Novel topological features of FhaC, the outer membrane transporter involved in the secretion of the Bordetella pertussis filamentous hemagglutinin

Many pathogenic Gram-negative bacteria secrete virulence factors across the cell envelope into the extracellular milieu. The secretion of filamentous hemagglutinin (FHA) by Bordetella pertussis depends on the pore-forming out er membrane protein FhaC, which belongs to a growing family of protein tran sporters. Protein alignment and secondary structure predictions indicated t hat FhaC is likely to be a beta-barrel protein with an odd number of transm embrane beta-strands connected by large surface loops and short periplasmic turns. The membrane topology of FhaC was investigated by random insertion of the c-Myc epitope and the tobacco etch virus protease-specific cleavage sequence. FhaC was fairly permissive to short linker insertions. Furthermor e, FhaC appeared to undergo conformational changes upon FHA secretion. Surf ace detection of the inserted sequences indicated that several predicted lo ops in the C-terminal moiety as well as the N terminus of the protein are e xposed. However, a large surface-predicted region in the N-terminal moiety of FhaC was inaccessible from the surface. In addition, the activity and th e stability of the protein were affected by insertions in that region, indi cating that it may have important structural and/or functional roles. The s urface exposure of the N terminus and the presence of an odd number of beta -strands are novel features for beta-barrel outer membrane proteins.