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Outer membrane translocation arrest of the TcpA pilin subunit in rfb mutants of Vibrio cholerae O1 strain 569B

The toxin-coregulated pilus (TCP) of Vibrio cholerae is a type 4-related fimbrial adhesin and a useful model for the study of type 4 pilus biogenesis and related bacterial macromolecular transport pathways. Transposon mutagenesis of the putative perosamine biosynthesis genes in the rfb operon of V. cholerae 569B eliminates lipopolysaccharide (LPS) O-antigen biosynthesis but also leads to a specific defect in TCP export. Localization of TcpA is made difficult by the hydrophobic nature of this bundle-forming pilin, which floats anomalously in sucrose density gradients, but the processed form of TcpA can be found in membrane and periplasmic fractions prepared from these strains. While TcpA cannot be detected by surface immunogold labelling in transmission electron microscope preparations, EDTA pretreatment facilitates immunofluorescent antibody labelling of whole cells, and ultrathin cryosectioning techniques confirm membrane and periplasmic accumulation of TcpA. Salt and detergent extraction, protease accessibility, and chemical cross-linking experiments suggest that although TcpA has not been assembled on the cell surface, subunit interactions are otherwise identical to those within TCP. In addition, TcpA-mediated fucose-resistant hemagglutination of murine erythrocytes is preserved in whole-cell lysates, suggesting that TcpA has obtained its mature conformation. These data localize a stage of type 4 pilin translocation to the outer membrane, at which stage export failure leads to the accumulation of pilin subunits in a configuration similar to that within the mature fiber. Possible candidates for the outer membrane defect are discussed.