Project Summary Streptococcal bacteria contribute to infections of vital organs through their surface proteins which facilitate their adherence, colonization, and biofilm formation. In the oral cavity, S. mutans use its surface adhesins AgI/II and GbpC to adhere to Gp340 for initial adherence, dextran to promote biofilm development, and in some cases other microbes which are incorporated into biofilms. More generally many members of the streptococcal species express an AgI/II-like homolog which they use to adhere to both shared and species-specific targets, implicating them in contributing to the disease state of infections involving these streptococci. At sites of physical damage to gums or gum disease, streptococcal species can infiltrate the blood stream and become systemic opportunistic pathogens. This proposal focuses on characterizing the molecular mechanisms of host-microbe and microbe-microbe interactions involving the AgI/II-family of proteins. In structural studies of SspB the Deivanayagam lab discovered a peptide-binding cleft within the V-region, a shared cleft housing a calcium ion. This peptide has nanomolar adherence with the V-regions of SspB, AgI/II and GbpC; the peptide inhibits the V-regions adherence to SRCR region of Gp340 and reduces biofilm formation. In studies with GbpC, this cleft was shown to also be involved in the V-regions adherence to dextran. Our preliminary studies have shown not only shared adherence targets, but also conserved areas of adherence. Both the apical V-region and cell-wall anchored C-terminal regions of AgI/II-family proteins have been shown to be involved in adherence host surfaces. In this proposal we plan to investigate the hypothesis that the Extracellular matrix (ECM) protein interactions among AgI/II-family proteins would share similar but distinct motifs that contribute to the initiation or progression of infections outside the oral cavity. The specific aims are Aim1: Characterize the interactions between AgI/II-family proteins and ECMs. Aim 2: Determine AgI/II proteins’ ability to interact with other pathogenic microorganisms. The proposed studies will elucidate shared areas of adherence to determine the potential for designing inhibitors to prevent the interaction which would reduce the ability of these streptococci to contribute to infection and disease.