Involvement of the twin-arginine translocation system in protein secretion via the type II pathway

The general secretory pathway (GSP) is a two-step process for the secretion of proteins by Gram-negative bacteria. The translocation across the outer membrane is carried out by the type II system, which involves machinery called the secreton. This step is considered to be an extension of the general export pathway, i.e. the export of proteins across the inner membrane by the See machinery. Here, we demonstrate that two substrates for the Pseudomonas aeruginosa secreton, both phospholipases, use the twin-arginine translocation (Tat) system, instead of the See system, for the first step of translocation across the inner membrane. These results challenge the previous vision of the GSP and suggest for the first time a mosaic model in which both the See and the Tat systems feed substrates into the secreton. Moreover, since P. aeruginosa phospholipases are secreted virulence factors, the Tat system appears to be a novel determinant of bacterial virulence.