Abstact

The periodontal pathogen Porphyromonas gingivalis secretes many potent virulence factors using the type IX secretion system (T9SS). T9SS cargo proteins that have been structurally determined by X-ray crystallography are composed of a signal peptide, functional domain(s), an immunoglobulin (Ig)-like domain and a C-terminal domain. Role of the Ig-like domains of cargo proteins in the T9SS has not been elucidated. Gingipain proteases, which are cargo proteins of the T9SS, were degraded when their Ig-like domains were lacking or truncated. The degradation was dependent on the activity of a quality control factor, HtrA protease. Another T9SS cargo protein, HBP35, which has a thioredoxin domain as a functional domain, was analyzed by X-ray crystallography, revealing that HBP35 has an Ig-like domain after the thioredoxin domain and that the hydrophobic regions of the thioredoxin domain and the Ig-like domain face each other. HBP35 with substitution of hydrophobic amino acids in the Ig-like domain was degraded depending on HtrA. These results suggest that the Ig-like domain mediates stability of the cargo proteins in the T9SS.