Polar confinement of a macromolecular machine by an SRP-type GTPase

Abstract

The SRP-type GTPase FlhF, along with its regulator FlhG, orchestrates the localization and quantity of flagella in bacteria. Our study reveals that FlhF anchors developing flagellar structures to the polar landmark protein HubP/FimV, thereby restricting their formation to the cell pole. Specifically, the GTPase domain of FlhF interacts with HubP, while an as-yet-uncharacterized structured domain at the N-terminus of FlhF binds to FliG. This FlhF-bound FliG subsequently engages with the MS-ring protein FliF, but not with the C-ring proteins FliM/FliN. Consequently, FlhF's interaction with HubP/FliG recruits a functional FliF/FliG complex to the pole, while FlhG's modulation of FlhF controls FliG's interaction with FliM/FliN, thereby regulating the progression of flagellar assembly at the pole.