1.A.30.1.2
The flagellar motor (smf-dependent) (PomAB; MotXY) (Okabe et al., 2005). PomB interacts with the third TMS of PomA in the Na⁺-driven polar flagellum (Yakushi et al. 2004). Sodium-powered stators of the flagellar motor can generate torque in the presence of the sodium channel blocker, phenamil, with mutations near the peptidoglycan-binding region of PomB (Ishida et al. 2019). FliL associates with the flagellar stator in the sodium-driven Vibrio motor (Lin et al. 2018). When the ion channel is closed, PomA and PomB interact strongly. When the ion channel opens, PomA interacts less tightly with PomB. The plug and loop between TMSs 1 and 2 regulate activation of the stator, which depends on the binding of sodium ion to the D24 residue of PomB (Nishikino et al. 2019). The PomA helices parallel to the inner membrane play roles in the hoop-like function in securing the stability of the stator complex and the ion conduction pathway (Nishikino et al. 2022). Na⁺-binding sites are formed by critical aspartic acid and threonine residues located in the TMSs of PomAB (Kojima et al. 2023). Vibrio alginolyticus forms a single flagellum at its cell pole. FlhF and FlhG are the main proteins responsible for the polar formation of the single flagellum. MS-ring formation in the flagellar basal body appears to be an initiation step for flagellar assembly. The MS-ring is formed by a single protein, FliF, which has two transmembrane (TM) segments and a large periplasmic region. FlhF is required for the polar localization of Vibrio FliF, and FlhF facilitated MS-ring formation when FliF was overexpressed in E. coli cells. These results suggest that FlhF interacts with FliF to facilitate MS-ring formation. Fukushima et al. 2023 detected this interaction using Vibrio FliF fragments fused to a tag of Glutathione S-transferase (GST) in E. coli. The N-terminal 108 residues of FliF, including the first TMS and the periplasmic region, could pull FlhF down. In the first step, Signal Recognition Particle (SRP) and its receptor are involved in the transport of membrane proteins to target them, which delivers them to the translocon. FlhF may have a similar or enhanced function as SRP, which binds to a region rich in hydrophobic residues (Fukushima et al. 2023).