Porphyromonas gingivalis Virulence Factor Gingipain RgpB Shows a Unique Zymogenic Mechanism for Cysteine Peptidases

摘要

Zymogenicity is a regulatory mechanism that prevents inadequate catalytic activity in the wrong context. It plays a central role in maintaining microbial virulence factors in an inactive form inside the pathogen until secretion. Among these virulence factors is the cysteine peptidase gingipain B (RgpB), which is the major virulence factor secreted by the periodontopathogen Porphyromonas gingivalis that attacks host vasculature and defense proteins. The structure of the complex between soluble mature RgpB, consisting of a catalytic domain and an immunoglobulin superfamily domain, and its 205-residue N-terminal prodomain, the largest structurally characterized to date for a cysteine peptidase, reveals a novel fold for the prodomain that is distantly related to sugar-binding lectins. It attaches laterally to the catalytic domain through a large concave surface. The main determinant for latency is a surface “inhibitory loop,” which approaches the active-site cleft of the enzyme on its non-primed side in a substrate-like manner. It inserts an arginine (Arg¹²⁶) into the S1 pocket, thus matching the substrate specificity of the enzyme. Downstream of Arg¹²⁶, the polypeptide leaves the cleft, thereby preventing cleavage. Moreover, the carbonyl group of Arg¹²⁶ establishes a very strong hydrogen bond with the co-catalytic histidine, His⁴⁴⁰, pulling it away from the catalytic cysteine, Cys⁴⁷³, and toward Glu³⁸¹, which probably plays a role in orienting the side chain of His⁴⁴⁰ during catalysis. The present results provide the structural determinants of zymogenic inhibition of RgpB by way of a novel inhibitory mechanism for peptidases in general and open the field for the design of novel inhibitory strategies in the treatment of human periodontal disease.