Does the importance of the C-terminal residues in the maturation of RgpB from Porphyromonas gingivalis reveal a novel mechanism for protein export in a subgroup of Gram-Negative bacteria?

The mature 507-residue RgpB protein belongs to an important class of extracellular outer membrane-associated proteases, the gingipains, from the oral pathogen Porphyromonas gingivalis that has been shown to play a central role in the virulence of the organism. The C termini of these gingipains along with other outer membrane proteins from the organism share homologous sequences and have been suggested to function in attachment of these proteins to the outer membrane. In this report, we have created a series of truncated and site-directed mutants of the C terminus from a representative member of this class, the RgpB protease, to investigate its role in the maturation of these proteins. Truncation of the last two residues (valyl-lysine) from the C terminus is sufficient to create an inactive version of the protein that lacks the posttranslational glycosylation seen in the wild type, and the protein remains trapped behind the outer membrane. Alanine scanning of the last five residues revealed the importance of the C-terminal motif in mediating correct posttranslational modification of the protein. This result may have a wider implication in a novel secretory pathway in distinct members of the Cytophaga-Flavobacterium-Bacteroidetes phylum.