Elucidating the molecular mechanisms of Vibrio cholerae toxin coregulated pilus assembly, bundling and CTXф interactions

Vibrio cholerae use toxin-coregulated pili (TCP) to colonize the human intestine and cause severe diarrheal disease, cholera. TCP are surface filaments that self-aggregate to hold the bacteria in microcolonies that protect them from host defences and provide concentrated amounts of secreted cholera toxin. Also, TCP are receptors for a filamentous bacteriophage, CTXф, which carries the cholera toxin genes. To understand TCP assembly, TCP-mediated microcolony formation, and TCP:CTXф interactions at the molecular level, structure-based mutagenesis and functional assays were employed. I show charge complementarity between pilin subunits is important for initiating TCP assembly and hydrophobic interactions between subunits are required for pilus stability. I propose a model for pilus interactions whereby protruding regions of the TCP intercalate into depressions in adjacent TCP to form specific interactions that hold cells in microcolonies. Mutagenesis, immuno-gold electron microscopy analysis and functional assays suggest that CTXф attachment occurs over a large surface of TCP.