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J. Bacteriol. doi:10.1128/JB.00205-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

XphA/XqhA, a novel GspCD subunit for type II secretion in Pseudomonas aeruginosa

Laboratoire d'Ingénierie des Systémes Macromolé;culaires, Institut de Biologie Structurale et Microbiologie, Centre National de la Recherche Scientifique, 31 Chemin Joseph Aiguier, 13402 Marseille Cedex 20, France

^* To whom correspondence should be addressed. Email: michel{at}ibsm.cnrs-mrs.fr.

	Abstract

The opportunistic human pathogen bacterium Pseudomonas aeruginosa secretes various exoproteins in its surrounding environment. Protein secretion involves different secretory systems including the type II secretion system, or T2SS, that is one of the most efficient secretory pathways of P. aeruginosa. There are two T2SS in this bacterium, the quorum-sensing regulated Xcp system and the Hxc system only present in phosphate-limiting conditions. Like T2SS of other bacteria, the Xcp T2SS is species-specific and this specificity mainly involves two proteins, XcpP (GspC family) and the secretin XcpQ (GspD family), which are the gatekeepers of the system. Interestingly, an orphan secretin, XqhA, was previously reported as being able to functionally replace the XcpQ secretin. In this study, we identified another gene, that we named xphA (xcpP homologue A) which is located next to xqhA. We showed that deletion of the xphA gene in a xcpP mutant caused the disappearance of the residual secretion observed in this mutant strain indicating that the protein XphA plays a role in the secretion process. Our results also revealed that complementation of a xcpP/xcpQ mutant can be obtained with the gene couple xphA/xqhA. The XphA and XqhA proteins (the P_AQ_A subunit) could thus form together with XcpR-Z a functional hybrid T2SS. A 2D-PAGE analysis showed that except for the aminopeptidase PaAP, which secretion is not restored by the P_AQ_A subunit in the xcpP/xcpQ deletion mutant, each major Xcp-dependent exoprotein is secreted by the new hybrid machinery. Our work contributes to support the idea that components of the GspC/GspD families such as XphA/XqhA or XcpP/XcpQ, are assembled as a specific tandem within the T2SS. Each of these pairs may thus confer a different level of secretion specificity as it is the case with respect to PaAP. Finally, using a chromosomal xphA-lacZ fusion, we showed that the xphA-xqhA genes are transcribed from an early stage of the bacterial growth. We thus suggest that the P_AQ_A subunit might be involved in the secretion process at a different growth stage as compared to XcpP/XcpQ.

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