Sodium-Dependent Conformational Change in Flagellar Stator Protein MotS from Bacillus subtilis

The bacterial flagellar motor consists of a rotor and stator units and is driven by ion flow through the stator. The activation of the ion flow is coupled with the anchoring of the stator units to the peptidoglycan layer by the stator B-subunit around the rotor. Gram-negative bacteria, such as Salmonella and Vibrio, change the conformation of the N-terminal helix of the periplasmic domain of the B-subunit to anchor the stator units. However, a recent high-speed atomic force microscopic study has suggested that the periplasmic domain of MotS, the stator B-subunit of the sodium (Na⁺)-driven stator of Bacillus subtilis, a gram-positive bacterium, unfolds at low external Na⁺ concentrations and folds at high Na⁺ concentrations to anchor the stator units. Here, we report the crystal structures of MotS_68-242, a periplasmic fragment of MotS, from B. subtilis at high and low Na⁺ concentrations. We also performed far-UV CD spectroscopic analysis of the wild-type MotS_68-242 and MotS_78-242 proteins and mutant variants of MotS_68-242 under high and low Na⁺ concentrations and found that the N-terminal disordered region of MotS_68-242 shows a Na⁺-dependent coil-helix transition. We propose a mechanism of the Na⁺-dependent structural transition of Bs-MotS to anchor the stator units.