Functional Defect and Restoration of Temperature-Sensitive Mutants of FlhA, a Subunit of the Flagellar Protein Export Apparatus

The flagellar axial component proteins are exported to the distal end of the growing flagellum for self-assembly by the flagellar type III export apparatus. FlhA is a key membrane protein of the export apparatus, and its C-terminal cytoplasmic domain (FlhAC) is a part of an assembly platform for the three soluble export components, FliH, FliI, and FliJ, as well as export substrates and chaperone–substrate complexes. FlhAC is composed of a flexible linker region and four compact domains (ACD1–ACD4). At 42 °C, a temperature-sensitive (TS) G368C mutation in FlhAC blocks the export process after the FliH–FliI–FliJ–substrate complex binds to the assembly platform, but it remains unknown how it does so. In this study, we analyzed a TS mutant variant, FlhAC(G368C), and its pseudorevertant variants FlhAC(G368C/L359F), FlhAC(G368C/G364R), FlhAC(G368C/R370S), and FlhAC(G368C/P550S) using far-ultraviolet circular dichroism. Whereas the denaturation of the wild-type FlhAC occurs in a single step, FlhAC(G368C) and its pseudorevertant variants showed thermal transitions, at least, in two steps. The first transition of FlhAC(G368C) can further be divided into reversible and following irreversible transitions, which correspond to the denaturation of ACD2 and ACD1, respectively. We show the relation between the reversible transition and the TS defect in the exporting function of FlhAC(G368C) and that the loss of function is caused by denaturation of ACD2. We suggest that ACD2 is directly involved in the translocation of export substrates.