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

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, FlhFs interaction with HubP/FliG recruits a functional FliF/FliG complex to the pole, while FlhGs modulation of FlhF controls FliGs interaction with FliM/FliN, thereby regulating the progression of flagellar assembly at the pole. Significance statementFlagella serve as bacterial locomotion organelles, with their number and location, known as the flagellation pattern, being species-specific and among the earliest taxonomic criteria in microbiology. Bacteria replicate their flagellation pattern with each cell division. Flagella localization and abundance depends on the SRP-type GTPase FlhF, together with its regulator FlhG. Our study clarifies the mechanism through which FlhF coordinates the polar positioning of the flagellum, working in tandem with the polar landmark protein HubP and aiding in the assembly of flagellar MS-ring/C-ring components at the cellular pole.