Topology determination and functional analysis of the Escherichia coli TatC protein.

The TatC protein is an essential component of the bacterial Tat system. By using alkaline phosphatase and beta-glucuronidase fusions we found that TatC contains four transmembrane helices. Three insertions of Ala-Ser dipeptide at the cytoplasmic N- and C-termini and in the cytoplasmic loop had no or only partial effect on the TatC function. In contrast, five of seven insertions in the two periplasmic loops abolished the Tat function. Four insertions analyzed had no effect on the stability of the altered TatC proteins or on membrane assembly of the TatA and TatB proteins. These data provide a novel base for more detailed studies of the mechanism of the Tat system.