Arg-gingipain (RGP) is an Arg-X-specific cysteine proteinase produced by the Gram-negative anaerobe Porphyromonas gingivalis and has been shown to be a potent virulence factor in progressive periodontal disease (Nakayama, K., Kadowaki, T., Okamoto, K., and Yamamoto, K. (1995) J. Biol. Chem. 270, 23619-23626). In this study, we provide evidence that RGP acts as a major processing enzyme for various cell surface and secretory proteins in P. gingivalis. Fimbrilin, a major component of fimbriae, remained in the precursor form in the RGP-null mutant. Prefimbrilin expressed in Escherichia coli was converted to the mature fimbrilin in vitro when incubated with purified RGP, but its conversion was suppressed by potent RGP inhibitors. The results were consistent with the electron microscopic observation indicating little or no fimbriation in the RGP-null mutant. The immunogenic 75-kDa cell surface protein was also shown to retain its proform in the RGP-null mutant. In addition, Lys-gingipain (KGP) was found to be abnormally processed in the RGP-null mutant. In contrast, both prefimbrilin and the 75-kDa protein precursor were processed to their respective mature forms in the KGP-null mutant, suggesting that KGP is not involved in the normal processing mechanisms of these proteins. These results suggest that RGP not only acts as a direct virulence factor but also makes a significant contribution as a major processing enzyme to the virulence of P. gingivalis.