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.
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Holm, Rikke ; Khandelwal, Jaanki ; Einholm, Anja P. ; Andersen, Jens P. ; Artigas, Pablo ; Vilsen, Bente ( , Proceedings of the National Academy of Sciences)