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

Abstract

The basal structure of the bacterial flagellum includes a membrane embedded MS-ring (formed by multiple copies of FliF) and a cytoplasmic C-ring (composed of proteins FliG, FliM and FliN). The SRP-type GTPase FlhF is required for directing the initial flagellar protein FliF to the cell pole, but the mechanisms are unclear. Here, we show 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 a structured domain at the N-terminus of FlhF binds to FliG. FlhF-bound FliG subsequently engages with the MS-ring protein FliF. Thus, the interaction of FlhF with HubP and FliG recruits a FliF-FliG complex to the cell pole. In addition, the modulation of FlhF activity by the MinD-type ATPase FlhG controls the interaction of FliG with FliM-FliN, thereby regulating the progression of flagellar assembly at the pole.

A conserved cell-pole determinant organizes proper polar flagellum formation

“…One candidate protein, the polar landmark HubP (FimV in Pseudomonas sp.) had been identified earlier and was shown to interact with FlhF in V. parahaemolyticus (Yamaichi et al, 2012;Dornes et al, 2024). Accordingly, polar localization of FlhF is decreased in mutants lacking hubP (this study ;Rossmann, Brenzinger et al, 2015), however, in a number of cells FlhF localized normally and the cells showed normal flagellation.…”

“…This is indicating a more important role for HubP in this species. Accordingly, it has been shown recently that in S. putrefaciens HubP directly interacts with HubP via its NG domain to recruit the initial flagellar building blocks (Dornes et al, 2024. However, also in S. putrefaciens active FlhF localizes to the cell pole and flagella are formed in the absence of HubP.…”

“…The FlhF-FliG complex is then recruited to the designated cell pole by HubP, where FlhF-bound FliG captures the transmembrane protein FliF and promotes formation of the MS-ring. This forms the scaffold from where further flagellar synthesis can occur (Dornes et al, 2024). However, in the absence of HubP, the majority of cells still forms normal polar flagella, indicating that HubP is not the only polarity factor in this process (Rossmann et al, 2015).…”