Abstract
Porphyromonas gingivalis, a periodontal pathogen, translocates many virulence factors including the cysteine proteases referred to as gingipains to the cell surface via the type IX secretion system (T9SS). Expression of the T9SS component proteins is regulated by the tandem signaling of the PorXY two-component system and the ECF sigma factor SigP. However, the details of this regulatory pathway are still unknown. We found that one of the T9SS conserved C-terminal domain-containing proteins, PGN_0123, which we have designated PorA, is involved in regulating expression of genes encoding T9SS structural proteins and that PorA can be translocated onto the cell surface without the T9SS translocation machinery. X-ray crystallography revealed that PorA has a domain similar to the mannose-binding domain of Escherichia coli FimH, the tip protein of Type 1 pilus. Mutations in the cytoplasmic domain of the sensor kinase PorY conferred phenotypic recovery on the ΔporA mutant. The SigP sigma factor, which is activated by the PorXY two-component system, markedly decreased in the ΔporA mutant. These results strongly support a potential role for PorA in relaying a signal from the cell surface to the PorXY-SigP signaling pathway.
Highlights
Porphyromonas gingivalis, a periodontal pathogen, translocates many virulence factors including the cysteine proteases referred to as gingipains to the cell surface via the type IX secretion system (T9SS)
PorA is translocated to the cell surface without the T9SS translocation machinery and localized on the cell surface in both anionic polysaccharide-containing lipopolysaccharides (A-LPS) bound diffuse form and 23-kDa C-terminal domain (CTD)-containing form, and PorA is a novel type of T9SS CTD-containing protein
We found in this study that PorA impacts the PorXY-SigP signaling pathway that regulates expression of the T9SS in P. gingivalis
Summary
Porphyromonas gingivalis, a periodontal pathogen, translocates many virulence factors including the cysteine proteases referred to as gingipains to the cell surface via the type IX secretion system (T9SS). We found that one of the T9SS conserved C-terminal domain-containing proteins, PGN_0123, which we have designated PorA, is involved in regulating expression of genes encoding T9SS structural proteins and that PorA can be translocated onto the cell surface without the T9SS translocation machinery. Group II proteins are associated with other proteins, not A-LPS on the cell surface after removal of their CTDs by PorU, which include Mfa[5]. Our exhaustive random transposon mutagenesis analysis on pigmentation has revealed that 19 genes are involved in the A-LPS synthesis, 15 genes are T9SS component proteins, and 3 genes, porX, porY and sigP are involved in gene regulation of the T9SS component proteins[28,29,30,31]. On the basis of the structure and the results of the functional analyses, we propose a plausible mechanism for how PorA influences the T9SS expression
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