Por Secretion System of Porphyromonas gingivalis

The virulence factors of pathogenic bacteria are major secretory proteins that are directly linked to their pathogenicity These secretory proteins are translocated across the membranes of bacterial cells by translocase nanomachines, which consists of integral membrane proteins. The periodontal pathogen, Porphyromonas gingivalis, secretes trypsin-like proteases (gingipains) either as a large complex on the bacterial cell surface or into the extracellular milieu. Gingipains are important virulence factors, because they degrade host proteins. They are responsible for the processing/maturation of other P. gingivalis virulence factors. At least six types of translocase nanomachines have been found in Gram-negative bacteria ; however, P. gingivalis does not have genes homologous to those coding these secretion systems in the bacterial genome and not much is known about the mechanism of gingipain secretion. In this study, the proteins responsible for gingipain secretion, i.e., PorK, PorL, PorM, PorN, and PorW, were identified by comparative genome analysis and genetic experiments. We named the gingipain secretion system the Por secretion system (PorSS). Genes encoding PorSS proteins are conserved among a group of bacteria including periodontal pathogens such as Tannellera forsythia and Prevotella intermedia in the phylum Bacteroidetes. In addition, homologous genes are involved in gliding motility and chitinase secretion in Flavobacterium johnsoniae, another member of the phylum Bacteroidetes. Two other genes, porX and porY, encoding the regulatory factors of PorSS gene expression were identified at the same time. The expression of the porT, porK, porL, porM, and porN genes was downregulated in PorX- or PorY-defective mutants. PorSS and its regulatory system appear to be associated with the pathogenicity of various bacteria in the phylum Bacteroidetes