InterPro

Secretion of virulence factors in Gram-negative bacteria involves transportation of the protein across two membranes to reach the cell exterior [PMID: 9618447]. There have been four secretion systems described in animal enteropathogens, such as Salmonella and Yersinia, with further sequence similarities in plant pathogens like Ralstonia and Erwinia [PMID: 9618447].

The type III secretion system is of great interest, as it is used to transport virulence factors from the pathogen directly into the host cell and is only triggered when the bacterium comes into close contact with the host. The protein subunits of the system are very similar to those of bacterial flagellar biosynthesis [PMID: 9618447]. However, while the latter forms a ring structure to allow secretion of flagellin and is an integral part of the flagellum itself, type III subunits in the outer membrane translocate secreted proteins through a channel-like structure.

Exotoxins secreted by the type III system do not possess a secretion signal, and are considered unique for this reason [PMID: 9618447]. Yersinia spp. secrete a serine/threonine kinase, YpkA, [PMID: 8441468, PMID: 8045884] that causes autophosphorylation of host cell components, although the exact targets are unknown at present. It has also been suggested that the YpkA protein is involved in interference of signal transduction in the target cell [PMID: 8736538].