NMR studies of the fifth transmembrane segment of Na+,K+-ATPase reveals a non-helical ion-binding region

The structure of a synthetic peptide corresponding to the fifth membrane-spanning segment (M5) in Na+,K+-ATPase in sodium dodecyl sulfate (SDS) micelles was determined using liquid-state nuclear magnetic resonance (NMR) spectroscopy. The spectra reveal that this peptide is substantially less α-helical than the corresponding M5 peptide of Ca2+-ATPase. A well-defined α-helix is shown in the C-terminal half of the peptide. Apart from a short helical stretch at the N-terminus, the N-terminal half contains a non-helical region with two proline residues and sequence similarity to a non-structured transmembrane element of the Ca2+-ATPase. Furthermore, this region spans the residues implicated in Na+ and K+ transport, where they are likely to offer the flexibility needed to coordinate Na+ as well as K+ during active transport.