Interaction of Mycobacterium tuberculosis Virulence Factor RipA with Chaperone MoxR1 Is Required for Transport through the TAT Secretion System

Modulators targeting protein-protein interactions in Mycobacterium tuberculosis

Tuberculosis (TB) is a chronic infectious disease caused by Mycobacterium tuberculosis (M. tuberculosis), mainly transmitted through droplets to infect the lungs, and seriously affecting patients' health and quality of life. Clinically, anti-TB drugs often entail side effects and lack efficacy against resistant strains. Thus, the exploration and development of novel targeted anti-TB medications are imperative. Currently, protein-protein interactions (PPIs) offer novel avenues for anti-TB drug development, and the study of targeted modulators of PPIs in M. tuberculosis has become a prominent research focus. Furthermore, a comprehensive PPI network has been constructed using computational methods and bioinformatics tools. This network allows for a more in-depth analysis of the structural biology of PPIs and furnishes essential insights for the development of targeted small-molecule modulators. Furthermore, this article provides a detailed overview of the research progress and regulatory mechanisms of PPI modulators in M. tuberculosis, the causative agent of TB. Additionally, it summarizes potential targets for anti-TB drugs and discusses the prospects of existing PPI modulators.

The Cytotoxic Mycobacteriophage Protein Phaedrus gp82 Interacts with and Modulates the Activity of the Host ATPase, MoxR

Approximately 70% of bacteriophage-encoded proteins are of unknown function. Elucidating these protein functions represents opportunities to discover new phage-host interactions and mechanisms by which the phages modulate host activities. Here, we describe a pipeline for prioritizing phage-encoded proteins for structural analysis and characterize the gp82 protein encoded by mycobacteriophage Phaedrus. Structural and solution studies of gp82 show it is a trimeric protein containing two domains. Co-precipitation studies with the host Mycobacterium smegmatis identified the ATPase MoxR as an interacting partner protein. Phaedrus gp82-MoxR interaction requires the presence of a loop sequence within gp82 that is highly exposed and disordered in the crystallographic structure. We show that Phaedrus gp82 overexpression in M. smegmatis retards the growth of M. smegmatis on solid medium, resulting in a small colony phenotype. Overexpression of gp82 containing a mutant disordered loop or the overexpression of MoxR both rescue this phenotype. Lastly, we show that recombinant gp82 reduces levels of MoxR-mediated ATPase activity in vitro that is required for its chaperone function, and that the disordered loop plays an important role in this phenotype. We conclude that Phaedrus gp82 binds to and reduces mycobacterial MoxR activity, leading to reduced function of host proteins that require MoxR chaperone activity for their normal activity.

Peptidoglycan NlpC/P60 peptidases in bacterial physiology and host interactions

The bacterial cell wall is composed of a highly crosslinked matrix of glycopeptide polymers known as peptidoglycan that dictates bacterial cell morphology and protects against environmental stresses. Regulation of peptidoglycan turnover is therefore crucial for bacterial survival and growth and is mediated by key protein complexes and enzyme families. Here, we review the prevalence, structure, and activity of NlpC/P60 peptidases, a family of peptidoglycan hydrolases that are crucial for cell wall turnover and division as well as interactions with antibiotics and different hosts. Understanding the molecular functions of NlpC/P60 peptidases should provide important insight into bacterial physiology, their interactions with different kingdoms of life, and the development of new therapeutic approaches.

PGRS Domain of Rv0297 of Mycobacterium tuberculosis Functions in A Calcium Dependent Manner

Mycobacterium tuberculosis (M.tb), the pathogen causing tuberculosis, is a major threat to human health worldwide. Nearly 10% of M.tb genome encodes for a unique family of PE/PPE/PGRS proteins present exclusively in the genus Mycobacterium. The functions of most of these proteins are yet unexplored. The PGRS domains of these proteins have been hypothesized to consist of Ca²⁺ binding motifs that help these intrinsically disordered proteins to modulate the host cellular responses. Ca²⁺ is an important secondary messenger that is involved in the pathogenesis of tuberculosis in diverse ways. This study presents the calcium-dependent function of the PGRS domain of Rv0297 (PE_PGRS5) in M.tb virulence and pathogenesis. Tandem repeat search revealed the presence of repetitive Ca²⁺ binding motifs in the PGRS domain of the Rv0297 protein (Rv0297PGRS). Molecular Dynamics simulations and fluorescence spectroscopy revealed Ca²⁺ dependent stabilization of the Rv0297PGRS protein. Calcium stabilized Rv0297PGRS enhances the interaction of Rv0297PGRS with surface localized Toll like receptor 4 (TLR4) of macrophages. The Ca²⁺ stabilized binding of Rv0297PGRS with the surface receptor of macrophages enhances its downstream consequences in terms of Nitric Oxide (NO) production and cytokine release. Thus, this study points to hitherto unidentified roles of calcium-modulated PE_PGRS proteins in the virulence of M.tb. Understanding the pathogenic potential of Ca²⁺ dependent PE_PGRS proteins can aid in targeting these proteins for therapeutic interventions.

Mycobacterium tuberculosis RipA Dampens TLR4-Mediated Host Protective Response Using a Multi-Pronged Approach Involving Autophagy, Apoptosis, Metabolic Repurposing, and Immune Modulation

Reductive evolution has endowed Mycobacterium tuberculosis (M. tb) with moonlighting in protein functions. We demonstrate that RipA (Rv1477), a peptidoglycan hydrolase, activates the NFκB signaling pathway and elicits the production of pro-inflammatory cytokines, TNF-α, IL-6, and IL-12, through the activation of an innate immune-receptor, toll-like receptor (TLR)4. RipA also induces an enhanced expression of macrophage activation markers MHC-II, CD80, and CD86, suggestive of M1 polarization. RipA harbors LC3 (Microtubule-associated protein 1A/1B-light chain 3) motifs known to be involved in autophagy regulation and indeed alters the levels of autophagy markers LC3BII and P62/SQSTM1 (Sequestosome-1), along with an increase in the ratio of P62/Beclin1, a hallmark of autophagy inhibition. The use of pharmacological agents, rapamycin and bafilomycin A1, reveals that RipA activates PI3K-AKT-mTORC1 signaling cascade that ultimately culminates in the inhibition of autophagy initiating kinase ULK1 (Unc-51 like autophagy activating kinase). This inhibition of autophagy translates into efficient intracellular survival, within macrophages, of recombinant Mycobacterium smegmatis expressing M. tb RipA. RipA, which also localizes into mitochondria, inhibits the production of oxidative phosphorylation enzymes to promote a Warburg-like phenotype in macrophages that favors bacterial replication. Furthermore, RipA also inhibited caspase-dependent programed cell death in macrophages, thus hindering an efficient innate antibacterial response. Collectively, our results highlight the role of an endopeptidase to create a permissive replication niche in host cells by inducing the repression of autophagy and apoptosis, along with metabolic reprogramming, and pointing to the role of RipA in disease pathogenesis.

Silver Nanoparticles for the Therapy of Tuberculosis

Rapid emergence of aggressive, multidrug-resistant Mycobacteria strain represents the main cause of the current antimycobacterial-drug crisis and status of tuberculosis (TB) as a major global health problem. The relatively low-output of newly approved antibiotics contributes to the current orientation of research towards alternative antibacterial molecules such as advanced materials. Nanotechnology and nanoparticle research offers several exciting new-concepts and strategies which may prove to be valuable tools in improving the TB therapy. A new paradigm in antituberculous therapy using silver nanoparticles has the potential to overcome the medical limitations imposed in TB treatment by the drug resistance which is commonly reported for most of the current organic antibiotics. There is no doubt that AgNPs are promising future therapeutics for the medication of mycobacterial-induced diseases but the viability of this complementary strategy depends on overcoming several critical therapeutic issues as, poor delivery, variable intramacrophagic antimycobacterial efficiency, and residual toxicity. In this paper, we provide an overview of the pathology of mycobacterial-induced diseases, andhighlight the advantages and limitations of silver nanoparticles (AgNPs) in TB treatment.

Convolutional neural network-based annotation of bacterial type IV secretion system effectors with enhanced accuracy and reduced false discovery

The type IV bacterial secretion system (SS) is reported to be one of the most ubiquitous SSs in nature and can induce serious conditions by secreting type IV SS effectors (T4SEs) into the host cells. Recent studies mainly focus on annotating new T4SE from the huge amount of sequencing data, and various computational tools are therefore developed to accelerate T4SE annotation. However, these tools are reported as heavily dependent on the selected methods and their annotation performance need to be further enhanced. Herein, a convolution neural network (CNN) technique was used to annotate T4SEs by integrating multiple protein encoding strategies. First, the annotation accuracies of nine encoding strategies integrated with CNN were assessed and compared with that of the popular T4SE annotation tools based on independent benchmark. Second, false discovery rates of various models were systematically evaluated by (1) scanning the genome of Legionella pneumophila subsp. ATCC 33152 and (2) predicting the real-world non-T4SEs validated using published experiments. Based on the above analyses, the encoding strategies, (a) position-specific scoring matrix (PSSM), (b) protein secondary structure & solvent accessibility (PSSSA) and (c) one-hot encoding scheme (Onehot), were identified as well-performing when integrated with CNN. Finally, a novel strategy that collectively considers the three well-performing models (CNN-PSSM, CNN-PSSSA and CNN-Onehot) was proposed, and a new tool (CNN-T4SE, https://idrblab.org/cnnt4se/) was constructed to facilitate T4SE annotation. All in all, this study conducted a comprehensive analysis on the performance of a collection of encoding strategies when integrated with CNN, which could facilitate the suppression of T4SS in infection and limit the spread of antimicrobial resistance.

The Cell Wall Hydrolytic NlpC/P60 Endopeptidases in Mycobacterial Cytokinesis: A Structural Perspective

In preparation for division, bacteria replicate their DNA and segregate the newly formed chromosomes. A division septum then assembles between the chromosomes, and the mother cell splits into two identical daughters due to septum degradation. A major constituent of bacterial septa and of the whole cell wall is peptidoglycan (PGN), an essential cell wall polymer, formed by glycan chains of β−(1-4)-linked-N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc), cross-linked by short peptide stems. Depending on the amino acid located at the third position of the peptide stem, PGN is classified as either Lys-type or meso-diaminopimelic acid (DAP)-type. Hydrolytic enzymes play a crucial role in the degradation of bacterial septa to split the cell wall material shared by adjacent daughter cells to promote their separation. In mycobacteria, a key PGN hydrolase, belonging to the NlpC/P60 endopeptidase family and denoted as RipA, is responsible for the degradation of septa, as the deletion of the gene encoding for this enzyme generates abnormal bacteria with multiple septa. This review provides an update of structural and functional data highlighting the central role of RipA in mycobacterial cytokinesis and the fine regulation of its catalytic activity, which involves multiple molecular partners.

Inner Membrane Translocases and Insertases

The inner membrane of Gram-negative bacteria is a ~6 nm thick phospholipid bilayer. It forms a semi-permeable barrier between the cytoplasm and periplasm allowing only regulated export and import of ions, sugar polymers, DNA and proteins. Inner membrane proteins, embedded via hydrophobic transmembrane α-helices, play an essential role in this regulated trafficking: they mediate insertion into the membrane (insertases) or complete crossing of the membrane (translocases) or both. The Gram-negative inner membrane is equipped with a variety of different insertases and translocases. Many of them are specialized, taking care of the export of only a few protein substrates, while others have more general roles. Here, we focus on the three general export/insertion pathways, the secretory (Sec) pathway, YidC and the twin-arginine translocation (TAT) pathway, focusing closely on the Escherichia coli (E. coli) paradigm. We only briefly mention dedicated export pathways found in different Gram-negative bacteria. The Sec system deals with the majority of exported proteins and functions both as a translocase for secretory proteins and an insertase for membrane proteins. The insertase YidC assists the Sec system or operates independently on membrane protein clients. Sec and YidC, in common with most export pathways, require their protein clients to be in soluble non-folded states to fit through the translocation channels and grooves. The TAT pathway is an exception, as it translocates folded proteins, some loaded with prosthetic groups.

Tat-exported peptidoglycan amidase-dependent cell division contributes to Salmonella Typhimurium fitness in the inflamed gut

Salmonella enterica serovar Typhimurium (S. Tm) is a cause of food poisoning accompanied with gut inflammation. Although mucosal inflammation is generally thought to be protective against bacterial infection, S. Tm exploits the inflammation to compete with commensal microbiota, thereby growing up to high densities in the gut lumen and colonizing the gut continuously at high levels. However, the molecular mechanisms underlying the beneficial effect of gut inflammation on S. Tm competitive growth are poorly understood. Notably, the twin-arginine translocation (Tat) system, which enables the transport of folded proteins outside bacterial cytoplasm, is well conserved among many bacterial pathogens, with Tat substrates including virulence factors and virulence-associated proteins. Here, we show that Tat and Tat-exported peptidoglycan amidase, AmiA- and AmiC-dependent cell division contributes to S. Tm competitive fitness advantage in the inflamed gut. S. Tm tatC or amiA amiC mutants feature a gut colonization defect, wherein they display a chain form of cells. The chains are attributable to a cell division defect of these mutants and occur in inflamed but not in normal gut. We demonstrate that attenuated resistance to bile acids confers the colonization defect on the S. Tm amiA amiC mutant. In particular, S. Tm cell chains are highly sensitive to bile acids as compared to single or paired cells. Furthermore, we show that growth media containing high concentrations of NaCl and sublethal concentrations of antimicrobial peptides induce the S. Tm amiA amiC mutant chain form, suggesting that gut luminal conditions such as high osmolarity and the presence of antimicrobial peptides impose AmiA- and AmiC-dependent cell division on S. Tm. Together, our data indicate that Tat and the Tat-exported amidases, AmiA and AmiC, are required for S. Tm luminal fitness in the inflamed gut, suggesting that these proteins might comprise effective targets for novel antibacterial agents against infectious diarrhea.

For proteins residing outside the bacterial cytoplasm, transport is an essential step for adequate function. The twin-arginine translocation (Tat) system enables the transport of folded proteins across the cytoplasmic membrane in prokaryotes. It has recently become clear that this system plays a pivotal role in the detrimental effects of many bacterial pathogens, suggesting Tat as a novel therapeutic target against their infection. In particular, the bacterial enteropathogen Salmonella Typhimurium causes foodborne diarrhea by colonizing the gut interior space. Here, we describe that the S. Typhimurium Tat system contributes to intestinal infection by facilitating colonization of the gut by this pathogen. We also identify that two Tat-exported enzymes, peptidoglycan amidase AmiA and AmiC, are responsible for the Tat-dependent colonization. S. Typhimurium strains having nonfunctional Tat systems or lacking these enzymes undergo filamentous growth in the gut interior owing to defective cell division. Notably, this chain form of S. Typhimurium cells is highly sensitive to bile acids, rendering it less competitive with native bacteria in the gut. The data presented here suggest that the Tat system and associated amidases may comprise promising therapeutic targets for Salmonella diarrhea, and that controlling bacterial shape might be new strategy for regulating intestinal enteropathogen infection.

MAP1203 Promotes Mycobacterium avium Subspecies paratuberculosis Binding and Invasion to Bovine Epithelial Cells

Mycobacterium avium subspecies paratuberculosis (MAP) is the causative agent of Johne's disease, chronic and ultimately fatal enteritis that affects ruminant populations worldwide. One mode of MAP transmission is oral when young animals ingest bacteria from the collostrum and milk of infected dams. The exposure to raw milk has a dramatic impact on MAP, resulting in a more invasive and virulent phenotype. The MAP1203 gene is upregulated over 28-fold after exposure of the bacterium to milk. In this study, the role of MAP1203 in binding and invasion of the bovine epithelial cells was investigated. By over-expressing the native MAP1203 gene and two clones of deletion mutant in the signal sequence and of missense mutations changing the integrin domain from RGD into RDE, we demonstrate that MAP1203 plays a role in increasing binding in more than 50% and invasion in 35% of bovine MDBK epithelial cells during early phase of infection. Furthermore, results obtained suggest that MAP1203 is a surface-exposed protein in MAP and the signal sequence is required for processing and expression of functional protein on the surface of the bacterium. Using the protein pull-down assay and far-Western blot, we also demonstrate that MAP1203 interacts with the host dihydropyrimidinase-related protein 2 and glyceraldehyde 3-phosphate dehydrogenase proteins, located on the membrane of epithelial cell and involved in the remodeling of the cytoskeleton. Our data suggests that MAP1203 plays a significant role in the initiation of MAP infection of the bovine epithelium.

Whole Genome Sequencing of Mycobacterium tuberculosis Isolates From Extrapulmonary Sites

Tuberculosis (TB) remains one of the leading causes of morbidity and mortality worldwide. Extrapulmonary tuberculosis (EPTB) constitutes around 15-20% of TB cases in immunocompetent individuals. Extrapulmonary sites that are affected by TB include bones, lymph nodes, meningitis, pleura, and genitourinary tract. Whole genome sequencing has emerged as a powerful tool to map genetic diversity among Mycobacterium tuberculosis (MTB) isolates and identify the genomic signatures associated with drug resistance, pathogenesis, and disease transmission. Several pulmonary isolates of MTB have been sequenced over the years. However, availability of whole genome sequences of MTB isolates from extrapulmonary sites is limited. Some studies suggest that genetic variations in MTB might contribute to disease presentation in extrapulmonary sites. This can be addressed if whole genome sequence data from large number of extrapulmonary isolates becomes available. In this study, we have performed whole genome sequencing of five MTB clinical isolates derived from EPTB sites using next-generation sequencing platform. We identified 1434 nonsynonymous single nucleotide variations (SNVs), 143 insertions and 105 deletions. This includes 279 SNVs that were not reported before in publicly available datasets. We found several mutations that are known to confer resistance to drugs. All the five isolates belonged to East-African-Indian lineage (lineage 3). We identified 9 putative prophage DNA integrations and 14 predicted clustered regularly interspaced short palindromic repeats (CRISPR) in MTB genome. Our analysis indicates that more work is needed to map the genetic diversity of MTB. Whole genome sequencing in conjunction with comprehensive drug susceptibility testing can reveal clinically relevant mutations associated with drug resistance.

Species-specific serine-threonine protein kinase Pkb2 of Bifidobacterium longum subsp. longum: Genetic environment and substrate specificity

The objective of this study was to determine for phosphorylated substrates of the species-specific serine-threonine protein kinase (STPK) Pkb2 from Bifidobacterium longum subsp. longum GT15. Two approaches were employed: analyses of phosphorylated membrane vesicles protein spectra following kinase reactions and analyses of the genes surrounding pkb2. A bioinformatics analysis of the genes surrounding pkb2 found a species-specific gene cluster PFNA in the genomes of 34 different bifidobacterial species. The identified cluster consisted of 5-8 genes depending on the species. The first five genes are characteristic for all considered species. These are the following genes encoding serine-threonine protein kinase (pkb2), fibronectin type III domain-containing protein (fn3), AAA-ATPase (aaa-atp), hypothetical protein with DUF58 domain (duf58) and transglutaminase (tgm). The sixth (protein phosphatase, prpC), seventh (hypothetical protein, BLGT_RS02790), and eighth (FHA domain-containing protein, fha) genes are included in this cluster, but they are not found in all species. The operon organization of the PFNA gene cluster was confirmed with transcriptional analysis. AAA-ATPase, which is encoded by a gene of the PFNA gene cluster, was found to be a substrate of the STPK Pkb2. Fourteen AAA-ATPase sites (seven serine, six threonine, and one tyrosine) phosphorylated by STPK Pkb2 were revealed. Analysis of the spectra of phosphorylated membrane vesicles proteins allowed us to identify eleven proteins that were considered as possible Pkb2 substrates. They belong to several functional classes: proteins involved in transcription and translation; proteins of the F1-domain of the FoF1-ATPase; ABC-transporters; molecular chaperone GroEL; and glutamine synthase, GlnA1. All identified proteins were considered moonlighting proteins. Three out of 11 proteins (glutamine synthetase GlnA1 and FoF1-ATPase alpha and beta subunits) were selected for further in vitro phosphorylation assays and were shown to be phosphorylated by Pkb2. Four phosphorylated substrates of the species-specific STPK Pkb2 from B. longum subsp. longum GT15 were identified for the first time. They included the moonlighting protein glutamine synthase GlnA, FoF1-ATPase alpha and beta subunits, and the chaperone MoxR family of AAA-ATPase. The ability of bifidobacterial STPK to phosphorylate the substrate on serine, threonine, and tyrosine residues was shown for the first time.

Role of Bacterioferritin & Ferritin in M. tuberculosis Pathogenesis and Drug Resistance: A Future Perspective by Interactomic Approach

Tuberculosis is caused by Mycobacterium tuberculosis, one of the most successful and deadliest human pathogen. Aminoglycosides resistance leads to emergence of extremely drug resistant strains of M. tuberculosis. Iron is crucial for the biological functions of the cells. Iron assimilation, storage and their utilization is not only involved in pathogenesis but also in emergence of drug resistance strains. We previously reported that iron storing proteins (bacterioferritin and ferritin) were found to be overexpressed in aminoglycosides resistant isolates. In this study we performed the STRING analysis of bacterioferritin & ferritin proteins and predicted their interactive partners [ferrochelatase (hemH), Rv1877 (hypothetical protein/probable conserved integral membrane protein), uroporphyrinogen decarboxylase (hemE) trigger factor (tig), transcriptional regulatory protein (MT3948), hypothetical protein (MT1928), glnA3 (glutamine synthetase), molecular chaperone GroEL (groEL1 & hsp65), and hypothetical protein (MT3947)]. We suggested that interactive partners of bacterioferritin and ferritin are directly or indirectly involved in M. tuberculosis growth, homeostasis, iron assimilation, virulence, resistance, and stresses.

Immunodominant protein MIP_05962 from Mycobacterium indicus pranii displays chaperone activity

Tuberculosis, a contagious disease of infectious origin is currently a major cause of deaths worldwide. Mycobacterium indicus pranii (MIP), a saprophytic nonpathogen and a potent immunomodulator is currently being investigated as an intervention against tuberculosis along with many other diseases with positive outcome. The apparent paradox of multiple chaperones in mycobacterial species and enigma about the cellular functions of the client proteins of these chaperones need to be explored. Chaperones are the known immunomodulators; thus, there is need to exploit the proteome of MIP for identification and characterization of putative chaperones. One of the immunogenic proteins, MIP_05962 is a member of heat shock protein (HSP) 20 family due to the presence of α-crystallin domain, and has amino acid similarity with Mycobacterium lepraeHSP18 protein. The diverse functions of M. lepraeHSP18 in stress conditions implicate MIP_05962 as an important protein that needs to be explored. Biophysical and biochemical characterization of the said protein proved it to be a chaperone. The observations of aggregation prevention and refolding of substrate proteins in the presence of MIP_05962 along with interaction with non-native proteins, surface hydrophobicity, formation of large oligomers, in-vivo thermal rescue of Escherichia coli expressing MIP_05962, enhancing solubility of insoluble protein maltodextrin glucosidase (MalZ) under in-vivo conditions, and thermal stability and reversibility confirmed MIP_05962 as a molecular chaperone.

Aggregation Prevention Assay for Chaperone Activity of Proteins Using Spectroflurometry

The ability to stabilize other proteins against thermal aggregation is one of the major characteristics of chaperone proteins. Molecular chaperones bind to nonnative conformations of proteins. Folding of the substrate is triggered by a dynamic association and dissociation cycles which keep the substrate protein on track of the folding pathway (Figure 1). Usually molecular chaperones exhibit differential affinities with different conformations of the substrate. With the exception of the sHsp family of molecular chaperones, the shift from a high-affinity binding state to the low-affinity release state is triggered by ATP binding and hydrolysis (Haselback and Buchner, 2015). Aggregation prevention assay is a simple, yet definitive assay to determine the chaperone activity of heat labile proteins such as Maltodextrin glucosidase (MalZ), Citrate Synthase (CS) and NdeI. This is based on the premise that proteins with chaperone like activity should prevent protein substrates (MalZ, CS and NdeI) from thermal aggregation. Here, we describe a detailed protocol for aggregation prevention assay using two different chaperone proteins, resistin and MoxR1, identified from our lab. Resistin, a human protein (hRes) and MoxR1 a Mycobacterium tuberculosis protein were analysed for their effect on prevention of MalZ/Citrate Synthase (CS)/NdeI aggregation. Figure 1.Mechanism of action of molecular chaperones. Citrate synthase folds via increasingly structured intermediates (I₁, I₂) from the unfolded state (U) to the folded state (N). Under heat shock conditions, this process is reversed.

Not just an antibiotic target: Exploring the role of type I signal peptidase in bacterial virulence

The looming antibiotic crisis has prompted the development of new strategies towards fighting infection. Traditional antibiotics target bacterial processes essential for viability, whereas proposed antivirulence approaches rely on the inhibition of factors that are required only for the initiation and propagation of infection within a host. Although antivirulence compounds have yet to prove their efficacy in the clinic, bacterial signal peptidase I (SPase) represents an attractive target in that SPase inhibitors exhibit broad-spectrum antibiotic activity, but even at sub-MIC doses also impair the secretion of essential virulence factors. The potential consequences of SPase inhibition on bacterial virulence have not been thoroughly examined, and are explored within this review. In addition, we review growing evidence that SPase has relevant biological functions outside of mediating secretion, and discuss how the inhibition of these functions may be clinically significant.

Mycobacterium tuberculosis Peptidyl-Prolyl Isomerases Also Exhibit Chaperone like Activity In-Vitro and In-Vivo

Peptidyl-prolyl cis-trans isomerases (Ppiases), also known as cyclophilins, are ubiquitously expressed enzymes that assist in protein folding by isomerization of peptide bonds preceding prolyl residues. Mycobacterium tuberculosis (M.tb) is known to possess two Ppiases, PpiA and PpiB. However, our understanding about the biological significance of mycobacterial Ppiases with respect to their pleiotropic roles in responding to stress conditions inside the macrophages is restricted. This study describes chaperone-like activity of mycobacterial Ppiases. We show that recombinant rPpiA and rPpiB can bind to non-native proteins in vitro and can prevent their aggregation. Purified rPpiA and rPpiB exist in oligomeric form as evident from gel filtration chromatography.E. coli cells overexpressing PpiA and PpiB of M.tb could survive thermal stress as compared to plasmid vector control. HEK293T cells transiently expressing M.tb PpiA and PpiB proteins show increased survival as compared to control cells in response to oxidative stress and hypoxic conditions generated after treatment with H₂O₂ and CoCl₂ thereby pointing to their likely role in adaption under host generated oxidative stress and conditions of hypoxia. The chaperone-like function of these M.tuberculosis cyclophilins may possibly function as a stress responder and consequently contribute to virulence.