Immunoelectron Microscopic Analysis of Dipeptidyl-Peptidases and Dipeptide Transporter Involved in Nutrient Acquisition in Porphyromonas gingivalis.

Porphyromonas gingivalis is an asaccharolytic, Gram-negative, anaerobic bacterium representing a keystone pathogen in chronic periodontitis. The bacterium's energy production depends on the metabolism of amino acids, which are predominantly incorporated as dipeptides via the proton-dependent oligopeptide transporter (Pot). In this study, the localization of dipeptidyl-peptidases (DPPs) and Pot was investigated for the first time in P. gingivalis using immunoelectron microscopy with specific antibodies for the bacterial molecules and gold-conjugated secondary antibodies on ultrathin sections. High-temperature protein G and hemin-binding protein 35 were used as controls, and the cytoplasmic localization of the former and outer membrane localization of the latter were confirmed. P. gingivalis DPP4, DPP5, DPP7, and DPP11, which are considered sufficient for complete dipeptide production, were detected in the periplasmic space. In contrast, DPP3 was localized in the cytoplasmic space in accord with the absence of a signal sequence. The inner membrane localization of Pot was confirmed. Thus, spatial integration of the nutrient acquisition system exists in P. gingivalis, in which where dipeptides are produced in the periplasmic space by DPPs and readily transported across the inner membrane via Pot.