FORMATION OF OLIGOMERIC RINGS BY XCPQ AND PILQ, WHICH ARE INVOLVED INPROTEIN-TRANSPORT ACROSS THE OUTER-MEMBRANE OF PSEUDOMONAS-AERUGINOSA

Pseudomonas aeruginosa is able to translocate proteins across both mem branes of the cell envelope. Many of these proteins are transported vi a the type II secretion pathway and adopt their tertiary conformation in the periplasm, which implies the presence of a large transport chan nel in the outer membrane. The outer membrane protein, XcpQ, which is involved in transport of folded proteins across the outer membrane of P. aeruginosa, was purified as a highly stable homomultimer. Insertion and deletion mutagenesis of xcpQ revealed that the C-terminal part of XcpQ is sufficient for the formation of the multimer. However, linker insertions in the N-terminal part can disturb complex formation compl etely. Furthermore, complex formation is strictly correlated with leth ality, caused by overexpression of xcpQ. Electron microscopic evaluati on of the XcpQ multimers revealed large, ring-shaped structures with a n apparent central cavity of 95 Angstrom. Purified PilQ, a homologue o f XcpQ involved in the biogenesis of type IV pili, formed similar stru ctures, However, the apparent cavity formed by PilQ was somewhat small er, 53 Angstrom. The size of this cavity could allow for the transport of intact type IV pili.