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Structural basis for substrate specificity switching revealed by the FlhA(A489E) suppressor mutation
(Motoshi Sakai, Miki Kinoshita, Norihiro Takekawa, Keiichi Namba, Katsumi Imada, Tohru Minamino, Biophysics and Physicobiology 23, e230030 (2026), DOI: 10.2142/biophysico.bppb-v23.0030)
The bacterial flagellar type III secretion system switches its substrate specificity during flagellar assembly, but the underlying structural mechanism remains poorly understood. FlhA and FlhB form a cytoplasmic substrate-docking platform that controls this switching process. Our structural and functional analyses suggest that FlhB stabilizes the hook-type state of the FlhA ring. Upon hook completion, FlhB dissociates from the FlhA ring, allowing the FlhA linker to interact with the neighboring FlhA subunit and stabilize the filament-type state. The FlhA(A489E) suppressor mutation provides structural insight into how conformational remodeling of the FlhA ring facilitates this critical transition during flagellar assembly.