Export of the pseudopilin XcpT of the Pseudomonas aeruginosa type II secretion system via the signal recognition particle-Sec pathway.

Type IV pilins and pseudopilins are found in various prokaryotic envelope protein complexes, including type IV pili and type II secretion machineries of gram-negative bacteria, competence systems of gram-positive bacteria, and flagella and sugar-binding structures in members of the archaeal kingdom. The precursors of these proteins have highly conserved N termini, consisting of a short, positively charged leader peptide, which is cleaved off by a dedicated peptidase during maturation, and a hydrophobic stretch of approximately 20 amino acid residues. Which pathway is involved in the inner membrane translocation of these proteins is unknown. We used XcpT, the major pseudopilin from the type II secretion machinery of Pseudomonas aeruginosa, as a model to study this process. Transport of an XcpT-PhoA hybrid was shown to occur in the absence of other Xcp components in P. aeruginosa and in Escherichia coli. Experiments with conditional sec mutants and reporter-protein fusions showed that this transport process involves the cotranslational signal recognition particle targeting route and is dependent on a functional Sec translocon.

Interaction domains in the Pseudomonas aeruginosa type II secretory apparatus component XcpS (GspF).

Pseudomonas aeruginosa is an opportunistic pathogen, which secretes a wide variety of enzymes and toxins into the extracellular medium. Most exoproteins are exported by the type II secretion machinery, the Xcp system, which encompasses 12 different proteins. One of the core components of the Xcp system is the inner-membrane protein XcpS (GspF), homologues of which can be identified in type II secretion machineries as well as in type IV piliation systems. In this study, XcpS was shown to be stabilized by co-expression of the XcpR (GspE) and XcpY (GspL) components of the machinery, demonstrating an interaction between these three proteins. By replacing segments of P. aeruginosa XcpS with the corresponding parts of its Pseudomonas putida counterpart, XcpS domains were identified that are important for species-specific functioning and thus represent putative interaction domains. The cytoplasmic loop of XcpS was found to be involved in the stabilization by XcpR and XcpY.