FliK, the protein responsible for flagellar hook length control in Salmonella, is exported during hook assembly

In wild-type Salmonella, the length of the flagellar hook, a structure consisting of subunits of the hook protein FlgE, is fairly tightly controlled at approximately 55 nm. Because fliK mutants produce abnormally elongated hook structures that lack the filament structure, FliK appears to be involved in both the termination of hook elongation and the initiation of filament formation. FliK, a soluble protein, is believed to function together with a membrane protein, FlhB, of the export apparatus to mediate the switching of export substrate specificity (from hook protein to flagellin) upon completion of hook assembly. We have examined the location of FliK during flagellar morphogenesis. FliK was found in the culture supernatants from the wild-type strain and from flgD (hook capping protein), flgE (hook protein) and flgK (hook-filament junction protein) mutants, but not in that from a flgB (rod protein) mutant. The amount of FliK in the culture supernatant from the flgE mutant was much higher than in that from the flgK mutant, indicating that FliK is most efficiently exported prior to the completion of hook assembly. Export was impaired by deletions within the N-terminal region of FliK, but not by C-terminal truncations. A decrease in the level of exported FliK resulted in elongated hook structures, sometimes with filaments attached. Our results suggest that the export of FliK during hook assembly is important for hook-length control and the switching of export substrate specificity.