Correlating structure, dynamics, and function in transmembrane segment VII of the Na⁺/H⁺ exchanger isoform 1

We place ¹⁵N nuclear magnetic resonance relaxation analysis and functional mutagenesis studies in the context of our previous structural and mutagenesis work to correlate structure, dynamics and function for the seventh transmembrane segment of the human Na⁺/H⁺ exchanger isoform 1. Although G261-S263 was previously identified as an interruption point in the helical structure of this isolated transmembrane peptide in dodecylphosphocholine micelles, and rapid conformational exchange was implicated in the NOE measurements, the six ¹⁵N labelled residues examined in this study all have similar dynamics on the ps-ns time scale. A mathematical model incorporating chemical exchange is the best fit for residues G261, L264, and A268. This implies that a segment of residues from G261 to A268 samples different conformations on the μs-ms time scale. Chemical exchange on an intermediate time scale is consistent with an alternating-access cycle where E262 is bent away from the cytosol during proton translocation by the exchanger. The functional importance of chemical exchange at G261-A268 is corroborated by the abrogated activity of the full-length exchanger with the bulky and restricting Ile substitutions F260I, G261I, E262I, S263I, and A268I.