Affinity and specificity of the interactions between Streptococcus mutans antigen I/II and salivary components

Adherence to salivary pellicle-coated tooth surfaces and aggregation by salivary components of Streptococcus mutans involves a major cell surface protein termed antigen (Ag) I/II. The objectives of this study were to evaluate the affinity and specificity of the interactions between AgI/II and human saliva in assays of 125I-AgI/II binding to saliva-coated hydroxyapatite (SHA) and of S. mutans aggregation by salivary agglutinin (SAG), monitored turbidimetrically. 125I-AgI/II binding to SHA followed saturation kinetics, and Scatchard plot analysis indicated two binding sites with dissociation constants of the order of 10(-10) mol/L and 10(-9) mol/L. The binding to SHA of the C-terminal one-third of AgI/II which corresponds to AgII was less than one-fifth that of the whole molecule and did not show evidence of saturation. The binding of 125I-AgI/II was inhibited by native or recombinant fragments that mapped in the N-terminal part of the molecule and that contained the alanine-rich repeat region, whereas fragments mapping at the central or C-terminal one-third had no effect. As with binding to SHA, the regions of AgI/II which inhibited aggregation mapped at the N-terminal part of the molecule, but, in addition, a recombinant segment mapping at the central part and containing the proline-rich repeat region was also inhibitory. The S. mutans-aggregating activity of SAG or whole saliva was inhibited by amino compounds, and most strongly by L-lysine and analogues possessing omega-primary amine groups. These data support the role of AgI/II as an adhesin with high-affinity binding for SHA receptors, mediated by the N-terminal part of the molecule. This region is also involved in SAG-induced S. mutans aggregation, which is sensitive to amino compounds.