A distinct sortase SrtB anchors and processes a streptococcal adhesin AbpA with a novel structural property

Abstract

Surface display of proteins by sortases in Gram-positive bacteria is crucial for bacterial fitness and virulence. We found a unique gene locus encoding an amylase-binding adhesin AbpA and a sortase B in oral streptococci. AbpA possesses a new distinct C-terminal cell wall sorting signal. We demonstrated that this C-terminal motif is required for anchoring AbpA to cell wall. In vitro and in vivo studies revealed that SrtB has dual functions, anchoring AbpA to the cell wall and processing AbpA into a ladder profile. Solution structure of AbpA determined by NMR reveals a novel structure comprising a small globular α/β domain and an extended coiled-coil heliacal domain. Structural and biochemical studies identified key residues that are crucial for amylase binding. Taken together, our studies document a unique sortase/adhesion substrate system in streptococci adapted to the oral environment rich in salivary amylase.

Comparative genomics and evolution of the amylase-binding proteins of oral streptococci

“…Our analyses of complete and nearly complete draft genomes of AbpA-encoding oral Streptococcus strains revealed that all contained the abpA - srtB gene locus. Sortase B is essential for initial covalent attachment of AbpA to the bacterial cell wall [18, 19]. In fact this gene locus was found in 15/18 strains sequenced in this study, supporting the role of AbpA as the predominate ABP.…”

Oral Biofilms: Pathogens, Matrix, and Polymicrobial Interactions in Microenvironments

“…In addition, the presence of amylase bound on bacterial (e.g. S. gordonii, S. parasanguinis ) and tooth surfaces produce a variety of starch hydrolysates in situ that can be metabolized into acids and incorporated into glucans synthesized by S. mutans Gtfs forming hybrid polymers [50, 51, 52]. Additional EPS produced by other oral bacteria may also contribute to biofilm matrix assembly even if the proportion of S. mutans is declining as the biofilm matures and the caries lesion worsens.…”

Genetics ofsanguinis-Group Streptococci in Health and Disease

“…Pili are relatively long and thin appendages and the pili of S. sanguinis can be as long as 1 µm, as shown by immune-gold staining of PilA (28). This poses a potentially interesting dynamic with AbpA, which is strictly confined to the outer cell surface, as shown for S. parasanguinis, also with immune-gold labeling (27). Taking into account that pili are flexible, one could hypothesize that binding to amylase in acquired enamel pellicle is possible even when the molecule is scarce, since the pili could serve as a flexible "arm", latching onto free amylase within saliva.…”

“…Mutation of AbpA results in deficient biofilm formation and bacterial adhesion in vitro (26). Although the sequenced reference strain S. sanguinis SK36 seems to encode an abpA homolog in a similar chromosomal context with its accessory sortase, srtB (27), its function is currently unknown. Interestingly, S. sanguinis is able to bind directly to surface-bound amylase and vice versa (24).…”

“…The pili also showed binding to other salivary proteins and their deletion diminished biofilm formation on saliva-coated surfaces (28). However, the mutant was still able to bind amylase, albeit with lower efficiency (28), suggesting that other surface proteins are also able to bind amylase, possibly the aforementioned AbpA homolog (27). A pilus-bound amylase also offers the advantage of retaining about 50% of its enzymatic function (30).…”