Novel insights into the mechanism of well-ordered assembly of bacterial flagellar proteins in Salmonella.

The FliI ATPase of the flagellar type III protein export apparatus forms the FliH2FliI complex along with its regulator FliH. The FliH2FliI complex is postulated to bring export substrates from the cytoplasm to the docking platform made of FlhA and FlhB although not essential for flagellar protein export. Here, to clarify the role of the FliH2FliI complex in flagellar assembly, we analysed the effect of FliH and FliI deletion on flagellar protein export and assembly. The hook length was not controlled properly in the ∆fliH-fliI flhB(P28T) mutant compared to wild-type cells, whose hook length is controlled to about 55 nm within 10% error. The FlhA(F459A) mutation increased the export level of the hook protein FlgE and the ruler protein FliK by about 10-fold and 3-fold, respectively, and improved the hook length control in the absence of FliH and FliI. However, the ∆fliH-fliI flhB(P28T) flhA(F459A) mutant did not produce flagellar filaments efficiently, and a large amount of flagellin monomers were leaked out into the culture media. Neither the hook length control nor flagellin leakage was affected by the FlhB(P28T) and FlhA(F459A) mutations. We will discuss a hierarchical protein export mechanism of the bacterial flagellum.