A novel type bacterial flagellar motor that can use divalent cations as a coupling ion

Abstract

The bacterial flagellar motor is a sophisticated nanomachine embedded in the cell envelope and powered by an electrochemical gradient of H+, Na+, or K+ across the cytoplasmic membrane. Here we describe a new member of the bacterial flagellar stator channel family (MotAB1 of Paenibacillus sp. TCA20 (TCA-MotAB1)) that is coupled to divalent cations (Ca2+ and Mg2+). In the absence of divalent cations of alkaline earth metals, no swimming was observed in Paenibacillus sp. TCA20, which grows optimally in Ca2+ -rich environments. This pattern was confirmed by swimming assays of a stator-free Bacillus subtilis mutant expressing TCA-MotAB1. Both a stator-free and major Mg2+ uptake system-deleted B. subtilis mutant expressing TCA-MotAB1 complemented both growth and motility deficiency under low Mg2+ conditions and exhibited [Mg2+]in identical to that of the wild-type. This is the first report of a flagellar motor that can use Ca2+ and Mg2+ as coupling ions. These findings will promote the understanding of the operating principles of flagellar motors and molecular mechanisms of ion selectivity.