MotA protein of E. coli is a proton-conducting component of the flagellar motor

A number of mutants of motA, a gene necessary for flagellar rotation in E. coli, were isolated and characterized. Many mutations were dominant, owing to competition between functional and nonfunctional MotA for a limited number of sites on the flagellar motor. A new class of mutant was discovered in which flagellar torque is normal at low speeds but reduced at high speeds. Hydrogen isotope effects on these mutants indicate that MotA catalyzes proton transfer.