Streptococcus mutans is recognized as the principal etiologic agent of dental caries. The most prominent virulence factors of S. mutans include its aciduricity, and its ability to synthesize glucans which promote adherence to the tooth surface. Recent studies have demonstrated the complexity of the mechanisms of adhesion and aggregation. S. mutans species have at least three glucosyltransferase (GTF) enzymes which are capable of synthesizing various forms of glucan. Additionally, the glucan-binding protein (GBP) of S. mutans, which appears to be unique to this species, has been hypothesized to contribute to adhesion and the formation of cohesive plaque. This proposal seeks to determine the role of GBP in binding glucans to the cell surface. In particular, the expression of GBP will be related to sucrose availability, and the binding ability of GBP at the cell surface and to various forms of glucan will be tested in order to evaluate the relative contributions of GBP, glucan polymers, and GTFs in adhesion and plaque formation. To meet these objectives the following specific aims are proposed: 1) Relate the level of GBP expression to sucrose availability and other environmental parameters such as pH and anaerobiosis. 2) Determine the functional similarity of the GBP and GTF repeat regions. Specifically, fusion proteins will be created by exchanging the repeat regions to determine the effects on glucan binding and GTF enzymatic activity. 3) Use synthesized oligopeptides to competitively inhibit the binding of GBP to glucans and thereby evaluate the relative importance of different domains of GBP to glucan binding. 4) Test the hypothesis that GBP and GTFs act as bridges between soluble and insoluble glucan adherence on the cell surface. 5) Clone and characterize the gene for the glucan-binding lectin (GBL) of Streptococcus downei. Determine the evolutionary relatedness of gbl and gbp, and the structural and functional similarities of their encoded protein products. The results from these experiments should lead to a better understanding of how the glucan binding proteins of the mutans streptococci contribute to plaque formation and the caries process.
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