Species-specific functioning of the Pseudomonas XcpQ secretin: role for the C-terminal homology domain and LPS

Department of Molecular Microbiology and Institute of Biomembranes, Utrecht University, 3584 CH Utrecht, The Netherlands ; Division of Immunochemistry, Research Center Borstel, Parkallee 22, D-23845 Borstel, Germany

^* To whom correspondence should be addressed. Email: m.c.koster{at}bio.uu.nl.

	Abstract

Secretins are oligomeric proteins that mediate the export of macromolecules across the bacterial outer membrane. The members of this superfamily possess a C-terminal homology domain that is important for oligomerization and channel formation, while their N-terminal halves are thought to be involved in system-specific interactions. The secretin XcpQ of Pseudomonas spp. is a component of the type II secretion pathway. XcpQ from Pseudomonas alcaligenes is not able to functionally replace the secretin of the closely related species Pseudomonas aeruginosa. By analysis of chimeric XcpQ proteins, a region important for species-specific functioning was mapped between amino acid residues 344 and 478 within the C-terminal homology domain. Two chromosomal suppressor mutations were obtained that resulted in the proper functioning in P. aeruginosa of P. alcaligenes XcpQ and inactive hybrids. These mutations caused a defect in the synthesis of the lipopolysaccharide (LPS) outer core region. Subsequent analysis of different LPS mutants showed that changes in the outer core and not the loss of O-antigen caused the suppressor phenotype. High concentrations of divalent cations in the growth medium also allowed P. alcaligenes XcpQ and inactive hybrids to function properly in P. aeruginosa. Since divalent cations are known to affect the structure of LPS, this observation supports a role of LPS in the functioning of secretins.