Structure-function analysis of XcpP, a component involved in general secretory pathway-dependent protein secretion in Pseudomonas aeruginosa

The general secretory pathway of Pseudomonas aeruginosa is required for the transport of signal peptide-containing exoproteins across the cell envelop e. After completion of the Sec-dependent translocation of exoproteins acros s the inner membrane and cleavage of the signal peptide, the Sep machinery mediates translocation across the outer membrane. This machinery consists o f 12 components, of which XcpQ (GspD) is the sole outer membrane protein. X cpQ forms a multimeric ring-shaped structure, with a central opening throug h which exoproteins could pass to reach the medium. Surprisingly, all of th e other Xcp proteins are located in or are associated with the cytoplasmic membrane. This study is focused on the characteristics of one such cytoplas mic membrane protein, XcpP. An xcpP mutant demonstrated that the product of this gene is indeed an essential element of the P. aeruginosa secretion ma chinery. Construction and analysis of truncated forms of XcpP made it possi ble to define essential domains for the function of the protein. Some of th ese domains, such as the N-terminal transmembrane domain and a coiled-coil structure identified at the C terminus of XcpP, may be involved in protein- protein interaction during the assembly of the secretory apparatus.