Substrate specificity switching of the flagellum-specific export apparatus during flagellar morphogenesis in Salmonella typhimurium.

During flagellar morphogenesis in Salmonella typhimurium, the flagellum-specific anti-sigma factor FlgM is exported out of the cells only after completion of hook assembly. In this study, we examined the export of the flagellar proteins, FlgD (hook capping protein), FlgE (hook protein), FlgK and FlgL (hook-filament junction proteins), FliD (filament capping protein), and FliC (flagellin), before and after completion of hook assembly. Like the FlgM protein, the FlgK, FlgL, FliD, and FliC proteins are exported efficiently only after completion of hook assembly. On the other hand, the FlgD and FlgE proteins are exported efficiently before, but poorly after, hook completion. These results indicate that the export properties are different between these two groups and that their export order exactly parallels the assembly order of the hook-filament structure. We propose that the substrate specificity switching occurs in the flagellum-specific export apparatus upon completion of hook assembly.