Interaction of FlhF, SRP-like GTPase with FliF, MS ring component assembling the initial structure of flagella in marine Vibrio

Vibrio alginolyticus forms a single flagellum at its cell pole. FlhF and FlhG are known to be the main proteins responsible for the polar formation of single flagellum. FlhF, which belongs to the signal recognition particle (SRP)-type GTPase family, localizes at the cell pole and initiates flagellar generation. In contrast, FlhG negatively regulates flagellar numbers. Furthermore, 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. We had shown that FlhF was 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. Here, we attempted to detect this interaction using Vibrio FliF fragments fused to a tag of Glutathione S-transferase (GST) in E. coli. We found that the N-terminal 108 residues of FliF, including the first TM segment and the periplasmic region, could pull FlhF down. In the first step, the 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. IMPORTANCEVibrio alginolyticus forms only a single flagellum at the cell pole by regulators of FlhF and FlhG. FlhF regulates positively the formation of flagella and is required for polar positioning of the flagellum. FliF, the two transmembrane (TM) segments and a large periplasmic region, forms the MS ring of flagellar basal body in the membrane. Previous studies suggest that FlhF interacts with FliF to facilitate MS ring formation at the cell pole, but the interaction has not been detected. Here, we show the evidence that FlhF interacts with FliF at residues including the first TM segment and following periplasmic region. The hydrophobic residues of this region seem to be important for the interaction.