Prior to 2008, all previously studied conventional bacterial flagellar motors appeared to utilize either H+ or Na+ as coupling ions. Membrane-embedded stator complexes support conversion of energy using transmembrane electrochemical ion gradients. The main H+-coupled stators, known as MotAB, differ from Na+-coupled stators, PomAB of marine bacteria, and MotPS of alkaliphilic Bacillus. However, in 2008, a MotAB-type flagellar motor of alkaliphilic Bacillus clausii KSM-K16 was revealed as an exception with the first dual-function motor. This bacterium was identified as the first bacterium with a single stator-rotor that can utilize both H+ and Na+ for ion-coupling at different pH ranges. Subsequently, another exception, a MotPS-type flagellar motor of alkaliphilic Bacillus alcalophilus AV1934, was reported to utilize Na+ plus K+ and Rb+ as coupling ions for flagellar rotation. In addition, the alkaline-tolerant bacterium Paenibacillus sp. TCA20, which can utilize divalent cations such as Ca2+, Mg2+, and Sr2+, was recently isolated from a hot spring in Japan, which contains a high Ca2+ concentration. These findings show that bacterial flagellar motors isolated from unique environments utilize unexpected coupling ions. This suggests that bacteria that grow in different extreme environments adapt to local conditions and evolve their motility machinery.
Keywords: Alkaliphiles; Divalent cation; Flagellar motor; MotPS; Stator.