Asp3 mediates multiple protein-protein interactions within the accessory Sec system of Streptococcus gordonii

Serine-rich repeat proteins: well-known yet little-understood bacterial adhesins

Serine-rich-repeat proteins (SRRPs) are large mucin-like glycoprotein adhesins expressed by a plethora of pathogenic and symbiotic Gram-positive bacteria. SRRPs play major functional roles in bacterial-host interactions, like adhesion, aggregation, biofilm formation, virulence, and pathogenesis. Through their functional roles, SRRPs aid in the development of host microbiomes but also diseases like infective endocarditis, otitis media, meningitis, and pneumonia. SRRPs comprise shared domains across different species, including two or more heavily O-glycosylated long stretches of serine-rich repeat regions. With loci that can be as large as ~40 kb and can encode up to 10 distinct glycosyltransferases that specifically facilitate SRRP glycosylation, the SRRP loci makes up a significant portion of the bacterial genome. The significance of SRRPs and their glycans in host-microbe communications is becoming increasingly evident. Studies are beginning to reveal the glycosylation pathways and mature O-glycans presented by SRRPs. Here we review the glycosylation machinery of SRRPs across species and discuss the functional roles and clinical manifestations of SRRP glycosylation.

Illuminating the oral microbiome and its host interactions: tools and approaches for molecular microbiology studies

Advancements in DNA sequencing technologies within the last decade have stimulated an unprecedented interest in the human microbiome, largely due the broad diversity of human diseases found to correlate with microbiome dysbiosis. As a direct consequence of these studies, a vast number of understudied and uncharacterized microbes have been identified as potential drivers of mucosal health and disease. The looming challenge in the field is to transition these observations into defined molecular mechanistic studies of symbiosis and dysbiosis. In order to meet this challenge, many of these newly identified microbes will need to be adapted for use in experimental models. Consequently, this review presents a comprehensive overview of the molecular microbiology tools and techniques that have played crucial roles in genetic studies of the bacteria found within the human oral microbiota. Here, we will use specific examples from the oral microbiome literature to illustrate the biology supporting these techniques, why they are needed in the field, and how such technologies have been implemented. It is hoped that this information can serve as a useful reference guide to help catalyze molecular microbiology studies of the many new understudied and uncharacterized species identified at different mucosal sites in the body.

O-acetylation controls the glycosylation of bacterial serine-rich repeat glycoproteins

The serine-rich repeat (SRR) glycoproteins of gram-positive bacteria are a family of adhesins that bind to a wide range of host ligands, and expression of SRR glycoproteins is linked with enhanced bacterial virulence. The biogenesis of these surface glycoproteins involves their intracellular glycosylation and export via the accessory Sec system. Although all accessory Sec components are required for SRR glycoprotein export, Asp2 of Streptococcus gordonii also functions as an O-acetyltransferase that modifies GlcNAc residues on the SRR adhesin gordonii surface protein B (GspB). Because these GlcNAc residues can also be modified by the glycosyltransferases Nss and Gly, it has been unclear whether the post-translational modification of GspB is coordinated. We now report that acetylation modulates the glycosylation of exported GspB. Loss of O-acetylation due to aps2 mutagenesis led to the export of GspB glycoforms with increased glucosylation of the GlcNAc moieties. Linkage analysis of the GspB glycan revealed that both O-acetylation and glucosylation occurred at the same C6 position on GlcNAc residues and that O-acetylation prevented Glc deposition. Whereas streptococci expressing nonacetylated GspB with increased glucosylation were significantly reduced in their ability to bind human platelets in vitro, deletion of the glycosyltransferases nss and gly in the asp2 mutant restored platelet binding to WT levels. These findings demonstrate that GlcNAc O-acetylation controls GspB glycosylation, such that binding via this adhesin is optimized. Moreover, because O-acetylation has comparable effects on the glycosylation of other SRR adhesins, acetylation may represent a conserved regulatory mechanism for the post-translational modification of the SRR glycoprotein family.

Streptococcal Serine-Rich Repeat Proteins in Colonization and Disease

Glycosylation of proteins, previously thought to be absent in prokaryotes, is increasingly recognized as important for both bacterial colonization and pathogenesis. For mucosal pathobionts, glycoproteins that function as cell wall-associated adhesins facilitate interactions with mucosal surfaces, permitting persistent adherence, invasion of deeper tissues and transition to disease. This is exemplified by Streptococcus pneumoniae and Streptococcus agalactiae, which can switch from being relatively harmless members of the mucosal tract microbiota to bona fide pathogens that cause life-threatening diseases. As part of their armamentarium of virulence factors, streptococci encode a family of large, glycosylated serine-rich repeat proteins (SRRPs) that facilitate binding to various tissue types and extracellular matrix proteins. This minireview focuses on the roles of S. pneumoniae and S. agalactiae SRRPs in persistent colonization and the transition to disease. The potential of utilizing SRRPs as vaccine targets will also be discussed.

Structural and functional insights into the Asp1/2/3 complex mediated secretion of pneumococcal serine-rich repeat protein PsrP

The accessory sec system consisting of seven conserved components is commonly distributed among pathogenic Gram-positive bacteria for the secretion of serine-rich-repeat proteins (SRRPs). Asp1/2/3 protein complex in the system is responsible for both the O-acetylation of GlcNAc and delivering SRRPs to SecA2. However, the molecular mechanism of how Asp1/2/3 transport SRRPs remains unknown. Here, we report the complex structure of Asp1/2/3 from Streptococcus pneumoniae at 2.9 Å. Further functional assays indicated that Asp1/2/3 can stimulate the ATPase activity of SecA2. In addition, the deletion of asp1/2/3 gene resulted in the accumulation of a secreted version of PsrP with an altered glycoform in protoplast fraction of the mutant cell, which suggested the modification/transport coupling of the substrate. Altogether, these findings not only provide structural basis for further investigations on the transport process of SRRPs, but also uncover the indispensable role of Asp1/2/3 in the accessory sec system.

The Two Distinct Types of SecA2-Dependent Export Systems

In addition to SecA of the general Sec system, many Gram-positive bacteria, including mycobacteria, express SecA2, a second, transport-associated ATPase. SecA2s can be subdivided into two mechanistically distinct types: (i) SecA2s that are part of the accessory Sec (aSec) system, a specialized transporter mediating the export of a family of serine-rich repeat (SRR) glycoproteins that function as adhesins, and (ii) SecA2s that are part of multisubstrate systems, in which SecA2 interacts with components of the general Sec system, specifically the SecYEG channel, to export multiple types of substrates. Found mainly in streptococci and staphylococci, the aSec system also contains SecY2 and novel accessory Sec proteins (Asps) that are required for optimal export. Asp2 also acetylates glucosamine residues on the SRR domains of the substrate during transport. Targeting of the SRR substrate to SecA2 and the aSec translocon is mediated by a specialized signal peptide. Multisubstrate SecA2 systems are present in mycobacteria, corynebacteria, listeriae, clostridia, and some bacillus species. Although most substrates for this SecA2 have canonical signal peptides that are required for export, targeting to SecA2 appears to depend on structural features of the mature protein. The feature of the mature domains of these proteins that renders them dependent on SecA2 for export may be their potential to fold in the cytoplasm. The discovery of aSec and multisubstrate SecA2 systems expands our appreciation of the diversity of bacterial export pathways. Here we present our current understanding of the mechanisms of each of these SecA2 systems.

Protein Transport Across the Bacterial Plasma Membrane by the Sec Pathway

More than a third of all bacterial polypeptides, comprising the 'exportome', are transported to extracytoplasmic locations. Most of the exportome is targeted and inserts into ('membranome') or crosses ('secretome') the plasma membrane. The membranome and secretome use distinct targeting signals and factors, and driving forces, but both use the ubiquitous and essential Sec translocase and its SecYEG protein-conducting channel. Membranome export is co-translational and uses highly hydrophobic N-terminal signal anchor sequences recognized by the signal recognition particle on the ribosome, that also targets C-tail anchor sequences. Translating ribosomes drive movement of these polypeptides through the lateral gate of SecY into the inner membrane. On the other hand, secretome export is post-translational and carries two types of targeting signals: cleavable N-terminal signal peptides and multiple short hydrophobic targeting signals in their mature domains. Secretome proteins remain translocation competent due to occupying loosely folded to completely non-folded states during targeting. This is accomplished mainly by the intrinsic properties of mature domains and assisted by signal peptides and/or chaperones. Secretome proteins bind to the dimeric SecA subunit of the translocase. SecA converts from a dimeric preprotein receptor to a monomeric ATPase motor and drives vectorial crossing of chains through SecY aided by the proton motive force. Signal peptides are removed by signal peptidases and translocated chains fold or follow subsequent trafficking.

Membrane trafficking of the bacterial adhesin GspB and the accessory Sec transport machinery

The serine-rich repeat (SRR) glycoproteins of Gram-positive bacteria are large, cell wall-anchored adhesins that mediate binding to many host cells and proteins and are associated with bacterial virulence. SRR glycoproteins are exported to the cell surface by the accessory Sec (aSec) system comprising SecA2, SecY2, and 3-5 additional proteins (Asp1 to Asp5) that are required for substrate export. These adhesins typically have a 90-amino acid-long signal peptide containing an elongated N-region and a hydrophobic core. Previous studies of GspB (the SRR adhesin of Streptococcus gordonii) have shown that a glycine-rich motif in its hydrophobic core is essential for selective, aSec-mediated transport. However, the role of this extended N-region in transport is poorly understood. Here, using protein-lipid co-flotation assays and site-directed mutagenesis, we report that the N-region of the GspB signal peptide interacts with anionic lipids through electrostatic forces and that this interaction is necessary for GspB preprotein trafficking to lipid membranes. Moreover, we observed that protein-lipid binding is required for engagement of GspB with SecA2 and for aSec-mediated transport. We further found that SecA2 and Asp1 to Asp3 also localize selectively to liposomes that contain anionic lipids. These findings suggest that the GspB signal peptide electrostatically binds anionic lipids at the cell membrane, where it encounters SecA2. After SecA2 engagement with the signal peptide, Asp1 to Asp3 promote SecA2 engagement with the mature domain, which activates GspB translocation.

Unraveling the sequence of cytosolic reactions in the export of GspB adhesin from Streptococcus gordonii

Many pathogenic bacteria, including Streptococcus gordonii, possess a pathway for the cellular export of a single serine-rich-repeat protein that mediates the adhesion of bacteria to host cells and the extracellular matrix. This adhesin protein is O-glycosylated by several cytosolic glycosyltransferases and requires three accessory Sec proteins (Asp1-3) for export, but how the adhesin protein is processed for export is not well understood. Here, we report that the S. gordonii adhesin GspB is sequentially O-glycosylated by three enzymes (GtfA/B, Nss, and Gly) that attach N-acetylglucosamine and glucose to Ser/Thr residues. We also found that modified GspB is transferred from the last glycosyltransferase to the Asp1/2/3 complex. Crystal structures revealed that both Asp1 and Asp3 are related to carbohydrate-binding proteins, suggesting that they interact with carbohydrates and bind glycosylated adhesin, a notion that was supported by further analyses. We further observed that Asp1 also has an affinity for phospholipids, which is attenuated by Asp2. In summary, our findings support a model in which the GspB adhesin is sequentially glycosylated by GtfA/B, Nss, and Gly and then transferred to the Asp1/2/3 complex in which Asp1 mediates the interaction of the Asp1/2/3 complex with the lipid bilayer for targeting of matured GspB to the export machinery.

O-acetylation of the serine-rich repeat glycoprotein GspB is coordinated with accessory Sec transport

The serine-rich repeat (SRR) glycoproteins are a family of adhesins found in many Gram-positive bacteria. Expression of the SRR adhesins has been linked to virulence for a variety of infections, including streptococcal endocarditis. The SRR preproteins undergo intracellular glycosylation, followed by export via the accessory Sec (aSec) system. This specialized transporter is comprised of SecA2, SecY2 and three to five accessory Sec proteins (Asps) that are required for export. Although the post-translational modification and transport of the SRR adhesins have been viewed as distinct processes, we found that Asp2 of Streptococcus gordonii also has an important role in modifying the SRR adhesin GspB. Biochemical analysis and mass spectrometry indicate that Asp2 is an acetyltransferase that modifies N-acetylglucosamine (GlcNAc) moieties on the SRR domains of GspB. Targeted mutations of the predicted Asp2 catalytic domain had no effect on transport, but abolished acetylation. Acetylated forms of GspB were only detected when the protein was exported via the aSec system, but not when transport was abolished by secA2 deletion. In addition, GspB variants rerouted to export via the canonical Sec pathway also lacked O-acetylation, demonstrating that this modification is specific to export via the aSec system. Streptococci expressing GspB lacking O-acetylated GlcNAc were significantly reduced in their ability bind to human platelets in vitro, an interaction that has been strongly linked to virulence in the setting of endocarditis. These results demonstrate that Asp2 is a bifunctional protein involved in both the post-translational modification and transport of SRR glycoproteins. In addition, these findings indicate that these processes are coordinated during the biogenesis of SRR glycoproteins, such that the adhesin is optimally modified for binding. This requirement for the coupling of modification and export may explain the co-evolution of the SRR glycoproteins with their specialized glycan modifying and export systems.

Bacterial Secretion Systems: An Overview

Bacterial pathogens utilize a multitude of methods to invade mammalian hosts, damage tissue sites, and thwart the immune system from responding. One essential component of these strategies for many bacterial pathogens is the secretion of proteins across phospholipid membranes. Secreted proteins can play many roles in promoting bacterial virulence, from enhancing attachment to eukaryotic cells, to scavenging resources in an environmental niche, to directly intoxicating target cells and disrupting their functions. Many pathogens use dedicated protein secretion systems to secrete virulence factors from the cytosol of the bacteria into host cells or the host environment. In general, bacterial protein secretion apparatuses can be divided into classes, based on their structures, functions, and specificity. Some systems are conserved in all classes of bacteria and secrete a broad array of substrates, while others are only found in a small number of bacterial species and/or are specific to only one or a few proteins. In this chapter, we review the canonical features of several common bacterial protein secretion systems, as well as their roles in promoting the virulence of bacterial pathogens. Additionally, we address recent findings that indicate that the innate immune system of the host can detect and respond to the presence of protein secretion systems during mammalian infection.

Composition and Activity of the Non-canonical Gram-positive SecY2 Complex

The accessory Sec system in Streptococcus gordonii DL1 is a specialized export system that transports a large serine-rich repeat protein, Hsa, to the bacterial surface. The system is composed of core proteins SecA2 and SecY2 and accessory Sec proteins Asp1–Asp5. Similar to canonical SecYEG, SecY2 forms a channel for translocation of the Hsa adhesin across the cytoplasmic membrane. Accessory Sec proteins Asp4 and Asp5 have been suggested to work alongside SecY2 to form the translocon, similar to the associated SecY, SecE, and SecG of the canonical system (SecYEG). To test this theory, S. gordonii secY2, asp4, and asp5 were co-expressed in Escherichia coli. The resultant complex was subsequently purified, and its composition was confirmed by mass spectrometry to be SecY2-Asp4-Asp5. Like SecYEG, the non-canonical complex activates the ATPase activity of the SecA motor (SecA2). This study also shows that Asp4 and Asp5 are necessary for optimal adhesion of S. gordonii to glycoproteins gp340 and fibronectin, known Hsa binding partners, as well as for early stage biofilm formation. This work opens new avenues for understanding the structure and function of the accessory Sec system.

Bacillus anthracis SlaQ Promotes S-Layer Protein Assembly

Bacillus anthracis vegetative forms assemble an S-layer comprised of two S-layer proteins, Sap and EA1. A hallmark of S-layer proteins are their C-terminal crystallization domains, which assemble into a crystalline lattice once these polypeptides are deposited on the bacterial surface via association between their N-terminal S-layer homology domains and the secondary cell wall polysaccharide. Here we show that slaQ, encoding a small cytoplasmic protein conserved among pathogenic bacilli elaborating S-layers, is required for the efficient secretion and assembly of Sap and EA1. S-layer protein precursors cosediment with SlaQ, and SlaQ appears to facilitate Sap assembly. Purified SlaQ polymerizes and when mixed with purified Sap promotes the in vitro formation of tubular S-layer structures. A model is discussed whereby SlaQ, in conjunction with S-layer secretion factors SecA2 and SlaP, promotes localized secretion and S-layer assembly in B. anthracis.

IMPORTANCE

S-layer proteins are endowed with the propensity for self-assembly into crystalline arrays. Factors promoting S-layer protein assembly have heretofore not been reported. We identified Bacillus anthracis SlaQ, a small cytoplasmic protein that facilitates S-layer protein assembly in vivo and in vitro.

Physiological adaptations of key oral bacteria

Oral colonising bacteria are highly adapted to the various environmental niches harboured within the mouth, whether that means while contributing to one of the major oral diseases of caries, pulp infections, or gingival/periodontal disease or as part of a commensal lifestyle. Key to these infections is the ability to adhere to surfaces via a range of specialised adhesins targeted at both salivary and epithelial proteins, their glycans and to form biofilm. They must also resist the various physical stressors they are subjected to, including pH and oxidative stress. Possibly most strikingly, they have developed the ability to harvest both nutrient sources provided by the diet and those derived from the host, such as protein and surface glycans. We have attempted to review recent developments that have revealed much about the molecular mechanisms at work in shaping the physiology of oral bacteria and how we might use this information to design and implement new treatment strategies.

Selective transport by SecA2: an expanding family of customized motor proteins

The SecA2 proteins are a special class of transport-associated ATPases that are related to the SecA component of the general Sec system, and are found in an increasingly large number of Gram-positive bacterial species. The SecA2 substrates are typically linked to the cell wall, but may be lipid-linked, peptidoglycan-linked, or non-covalently associated S-layer proteins. These substrates can have a significant impact on virulence of pathogenic organisms, but may also aid colonization by commensals. The SecA2 orthologues range from being highly similar to their SecA paralogues, to being distinctly different in apparent structure and function. Two broad classes of SecA2 are evident. One transports multiple substrates, and may interact with the general Sec system, or with an as yet unidentified transmembrane channel. The second type transports a single substrate, and is a component of the accessory Sec system, which includes the SecY paralogue SecY2 along with the accessory Sec proteins Asp1-3. Recent studies indicate that the latter three proteins may have a unique role in coordinating post-translational modification of the substrate with transport by SecA2. Comparative functional and phylogenetic analyses suggest that each SecA2 may be uniquely adapted for a specific type of substrate. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Leaving home ain't easy: protein export systems in Gram-positive bacteria

Transport of proteins into or across biological membranes is catalyzed by membrane-bound transport machineries. In Gram-positive bacteria, the vast majority of proteins are exported out of the cytosol by the conserved general secretion (Sec) system or, alternatively, by the twin-arginine translocation (Tat) system, that closely resemble their well-studied counterparts in Gram-negative bacteria. Besides these common major export routes, additional unique protein export systems (such as accessory Sec2 systems and/or type VII/WXG100 secretion systems) exist in some Gram-positive bacteria that are specifically involved in the secretion of limited subsets of proteins.

Gap2 promotes the formation of a stable protein complex required for mature Fap1 biogenesis

Serine-rich repeat glycoproteins (SRRPs) are important bacterial adhesins conserved in streptococci and staphylococci. Fap1, a SRRP identified in Streptococcus parasanguinis, is the major constituent of bacterial fimbriae and is required for adhesion and biofilm formation. An 11-gene cluster is required for Fap1 glycosylation and secretion; however, the exact mechanism of Fap1 biogenesis remains a mystery. Two glycosylation-associated proteins within this cluster--Gap1 and Gap3--function together in Fap1 biogenesis. Here we report the role of the third glycosylation-associated protein, Gap2. A gap2 mutant exhibited the same phenotype as the gap1 and gap3 mutants in terms of Fap1 biogenesis, fimbrial assembly, and bacterial adhesion, suggesting that the three proteins interact. Indeed, all three proteins interacted with each other independently and together to form a stable protein complex. Mechanistically, Gap2 protected Gap3 from degradation by ClpP protease, and Gap2 required the presence of Gap1 for expression at the wild-type level. Gap2 augmented the function of Gap1 in stabilizing Gap3; this function was conserved in Gap homologs from Streptococcus agalactiae. Our studies demonstrate that the three Gap proteins work in concert in Fap1 biogenesis and reveal a new function of Gap2. This insight will help us elucidate the molecular mechanism of SRRP biogenesis in this bacterium and in pathogenic species.

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.

The accessory Sec protein Asp2 modulates GlcNAc deposition onto the serine-rich repeat glycoprotein GspB

The accessory Sec system is a specialized transport system that exports serine-rich repeat (SRR) glycoproteins of Gram-positive bacteria. This system contains two homologues of the general secretory (Sec) pathway (SecA2 and SecY2) and several other essential proteins (Asp1 to Asp5) that share no homology to proteins of known function. In Streptococcus gordonii, Asp2 is required for the transport of the SRR adhesin GspB, but its role in export is unknown. Tertiary structure predictions suggest that the carboxyl terminus of Asp2 resembles the catalytic region of numerous enzymes that function through a Ser-Asp-His catalytic triad. Sequence alignment of all Asp2 homologues identified a highly conserved pentapeptide motif (Gly-X-Ser(362)-X-Gly) typical of most Ser-Asp-His catalytic triads, where Ser forms the reactive residue. Site-directed mutagenesis of residues comprising the predicted catalytic triad of Asp2 of S. gordonii had no effect upon GspB transport but did result in a marked change in the electrophoretic mobility of the protein. Lectin-binding studies and monosaccharide content analysis of this altered glycoform revealed an increase in glucosamine deposition. Random mutagenesis of the Asp2 region containing this catalytic domain also disrupted GspB transport. Collectively, our findings suggest that Asp2 is a bifunctional protein that is essential for both GspB transport and correct glycosylation. The catalytic domain may be responsible for controlling the glycosylation of GspB, while other surrounding regions are functionally required for glycoprotein transport.

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.

A Specific interaction between SecA2 and a region of the preprotein adjacent to the signal peptide occurs during transport via the accessory Sec system

The accessory Sec systems of streptococci and staphylococci mediate the transport of a family of large, serine-rich glycoproteins to the bacterial cell surface. These systems are comprised of SecA2, SecY2, and three core accessory Sec proteins (Asp1-3). In Streptococcus gordonii, transport of the serine-rich glycoprotein GspB requires both a unique 90-residue N-terminal signal peptide and an adjacent 24-residue segment (the AST domain). We used in vivo site-specific photo-cross-linking to identify proteins that interact with the AST domain during transport. To facilitate this analysis, the entire accessory Sec system of S. gordonii was expressed in Escherichia coli. The determinants of GspB trafficking to the accessory Sec system in E. coli matched those in S. gordonii, establishing the validity of this approach. When the photo-cross-linker was placed within the AST domain, the preprotein was found to cross-link to SecA2. Importantly, no cross-linking to SecA was detected. Cross-linking of the N-terminal end of the AST domain to SecA2 occurred regardless of whether Asp1-3 were present. However, cross-linking to the C-terminal end was dependent on the Asps. The combined results indicate that full engagement of the AST domain by SecA2 is modulated by one or more of the Asps, and suggest that this process is important for initiating transport.

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.

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.

Gap1 functions as a molecular chaperone to stabilize its interactive partner Gap3 during biogenesis of serine-rich repeat bacterial adhesin

Serine-rich repeat glycoproteins (SRRPs) are important bacterial adhesins that are conserved in streptococci and staphylococci. Fimbriae-associated protein (Fap1) from Streptococcus parasanguinis, was the first SRRP identified; it plays an important role in bacterial biofilm formation. A gene cluster encoding glycosyltransferases and accessory secretion components is required for Fap1 biogenesis. Two glycosylation-associated proteins, Gap1 and Gap3 within the cluster, interact with each other and function in concert in Fap1 biogenesis. Here we report the new molecular events underlying contribution of the interaction to Fap1 biogenesis. The Gap1-deficient mutant rendered Gap3 unstable and degraded in vitro and in vivo. Inactivation of a gene encoding protease ClpP reversed the phenotype of the gap1 mutant, suggesting that ClpP is responsible for degradation of Gap3. Molecular chaperone GroEL was co-purified with Gap3 only when Gap1 was absent and also reacted with Gap1 monoclonal antibody, suggesting that Gap1 functions as a specific chaperone for Gap3. The N-terminal interacting domains of Gap1 mediated the Gap3 stability and Fap1 biogenesis. Gap1 homologues from Streptococcus agalactiae and Staphylococcus aureus also interacted with and stabilized corresponding Gap3 homologues, suggesting that the chaperone activity of the Gap1 homologues is common in biogenesis of SRRPs.

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.

A molecular chaperone mediates a two-protein enzyme complex and glycosylation of serine-rich streptococcal adhesins

Serine-rich repeat glycoproteins identified from streptococci and staphylococci are important for bacterial adhesion and biofilm formation. Two putative glycosyltransferases, Gtf1 and Gtf2, from Streptococcus parasanguinis form a two-protein enzyme complex that is required for glycosylation of a serine-rich repeat adhesin, Fap1. Gtf1 is a glycosyltransferase; however, the function of Gtf2 is unknown. Here, we demonstrate that Gtf2 enhances the enzymatic activity of Gtf1 by its chaperone-like property. Gtf2 interacted with Gtf1, mediated the subcellular localization of Gtf1, and stabilized Gtf1. Deletion of invariable amino acid residues in a conserved domain of unknown function (DUF1975) at the N terminus of Gtf2 had a greater impact on Fap1 glycosylation than deletion of the C-terminal non-DUF1975 residues. The DUF1975 deletions concurrently reduced the interaction between Gtf1 and Gtf2, altered the subcellular localization of Gtf1, and destabilized Gtf1, suggesting that DUF1975 is crucial for the chaperone activity of Gtf2. Homologous GtfA and GtfB from Streptococcus agalactiae rescued the glycosylation defect in the gtf1gtf2 mutant; like Gtf2, GtfB also possesses chaperone-like activity. Taken together, our studies suggest that Gtf2 and its homologs possess the conserved molecular chaperone activity that mediates protein glycosylation of bacterial adhesins.

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.