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Distinct effects of Q925 mutation on intracellular and extracellular Na+ and K+ binding to the Na+, K+-ATPase.
Nielsen HN
,
Spontarelli K
,
Holm R
,
Andersen JP
,
Einholm AP
,
Artigas P
,
Vilsen B
.
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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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