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

Biochemical characterization of a glycosyltransferase Gtf3 from Mycobacterium smegmatis: a case study of improved protein solubilization

Glycosyltransferases (GTs) are widely present in several organisms. These enzymes specifically transfer sugar moieties to a range of substrates. The processes of bacterial glycosylation of the cell wall and their relations with host-pathogen interactions have been studied extensively, yet the majority of mycobacterial GTs involved in the cell wall synthesis remain poorly characterized. Glycopeptidolipids (GPLs) are major class of glycolipids present on the cell wall of various mycobacterial species. They play an important role in drug resistance and host-pathogen interaction virulence. Gtf3 enzyme performs a key step in the biosynthesis of triglycosylated GPLs. Here, we describe a general procedure to achieve expression, purification, and crystallization of recombinant protein Gtf3 from Mycobacterium smegmatis using an E. coli expression system. We reported also a combined bioinformatics and biochemical methods to predict aggregation propensity and improve protein solubilization of recombinant Gtf3. NVoy, a carbohydrate-based polymer reagent, was added to prevent protein aggregation by binding to hydrophobic protein surfaces of Gtf3. Using intrinsic tryptophan fluorescence quenching experiments, we also demonstrated that Gtf3-NVoy enzyme interacted with TDP and UDP nucleotide ligands. This case report proposes useful tools for the study of other glycosyltransferases which are rather difficult to characterize and crystallize.

Serine-rich repeat protein adhesins from Lactobacillus reuteri display strain specific glycosylation profiles

Lactobacillus reuteri is a gut symbiont inhabiting the gastrointestinal tract of numerous vertebrates. The surface-exposed serine-rich repeat protein (SRRP) is a major adhesin in Gram-positive bacteria. Using lectin and sugar nucleotide profiling of wild-type or L. reuteri isogenic mutants, MALDI-ToF-MS, LC-MS and GC-MS analyses of SRRPs, we showed that L. reuteri strains 100-23C (from rodent) and ATCC 53608 (from pig) can perform protein O-glycosylation and modify SRRP100-23 and SRRP53608 with Hex-Glc-GlcNAc and di-GlcNAc moieties, respectively. Furthermore, in vivo glycoengineering in E. coli led to glycosylation of SRRP53608 variants with α-GlcNAc and GlcNAcβ(1→6)GlcNAcα moieties. The glycosyltransferases involved in the modification of these adhesins were identified within the SecA2/Y2 accessory secretion system and their sugar nucleotide preference determined by saturation transfer difference NMR spectroscopy and differential scanning fluorimetry. Together, these findings provide novel insights into the cellular O-protein glycosylation pathways of gut commensal bacteria and potential routes for glycoengineering applications.

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.

How Sweet Are Our Gut Beneficial Bacteria? A Focus on Protein Glycosylation in Lactobacillus

Protein glycosylation is emerging as an important feature in bacteria. Protein glycosylation systems have been reported and studied in many pathogenic bacteria, revealing an important diversity of glycan structures and pathways within and between bacterial species. These systems play key roles in virulence and pathogenicity. More recently, a large number of bacterial proteins have been found to be glycosylated in gut commensal bacteria. We present an overview of bacterial protein glycosylation systems (O- and N-glycosylation) in bacteria, with a focus on glycoproteins from gut commensal bacteria, particularly Lactobacilli. These emerging studies underscore the importance of bacterial protein glycosylation in the interaction of the gut microbiota with the host.

Structure-Based Discovery of Small Molecule Inhibitors of Cariogenic Virulence

Streptococcus mutans employs a key virulence factor, three glucosyltransferase (GtfBCD) enzymes to establish cariogenic biofilms. Therefore, the inhibition of GtfBCD would provide anti-virulence therapeutics. Here a small molecule library of 500,000 small molecule compounds was screened in silico against the available crystal structure of the GtfC catalytic domain. Based on the predicted binding affinities and drug-like properties, small molecules were selected and evaluated for their ability to reduce S. mutans biofilms, as well as inhibit the activity of Gtfs. The most potent inhibitor was further characterized for Gtf binding using OctetRed instrument, which yielded low micromolar K_D against GtfB and nanomolar K_D against GtfC, demonstrating selectivity towards GtfC. Additionally, the lead compound did not affect the overall growth of S. mutans and commensal oral bacteria, and selectively inhibit the biofilm formation by S. mutans, indicative of its selectivity and non-bactericidal nature. The lead compound also effectively reduced cariogenicity in vivo in a rat model of dental caries. An analog that docked poorly in the GtfC catalytic domain failed to inhibit the activity of Gtfs and S. mutans biofilms, signifying the specificity of the lead compound. This report illustrates the validity and potential of structure-based design of anti-S. mutans virulence inhibitors.

Engineering and Dissecting the Glycosylation Pathway of a Streptococcal Serine-rich Repeat Adhesin

Serine-rich repeat glycoproteins (SRRPs) are conserved in Gram-positive bacteria. They are crucial for modulating biofilm formation and bacterial-host interactions. Glycosylation of SRRPs plays a pivotal role in the process; thus understanding the glycosyltransferases involved is key to identifying new therapeutic drug targets. The glycosylation of Fap1, an SRRP of Streptococcus parasanguinis, is mediated by a gene cluster consisting of six genes: gtf1, gtf2, gly, gtf3, dGT1, and galT2. Mature Fap1 glycan possesses the sequence of Rha1–3Glc1-(Glc1–3GlcNAc1)-2,6-Glc1–6GlcNAc. Gtf12, Gtf3, and dGT1 are responsible for the first four steps of the Fap1 glycosylation, catalyzing the transfer of GlcNAc, Glc, Glc, and GlcNAc residues to the protein backbone sequentially. The role of GalT2 and Gly in the Fap1 glycosylation is unknown. In the present study, we synthesized the fully modified Fap1 glycan in Escherichia coli by incorporating all six genes from the cluster. This study represents the first reconstitution of an exogenous stepwise O-glycosylation synthetic pathway in E. coli. In addition, we have determined that GalT2 mediates the fifth step of the Fap1 glycosylation by adding a rhamnose residue, and Gly mediates the final glycosylation step by transferring glucosyl residues. Furthermore, inactivation of each glycosyltransferase gene resulted in differentially impaired biofilms of S. parasanguinis, demonstrating the importance of Fap1 glycosylation in the biofilm formation. The Fap1 glycosylation system offers an excellent model to engineer glycans using different permutations of glycosyltransferases and to investigate biosynthetic pathways of SRRPs because SRRP genetic loci are highly conserved.

Transcriptional organization of pneumococcal psrP-secY2A2 and impact of GtfA and GtfB deletion on PsrP-associated virulence properties

Pneumococcal serine-rich repeat protein (PsrP) is a glycoprotein that mediates Streptococcus pneumoniae attachment to lung cells and promotes biofilm formation. Herein, we investigated the transcriptional organization of psrP-secY2A2, the 37-kbp pathogenicity island encoding PsrP and its accessory genes. PCR amplification of cDNA and RNA-seq analysis found psrP-secY2A2 to be minimally composed of three operons: psrP-glyA, glyB, and glyC-asp5. Transcription of all three operons was greatest during biofilm growth and immunoblot analyses confirmed increased PsrP production by biofilm pneumococci. Using gas chromatography-mass spectrometry we identified monomeric N-acetylglucosamine as the primary glycoconjugate present on a recombinant intracellular version of PsrP, i.e. PsrP1-734. This finding was validated by immunoblot using lectins with known carbohydrate specificities. We subsequently deleted gtfA and gtfB, the GTFs thought to be responsible for addition of O-linked N-acetylglucosamine, and tested for PsrP and its associated virulence properties. These deletions negatively affected our ability to detect PsrP1-734 in bacterial whole cell lysates. Moreover, S. pneumoniae mutants lacking these genes pheno-copied the psrP mutant and were attenuated for: biofilm formation, adhesion to lung epithelial cells, and pneumonia in mice. Our studies identify the transcriptional organization of psrP-secY2A2 and show the indispensable role of GtfA and GtfB on PsrP-mediated pneumococcal virulence.

Defining the enzymatic pathway for polymorphic O-glycosylation of the pneumococcal serine-rich repeat protein PsrP

Protein O-glycosylation is an important post-translational modification in all organisms, but deciphering the specific functions of these glycans is difficult due to their structural complexity. Understanding the glycosylation of mucin-like proteins presents a particular challenge as they are modified numerous times with both the enzymes involved and the glycosylation patterns being poorly understood. Here we systematically explored the O-glycosylation pathway of a mucin-like serine-rich repeat protein PsrP from the human pathogen Streptococcus pneumoniae TIGR4. Previous works have assigned the function of 3 of the 10 glycosyltransferases thought to modify PsrP, GtfA/B, and Gtf3 as catalyzing the first two reactions to form a unified disaccharide core structure. We now use in vivo and in vitro glycosylation assays combined with hydrolytic activity assays to identify the glycosyltransferases capable of decorating this core structure in the third and fourth steps of glycosylation. Specifically, the full-length GlyE and GlyG proteins and the GlyD DUF1792 domain participate in both steps, whereas full-length GlyA and the GlyD GT8 domain catalyze only the fourth step. Incorporation of different sugars to the disaccharide core structure at multiple sites along the serine-rich repeats results in a highly polymorphic product. Furthermore, crystal structures of apo- and UDP-complexed GlyE combined with structural analyses reveal a novel Rossmann-fold "add-on" domain that we speculate to function as a universal module shared by GlyD, GlyE, and GlyA to forward the peptide acceptor from one enzyme to another. These findings define the complete glycosylation pathway of a bacterial glycoprotein and offer a testable hypothesis of how glycosyltransferase coordination facilitates glycan assembly.

New Helical Binding Domain Mediates a Glycosyltransferase Activity of a Bifunctional Protein

Serine-rich repeat glycoproteins (SRRPs) conserved in streptococci and staphylococci are important for bacterial colonization and pathogenesis. Fap1, a well studied SRRP is a major surface constituent of Streptococcus parasanguinis and is required for bacterial adhesion and biofilm formation. Biogenesis of Fap1 is a multistep process that involves both glycosylation and secretion. A series of glycosyltransferases catalyze sequential glycosylation of Fap1. We have identified a unique hybrid protein dGT1 (dual glycosyltransferase 1) that contains two distinct domains. N-terminal DUF1792 is a novel GT-D-type glycosyltransferase, transferring Glc residues to Glc-GlcNAc-modified Fap1. C-terminal dGT1 (CgT) is predicted to possess a typical GT-A-type glycosyltransferase, however, the activity remains unknown. In this study, we determine that CgT is a distinct glycosyltransferase, transferring GlcNAc residues to Glc-Glc-GlcNAc-modified Fap1. A 2.4-Å x-ray crystal structure reveals that CgT has a unique binding domain consisting of three α helices in addition to a typical GT-A-type glycosyltransferase domain. The helical domain is crucial for the oligomerization of CgT. Structural and biochemical studies revealed that the helix domain is required for the protein-protein interaction and crucial for the glycosyltransferase activity of CgT in vitro and in vivo. As the helix domain presents a novel structural fold, we conclude that CgT represents a new member of GT-A-type glycosyltransferases.

Collagen-binding proteins of Streptococcus mutans and related streptococci

The ability of Streptococcus mutans to interact with collagen through the expression of collagen-binding proteins (CBPs) bestows this oral pathogen with an alternative to the sucrose-dependent mechanism of colonization classically attributed to caries development. Based on the abundance and distribution of collagen throughout the human body, stringent adherence to this molecule grants S. mutans with the opportunity to establish infection at different host sites. Surface proteins, such as SpaP, WapA, Cnm and Cbm, have been shown to bind collagen in vitro, and it has been suggested that these molecules play a role in colonization of oral and extra-oral tissues. However, robust collagen binding is not achieved by all strains of S. mutans, particularly those that lack Cnm or Cbm. These observations merit careful dissection of the contribution from these different CBPs towards tissue colonization and virulence. In this review, we will discuss the current understanding of mechanisms used by S. mutans and related streptococci to colonize collagenous tissues, and the possible contribution of CBPs to infections in different sites of the host.

Insights into bacterial protein glycosylation in human microbiota

The study of human microbiota is an emerging research topic. The past efforts have mainly centered on studying the composition and genomic landscape of bacterial species within the targeted communities. The interaction between bacteria and hosts is the pivotal event in the initiation and progression of infectious diseases. There is a great need to identify and characterize the molecules that mediate the bacteria-host interaction. Bacterial surface exposed proteins play an important role in the bacteria- host interaction. Numerous surface proteins are glycosylated, and the glycosylation is crucial for their function in mediating the bacterial interaction with hosts. Here we present an overview of surface glycoproteins from bacteria that inhabit three major mucosal environments across human body: oral, gut and skin. We describe the important enzymes involved in the process of protein glycosylation, and discuss how the process impacts the bacteria-host interaction. Emerging molecular details underlying glycosylation of bacterial surface proteins may lead to new opportunities for designing anti-infective small molecules, and developing novel vaccines in order to treat or prevent bacterial infection.

The sweet tooth of bacteria: common themes in bacterial glycoconjugates

Humans have been increasingly recognized as being superorganisms, living in close contact with a microbiota on all their mucosal surfaces. However, most studies on the human microbiota have focused on gaining comprehensive insights into the composition of the microbiota under different health conditions (e.g., enterotypes), while there is also a need for detailed knowledge of the different molecules that mediate interactions with the host. Glycoconjugates are an interesting class of molecules for detailed studies, as they form a strain-specific barcode on the surface of bacteria, mediating specific interactions with the host. Strikingly, most glycoconjugates are synthesized by similar biosynthesis mechanisms. Bacteria can produce their major glycoconjugates by using a sequential or an en bloc mechanism, with both mechanistic options coexisting in many species for different macromolecules. In this review, these common themes are conceptualized and illustrated for all major classes of known bacterial glycoconjugates, with a special focus on the rather recently emergent field of glycosylated proteins. We describe the biosynthesis and importance of glycoconjugates in both pathogenic and beneficial bacteria and in both Gram-positive and -negative organisms. The focus lies on microorganisms important for human physiology. In addition, the potential for a better knowledge of bacterial glycoconjugates in the emerging field of glycoengineering and other perspectives is discussed.

NMR assignment of the amylase-binding protein A from Streptococcus parasanguinis

Streptococcus parasanguinis is a primary colonizer of tooth surfaces in the oral cavity. Amylase-binding protein A (AbpA) from S. parasanguinis is responsible for the recruitment of salivary amylase to bacterial surface, which plays an important role in the development of oral biofilms. Here, we describe the essentially complete NMR assignments for AbpA.

Glycosyltransferase-mediated Sweet Modification in Oral Streptococci

Bacterial glycosyltransferases play important roles in bacterial fitness and virulence. Oral streptococci have evolved diverse strategies to survive and thrive in the carbohydrate-rich oral cavity. In this review, we discuss 2 important biological processes mediated by 2 distinct groups of glycosyltransferases in oral streptococci that are important for bacterial colonization and virulence. The first process is the glycosylation of highly conserved serine-rich repeat adhesins by a series of glycosyltransferases. Using Streptococcus parasanguinis as a model, we highlight new features of several glycosyltransferases that sequentially modify the serine-rich glycoprotein Fap1. Distinct features of a novel glycosyltransferase fold from a domain of unknown function 1792 are contrasted with common properties of canonical glycosyltransferases. The second biological process we cover is involved in building sticky glucan matrix to establish cariogenic biofilms by an important opportunistic pathogen Streptococcus mutans through the action of a family of 3 glucosyltransferases. We focus on discussing the structural feature of this family as a glycoside hydrolase family of enzymes. While the 2 processes are distinct, they all produce carbohydrate-coated biomolecules, which enable bacteria to stick better in the complex oral microbiome. Understanding the making of the sweet modification presents a unique opportunity to develop novel antiadhesion and antibiofilm strategies to fight infections by oral streptococci and beyond.

A conserved domain is crucial for acceptor substrate binding in a family of glucosyltransferases

Serine-rich repeat glycoproteins (SRRPs) are highly conserved in streptococci and staphylococci. Glycosylation of SRRPs is important for bacterial adhesion and pathogenesis. Streptococcus agalactiae is the leading cause of bacterial sepsis and meningitis among newborns. Srr2, an SRRP from S. agalactiae strain COH1, has been implicated in bacterial virulence. Four genes (gtfA, gtfB, gtfC, and gtfD) located downstream of srr2 share significant homology with genes involved in glycosylation of other SRRPs. We have shown previously that gtfA and gtfB encode two glycosyltransferases, GtfA and GtfB, that catalyze the transfer of GlcNAc residues to the Srr2 polypeptide. However, the function of other glycosyltransferases in glycosylation of Srr2 is unknown. In this study, we determined that GtfC catalyzed the direct transfer of glucosyl residues to Srr2-GlcNAc. The GtfC crystal structure was solved at 2.7 Å by molecular replacement. Structural analysis revealed a loop region at the N terminus as a putative acceptor substrate binding domain. Deletion of this domain rendered GtfC unable to bind to its substrate Srr2-GlcNAc, concurrently abolished the glycosyltransferase activity of GtfC, and also altered glycosylation of Srr2. Furthermore, deletion of the corresponding regions from GtfC homologs also abolished their substrate binding and enzymatic activity, indicating that this region is functionally conserved. In summary, we have determined that GtfC is important for the glycosylation of Srr2 and identified a conserved loop region that is crucial for acceptor substrate binding from GtfC homologs in streptococci. These findings shed new mechanistic insight into this family of glycosyltransferases.

Advances in understanding glycosyltransferases from a structural perspective

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Glycosyltransferases are the enzymes that catalyse glycosidic bond formation.

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Structural and kinetic studies are important for understanding function.

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Bacterial oligosaccharyltransferase structure aids understanding of N-linked glycosylation.

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Structure of human O-GlcNAc transferase gives mechanistic insights.

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Landmark structure of cellulose synthase membrane protein complex.

Glycosyltransferases (GTs), the enzymes that catalyse glycosidic bond formation, create a diverse range of saccharides and glycoconjugates in nature. Understanding GTs at the molecular level, through structural and kinetic studies, is important for gaining insights into their function. In addition, this understanding can help identify those enzymes which are involved in diseases, or that could be engineered to synthesize biologically or medically relevant molecules. This review describes how structural data, obtained in the last 3–4 years, have contributed to our understanding of the mechanisms of action and specificity of GTs. Particular highlights include the structure of a bacterial oligosaccharyltransferase, which provides insights into N-linked glycosylation, the structure of the human O-GlcNAc transferase, and the structure of a bacterial integral membrane protein complex that catalyses the synthesis of cellulose, the most abundant organic molecule in the biosphere.

The highly conserved domain of unknown function 1792 has a distinct glycosyltransferase fold

More than 33,000 glycosyltransferases have been identified. Structural studies, however, have only revealed two distinct glycosyltransferase (GT) folds, GT-A and GT-B. Here we report a 1.34-Å resolution X-ray crystallographic structure of a previously uncharacterized 'domain of unknown function' 1792 (DUF1792) and show that the domain adopts a new fold and is required for glycosylation of a family of serine-rich repeat streptococcal adhesins. Biochemical studies reveal that the domain is a glucosyltransferase, and it catalyses the transfer of glucose to the branch point of the hexasaccharide O-linked to the serine-rich repeat of the bacterial adhesin, Fap1 of Streptococcus parasanguinis. DUF1792 homologues from both Gram-positive and Gram-negative bacteria also exhibit the activity. Thus, DUF1792 represents a new family of glycosyltransferases; therefore, we designate it as a GT-D glycosyltransferase fold. As the domain is highly conserved in bacteria and not found in eukaryotes, it can be explored as a new antibacterial target.

Preliminary X-ray crystallographic studies of an N-terminal domain of unknown function from a putative glycosyltransferase from Streptococcus parasanguinis

Serine-rich repeat glycoproteins (SRRPs) belong to a growing family of bacterial adhesins; they play important roles in bacterial virulence. Fap1, the first SRRP protein to be identified, is glycosylated; while the first two steps of its glycosylation have been determined, the remaining glycosylation steps are unknown. In a search for proteins that might be relevant to the glycosylation of Fap1, a putative glycosyltransferase (GalT1) from Streptococcus parasanguinis was identified. GalT1 possesses a domain of unknown function at the N-terminus. This domain is highly conserved in bacteria and is a member of a broad superfamily. However, the structure of this domain has not been determined. Here, the conditions used to produce a recombinant version of this protein domain and to grow protein crystals are reported. The crystals obtained belonged to space group C2, with unit-cell parameters a = 71.0, b = 45.1, c = 78.6 Å, β = 109.6°, and diffracted to 1.55 Å resolution at a synchrotron X-ray source. This domain does not share sequence identity with proteins of known structures above a level of 12%.

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.

Structural and biochemical analysis of a bacterial glycosyltransferase

Glycosyltransferases (GTs) are a large family of enzymes that specifically transfer sugar moieties to a diverse range of substrates. The process of bacterial glycosylation (such as biosynthesis of glycolipids, glycoproteins, and polysaccharides) has been studied extensively, yet the majority of GTs involved remains poorly characterized. Besides predicting enzymatic parameters of GTs, the resolution of three-dimensional structures of GTs can help to determine activity, donor sugar binding, and acceptor substrate binding sites. It also facilitates amino acid sequence-based structural modeling and biochemical characterization of their homologues. Here we describe a general procedure to accomplish expression and purification of soluble and active recombinant GTs. Enzymatic characterization, and crystallization of GTs, and data refinement for structural analysis are also covered in this protocol.

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.

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.