A new twist on an old pathway--accessory Sec [corrected] systems

Differential localization of the streptococcal accessory sec components and implications for substrate export

The accessory Sec system of Streptococcus gordonii is comprised of SecY2, SecA2, and five proteins (Asp1 through -5) that are required for the export of a serine-rich glycoprotein, GspB. We have previously shown that a number of the Asps interact with GspB, SecA2, or each other. To further define the roles of these Asps in export, we examined their subcellular localization in S. gordonii and in Escherichia coli expressing the streptococcal accessory Sec system. In particular, we assessed how the locations of these accessory Sec proteins were altered by the presence of other components. Using fluorescence microscopy, we found in E. coli that SecA2 localized within multiple foci at the cell membrane, regardless of whether other accessory Sec proteins were expressed. Asp2 alone localized to the cell poles but formed a similar punctate pattern at the membrane when SecA2 was present. Asp1 and Asp3 localized diffusely in the cytosol when expressed alone or with SecA2. However, these proteins redistributed to the membrane in a punctate arrangement when all of the accessory Sec components were present. Cell fractionation studies with S. gordonii further corroborated these microscopy results. Collectively, these findings indicate that Asp1 to -3 are not integral membrane proteins that form structural parts of the translocation channel. Instead, SecA2 serves as a docking site for Asp2, which in turn attracts a complex of Asp1 and Asp3 to the membrane. These protein interactions may be important for the trafficking of GspB to the cell membrane and its subsequent translocation.

Emerging themes in SecA2-mediated protein export

The conserved general secretion (Sec) pathway carries out most protein export in bacteria and is powered by the essential ATPase SecA. Interestingly, mycobacteria and some Gram-positive bacteria possess two SecA proteins: SecA1 and SecA2. In these species, SecA1 is responsible for exporting most proteins, whereas SecA2 exports only a subset of substrates and is implicated in virulence. However, despite the impressive body of knowledge about the canonical SecA1, less is known concerning SecA2 function. Here, we review our current understanding of the different types of SecA2 systems and outline future directions for their study.

Subcellular localization of extracytoplasmic proteins in monoderm bacteria: rational secretomics-based strategy for genomic and proteomic analyses

Genome-scale prediction of subcellular localization (SCL) is not only useful for inferring protein function but also for supporting proteomic data. In line with the secretome concept, a rational and original analytical strategy mimicking the secretion steps that determine ultimate SCL was developed for Gram-positive (monoderm) bacteria. Based on the biology of protein secretion, a flowchart and decision trees were designed considering (i) membrane targeting, (ii) protein secretion systems, (iii) membrane retention, and (iv) cell-wall retention by domains or post-translocational modifications, as well as (v) incorporation to cell-surface supramolecular structures. Using Listeria monocytogenes as a case study, results were compared with known data set from SCL predictors and experimental proteomics. While in good agreement with experimental extracytoplasmic fractions, the secretomics-based method outperforms other genomic analyses, which were simply not intended to be as inclusive. Compared to all other localization predictors, this method does not only supply a static snapshot of protein SCL but also offers the full picture of the secretion process dynamics: (i) the protein routing is detailed, (ii) the number of distinct SCL and protein categories is comprehensive, (iii) the description of protein type and topology is provided, (iv) the SCL is unambiguously differentiated from the protein category, and (v) the multiple SCL and protein category are fully considered. In that sense, the secretomics-based method is much more than a SCL predictor. Besides a major step forward in genomics and proteomics of protein secretion, the secretomics-based method appears as a strategy of choice to generate in silico hypotheses for experimental testing.

Mycobacterium marinum SecA2 promotes stable granulomas and induces tumor necrosis factor alpha in vivo

SecA2 is an ATPase present in some pathogenic Gram-positive bacteria, is required for translocation of a limited set of proteins across the cytosolic membrane, and plays an important role in virulence in several bacteria, including mycobacteria that cause diseases such as tuberculosis and leprosy. However, the mechanisms by which SecA2 affects virulence are incompletely understood. To investigate whether SecA2 modulates host immune responses in vivo, we studied Mycobacterium marinum infection in two different hosts: an established zebrafish model and a recently described mouse model. Here we show that M. marinum ΔsecA2 was attenuated for virulence in both host species and SecA2 was needed for normal granuloma numbers and for optimal tumor necrosis factor alpha response in both zebrafish and mice. M. marinum ΔsecA2 was more sensitive to SDS and had unique protrusions from its cell envelope when examined by cryo-electron tomography, suggesting that SecA2 is important for bacterial cell wall integrity. These results provide evidence that SecA2 induces granulomas and is required for bacterial modulation of the host response because it affects the mycobacterial cell envelope.

Intra-plastid protein trafficking: how plant cells adapted prokaryotic mechanisms to the eukaryotic condition

Protein trafficking and localization in plastids involve a complex interplay between ancient (prokaryotic) and novel (eukaryotic) translocases and targeting machineries. During evolution, ancient systems acquired new functions and novel translocation machineries were developed to facilitate the correct localization of nuclear encoded proteins targeted to the chloroplast. Because of its post-translational nature, targeting and integration of membrane proteins posed the biggest challenge to the organelle to avoid aggregation in the aqueous compartments. Soluble proteins faced a different kind of problem since some had to be transported across three membranes to reach their destination. Early studies suggested that chloroplasts addressed these issues by adapting ancient-prokaryotic machineries and integrating them with novel-eukaryotic systems, a process called 'conservative sorting'. In the last decade, detailed biochemical, genetic, and structural studies have unraveled the mechanisms of protein targeting and localization in chloroplasts, suggesting a highly integrated scheme where ancient and novel systems collaborate at different stages of the process. In this review we focus on the differences and similarities between chloroplast ancestral translocases and their prokaryotic relatives to highlight known modifications that adapted them to the eukaryotic situation. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Protein secretion and surface display in Gram-positive bacteria

The cell wall peptidoglycan of Gram-positive bacteria functions as a surface organelle for the transport and assembly of proteins that interact with the environment, in particular, the tissues of an infected host. Signal peptide-bearing precursor proteins are secreted across the plasma membrane of Gram-positive bacteria. Some precursors carry C-terminal sorting signals with unique sequence motifs that are cleaved by sortase enzymes and linked to the cell wall peptidoglycan of vegetative forms or spores. The sorting signals of pilin precursors are cleaved by pilus-specific sortases, which generate covalent bonds between proteins leading to the assembly of fimbrial structures. Other precursors harbour surface (S)-layer homology domains (SLH), which fold into a three-pronged spindle structure and bind secondary cell wall polysaccharides, thereby associating with the surface of specific Gram-positive microbes. Type VII secretion is a non-canonical secretion pathway for WXG100 family proteins in mycobacteria. Gram-positive bacteria also secrete WXG100 proteins and carry unique genes that either contribute to discrete steps in secretion or represent distinctive substrates for protein transport reactions.

A role for glycosylated serine-rich repeat proteins in gram-positive bacterial pathogenesis

Bacterial attachment to host surfaces is a pivotal event in the biological and infectious processes of both commensal and pathogenic bacteria, respectively. Serine-rich repeat proteins (SRRPs) are a family of adhesins in Gram-positive bacteria that mediate attachment to a variety of host and bacterial surfaces. As such, they contribute towards a wide-range of diseases including sub-acute bacterial endocarditis, community-acquired pneumonia, and meningitis. SRRPs are unique in that they are glycosylated, require a non-canonical Sec-translocase for transport, and are largely composed of a domain containing hundreds of alternating serine residues. These serine-rich repeats are thought to extend a unique non-repeat (NR) domain outward away from the bacterial surface to mediate adhesion. So far, NR domains have been determined to bind to sialic acid moieties, keratins, or other NR domains of a similar SRRP. This review summarizes how this important family of bacterial adhesins mediates bacterial attachment to host and bacterial cells, contributes to disease pathogenesis, and might be targeted for pharmacological intervention or used as novel protective vaccine antigens. This review also highlights recent structural findings on the NR domains of these proteins.

Secretion genes as determinants of Bacillus anthracis chain length

Bacillus anthracis grows in chains of rod-shaped cells, a trait that contributes to its escape from phagocytic clearance in host tissues. Using a genetic approach to search for determinants of B. anthracis chain length, we identified mutants with insertional lesions in secA2. All isolated secA2 mutants exhibited an exaggerated chain length, whereas the dimensions of individual cells were not changed. Complementation studies revealed that slaP (S-layer assembly protein), a gene immediately downstream of secA2 on the B. anthracis chromosome, is also a determinant of chain length. Both secA2 and slaP are required for the efficient secretion of Sap and EA1 (Eag), the two S-layer proteins of B. anthracis, but not for the secretion of S-layer-associated proteins or of other secreted products. S-layer assembly via secA2 and slaP contributes to the proper positioning of BslO, the S-layer-associated protein, and murein hydrolase, which cleaves septal peptidoglycan to separate chains of bacilli. SlaP was found to be both soluble in the bacterial cytoplasm and associated with the membrane. The purification of soluble SlaP from B. anthracis-cleared lysates did not reveal a specific ligand, and the membrane association of SlaP was not dependent on SecA2, Sap, or EA1. We propose that SecA2 and SlaP promote the efficient secretion of S-layer proteins by modifying the general secretory pathway of B. anthracis to transport large amounts of Sap and EA1.

Mycobacterium tuberculosis: success through dormancy

Tuberculosis (TB) remains a major health threat, killing nearly 2 million individuals around this globe, annually. The only vaccine, developed almost a century ago, provides limited protection only during childhood. After decades without the introduction of new antibiotics, several candidates are currently undergoing clinical investigation. Curing TB requires prolonged combination of chemotherapy with several drugs. Moreover, monitoring the success of therapy is questionable owing to the lack of reliable biomarkers. To substantially improve the situation, a detailed understanding of the cross-talk between human host and the pathogen Mycobacterium tuberculosis (Mtb) is vital. Principally, the enormous success of Mtb is based on three capacities: first, reprogramming of macrophages after primary infection/phagocytosis to prevent its own destruction; second, initiating the formation of well-organized granulomas, comprising different immune cells to create a confined environment for the host-pathogen standoff; third, the capability to shut down its own central metabolism, terminate replication, and thereby transit into a stage of dormancy rendering itself extremely resistant to host defense and drug treatment. Here, we review the molecular mechanisms underlying these processes, draw conclusions in a working model of mycobacterial dormancy, and highlight gaps in our understanding to be addressed in future research.

Involvement of two latex-clearing proteins during rubber degradation and insights into the subsequent degradation pathway revealed by the genome sequence of Gordonia polyisoprenivorans strain VH2

The increasing production of synthetic and natural poly(cis-1,4-isoprene) rubber leads to huge challenges in waste management. Only a few bacteria are known to degrade rubber, and little is known about the mechanism of microbial rubber degradation. The genome of Gordonia polyisoprenivorans strain VH2, which is one of the most effective rubber-degrading bacteria, was sequenced and annotated to elucidate the degradation pathway and other features of this actinomycete. The genome consists of a circular chromosome of 5,669,805 bp and a circular plasmid of 174,494 bp with average GC contents of 67.0% and 65.7%, respectively. It contains 5,110 putative protein-coding sequences, including many candidate genes responsible for rubber degradation and other biotechnically relevant pathways. Furthermore, we detected two homologues of a latex-clearing protein, which is supposed to be a key enzyme in rubber degradation. The deletion of these two genes for the first time revealed clear evidence that latex-clearing protein is essential for the microbial utilization of rubber. Based on the genome sequence, we predict a pathway for the microbial degradation of rubber which is supported by previous and current data on transposon mutagenesis, deletion mutants, applied comparative genomics, and literature search.

DivIVA affects secretion of virulence-related autolysins in Listeria monocytogenes

DivIVA is a well-conserved coiled-coil protein present in most Gram-positive bacteria and has been implicated in division site selection, peptidoglycan biosynthesis and sporulation. DivIVA proteins bind lipid membranes and characteristically accumulate at curved membrane areas, i.e. the cell poles and the division site, to which they recruit various interaction partners. We have studied the role of this morphogen in the human pathogen Listeria monocytogenes and our results suggest a novel mechanism by which DivIVA contributes to cell division. Contrary to expectation a ΔdivIVA mutant exhibited a pronounced chaining phenotype rather than a defect in cell division which we attributed to reduced extracellular levels of the autolytic enzymes p60 and MurA. We demonstrate that this is due to a malfunction in secretion of these autolysins and phenotypic comparison of the ΔdivIVA strain with a ΔsecA2 mutant suggests that DivIVA influences the activity of the SecA2 secretion route in L. monocytogenes. Also from the phenotypic analysis it was clear that divIVA affected swarming motility, biofilm formation, invasiveness and cell-to-cell spread in cell culture infection models. Thus, our experiments show that DivIVA is an important factor for various listerial traits that are essential for the pathogenicity of this organism.

The Mycobacterium tuberculosis SecA2 system subverts phagosome maturation to promote growth in macrophages

The ability of Mycobacterium tuberculosis to grow in macrophages is critical to the virulence of this important pathogen. One way M. tuberculosis is thought to maintain a hospitable niche in macrophages is by arresting the normal process of phagosomes maturing into acidified phagolysosomes. The process of phagosome maturation arrest by M. tuberculosis is not fully understood, and there has remained a need to firmly establish a requirement for phagosome maturation arrest for M. tuberculosis growth in macrophages. Other intracellular pathogens that control the phagosomal environment use specialized protein export systems to deliver effectors of phagosome trafficking to the host cell. In M. tuberculosis, the accessory SecA2 system is a specialized protein export system that is required for intracellular growth in macrophages. In studying the importance of the SecA2 system in macrophages, we discovered that SecA2 is required for phagosome maturation arrest. Shortly after infection, phagosomes containing a ΔsecA2 mutant of M. tuberculosis were more acidified and showed greater association with markers of late endosomes than phagosomes containing wild-type M. tuberculosis. We further showed that inhibitors of phagosome acidification rescued the intracellular growth defect of the ΔsecA2 mutant, which demonstrated that the phagosome maturation arrest defect of the ΔsecA2 mutant is responsible for the intracellular growth defect. This study demonstrates the importance of phagosome maturation arrest for M. tuberculosis growth in macrophages, and it suggests there are effectors of phagosome maturation that are exported into the host environment by the accessory SecA2 system.

The ins and outs of Mycobacterium tuberculosis protein export

Mycobacterium tuberculosis is an important pathogen that infects approximately one-third of the world's population and kills almost two million people annually. An important aspect of M. tuberculosis physiology and pathogenesis is its ability to export proteins into and across the thick mycobacterial cell envelope, where they are ideally positioned to interact with the host. In addition to the specific proteins that are exported by M. tuberculosis, the systems through which these proteins are exported represent potential targets for future drug development. M. tuberculosis possesses two well-known and conserved export systems: the housekeeping Sec pathway and the Tat pathway. In addition, M. tuberculosis possesses specialized export systems including the accessory SecA2 pathway and five ESX pathways. Here we review the current understanding of each of these export systems, with a focus on M. tuberculosis, and discuss the contribution of each system to disease and physiology.

Clostridium difficile cell wall protein CwpV undergoes enzyme-independent intramolecular autoproteolysis

Clostridium difficile infection is a leading cause of antibiotic-associated diarrhea, placing considerable economic pressure on healthcare systems and resulting in significant morbidity and mortality. The pathogen produces a proteinaceous array on its cell surface known as the S-layer, consisting primarily of the major S-layer protein SlpA and a family of SlpA homologs. CwpV is the largest member of this family and is expressed in a phase-variable manner. The protein is post-translationally processed into two fragments that form a noncovalent, heterodimeric complex. To date, no specific proteases capable of cleaving CwpV have been identified. Using site-directed mutagenesis we show that CwpV undergoes intramolecular autoproteolysis, most likely facilitated by a N-O acyl shift, with Thr-413 acting as the source of a nucleophile driving this rearrangement. We demonstrate that neighboring residues are also important for correct processing of CwpV. Based on protein structural predictions and analogy to the glycosylasparaginase family of proteins, it appears likely that these residues play key roles in determining the correct protein fold and interact directly with Thr-413 to promote nucleophilic attack. Furthermore, using a cell-free protein synthesis assay we show that CwpV maturation requires neither cofactors nor auxiliary enzymes.

Complete genome sequence of Corynebacterium variabile DSM 44702 isolated from the surface of smear-ripened cheeses and insights into cheese ripening and flavor generation

Background

Corynebacterium variabile is part of the complex microflora on the surface of smear-ripened cheeses and contributes to the development of flavor and textural properties during cheese ripening. Still little is known about the metabolic processes and microbial interactions during the production of smear-ripened cheeses. Therefore, the gene repertoire contributing to the lifestyle of the cheese isolate C. variabile DSM 44702 was deduced from the complete genome sequence to get a better understanding of this industrial process.

Results

The chromosome of C. variabile DSM 44702 is composed of 3, 433, 007 bp and contains 3, 071 protein-coding regions. A comparative analysis of this gene repertoire with that of other corynebacteria detected 1, 534 predicted genes to be specific for the cheese isolate. These genes might contribute to distinct metabolic capabilities of C. variabile, as several of them are associated with metabolic functions in cheese habitats by playing roles in the utilization of alternative carbon and sulphur sources, in amino acid metabolism, and fatty acid degradation. Relevant C. variabile genes confer the capability to catabolize gluconate, lactate, propionate, taurine, and gamma-aminobutyric acid and to utilize external caseins. In addition, C. variabile is equipped with several siderophore biosynthesis gene clusters for iron acquisition and an exceptional repertoire of AraC-regulated iron uptake systems. Moreover, C. variabile can produce acetoin, butanediol, and methanethiol, which are important flavor compounds in smear-ripened cheeses.

Conclusions

The genome sequence of C. variabile provides detailed insights into the distinct metabolic features of this bacterium, implying a strong adaption to the iron-depleted cheese surface habitat. By combining in silico data obtained from the genome annotation with previous experimental knowledge, occasional observations on genes that are involved in the complex metabolic capacity of C. variabile were integrated into a global view on the lifestyle of this species.

Traffic jam at the bacterial sec translocase: targeting the SecA nanomotor by small-molecule inhibitors

The rapid rise of drug-resistant bacteria is one of the most serious unmet medical needs facing the world. Despite this increasing problem of antibiotic resistance, the number of different antibiotics available for the treatment of serious infections is dwindling. Therefore, there is an urgent need for new antibacterial drugs, preferably with novel modes of action to potentially avoid cross-resistance with existing antibacterial agents. In recent years, increasing attention has been paid to bacterial protein secretion as a potential antibacterial target. Among the different protein secretion pathways that are present in bacterial pathogens, the general protein secretory (Sec) pathway is widely considered as an attractive target for antibacterial therapy. One of the key components of the Sec pathway is the peripheral membrane ATPase SecA, which provides the energy for the translocation of preproteins across the bacterial cytoplasmic membrane. In this review, we will provide an overview of research efforts on the discovery and development of small-molecule SecA inhibitors. Furthermore, recent advances on the structure and function of SecA and their potential impact on antibacterial drug discovery will be discussed.

Dynamic distribution of the SecA and SecY translocase subunits and septal localization of the HtrA surface chaperone/protease during Streptococcus pneumoniae D39 cell division

The Sec translocase pathway is the major route for protein transport across and into the cytoplasmic membrane of bacteria. Previous studies reported that the SecA translocase ATP-binding subunit and the cell surface HtrA protease/chaperone formed a single microdomain, termed “ExPortal,” in some species of ellipsoidal (ovococcus) Gram-positive bacteria, including Streptococcus pyogenes. To investigate the generality of microdomain formation, we determined the distribution of SecA and SecY by immunofluorescent microscopy in Streptococcus pneumoniae (pneumococcus), which is an ovococcus species evolutionarily distant from S. pyogenes. In the majority (≥75%) of exponentially growing cells, S. pneumoniae SecA (SecA_Spn) and SecY_Spn located dynamically in cells at different stages of division. In early divisional cells, both Sec subunits concentrated at equators, which are future sites of constriction. Further along in division, SecA_Spn and SecY_Spn remained localized at mid-cell septa. In late divisional cells, both Sec subunits were hemispherically distributed in the regions between septa and the future equators of dividing cells. In contrast, the HtrA_Spn homologue localized to the equators and septa of most (>90%) dividing cells, whereas the SrtA_Spn sortase located over the surface of cells in no discernable pattern. This dynamic pattern of Sec distribution was not perturbed by the absence of flotillin family proteins, but was largely absent in most cells in early stationary phase and in ∆cls mutants lacking cardiolipin synthase. These results do not support the existence of an ExPortal microdomain in S. pneumoniae. Instead, the localization of the pneumococcal Sec translocase depends on the stage of cell division and anionic phospholipid content.

Two patterns of Sec translocase distribution, an ExPortal microdomain in certain ovococcus-shaped species like Streptococcus pyogenes and a spiral pattern in rod-shaped species like Bacillus subtilis, have been reported for Gram-positive bacteria. This study provides evidence for a third pattern of Sec localization in the ovococcus human pathogen Streptococcus pneumoniae. The SecA motor and SecY channel subunits of the Sec translocase localize dynamically to different places in the mid-cell region during the division cycle of exponentially growing, but not stationary-phase, S. pneumoniae. Unexpectedly, the S. pneumoniae HtrA (HtrA_Spn) protease/chaperone principally localizes to cell equators and division septa. The coincident localization of SecA_Spn, SecY_Spn, and HtrA_Spn to regions of peptidoglycan (PG) biosynthesis in unstressed, growing cells suggests that the pneumococcal Sec translocase directs assembly of the PG biosynthesis apparatus to regions where it is needed during division and that HtrA_Spn may play a general role in quality control of proteins exported by the Sec translocase.

Canonical SecA associates with an accessory secretory protein complex involved in biogenesis of a streptococcal serine-rich repeat glycoprotein

Fap1, a serine-rich repeat glycoprotein (SRRP), is required for bacterial biofilm formation of Streptococcus parasanguinis. Fap1-like SRRPs are found in many gram-positive bacteria and have been implicated in bacterial fitness and virulence. A conserved five-gene cluster, secY2-gap1-gap2-gap3-secA2, located immediately downstream of fap1, is required for Fap1 biogenesis. secA2, gap1, and gap3 encode three putative accessory Sec proteins. SecA2 mediates export of mature Fap1, and Gap1 and Gap3 are required for Fap1 biogenesis. Interestingly, gap1 and gap3 mutants exhibited the same phenotype as a secA2 mutant, implying that Gap1 and Gap3 may interact with SecA2 to mediate Fap1 biogenesis. Glutathione S-transferase pulldown experiments revealed a direct interaction between SecA2, Gap1, and Gap3 in vitro. Coimmunoprecipitation analysis demonstrated the formation of a SecA2-Gap1-Gap3 complex. Homologues of SecA2, Gap1, and Gap3 are conserved in many streptococci and staphylococci. The corresponding homologues from Streptococcus agalactiae also interacted with each other and formed a protein complex. Furthermore, the Gap1 homologues from S. agalactiae and Streptococcus sanguinis rescued the Fap1 defect in the Gap1 mutant, indicating the functional conservation of the accessory Sec complex. Importantly, canonical SecA interacted with the accessory Sec protein complex, suggesting that the biogenesis of SRRPs mediated by the accessory Sec system is linked to the canonical Sec system.

The superoxide dismutase SodA is targeted to the periplasm in a SecA-dependent manner by a novel mechanism

The manganese/iron-type superoxide dismutase (SodA) of Rhizobium leguminosarum bv. viciae 3841 is exported to the periplasm of R. l. bv. viciae and Escherichia coli. However, it does not possess a hydrophobic cleaved N-terminal signal peptide typically present in soluble proteins exported by the Sec-dependent (Sec) pathway or the twin-arginine translocation (TAT) pathway. A tatC mutant of R. l. bv. viciae exported SodA to the periplasm, ruling out export of SodA as a complex with a TAT substrate as a chaperone. The export of SodA was unaffected in a secB mutant of E. coli, but its export from R. l. bv. viciae was inhibited by azide, an inhibitor of SecA ATPase activity. A temperature-sensitive secA mutant of E. coli was strongly reduced for SodA export. The 10 N-terminal amino acid residues of SodA were sufficient to target the reporter protein alkaline phosphatase to the periplasm. Our results demonstrate the export of a protein lacking a classical signal peptide to the periplasm by a SecA-dependent, but SecB-independent targeting mechanism. Export of the R. l. bv. viciae SodA to the periplasm was not limited to the genus Rhizobium, but was also observed in other proteobacteria.

Clostridium difficile has two parallel and essential Sec secretion systems

Protein translocation across the cytoplasmic membrane is an essential process in all bacteria. The Sec system, comprising at its core an ATPase, SecA, and a membrane channel, SecYEG, is responsible for the majority of this protein transport. Recently, a second parallel Sec system has been described in a number of gram-positive species. This accessory Sec system is characterized by the presence of a second copy of the energizing ATPase, SecA2; where it has been studied, SecA2 is responsible for the translocation of a subset of Sec substrates. In common with many pathogenic gram-positive species, Clostridium difficile possesses two copies of SecA. Here, we describe the first characterization of the C. difficile accessory Sec system and the identification of its major substrates. Using inducible antisense RNA expression and dominant-negative alleles of secA1 and secA2, we demonstrate that export of the S-layer proteins (SLPs) and an additional cell wall protein (CwpV) is dependent on SecA2. Accumulation of the cytoplasmic precursor of the SLPs SlpA and other cell wall proteins was observed in cells expressing dominant-negative secA1 or secA2 alleles, concomitant with a decrease in the levels of mature SLPs in the cell wall. Furthermore, expression of either dominant-negative allele or antisense RNA knockdown of SecA1 or SecA2 dramatically impaired growth, indicating that both Sec systems are essential in C. difficile.

Structural and functional analysis of a new subfamily of glycosyltransferases required for glycosylation of serine-rich streptococcal adhesins

Serine-rich repeat glycoproteins (SRRPs) are a growing family of bacterial adhesins found in many streptococci and staphylococci; they play important roles in bacterial biofilm formation and pathogenesis. Glycosylation of this family of adhesins is essential for their biogenesis. A glucosyltransferase (Gtf3) catalyzes the second step of glycosylation of a SRRP (Fap1) from an oral streptococcus, Streptococcus parasanguinis. Although Gtf3 homologs are highly conserved in SRRP-containing streptococci, they share minimal homology with functionally known glycosyltransferases. We report here the 2.3 Å crystal structure of Gtf3. The structural analysis indicates that Gtf3 forms a tetramer and shares significant structural homology with glycosyltransferases from GT4, GT5, and GT20 subfamilies. Combining crystal structural analysis with site-directed mutagenesis and in vitro glycosyltransferase assays, we identified residues that are required for UDP- or UDP-glucose binding and for oligomerization of Gtf3 and determined their contribution to the enzymatic activity of Gtf3. Further in vivo studies revealed that the critical amino acid residues identified by the structural analysis are crucial for Fap1 glycosylation in S. parasanguinis in vivo. Moreover, Gtf3 homologs from other streptococci were able to rescue the gtf3 knock-out mutant of S. parasanguinis in vivo and catalyze the sugar transfer to the modified SRRP substrate in vitro, demonstrating the importance and conservation of the Gtf3 homologs in glycosylation of SRRPs. As the Gtf3 homologs only exist in SRRP-containing streptococci, we conclude that the Gtf3 homologs represent a unique subfamily of glycosyltransferases.

The production of extracellular proteins is regulated by ribonuclease III via two different pathways in Staphylococcus aureus

Staphylococcus aureus ribonuclease III belongs to the enzyme family known to degrade double-stranded RNAs. It has previously been reported that RNase III cannot influence cell growth but regulates virulence gene expression in S. aureus. Here we constructed an RNase III inactivation mutant (Δrnc) from S. aureus 8325-4. It was found that the extracellular proteins of Δrnc were decreased. Furthermore, we explored how RNase III regulated the production of the extracellular proteins in S. aureus. We found during the lag phase of the bacterial growth cycle RNase III could influence the extracellular protein secretion via regulating the expression of secY2, one component of accessory secretory (sec) pathway. After S. aureus cells grew to exponential phase, RNase III can regulate the expression of extracellular proteins by affecting the level of RNAIII. Further investigation showed that the mRNA stability of secY2 and RNAIII was affected by RNase III. Our results suggest that RNase III could regulate the pathogenicity of S. aureus by influencing the level of extracellular proteins via two different ways respectively at different growth phases.

Asp2 and Asp3 interact directly with GspB, the export substrate of the Streptococcus gordonii accessory Sec System

GspB is a serine-rich glycoprotein adhesin of Streptococcus gordonii that is exported to the bacterial surface by the accessory Sec system. This dedicated export pathway is comprised of seven components (SecA2, SecY2, and five accessory Sec proteins [Asp1 to Asp5]). The latter proteins have no known homologs beyond the Asps of other species. Asp1 to Asp3 are absolutely required for export of the substrate GspB, but their roles in this process are unknown. Using copurification analysis and far-Western blotting, we found that Asp2 and Asp3 could individually bind the serine-rich repeat (SRR) domains of GspB. Deletion of both SRR regions of GspB led to a decrease in its export, suggesting that binding of the Asps to the SRR regions is important for GspB transport by the accessory Sec system. The Asps also bound a heterologous substrate for the accessory Sec system containing a slow-folding MalE variant, but they did not bind wild-type MalE. The combined results indicate that the Asps may recognize the export substrate through preferential interactions with its unstructured or unfolded regions. Glycosylation of the SRR domains on GspB prevented Asp binding, suggesting that binding of the Asps to the preprotein occurs prior to its full glycosylation. Together, these findings suggest that Asp2 and Asp3 are likely to function in part as chaperones in the early phase of GspB transport.

Proteomic profile of culture filtrate from the Brazilian vaccine strain Mycobacterium bovis BCG Moreau compared to M. bovis BCG Pasteur

Background

Bacille Calmette-Guerin (BCG) is currently the only available vaccine against tuberculosis (TB) and comprises a heterogeneous family of sub-strains with genotypic and phenotypic differences. The World Health Organization (WHO) affirms that the characterization of BCG sub-strains, both on genomic and proteomic levels, is crucial for a better comprehension of the vaccine. In addition, these studies can contribute in the development of a more efficient vaccine against TB. Here, we combine two-dimensional electrophoresis (2DE) and mass spectrometry to analyse the proteomic profile of culture filtrate proteins (CFPs) from M. bovis BCG Moreau, the Brazilian vaccine strain, comparing it to that of BCG Pasteur. CFPs are considered of great importance given their dominant immunogenicity and role in pathogenesis, being available for interaction with host cells since early infection.

Results

The 2DE proteomic map of M. bovis BCG Moreau CFPs in the pH range 3 - 8 allowed the identification of 158 spots corresponding to 101 different proteins, identified by MS/MS. Comparison to BCG Pasteur highlights the great similarity between these BCG strains. However, quantitative analysis shows a higher expression of immunogenic proteins such as Rv1860 (BCG1896, Apa), Rv1926c (BCG1965c, Mpb63) and Rv1886c (BCG1923c, Ag85B) in BCG Moreau when compared to BCG Pasteur, while some heat shock proteins, such as Rv0440 (BCG0479, GroEL2) and Rv0350 (BCG0389, DnaK), show the opposite pattern.

Conclusions

Here we report the detailed 2DE profile of CFPs from M. bovis BCG Moreau and its comparison to BCG Pasteur, identifying differences that may provide relevant information on vaccine efficacy. These findings contribute to the detailed characterization of the Brazilian vaccine strain against TB, revealing aspects that may lead to a better understanding of the factors leading to BCG's variable protective efficacy against TB.

Protein export systems of Mycobacterium tuberculosis: novel targets for drug development?

Protein export is essential in all bacteria and many bacterial pathogens depend on specialized protein export systems for virulence. In Mycobacterium tuberculosis, the etiological agent of the disease tuberculosis, the conserved general secretion (Sec) and twin-arginine translocation (Tat) pathways perform the bulk of protein export and are both essential. M. tuberculosis also has specialized export pathways that transport specific subsets of proteins. One such pathway is the accessory SecA2 system, which is important for M. tuberculosis virulence. There are also specialized ESX export systems that function in virulence (ESX-1) or essential physiologic processes (ESX-3). The increasing prevalence of drug-resistant M. tuberculosis strains makes the development of novel drugs for tuberculosis an urgent priority. In this article, we discuss our current understanding of the protein export systems of M. tuberculosis and consider the potential of these pathways to be novel targets for tuberculosis drugs.

The evolution of host specialization in the vertebrate gut symbiont Lactobacillus reuteri

Recent research has provided mechanistic insight into the important contributions of the gut microbiota to vertebrate biology, but questions remain about the evolutionary processes that have shaped this symbiosis. In the present study, we showed in experiments with gnotobiotic mice that the evolution of Lactobacillus reuteri with rodents resulted in the emergence of host specialization. To identify genomic events marking adaptations to the murine host, we compared the genome of the rodent isolate L. reuteri 100-23 with that of the human isolate L. reuteri F275, and we identified hundreds of genes that were specific to each strain. In order to differentiate true host-specific genome content from strain-level differences, comparative genome hybridizations were performed to query 57 L. reuteri strains originating from six different vertebrate hosts in combination with genome sequence comparisons of nine strains encompassing five phylogenetic lineages of the species. This approach revealed that rodent strains, although showing a high degree of genomic plasticity, possessed a specific genome inventory that was rare or absent in strains from other vertebrate hosts. The distinct genome content of L. reuteri lineages reflected the niche characteristics in the gastrointestinal tracts of their respective hosts, and inactivation of seven out of eight representative rodent-specific genes in L. reuteri 100-23 resulted in impaired ecological performance in the gut of mice. The comparative genomic analyses suggested fundamentally different trends of genome evolution in rodent and human L. reuteri populations, with the former possessing a large and adaptable pan-genome while the latter being subjected to a process of reductive evolution. In conclusion, this study provided experimental evidence and a molecular basis for the evolution of host specificity in a vertebrate gut symbiont, and it identified genomic events that have shaped this process.

The gastrointestinal microbiota of vertebrates is important for nutrient utilization, resistance against pathogens, and immune maturation of its host, but little is known about the evolutionary relationships between vertebrates and individual bacterial members of these communities. Here we provide robust evidence that the evolution of the gut symbiont Lactobacillus reuteri with vertebrates resulted in the emergence of host specialization. Genomic approaches using a combination of genome sequence comparisons and microarray analysis were used to identify the host-specific genome content in rodent and human strains and the evolutionary events that resulted in host adaptation. The study revealed divergent patterns of genome evolution in rodent and human lineages and a distinct genome inventory in host-restricted sub-populations of L. reuteri that reflected the niche characteristics in the gut of their particular vertebrate hosts. The ecological significance of representative rodent-specific genes was demonstrated in gnotobiotic mice. In conclusion, this work provided evidence that the vertebrate gut symbiont Lactobacillus reuteri, despite the likelihood of horizontal transmission, has remained stably associated with related groups of vertebrate hosts over evolutionary time and has evolved a lifestyle specialized to these host animals.

Plastids contain a second sec translocase system with essential functions

Proteins that are synthesized on cytoplasmic ribosomes but function within plastids must be imported and then targeted to one of six plastid locations. Although multiple systems that target proteins to the thylakoid membranes or thylakoid lumen have been identified, a system that can direct the integration of inner envelope membrane proteins from the stroma has not been previously described. Genetics and localization studies were used to show that plastids contain two different Sec systems with distinct functions. Loss-of-function mutations in components of the previously described thylakoid-localized Sec system, designated as SCY1 (At2g18710), SECA1 (At4g01800), and SECE1 (At4g14870) in Arabidopsis (Arabidopsis thaliana), result in albino seedlings and sucrose-dependent heterotrophic growth. Loss-of-function mutations in components of the second Sec system, designated as SCY2 (At2g31530) and SECA2 (At1g21650) in Arabidopsis, result in arrest at the globular stage and embryo lethality. Promoter-swap experiments provided evidence that SCY1 and SCY2 are functionally nonredundant and perform different roles in the cell. Finally, chloroplast import and fractionation assays and immunogold localization of SCY2-green fluorescent protein fusion proteins in root tissues indicated that SCY2 is part of an envelope-localized Sec system. Our data suggest that SCY2 and SECA2 function in Sec-mediated integration and translocation processes at the inner envelope membrane.

Genome-wide identification of Mycobacterium tuberculosis exported proteins with roles in intracellular growth

The exported proteins of Mycobacterium tuberculosis that are localized at the bacterial cell surface or secreted into the environment are ideally situated to interact with host factors and to function in virulence. In this study, we constructed a novel β-lactamase reporter transposon and used it directly in M. tuberculosis for genome-wide identification of exported proteins. From 177 β-lactam-resistant transposon mutants, we identified 111 different exported proteins. The majority of these proteins have no known function, and for nearly half of the proteins, our demonstration that they are exported when fused to a β-lactamase reporter is the first experimental proof of their extracytoplasmic localization. The transposon mutants in our banked library were of further value as a collection of mutants lacking individual exported proteins. By individually testing each of 111 mutants for growth in macrophages, six attenuated mutants with insertions in mce1A, mce1B, mce2F, rv0199, ctaC, and lppX were identified. Given that much of the M. tuberculosis genome encodes proteins of unknown function, our library of mapped transposon mutants is a valuable resource for efforts in functional genomics. This work also demonstrates the power of a β-lactamase reporter transposon that could be applied similarly to other bacterial pathogens.

Comprehensive appraisal of the extracellular proteins from a monoderm bacterium: theoretical and empirical exoproteomes of Listeria monocytogenes EGD-e by secretomics

Defined as proteins actively transported via secretion systems, secreted proteins can have radically different subcellular destinations in monoderm (Gram-positive) bacteria. From degradative enzymes in saprophytes to virulence factors in pathogens, secreted proteins are the main tools used by bacteria to interact with their surroundings. The etiological agent of listeriosis, Listeria monocytogenes, is a Gram-positive facultative intracellular foodborne pathogen, whose ecological niche is the soil and as such should be primarily considered as a ubiquitous saprophyte. Recent advances on protein secretion systems in this species prompted us to investigate the exoproteome. First, an original and rational bioinformatic strategy was developed to mimic the protein exportation steps leading to the extracellular localization of secreted proteins; 79 exoproteins were predicted as secreted via Sec, 1 exoprotein via Tat, 4 bacteriocins via ABC exporters, 3 exoproteins via holins, and 3 exoproteins via the WXG100 system. This bioinformatic analysis allowed for defining a databank of the mature protein set in L. monocytogenes, which was used for generating the theoretical exoproteome and for subsequent protein identification by proteomics. 2-DE proteomic analyses were performed over a wide pI range to experimentally cover the largest protein spectrum possible. A total of 120 spots could be resolved and identified, which corresponded to 50 distinct proteins. These exoproteins were essentially virulence factors, degradative enzymes, and proteins of unknown functions, which exportation would essentially rely on the Sec pathway or nonclassical secretion. This investigation resulted in the first comprehensive appraisal of the exoproteome of L. monocytogenes EGD-e based on theoretical and experimental secretomic analyses, which further provided indications on listerial physiology in relation with its habitat and lifestyle. The novel and rational strategy described here is generic and has been purposely designed for the prediction of proteins localized extracellularly in monoderm bacteria.

Structural insights into serine-rich fimbriae from Gram-positive bacteria

The serine-rich repeat family of fimbriae play important roles in the pathogenesis of streptococci and staphylococci. Despite recent attention, their finer structural details and precise adhesion mechanisms have yet to be determined. Fap1 (Fimbriae-associated protein 1) is the major structural subunit of serine-rich repeat fimbriae from Streptococcus parasanguinis and plays an essential role in fimbrial biogenesis, adhesion, and the early stages of dental plaque formation. Combining multidisciplinary, high resolution structural studies with biological assays, we provide new structural insight into adhesion by Fap1. We propose a model in which the serine-rich repeats of Fap1 subunits form an extended structure that projects the N-terminal globular domains away from the bacterial surface for adhesion to the salivary pellicle. We also uncover a novel pH-dependent conformational change that modulates adhesion and likely plays a role in survival in acidic environments.

Lipoic acid metabolism in microbial pathogens

Lipoic acid [(R)-5-(1,2-dithiolan-3-yl)pentanoic acid] is an enzyme cofactor required for intermediate metabolism in free-living cells. Lipoic acid was discovered nearly 60 years ago and was shown to be covalently attached to proteins in several multicomponent dehydrogenases. Cells can acquire lipoate (the deprotonated charge form of lipoic acid that dominates at physiological pH) through either scavenging or de novo synthesis. Microbial pathogens implement these basic lipoylation strategies with a surprising variety of adaptations which can affect pathogenesis and virulence. Similarly, lipoylated proteins are responsible for effects beyond their classical roles in catalysis. These include roles in oxidative defense, bacterial sporulation, and gene expression. This review surveys the role of lipoate metabolism in bacterial, fungal, and protozoan pathogens and how these organisms have employed this metabolism to adapt to niche environments.

The genome of a Bacillus isolate causing anthrax in chimpanzees combines chromosomal properties of B. cereus with B. anthracis virulence plasmids

Anthrax is a fatal disease caused by strains of Bacillus anthracis. Members of this monophyletic species are non motile and are all characterized by the presence of four prophages and a nonsense mutation in the plcR regulator gene. Here we report the complete genome sequence of a Bacillus strain isolated from a chimpanzee that had died with clinical symptoms of anthrax. Unlike classic B. anthracis, this strain was motile and lacked the four prohages and the nonsense mutation. Four replicons were identified, a chromosome and three plasmids. Comparative genome analysis revealed that the chromosome resembles those of non-B. anthracis members of the Bacillus cereus group, whereas two plasmids were identical to the anthrax virulence plasmids pXO1 and pXO2. The function of the newly discovered third plasmid with a length of 14 kbp is unknown. A detailed comparison of genomic loci encoding key features confirmed a higher similarity to B. thuringiensis serovar konkukian strain 97-27 and B. cereus E33L than to B. anthracis strains. For the first time we describe the sequence of an anthrax causing bacterium possessing both anthrax plasmids that apparently does not belong to the monophyletic group of all so far known B. anthracis strains and that differs in important diagnostic features. The data suggest that this bacterium has evolved from a B. cereus strain independently from the classic B. anthracis strains and established a B. anthracis lifestyle. Therefore we suggest to designate this isolate as "B. cereus variety (var.) anthracis".

The mycobacteriophage Ms6 encodes a chaperone-like protein involved in the endolysin delivery to the peptidoglycan

Like most double-stranded (ds) DNA phages, mycobacteriophage Ms6 uses the holin-endolysin system to achieve lysis of its host. In addition to endolysin (lysA) and holin (hol) genes, Ms6 encodes three accessory lysis proteins. In this study we investigated the lysis function of Gp1, which is encoded by the gp1 gene that lies immediately upstream of lysA. Escherichia coli lysis was observed after coexpression of LysA and Gp1 in the absence of Ms6 holin. Gp1 does not belong to the holin class of proteins, and we provide evidence that it shares several characteristics with molecular chaperones. We show that Gp1 interacts with LysA, and that this interaction is necessary for LysA delivery to its target. In addition, PhoA fusions showed that, in Mycobacterium smegmatis, LysA is exported to the extracytoplasmic environment in the presence of Gp1. We also show that Gp1 is necessary for efficient M. smegmatis lysis, as Ms6 gp1 deletion results in host lysis defects. We propose that delivery of Ms6 endolysin to the murein layer is assisted by Gp1, a chaperone-like protein, in a holin-independent manner.

The twin arginine protein transport pathway exports multiple virulence proteins in the plant pathogen Streptomyces scabies

Summary Streptomyces scabies is one of a group of organisms that causes the economically important disease potato scab. Analysis of the S. scabies genome sequence indicates that it is likely to secrete many proteins via the twin arginine protein transport (Tat) pathway, including several proteins whose coding sequences may have been acquired through horizontal gene transfer and share a common ancestor with proteins in other plant pathogens. Inactivation of the S. scabies Tat pathway resulted in pleiotropic phenotypes including slower growth rate and increased permeability of the cell envelope. Comparison of the extracellular proteome of the wild type and DeltatatC strains identified 73 predicted secretory proteins that were present in reduced amounts in the tatC mutant strain, and 47 Tat substrates were verified using a Tat reporter assay. The DeltatatC strain was almost completely avirulent on Arabidopsis seedlings and was delayed in attaching to the root tip relative to the wild-type strain. Genes encoding 14 candidate Tat substrates were individually inactivated, and seven of these mutants were reduced in virulence compared with the wild-type strain. We conclude that the Tat pathway secretes multiple proteins that are required for full virulence.

Identification of sortase A (SrtA) substrates in Streptococcus uberis: evidence for an additional hexapeptide (LPXXXD) sorting motif

Sortase (a transamidase) has been shown to be responsible for the covalent attachment of proteins to the bacterial cell wall. Anchoring is effected on secreted proteins containing a specific cell wall motif toward their C-terminus; that for sortase A (SrtA) in Gram-positive bacteria often incorporates the sequence LPXTG. Such surface proteins are often characterized as virulence determinants and play important roles during the establishment and persistence of infection. Intramammary infection with Streptococcus uberis is a common cause of bovine mastitis, which impacts on animal health and welfare and the economics of milk production. Comparison of stringently produced cell wall fractions from S. uberis and an isogenic mutant strain lacking SrtA permitted identification of 9 proteins likely to be covalently anchored at the cell surface. Analysis of these sequences implied the presence of two anchoring motifs for S. uberis, the classical LPXTG motif and an additional LPXXXD motif.

To catch a killer. What can mycobacterial models teach us about Mycobacterium tuberculosis pathogenesis?

Mycobacterium tuberculosis is the causative agent of the global tuberculosis epidemic. To combat this successful human pathogen we need a better understanding of the basic biology of mycobacterial pathogenesis. The use of mycobacterial model systems has the potential to greatly facilitate our understanding of how M. tuberculosis causes disease. Recently, studies using mycobacterial models, including M. bovis BCG, M. marinum, and M. smegmatis have significantly contributed to understanding M. tuberculosis. Specifically, there have been advances in genetic manipulation of M. tuberculosis using inducible promoters and recombineering that alleviate technical limitations in working with mycobacteria. Model systems have helped elucidate how secretion systems function at both the molecular level and during virulence. Mycobacterial models have also led to interesting hypotheses about how M. tuberculosis mediates latent infection and host response. While there is utility in using model systems to understand tuberculosis, each of these models represent distinct mycobacterial species with unique environmental adaptations. Directly comparing findings in model mycobacteria to those in M. tuberculosis will illuminate the similarities and differences between these species and increase our understanding of why M. tuberculosis is such a potent human pathogen.

Protein transport across and into cell membranes in bacteria and archaea

In the three domains of life, the Sec, YidC/Oxa1, and Tat translocases play important roles in protein translocation across membranes and membrane protein insertion. While extensive studies have been performed on the endoplasmic reticular and Escherichia coli systems, far fewer studies have been done on archaea, other Gram-negative bacteria, and Gram-positive bacteria. Interestingly, work carried out to date has shown that there are differences in the protein transport systems in terms of the number of translocase components and, in some cases, the translocation mechanisms and energy sources that drive translocation. In this review, we will describe the different systems employed to translocate and insert proteins across or into the cytoplasmic membrane of archaea and bacteria.

A ribosome-nascent chain sensor of membrane protein biogenesis in Bacillus subtilis

Proteins in the YidC/Oxa1/Alb3 family have essential functions in membrane protein insertion and folding. Bacillus subtilis encodes two YidC homologs, one that is constitutively expressed (spoIIIJ/yidC1) and a second (yqjG/yidC2) that is induced in spoIIIJ mutants. Regulated induction of yidC2 allows B. subtilis to maintain capacity of the membrane protein insertion pathway. We here show that a gene located upstream of yidC2 (mifM/yqzJ) serves as a sensor of SpoIIIJ activity that regulates yidC2 translation. Decreased SpoIIIJ levels or deletion of the MifM transmembrane domain arrests mifM translation and unfolds an mRNA hairpin that otherwise blocks initiation of yidC2 translation. This regulated translational arrest and yidC2 induction require a specific interaction between the MifM C-terminus and the ribosomal polypeptide exit tunnel. MifM therefore acts as a ribosome-nascent chain complex rather than as a fully synthesized protein. B. subtilis MifM and the previously described secretion monitor SecM in Escherichia coli thereby provide examples of the parallel evolution of two regulatory nascent chains that monitor different protein export pathways by a shared molecular mechanism.

Streptococcus adherence and colonization

Streptococci readily colonize mucosal tissues in the nasopharynx; the respiratory, gastrointestinal, and genitourinary tracts; and the skin. Each ecological niche presents a series of challenges to successful colonization with which streptococci have to contend. Some species exist in equilibrium with their host, neither stimulating nor submitting to immune defenses mounted against them. Most are either opportunistic or true pathogens responsible for diseases such as pharyngitis, tooth decay, necrotizing fasciitis, infective endocarditis, and meningitis. Part of the success of streptococci as colonizers is attributable to the spectrum of proteins expressed on their surfaces. Adhesins enable interactions with salivary, serum, and extracellular matrix components; host cells; and other microbes. This is the essential first step to colonization, the development of complex communities, and possible invasion of host tissues. The majority of streptococcal adhesins are anchored to the cell wall via a C-terminal LPxTz motif. Other proteins may be surface anchored through N-terminal lipid modifications, while the mechanism of cell wall associations for others remains unclear. Collectively, these surface-bound proteins provide Streptococcus species with a "coat of many colors," enabling multiple intimate contacts and interplays between the bacterial cell and the host. In vitro and in vivo studies have demonstrated direct roles for many streptococcal adhesins as colonization or virulence factors, making them attractive targets for therapeutic and preventive strategies against streptococcal infections. There is, therefore, much focus on applying increasingly advanced molecular techniques to determine the precise structures and functions of these proteins, and their regulatory pathways, so that more targeted approaches can be developed.

Characterization of a Mycobacterium tuberculosis ESX-3 conditional mutant: essentiality and rescue by iron and zinc

Recently, a novel type of secretory pathway, type VII secretion systems (T7SSs), has been characterized in mycobacteria. The chromosomes of Mycobacterium tuberculosis and Mycobacterium bovis encode five T7SSs (ESX-1 to ESX-5). The best characterized of them, ESX-1, is involved in host-pathogen interactions, and its deletion is one of the main causes of M. bovis BCG attenuation. Another T7SS, ESX-3, has been previously shown to be transcriptionally controlled by the zinc uptake repressor (Zur) and by the iron-dependent transcriptional repressor (IdeR), suggesting that it might be involved in zinc and iron homeostasis. In this study, we characterized an M. tuberculosis conditional mutant in which transcription of the ESX-3 gene cluster can be downregulated by anhydrotetracycline. We showed that this T7SS is essential for growth and that this phenotype can be complemented by zinc, iron, or supernatant from a wild-type parental strain culture, demonstrating that the ESX-3 secretion system is responsible for the secretion of some soluble factor(s) required for growth that is probably involved in optimal iron and zinc uptake.

Mycobacterial survival strategies in the phagosome: defence against host stresses

Infections with Mycobacterium tuberculosis remain a major cause of disease and death in humans. Among the factors that contribute to M. tuberculosis's success as a pathogen is its ability to withstand potentially bactericidal host defences and to resist elimination by an activated immune system. This resistance to killing by the host is in part due to the low permeability of the mycobacterial cell envelope for many toxic molecules. In addition, it depends upon the detoxification of reactive oxygen and reactive nitrogen molecules produced by the host, the repair of the damage these molecules cause and maintenance of a neutral intrabacterial pH within acidic environments. The latter three mechanisms are the focus of this review.

[Bacterial secretion systems: their function and contribution to disease process]

Bacterial pathogens possesses certain secretion systems to maintain their homeostasis and to exert full virulence. Currently, two translocons including Sec and Tat, and 7 secretion systems are found in Gram-positive and -negative bacteria. Some virulence factors, which are referred to as effectors, are directly translocated into the host cell via an injection apparatus, i.e., the type III secretion system. Thus, characterization of secretion systems and their delivered proteins into extracellular milieus is required for understanding of strategies of bacterial pathogens.

Molecular dissection of the secA2 locus of group B Streptococcus reveals that glycosylation of the Srr1 LPXTG protein is required for full virulence

In streptococci, the secA2 locus includes genes encoding the following: (i) the accessory Sec components (SecA2, SecY2, and at least three accessory secretion proteins), (ii) two essential glycosyltranferases (GTs) (GtfA and GtfB), (iii) a variable number of dispensable additional GTs, and (iv) a secreted serine-rich LPXTG protein which is glycosylated in the cytoplasm and transported to the cell surface by this accessory Sec system. The secA2 locus of Streptococcus agalactiae strain NEM316 is structurally related to those found in other streptococci and encodes the serine-rich surface protein Srr1. We demonstrated that expression of Srr1 but not that of the SecA2 components and the associated GTs is regulated by the standalone transcriptional regulator Rga. Srr1 is synthesized as a glycosylated precursor, secreted by the SecA2 system, and anchored to the cell wall by the housekeeping sortase A. Srr1 was localized preferentially at the old poles. GtfA and/or GtfB, but not the six additional GTs, is essential for the production of Srr1. These GTs are involved in the attachment of GlcNac and sialic acid to Srr1. Full glycosylation of Srr1 is associated with the cell surface display of a protein that is more resistant to proteolytic attack. Srr1 contributes to bacterial adherence to human epithelial cell lines and virulence in a neonatal rat model. The extent of Srr1 glycosylation by GtfC to -H modulates bacterial adherence and virulence.

Characterization of Streptococcus gordonii SecA2 as a paralogue of SecA

The accessory Sec system of Streptococcus gordonii is essential for transport of the glycoprotein GspB to the bacterial cell surface. A key component of this dedicated transport system is SecA2. The SecA2 proteins of streptococci and staphylococci are paralogues of SecA and are presumed to have an analogous role in protein transport, but they may be specifically adapted for the transport of large, serine-rich glycoproteins. We used a combination of genetic and biochemical methods to assess whether the S. gordonii SecA2 functions similarly to SecA. Although mutational analyses demonstrated that conserved amino acids are essential for the function of SecA2, replacing such residues in one of two nucleotide binding folds had only minor effects on SecA2 function. SecA2-mediated transport is highly sensitive to azide, as is SecA-mediated transport. Comparison of the S. gordonii SecA and SecA2 proteins in vitro revealed that SecA2 can hydrolyze ATP at a rate similar to that of SecA and is comparably sensitive to azide but that the biochemical properties of these enzymes are subtly different. That is, SecA2 has a lower solubility in aqueous solutions and requires higher Mg(2+) concentrations for maximal activity. In spite of the high degree of similarity between the S. gordonii paralogues, analysis of SecA-SecA2 chimeras indicates that the domains are not readily interchangeable. This suggests that specific, unique contacts between SecA2 and other components of the accessory Sec system may preclude cross-functioning with the canonical Sec system.

The Accessory SecA2 System of Mycobacteria Requires ATP Binding and the Canonical SecA1

In bacteria, the majority of exported proteins are transported by the general Sec pathway from their site of synthesis in the cytoplasm across the cytoplasmic membrane. The essential SecA ATPase powers this Sec-mediated export. Mycobacteria possess two nonredundant SecA homologs: SecA1 and SecA2. In pathogenic Mycobacterium tuberculosis and the nonpathogenic model mycobacterium Mycobacterium smegmatis, SecA1 is essential for protein export and is the "housekeeping" SecA, whereas SecA2 is an accessory SecA that exports a specific subset of proteins. In M. tuberculosis the accessory SecA2 pathway plays a role in virulence. In this study, we uncovered basic properties of the mycobacterial SecA2 protein and its pathway for exporting select proteins. By constructing secA2 mutant alleles that encode proteins defective in ATP binding, we showed that ATP binding is required for SecA2 function. SecA2 mutant proteins unable to bind ATP were nonfunctional and dominant negative. By evaluating the subcellular distribution of each SecA, SecA1 was shown to be equally divided between cytosolic and cell envelope fractions, whereas SecA2 was predominantly localized to the cytosol. Finally, we showed that the canonical SecA1 has a role in the process of SecA2-dependent export. The accessory SecA2 export system is important to the physiology and virulence of mycobacteria. These studies help establish the mechanism of this new type of specialized protein export pathway.

LpqM, a mycobacterial lipoprotein-metalloproteinase, is required for conjugal DNA transfer in Mycobacterium smegmatis

We have previously described a novel conjugal DNA transfer process that occurs in Mycobacterium smegmatis. To identify donor genes required for transfer, we have performed a transposon mutagenesis screen; we report here that LpqM, a putative lipoprotein-metalloproteinase, is essential for efficient DNA transfer. Bioinformatic analyses predict that LpqM contains a signal peptide necessary for the protein's targeting to the cell envelope and a metal ion binding motif, the likely catalytic site for protease activity. Using targeted mutagenesis, we demonstrate that each of these motifs is necessary for DNA transfer and that LpqM is located in the cell envelope. The requirement for transfer is specific to the donor strain; an lpqM knockout mutant in the recipient is still proficient in transfer assays. The activity of LpqM is conserved among mycobacteria; homologues from both Mycobacterium tuberculosis and Mycobacterium avium can complement lpqM donor mutants, suggesting that the homologues recognize and process similar proteins. Lipoproteins constitute a significant proportion of the mycobacterial cell wall, but despite their abundance, very few have been assigned an activity. We discuss the potential role of LpqM in DNA transfer and the implications of the conservation of LpqM activity in M. tuberculosis.

Glycosylation and biogenesis of a family of serine-rich bacterial adhesins

Glycosylation of bacterial proteins is an important process for bacterial physiology and pathophysiology. Both O- and N-linked glycan moieties have been identified in bacterial glycoproteins. The N-linked glycosylation pathways are well established in Gram-negative bacteria. However, the O-linked glycosylation pathways are not well defined due to the complex nature of known O-linked glycoproteins in bacteria. In this review, we examine a new family of serine-rich O-linked glycoproteins which are represented by fimbriae-associated adhesin Fap1 of Streptococcus parasanguinis and human platelet-binding protein GspB of Streptococcus gordonii. This family of glycoproteins is conserved in streptococcal and staphylococcal species. A gene cluster coding for glycosyltransferases and accessory Sec proteins has been implicated in the protein glycosylation. A two-step glycosylation model is proposed. Two glycosyltransferases interact with each other and catalyse the first step of the protein glycosylation in the cytoplasm; the cross-talk between glycosylation-associated proteins and accessory Sec components mediates the second step of the protein glycosylation, an emerging mechanism for bacterial O-linked protein glycosylation. Dissecting the molecular mechanism of this conserved biosynthetic pathway offers opportunities to develop new therapeutic strategies targeting this previously unrecognized pathway, as serine-rich glycoproteins have been shown to play a role in bacterial pathogenesis.