Characterization of PomA periplasmic loop and sodium ion entering in stator complex of sodium-driven flagellar motor.

The bacterial flagellar motor is a rotary nanomachine driven by ion flow. The flagellar stator complex, which is composed of two proteins, PomA and PomB, performs energy transduction in marine Vibrio. PomA is a four transmembrane (TM) protein and the cytoplasmic region between TM2 and TM3 (loop(2-3)) interacts with the rotor protein FliG to generate torque. The periplasmic regions between TM1 and TM2 (loop(1-2)) and TM3 and TM4 (loop(3-4)) are candidates to be at the entrance to the transmembrane ion channel of the stator. In this study, we purified the stator complex with cysteine replacements in the periplasmic loops and assessed the reactivity of the protein with biotin maleimide (BM). BM easily modified Cys residues in loop(3-4) but hardly labelled Cys residues in loop(1-2). We could not purify the plug deletion stator (AL stator) composed of PomBA41-120 and WT-PomA but could do the AL stator with PomA-D31C of loop(1-2) or with PomB-D24N of TM. 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(1-2) region regulate this activation of the stator, which depends on the binding of sodium ion to the D24 residue of PomB.