Distinct effects of Q925 mutation on intracellular and extracellular Na + and K + binding to the Na + , K + -ATPase

Three Na + sites are defined in the Na + -bound crystal structure of Na + , K + -ATPase. Sites I and II overlap with two K + sites in the K + -bound structure, whereas site III is unique and Na + specific. A glutamine in transmembrane helix M8 (Q925) appears from the crystal structures to coordinate Na + at site III, but does not contribute to K + coordination at sites I and II. Here we address the functional role of Q925 in the various conformational states of Na + , K + -ATPase by examining the mutants Q925A/G/E/N/L/I/Y. We characterized these mutants both enzymatically and electrophysiologically, thereby revealing their Na + and K + binding properties. Remarkably, Q925 substitutions had minor effects on Na + binding from the intracellular side of the membrane – in fact, mutations Q925A and Q925G increased the apparent Na + affinity – but caused dramatic reductions of the binding of K + as well as Na + from the extracellular side of the membrane. These results provide insight into the changes taking place in the Na + -binding sites, when they are transformed from intracellular- to extracellular-facing orientation in relation to the ion translocation process, and demonstrate the interaction between sites III and I and a possible gating function of Q925 in the release of Na + at the extracellular side.