Involvement of the twin-arginine translocation system in protein secretionvia 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 cal led 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 t he See machinery. Here, we demonstrate that two substrates for the Pseudomo nas aeruginosa secreton, both phospholipases, use the twin-arginine translo cation (Tat) system, instead of the See system, for the first step of trans location across the inner membrane. These results challenge the previous vi sion 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, s ince P. aeruginosa phospholipases are secreted virulence factors, the Tat s ystem appears to be a novel determinant of bacterial virulence.