Exchange of Xcp (Gsp) secretion machineries between Pseudomonas aeruginosaand Pseudomonas alcaligenes: Species specificity unrelated to substrate recognition

Pseudomonas aeruginosa and Pseudomonas alcaligenes are gram-negative bacter ia that secrete proteins using the type II or general secretory pathway, wh ich requires at least 12 xcp gene products (XcpA and XcpP to -Z), Despite s trong conservation of this secretion pathway, gram-negative bacteria usuall y cannot secrete exoproteins from other species. Based on results obtained with Erwinia, it has been proposed that the XcpP and/or XcpQ homologs deter mine this secretion specificity (M. Linderberg, G. P. Salmond, and A. Collm er, Mel. Microbiol. 20:175-190, 1996). In the present study, we report that XcpP and XcpQ of P. alcaligenes could not substitute for their respective P. aeruginosa counterparts. However, these complementation failures could n ot be correlated to species specific recognition of exoproteins, since thes e bacteria could secrete exoproteins of each other. Moreover, when P. alcal igenes xcpP and xcpQ were expressed simultaneously in a P. aeruginosa xcpPQ deletion mutant, complementation was observed, albeit only on agar plates and not in liquid cultures. After growth in liquid culture the heat-stable P. alcaligenes XcpQ multimers were not detected, whereas monomers were clea rly visible. Together, our results indicate that the assembly of a function al Xcp machinery requires species-specific interactions between XcpP and Xc pQ and between XcpP or XcpQ and another, as yet uncharacterized component(s ).