Transmembrane Segment XI of the Na(+)/H(+) Antiporter of S. pombe is a Critical Part of the Ion Translocation Pore

The Na(+)/H(+) exchanger of the plasma membrane of S. pombe (SpNHE1) removes intracellular sodium in exchange for an extracellular proton. We examined the structure and functional role of amino acids 360–393 of putative transmembrane (TM) segment XI of SpNHE1. Structural analysis suggested that it had a helical propensity over amino acids 360–368, an extended region from 369–378 and was helical over amino acids 379–386. TM XI was sensitive to side chain alterations. Mutation of eight amino acids to alanine resulted in loss of one or both of LiCl or NaCl tolerance when re-introduced into SpNHE1 deficient S. pombe. Mutation of seven other amino acids had minor effects. Analysis of structure and functional mutations suggested that Glu(361) may be involved in cation coordination on the cytoplasmic face of the protein with a negative charge in this position being important. His(367), Ile(371) and Gly(372) were important in function. Ile(371) may have important hydrophobic interactions with other residues and Gly(372) may be important in maintaining an extended conformation. Several residues from Val(377) to Leu(384) are important in function possibly involved in hydrophobic interactions with other amino acids. We suggest that TM XI forms part of the ion translocation core of this Na(+)/H(+) exchanger.