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Crystal Structure of Na+, K+-ATPase in the Na+-Bound State

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Science  04 Oct 2013:
Vol. 342, Issue 6154, pp. 123-127
DOI: 10.1126/science.1243352

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Pumping Out Sodium

Mammalian cells contain relatively high concentrations of potassium but low concentrations of sodium. This balance is maintained by an ion pump, the Na+, K+–adenosine triphosphatase, in an adenosine triphosphate–driven transport cycle that results in the export of three sodium ions and the import of two potassium ions. Structures of potassium-bound conformations of the pump have been determined. Now, Nyblom et al. (p. 123, published online 19 September) report on the high-resolution crystal structure of a Na+-bound conformation, which reveals conformational changes associated with Na+ binding.

Abstract

The Na+, K+–adenosine triphosphatase (ATPase) maintains the electrochemical gradients of Na+ and K+ across the plasma membrane—a prerequisite for electrical excitability and secondary transport. Hitherto, structural information has been limited to K+-bound or ouabain-blocked forms. We present the crystal structure of a Na+-bound Na+, K+-ATPase as determined at 4.3 Å resolution. Compared with the K+-bound form, large conformational changes are observed in the α subunit whereas the β and γ subunit structures are maintained. The locations of the three Na+ sites are indicated with the unique site III at the recently suggested IIIb, as further supported by electrophysiological studies on leak currents. Extracellular release of the third Na+ from IIIb through IIIa, followed by exchange of Na+ for K+ at sites I and II, is suggested.

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