A conditional gene expression system in Porphyromonas gingivalis for study of the secretion mechanisms of lipoproteins and T9SS cargo proteins

The Gram-negative anaerobe, Porphyromonas gingivalis, is known to be a pathogen associated with chronic periodontitis. P. gingivalis possesses virulence factors such as fimbriae and gingipain proteinases. Fimbrial proteins are secreted to the cell surface as lipoproteins. In contrast, gingipain proteinases are secreted into the bacterial cell surface via the type IX secretion system (T9SS). The transport mechanisms of lipoproteins and T9SS cargo proteins are entirely different and remain unknown. Therefore, using the Tet-on system developed for the genus Bacteroides, we newly created a conditional gene expression system in P. gingivalis. We succeeded in establishing conditional expression of nanoluciferase and its derivatives for lipoprotein export, of FimA for a representative of lipoprotein export, and of T9SS cargo proteins such as Hbp35 and PorA for representatives of type 9 protein export. Using this system, we showed that the lipoprotein export signal, which has recently been found in other species in the phylum Bacteroidota, is also functional in FimA, and that a proton motive force inhibitor can affect type 9 protein export. Collectively, our conditional protein expression method is useful for screening inhibitors of virulence factors, and may be used to investigate the role of proteins essential to bacterial survival in vivo.

Secreted peptidases contribute to virulence of fish pathogen Flavobacterium columnare

Flavobacterium columnare causes columnaris disease in freshwater fish in both natural and aquaculture settings. This disease is often lethal, especially when fish population density is high, and control options such as vaccines are limited. The type IX secretion system (T9SS) is required for F. columnare virulence, but secreted virulence factors have not been fully identified. Many T9SS-secreted proteins are predicted peptidases, and peptidases are common virulence factors of other pathogens. T9SS-deficient mutants, such as ΔgldN and ΔporV, exhibit strong defects in secreted proteolytic activity. The F. columnare genome has many peptidase-encoding genes that may be involved in nutrient acquisition and/or virulence. Mutants lacking individual peptidase-encoding genes, or lacking up to ten peptidase-encoding genes, were constructed and examined for extracellular proteolytic activity, for growth defects, and for virulence in zebrafish and rainbow trout. Most of the mutants retained virulence, but a mutant lacking 10 peptidases, and a mutant lacking the single peptidase TspA exhibited decreased virulence in rainbow trout fry, suggesting that peptidases contribute to F. columnare virulence.

Quantitative proteomic analysis of the type IX secretion system mutants in Porphyromonas gingivalis

Porphyromonas gingivalis is an anaerobic, gram-negative human oral pathogen highly associated with chronic periodontitis. P. gingivalis utilizes the type IX secretion system (T9SS) to transport many of its virulence factors including the gingipains to the cell surface. The T9SS is comprised of at least 16 proteins and the involvement of these 16 proteins in the T9SS has been verified by creating gene deletion mutants in P. gingivalis. These T9SS mutants are regularly utilized to understand how these proteins function together to allow the secretion of the T9SS substrates. We performed label-free quantitative proteomic analysis on the T9SS protein mutants in P. gingivalis to understand the relative abundance of each T9SS component in different mutants. The T9SS components were reduced in abundance in the porK, porL, porM, porN, sov and porT mutants, whereas they were increased in the porE, porU, porV, porZ and porQ mutants. Sov and PorW appear to be the lowest in abundance and PorV the highest amongst all the T9SS components in P. gingivalis wild-type strain. These results are consistent with the proposed role of Sov as the translocation pore in the outer membrane and PorV as the shuttle protein that transports the T9SS substrates between sub-complexes. Together, the label-free quantitative proteomics analyses showed that different T9SS mutants have vastly different abundances of the T9SS components. This knowledge will greatly assist in interpreting the phenotype of the T9SS mutants as well as selecting the right mutant for exploring the role of an individual component.