Binding of the Streptococcus gordonii surface glycoproteins GspB and Hsa to specific carbohydrate structures on platelet membrane glycoprotein Ibα

GspB and Hsa are homologous serine-rich surface glycoproteins of Streptococcus gordonii strains M99 and Challis, respectively, that mediate the binding of these organisms to platelet membrane glycoprotein (GP) Ib alpha. Both GspB and Hsa consist of an N-terminal putative signal peptide, a short serine-rich region, a region (BR) that is rich in basic amino acids, a longer serine-rich region and a C-terminal cell wall anchoring domain. To further assess the mechanisms for GspB and Hsa binding, we investigated the binding of the BRs of GspB and Hsa (expressed as glutathione S-tranferase fusion proteins) to sialylated glycoproteins in vitro. Both fusion proteins showed significant levels of binding to sialylated moieties on fetuin and GPIb alpha. In contrast, the corresponding region of a GspB homologue of Streptococcus agalactiae, which is acidic rather than basic, showed no binding to either fetuin or GPIb alpha. As measured by surface plasmon resonance kinetic analysis, GspB- and Hsa-derived fusion proteins had high affinity for GPIb alpha, but with somewhat different dissociation constants. Dot blot analysis using a panel of synthesized oligosaccharides revealed that the BR of Hsa can bind both alpha(2-3) sialyllactosamine [NeuAc alpha(2-3)Gal beta(1-4)GlcNAc] and sialyl-T antigen [NeuAc alpha(2-3)Gal beta(1-3)GalNAc], whereas the BR of GspB only bound sialyl-T antigen. Moreover, far Western blotting using platelet membrane proteins revealed that GPIb alpha is the principal receptor for GspB and Hsa on human platelets. The combined results indicate that the BRs of GspB and Hsa are the binding domains of these adhesins. However, the subsets of carbohydrate structures on GPIb alpha recognized by the binding domains appear to be different between the two proteins.