OmpA_C‐Like Domain of PorE Is Essential for PorE Function in the Type IX Secretion System (T9SS) of Porphyromonas gingivalis and Some T9SS Cargo Proteins Are Secreted in a PorE‐Independent Manner

The periodontal pathogen Porphyromonas gingivalis secretes gingipains, highly hydrolytic proteases, to the cell surface or external environment via a type IX secretion system (T9SS). PorE is one of the T9SS component proteins essential for gingipain secretion. It consists of four domains: TPR, WD40, CRD, and OmpA_C‐like. In this study, we investigated the contribution and function of these PorE domains to gingipain secretion by generating PorE mutants with domain deletions and residue substitutions. We found that (i) the OmpA_C‐like domain is essential for gingipain secretion, (ii) the peptidoglycan binding sites (Asp 576 and Asn 584 ) in the OmpA_C‐like domain are a necessity for gingipain secretion, (iii) the WD40 domain is likely involved in interaction with PorP, and (iv) the CRD domain is not essential for gingipain secretion. The function of the TPR domain could not be assessed because no PorE protein of the predicted molecular mass was detected in the TPR domain‐deleted PorE mutant. We also demonstrated that some T9SS cargo proteins, such as PorA and Hbp35, do not require the PorE‐PorP complex for secretion, suggesting that the PorE–PorP complex may occupy a unique position within the T9SS that is distinct from other components.