Structures of the Streptococcus sanguinis SrpA Binding Region with Human Sialoglycans Suggest Features of the Physiological Ligand

Abstract

Streptococcus sanguinis is a leading cause of bacterial infective endocarditis, a life threatening infection of heart valves. S. sanguinis binds to human platelets with high avidity, and this adherence is likely to enhance virulence. Previous studies suggest that a serine-rich repeat adhesin termed SrpA mediates the binding of S. sanguinis to human platelets via its interaction with sialoglycans on the receptor GPIbα. However, in vitro binding assays between SrpA and defined sialoglycans failed to identify specific high-affinity ligands. To better understand the interaction between SrpA and human platelets, we determined cocrystal structures of the SrpA sialoglycan binding region (SrpABR) with five low-affinity ligands: three sialylated trisaccharides (sialyl-T antigen, 3'-sialyllactose, and 3'-sialyl-N-acetyllactosamine), a sialylated tetrasaccharide (sialyl-LewisX) and a sialyl galactose disaccharide component common to these sialoglyans. We then combined structural analysis with mutagenesis to further determine whether our observed interactions between SrpABR and glycans are important for binding to platelets and to better map the binding site for the physiological receptor. We found that the sialoglycan binding site of SrpABR is significantly larger than the sialoglycans cocrystallized in this study, which suggests that SrpA binding to platelets is either multivalent or occurs via a larger, disialylated glycan.

Streptococcus oralis subsp. dentisani Produces Monolateral Serine-Rich Repeat Protein Fibrils, One of Which Contributes to Saliva Binding via Sialic Acid

“…5E). This was unexpected and in contrast to the findings of previous SRRP adhesin studies (53)(54)(55). Interactions in homologous structures suggest that hydrogen bonds form between the arginine residue and the glycerol group of sialic acid.…”

“…While the majority of Siglec-containing SRRPs contain a single Siglec-like domain and a single unique domain, FapC contains two of each. One of the two putative Siglec-like domains within FapC was identified as containing the conserved arginine residue essential for sialic acid binding in other bacterial Siglec-like domains (53)(54)(55). These data support a role for FapC in sialic acid binding.…”

“…FapC shares predicted structural similarity with sialic acid binding immunoglobulinlike lectin (Siglec)-like and unique domains from S. sanguinis and S. gordonii (PDB accession numbers 3QC5 and 5KIQ, respectively) (Ͼ99% probability; E value, Ͼ1 ϫ 10 Ϫ14 ) (50,52,53). While the majority of Siglec-containing SRRPs contain a single Siglec-like domain and a single unique domain, FapC contains two of each.…”

Glycan recognition at the saliva – oral microbiome interface

“…Through binding of whole bacteria and of their recombinant ligand binding domains to a sialoglycan array, it was found that each unique sialic acid binding SRR protein adhesin has a distinctive binding profile that is influenced by both the subtype of sialic acid, its linkage, and the underlying glycan chain [137–139]. For example, Hsa of S. gordonii DL1 showed broad specificity for a sialic acids α2–3-lined to a wide range of subterminal glycans, but GspB of S. gordonii M99 showed a narrower specificity, binding primarily sialyl-T antigen (Neu5Acα2–3Galβ1–3GalNAc) terminating glycans [137].…”

Serine-rich repeat proteins from gut microbes

“…59,71,[79][80][81][82][83][84][85] SraP specifically recognizes N-acetylneuraminic acid. 79,86 Recently, a novel SRRP, SssP1, associated with a SecA2/Y2 gene cluster has been identified in strain CZ130302 of Strep. suis, an important Gram-positive pathogen in the swine industry and emerging zoonotic pathogen for humans.…”