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The oral anaerobic bacterium Porphyromonas gingivalis has been implicated as a major pathogen for chronic periodontitis. The microorganism produces strong proteinases, Arg-gingipain (Rgp) and Lys-gingipain (Kgp), on the cell surface and in extracellular milieu. Gingipain genes encode polyproteins consisting of four parts: signal peptide, propeptide, proteinase, and a C-terminal adhesin domain region. The C-terminal adhesin domain region encoded by Rgp-encoding gene A (rgpA) comprises four domains (Hgp44, Hgp15, Hgp17, and Hgp27). A number of studies have indicated that a major hemagglutinin is derived from the gingipain genes; however, direct evidence has not yet been obtained. We showed that a fully processed recombinant Hgp44 had hemagglutinating activity, and that Hgp44 hemagglutinin bound to glycophorin on the erythrocyte membrane. P. gingivalis cells have the ability to aggregate platelets. We showed that P. gingivalis cell-mediated platelet aggregation required Hgp44 hemagglutinin on the bacterial cell surface, P. gingivalis-reactive antibody in plasma, and FcνRIIa, GPIIa/IIIb and GPIb-IX-V receptors on platelets. We also showed that a major protein of the culture supernatant of P. gingivalis, Hgp15, suppressed in vitro receptor activator of nuclear factor-ΚB (NF-ΚB) ligand (RANKL)-mediated osteoclastogenesis. Hgp15 inhibited RANKL-mediated induction of c-Fos and NFATc1. These results suggest that the C-terminal adhesin domains have multiple functions associated with virulence of P. gingivalis. [ABSTRACT FROM AUTHOR]