AFFINITY AND SPECIFICITY OF THE INTERACTIONS BETWEEN STREPTOCOCCUS-MUTANS ANTIGEN I II AND SALIVARY COMPONENTS/

Adherence to salivary pellicle-coated tooth surfaces and aggregation b y salivary components of Streptococcus mutans involves a major cell su rface protein termed antigen (Ag) I/II, The objectives of this study w ere to evaluate the affinity and specificity of the interactions betwe en AgI/II and human saliva in assays of I-125-AgI/II binding to saliva -coated hydroxyapatite (SHA) and of S. mutans aggregation by salivary agglutinin (SAG), monitored turbidimetrically. I-125-AgI/II binding to SHA followed saturation kinetics, and Scatchard plot analysis indicat ed two binding sites with dissociation constants of the order of 10(-1 0) mol/L and 10(-9) mol/L. The binding to SHA of the C-terminal one-th ird of AgI/II which corresponds to AgII was less than one-fifth that o f the whole molecule and did not show evidence of saturation. The bind ing of I-125-AgI/II was inhibited by native or recombinant fragments t hat mapped in the N-terminal part of the molecule and that contained t he alanine-rich repeat region, whereas fragments mapping at the centra l or C-terminal one-third had no effect. As with binding to SHA, the r egions 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 sa liva was inhibited by amino compounds, and most strongly by L-lysine a nd analogues possessing omega-primary amine groups. These data support the role of AgI/II as an adhesin with high-affinity binding for SHA r eceptors, mediated by the N-terminal part of the molecule. This region is also involved in SAG-induced S. mutans aggregation, which is sensi tive to amino compounds.