional analyses challenging. In addition, the number of modified enzymes in bacterial species differs widely, and degrees of proteome modification depend on environmental conditions. Nevertheless, evidence suggests that protein PTMs play essential roles in various cellular processes, including nitrogen metabolism, protein synthesis and turnover, the cell cycle, dormancy, spore germination, sporulation, persistence, and virulence. Additional investigations on protein post-translational changes will undoubtedly close knowledge gaps in bacterial physiology and create new means of treating infectious diseases. Here, we describe the role of the post-translation phosphorylation of major bacterial proteins and review the progress of research on phosphorylated proteins depending on bacterial species.

The crystal structure of CbpD clarifies substrate-specificity motifs in chitin-active lytic polysaccharide monooxygenases

Pseudomonas aeruginosa secretes diverse proteins via its type 2 secretion system, including a 39 kDa chitin-binding protein, CbpD. CbpD has recently been shown to be a lytic polysaccharide monooxygenase active on chitin and to contribute substantially to virulence. To date, no structure of this virulence factor has been reported. Its first two domains are homologous to those found in the crystal structure of Vibrio cholerae GbpA, while the third domain is homologous to the NMR structure of the CBM73 domain of Cellvibrio japonicus CjLPMO10A. Here, the 3.0 Å resolution crystal structure of CbpD solved by molecular replacement is reported, which required ab initio models of each CbpD domain generated by the artificial intelligence deep-learning structure-prediction algorithm RoseTTAFold. The structure of CbpD confirms some previously reported substrate-specificity motifs among LPMOAA10s, while challenging the predictive power of others. Additionally, the structure of CbpD shows that post-translational modifications occur on the chitin-binding surface. Moreover, the structure raises interesting possibilities about how type 2 secretion-system substrates may interact with the secretion machinery and demonstrates the utility of new artificial intelligence protein structure-prediction algorithms in making challenging structural targets tractable.

Quantitative phosphoproteomics reveals the effect of baeSR and acrB genes on protein phosphorylation in Salmonella enterica serovar Typhimurium

The BaeSR two-component system and the AcrB efflux pump are closely associated with Salmonella resistance to antibiotics. However, the relationship between the two-component system, efflux pumps and protein phosphorylation of Salmonella is poorly understood. In this study, S. typhimurium ciprofloxacin-resistant strain CR, baeSR gene deletion strain CR^ΔbaeSR, acrB gene deletion strain CR^ΔacrB, and double gene deletion strain CR^{ΔbaeSRΔacrB} were used to explore phosphorylated proteins with significant difference, based on non-marker, quantitative phosphorylation modified proteomics technique. Consequently, 363 phosphosites of 213 phosphoproteins were identified in the four strains. More than 70% of the phosphosites were serine phosphorylation. In the CR^ΔbaeSR/CR, CR^ΔacrB/CR and CR^{ΔbaeSRΔacrB}/CR comparison groups, 36, 37 and 49 phosphosites were significantly altered, respectively. Bioinformatic analysis revealed that the main enrichment pathways of these differentially phosphorylated proteins were metabolic pathways, biosynthesis of antibiotics, phosphotransferase system (PTS), ABC transporters, and lipopolysaccharide biosynthesis. Furthermore, 21 differentially phosphorylated proteins were identified to be associated with antibiotic resistance. These results suggest that the BaeSR two-component system and the AcrB efflux pump affect the phosphorylation of proteins in S. typhimurium and may influence the drug resistance and virulence of S. typhimurium by affecting protein phosphorylation, providing a new idea to explore the mechanism of drug resistance in Salmonella.

Phosphorylation of Extracellular Proteins in Acinetobacter baumannii in Sessile Mode of Growth

Acinetobacter baumannii is a problematic nosocomial pathogen owing to its increasing resistance to antibiotics and its great ability to survive in the hospital environment, which is linked to its capacity to form biofilms. Structural and functional investigations of post-translational modifications, such as phosphorylations, may lead to identification of candidates for therapeutic targets against this pathogen. Here, we present the first S/T/Y phosphosecretome of two A. baumannii strains, the reference strain ATCC 17978 and the virulent multi-drug resistant strain AB0057, cultured in two modes of growth (planktonic and biofilm) using TiO2 chromatography followed by high resolution mass spectrometry. In ATCC 17978, we detected a total of 137 (97 phosphoproteins) and 52 (33 phosphoproteins) phosphosites in biofilm and planktonic modes of growth, respectively. Similarly, in AB0057, 155 (119 phosphoproteins) and 102 (74 phosphoproteins) phosphosites in biofilm and planktonic modes of growth were identified, respectively. Both strains in the biofilm mode of growth showed a higher number of phosphosites and phosphoproteins compared to planktonic growth. Several phosphorylated sites are localized in key regions of proteins involved in either drug resistance (β-lactamases), adhesion to host tissues (pilins), or protein secretion (Hcp). Site-directed mutagenesis of the Hcp protein, essential for type VI secretion system-mediated interbacterial competition, showed that four of the modified residues are essential for type VI secretion system activity.

The lytic polysaccharide monooxygenase CbpD promotes Pseudomonas aeruginosa virulence in systemic infection

The recently discovered lytic polysaccharide monooxygenases (LPMOs), which cleave polysaccharides by oxidation, have been associated with bacterial virulence, but supporting functional data is scarce. Here we show that CbpD, the LPMO of Pseudomonas aeruginosa, is a chitin-oxidizing virulence factor that promotes survival of the bacterium in human blood. The catalytic activity of CbpD was promoted by azurin and pyocyanin, two redox-active virulence factors also secreted by P. aeruginosa. Homology modeling, molecular dynamics simulations, and small angle X-ray scattering indicated that CbpD is a monomeric tri-modular enzyme with flexible linkers. Deletion of cbpD rendered P. aeruginosa unable to establish a lethal systemic infection, associated with enhanced bacterial clearance in vivo. CbpD-dependent survival of the wild-type bacterium was not attributable to dampening of pro-inflammatory responses by CbpD ex vivo or in vivo. Rather, we found that CbpD attenuates the terminal complement cascade in human serum. Studies with an active site mutant of CbpD indicated that catalytic activity is crucial for virulence function. Finally, profiling of the bacterial and splenic proteomes showed that the lack of this single enzyme resulted in substantial re-organization of the bacterial and host proteomes. LPMOs similar to CbpD occur in other pathogens and may have similar immune evasive functions.

Arming the troops: Post-translational modification of extracellular bacterial proteins

Protein secretion is almost universally employed by bacteria. Some proteins are retained on the cell surface, whereas others are released into the extracellular milieu, often playing a key role in virulence. In this review, we discuss the diverse types and potential functions of post-translational modifications (PTMs) occurring to extracellular bacterial proteins.

Exoproteomics for Better Understanding Pseudomonas aeruginosa Virulence

Pseudomonas aeruginosa is the most common human opportunistic pathogen associated with nosocomial diseases. In 2017, the World Health Organization has classified P. aeruginosa as a critical agent threatening human health, and for which the development of new treatments is urgently necessary. One interesting avenue is to target virulence factors to understand P. aeruginosa pathogenicity. Thus, characterising exoproteins of P. aeruginosa is a hot research topic and proteomics is a powerful approach that provides important information to gain insights on bacterial virulence. The aim of this review is to focus on the contribution of proteomics to the studies of P. aeruginosa exoproteins, highlighting its relevance in the discovery of virulence factors, post-translational modifications on exoproteins and host-pathogen relationships.

Importance of protein Ser/Thr/Tyr phosphorylation for bacterial pathogenesis

Protein phosphorylation regulates a large variety of biological processes in all living cells. In pathogenic bacteria, the study of serine, threonine, and tyrosine (Ser/Thr/Tyr) phosphorylation has shed light on the course of infectious diseases, from adherence to host cells to pathogen virulence, replication, and persistence. Mass spectrometry (MS)-based phosphoproteomics has provided global maps of Ser/Thr/Tyr phosphosites in bacterial pathogens. Despite recent developments, a quantitative and dynamic view of phosphorylation events that occur during bacterial pathogenesis is currently lacking. Temporal, spatial, and subpopulation resolution of phosphorylation data is required to identify key regulatory nodes underlying bacterial pathogenesis. Herein, we discuss how technological improvements in sample handling, MS instrumentation, data processing, and machine learning should improve bacterial phosphoproteomic datasets and the information extracted from them. Such information is expected to significantly extend the current knowledge of Ser/Thr/Tyr phosphorylation in pathogenic bacteria and should ultimately contribute to the design of novel strategies to combat bacterial infections.

Goals and Challenges in Bacterial Phosphoproteomics

Reversible protein phosphorylation at serine, threonine and tyrosine is a well-known dynamic post-translational modification with stunning regulatory and signalling functions in eukaryotes. Shotgun phosphoproteomic analyses revealed that this post-translational modification is dramatically lower in bacteria than in eukaryotes. However, Ser/Thr/Tyr phosphorylation is present in all analysed bacteria (24 eubacteria and 1 archaea). It affects central processes, such as primary and secondary metabolism development, sporulation, pathogenicity, virulence or antibiotic resistance. Twenty-nine phosphoprotein orthologues were systematically identified in bacteria: ribosomal proteins, enzymes from glycolysis and gluconeogenesis, elongation factors, cell division proteins, RNA polymerases, ATP synthases and enzymes from the citrate cycle. While Ser/Thr/Tyr phosphorylation exists in bacteria, there is a consensus that histidine phosphorylation is the most abundant protein phosphorylation in prokaryotes. Unfortunately, histidine shotgun phosphorproteomics is not possible due to the reduced phosphohistidine half-life under the acidic pH conditions used in standard LC-MS/MS analysis. However, considering the fast and continuous advances in LC-MS/MS-based phosphoproteomic methodologies, it is expected that further innovations will allow for the study of His phosphoproteomes and a better coverage of bacterial phosphoproteomes. The characterisation of the biological role of bacterial Ser/Thr/Tyr and His phosphorylations might revolutionise our understanding of prokaryotic physiology.

Advanced Proteomics as a Powerful Tool for Studying Toxins of Human Bacterial Pathogens

Exotoxins contribute to the infectious processes of many bacterial pathogens, mainly by causing host tissue damages. The production of exotoxins varies according to the bacterial species. Recent advances in proteomics revealed that pathogenic bacteria are capable of simultaneously producing more than a dozen exotoxins. Interestingly, these toxins may be subject to post-transcriptional modifications in response to environmental conditions. In this review, we give an outline of different bacterial exotoxins and their mechanism of action. We also report how proteomics contributed to immense progress in the study of toxinogenic potential of pathogenic bacteria over the last two decades.

Protein post-translational modifications in bacteria

Over the past decade the number and variety of protein post-translational modifications that have been detected and characterized in bacteria have rapidly increased. Most post-translational protein modifications occur in a relatively low number of bacterial proteins in comparison with eukaryotic proteins, and most of the modified proteins carry low, substoichiometric levels of modification; therefore, their structural and functional analysis is particularly challenging. The number of modifying enzymes differs greatly among bacterial species, and the extent of the modified proteome strongly depends on environmental conditions. Nevertheless, evidence is rapidly accumulating that protein post-translational modifications have vital roles in various cellular processes such as protein synthesis and turnover, nitrogen metabolism, the cell cycle, dormancy, sporulation, spore germination, persistence and virulence. Further research of protein post-translational modifications will fill current gaps in the understanding of bacterial physiology and open new avenues for treatment of infectious diseases.

LasB and CbpD Virulence Factors of Pseudomonas aeruginosa Carry Multiple Post-Translational Modifications on Their Lysine Residues

Pseudomonas aeruginosa is a multi-drug resistant human pathogen largely involved in nosocomial infections. Today, effective antibacterial agents are lacking. Exploring the bacterial physiology at the post-translational modifications (PTM) level may contribute to the renewal of fighting strategies. Indeed, some correlations between PTMs and the bacterial virulence, adaptation, and resistance have been shown. In a previous study performed in P. aeruginosa, we reported that many virulence factors like chitin-binding protein CbpD and elastase LasB were multiphosphorylated. Besides phosphorylation, other PTMs, like those occurring on lysine, seem to play key roles in bacteria. In the present study, we investigated for the first time the lysine succinylome and acetylome of the extracellular compartment of P. aeruginosa by using a two-dimensional immunoaffinity approach. Some virulence factors were identified as multimodified on lysine residues, among them, LasB and CbpD. Lysine can be modified by a wide range of chemical groups. In order to check the presence of other chemical groups on modified lysines identified on LasB and CbpD, we used 1- and 2- dimensional gel electrophoresis approaches to target lysine modified by 7 other modifications: butyrylation, crotonylation, dimethylation, malonylation, methylation, propionylation, and trimethylation. We showed that some lysines of these two virulence factors were modified by these 9 different PTMs. Interestingly, we found that the PTMs recovered on these two virulence factors were different than those previously reported in the intracellular compartment.

Serine/threonine protein kinase PpkA contributes to the adaptation and virulence in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a leading cause of nosocomial infections worldwide and has become a serious public health threat, which is attributed to a large extent to its extraordinary environmental adaptability and diverse virulence factors that result in infection and progression of pathogenesis. The eukaryote-type serine/threonine (Ser/Thr) protein kinases, known for playing major regulatory roles in eukaryotes, have been demonstrated to play a central role in regulating various bacterial cellular processes via catalyzing protein phosphorylation. Although PpkA, a Ser/Thr protein kinase first identified in P. aeruginosa, has been implicated in association with bacterial virulence, little is known about the protein. Therefore, in this study, to assess the potential role of PpkA in the regulation of P. aeruginosa environmental adaptation and virulence, variations of biofilm formation, pyocyanin production, tolerance to stress, cell invasion and plant virulence were determined in wild type PAO1, ppkA gene-deleted and complemented mutant strains. Our results indicate that the mutant strain lacking ppkA exhibited a significant decrease of biofilm formation and pyocyanin production, less tolerance to oxidative and osmotic stresses, inefficient invasion of host cells and a reduction of bacterial virulence. These findings provide new insight into the regulation of various cellular processes by PpkA; this is an important mechanism for adaptation and virulence in P. aeruginosa.

Proteomic characterization of Nα- and Nε-acetylation in Acinetobacter baumannii

Nα- and Nε-acetylation represent a pivotal post-translational modification used by both eukaryotes and prokaryotes to modulate diverse biological processes. Acinetobacter baumannii has been described as an important nosocomial pathogen for the past 30 years, frequently involved in ventilator-associated pneumonia, bloodstream and urinary tract infections. Many aspects of the biology of A. baumannii remain elusive, in particular the extent and function of N-acetylation. We investigated here N-acetylation in A. baumannii strain ATCC 17978 by proteomic analysis, and we showed the usefulness of using different analytical approaches. Overall, we identified 525 N-acetylated proteins in which, 145 were Nα-acetylated and 411 were Nε-acetylated. Among them, 41 proteins carried both types of N-acetylation. We found that N-acetylation may play a role in biofilm formation, bacterial virulence (e.g. in several iron acquisition pathways), as well as a number of phenotypes, such as, stress adaptation and drug resistance.

BIOLOGICAL SIGNIFICANCE

This study is the first to perform the N-acetylome of A. baumannii using different analytical approaches. Each analytical tool permitted to characterize distinctive modified peptides. The combination of all these methods allowed us to identify 145 and 411 Nα- and Nε-acetylated proteins. Besides the fact that acetylation was involved in central metabolism as previously described in other bacteria, some N-acetylated proteins showed interesting role in bacterial virulence (iron acquisition), biofilm formation, stress adaptation and drug resistance of A. baumannii.

Post-translational modifications in Pseudomonas aeruginosa revolutionized by proteomic analysis

Pseudomonas aeruginosa is an opportunistic pathogen that causes severe infections in vulnerable individuals. It is known that post-translational modifications (PTMs) play a key role in bacterial physiology. Their characterization is still challenging and the recent advances in proteomics allow large-scale and high-throughput analyses of PTMs. Here, we provide an overview of proteomic data about the modified proteins in P. aeruginosa. We emphasize the significant contribution of proteomics in knowledge enhancement of PTMs (phosphorylation, N-acetylation and glycosylation) and we discuss their importance in P. aeruginosa physiology.

Proteome Profiles of Outer Membrane Vesicles and Extracellular Matrix of Pseudomonas aeruginosa Biofilms

In the present work, two different proteomic platforms, gel-based and gel-free, were used to map the matrix and outer membrane vesicle exoproteomes of Pseudomonas aeruginosa PAO1 biofilms. These two proteomic strategies allowed us a confident identification of 207 and 327 proteins from enriched outer membrane vesicles and whole matrix isolated from biofilms. Because of the physicochemical characteristics of these subproteomes, the two strategies showed complementarity, and thus, the most comprehensive analysis of P. aeruginosa exoproteome to date was achieved. Under our conditions, outer membrane vesicles contribute approximately 20% of the whole matrix proteome, demonstrating that membrane vesicles are an important component of the matrix. The proteomic profiles were analyzed in terms of their biological context, namely, a biofilm. Accordingly relevant metabolic processes involved in cellular adaptation to the biofilm lifestyle as well as those related to P. aeruginosa virulence capabilities were a key feature of the analyses. The diversity of the matrix proteome corroborates the idea of high heterogeneity within the biofilm; cells can display different levels of metabolism and can adapt to local microenvironments making this proteomic analysis challenging. In addition to analyzing our own primary data, we extend the analysis to published data by other groups in order to deepen our understanding of the complexity inherent within biofilm populations.