Verification of a topology model of PorT as an integral outer-membrane protein in Porphyromonas gingivalis

Abstract

PorT is a membrane-associated protein shown to be essential for the maturation and secretion of a class of cysteine proteinases, the gingipains, from the periodontal pathogenPorphyromonas gingivalis. It was previously reported that PorT is located on the periplasmic surface of the inner membrane to function as a chaperone for the maturing proteinases. Our modelling suggested it to be an integral outer-membrane protein with eight anti-parallel, membrane-traversingβ-strands. In this report, the outer-membrane localization model was confirmed by the structural and functional tolerance of PorT to hexahistidine (6×His) tag insertions at selected locations within the protein using site-directed mutagenesis. Interestingly, those PorT mutations adversely affecting gingipain secretion enhanced expression of theporTgene but at the same time suppressed the transcription of the gingipainrgpBgene. Further, PorT mutants deficient in gingipain activities produced significantly more di- and triaminopeptidase activities. PorT homologues have been found in restricted members of theBacteroidetesphylum where there is potential for PorT to participate in the maturation and secretion of proteins with characteristic C-terminal domains (CTDs). Knowledge of the cellular localization of PorT will enable analysis of the role of this protein in a new secretory pathway for the export of gingipains and other CTD-class proteins.