Assembly of XcpR in the cytoplasmic membrane is required for extracellularprotein secretion in Pseudomonas aeruginosa

A broad range of extracellular proteins secreted by Pseudomonas aeruginosa use the type II or general secretory pathway (GSP) to reach the medium. Thi s pathway requires the expression of at least 12 xcp gene products. XcpR, a putative nucleotide-binding protein, is essential for the secretion proces s across the outer membrane even though the protein contains no hydrophobic sequence that could target or anchor it to the bacterial envelope. For a b etter understanding of the relationship between XcpR and the other Xcp prot eins which are located in the envelope, we have studied its subcellular loc alization. In a wild-type P. aeruginosa strain, XcpR was found associated w ith the cytoplasmic membrane. This association depends on the presence of t he XcpY protein, which also appears to be necessary for XcpR stability. Fun ctional complementation of an xcpY mutant required the XcpY protein to be e xpressed at a low level. Higher expression precluded the complementing acti vity of XcpY, although membrane association of XcpR was restored. This beha vior suggested that an excess of free XcpY might interfere with the secreti on by formation of inactive XcpR-XcpY complexes which cannot properly inter act with their natural partners in the secretion machinery. These data show that a precise stoichiometric ratio between several components may be cruc ial for the functioning of the GSP.