Abstract

The bacterial flagellar motor consists of a rotor and a dozen stator units and regulates the number of active stator units around the rotor in response to changes in the environment. The MotPS complex is a Na⁺-type stator unit in the Bacillus subtilis flagellar motor and binds to the peptidoglycan layer through the peptidoglycan binding (PGB) domain of MotS to act as the stator. The MotPS complex is activated in response to an increase in the Na⁺ concentration in the environment, but the mechanism of the activation has remained unknown. Here we report that activation occurs by Na⁺-induced folding and dimer formation of the PGB domain of MotS, as revealed in real-time imaging by high-speed atomic force microscopy. The MotPS complex showed two distinct ellipsoid domains connected by a flexible linker. A smaller domain, corresponding to the PGB domain, became structured and unstructured in the presence and absence of 150 mM NaCl, respectively. When the N-terminal portion of the PGB domain adopted a partially stretched conformation in the presence of NaCl, the center-to-center distance between these two domains increased by up to 5 nm, allowing the PGB domain to reach and bind to the peptidoglycan layer. We propose that assembly of the MotPS complex into a motor proceeds by means of Na⁺-induced structural transitions in its PGB domain.