FliK-Driven Conformational Rearrangements of FlhA and FlhB Are Required for Export Switching of the Flagellar Protein Export Apparatus

Abstract

The flagellar type III protein export apparatus coordinates protein export with assembly, which allows the flagellum to be efficiently built at the cell surface. Hook completion is an important morphological checkpoint for the sequential flagellar assembly process. The protein export apparatus switches its substrate specificity from the hook protein to the filament protein upon hook completion. FliK, FlhB, and FlhA are involved in the export-switching process, but the mechanism remains a mystery. By analyzing a slow-cleaving flhB ( P270A ) mutant, we provide evidence that an interaction between FliK and FlhB induces conformational rearrangements in FlhB, followed by a structural remodeling of the FlhA ring structure that terminates hook assembly and initiates filament formation.