Localization of TraC, a protein involved in assembly of the F conjugative pilus

TraC is one of the proteins encoded by the F transfer region of the F conjugative plasmid which is required for the assembly of F pilin into the mature F pilus structure. Overproduction of this protein from the plasmid pKAS2, which carries only traC, resulted in the formation of inclusion bodies from which soluble TraC was purified. When small amounts of TraC were produced from pKAS2, the protein was localized to the cytoplasm by using anti-TraC antibodies. Similar analysis of a set of TraC-alkaline phosphatase fusion proteins localized all of these fusion proteins to the cytoplasm. However, when TraC was expressed from the F plasmid, much of it appeared associated with the bacterial membrane fraction. Under these conditions, TraC does not appear to be part of the tip of the F pilus, as neither anti-TraC antibodies nor purified TraC had any effect on the infection of F-containing bacteria by the filamentous bacteriophage f1. These data suggest that TraC is normally associated with the membrane through interactions with other proteins specified by the tra region. This interaction may be via the carboxyl-terminal region of the TraC protein, as a mutant TraC protein containing an Arg-Cys substitution at amino acid 811 exhibits an interaction with the membrane weaker than that of the wild-type protein in the presence of the other Tra proteins.