Conformational dynamics of a histidine molecular switch in a cation/proton antiporter

Multisubunit Mrp (multiple resistance and pH adaptation) type sodium proton antiporters are indispensable for the growth of alkali and salt tolerant bacteria and archaea. They share sequence and structural similarity with the membrane domain of respiratory complex I, a key mitochondrial enzyme. The molecular mechanism of complex I and Mrp antiporters has remained largely unknown and is the subject of intense debate. Here, by combining site-directed mutagenesis with large-scale molecular dynamics simulations, we explore the conformational dynamics of a key histidine residue in the MrpA subunit of the antiporter. We show that point mutations perturbing the conformational mobility of the histidine sidechain directly affect the transport activity of the antiporter. We identify that protonation state variations in conserved lysine residues around the histidine drive hydrogen bonding rearrangements and hydration changes coupled to sidechain and backbone conformational dynamics. Finally, we develop detailed and testable mechanistic models of proton transfer in Mrp antiporter and complex I, in which the histidine switch functions as a unique gating element.