title: Structural and functional characterization of transmembrane segment VII of the Na+/H+ exchanger isoform 1.
abstract: The Na(+)/H(+) exchanger isoform 1 is an integral membrane protein that regulates intracellular pH by exchanging one intracellular H(+) for one extracellular Na(+). It is composed of an N-terminal membrane domain of 12 transmembrane segments and an intracellular C-terminal regulatory domain. We characterized the structural and functional aspects of the critical transmembrane segment VII (TM VII, residues 251-273) by using alanine scanning mutagenesis and high resolution NMR. Each residue of TM VII was mutated to alanine, the full-length protein expressed, and its activity characterized. TM VII was sensitive to mutation. Mutations at 13 of 22 residues resulted in severely reduced activity, whereas other mutants exhibited varying degrees of decreases in activity. The impaired activities sometimes resulted from low expression and/or low surface targeting. Three of the alanine scanning mutant proteins displayed increased, and two displayed decreased resistance to the Na(+)/H(+) exchanger isoform 1 inhibitor EMD87580. The structure of a peptide of TM VII was determined by using high resolution NMR in dodecylphosphocholine micelles. TM VII is predominantly alpha-helical, with a break in the helix at the functionally critical residues Gly(261)-Glu(262). The relative positions and orientations of the N- and C-terminal helical segments are seen to vary about this extended segment in the ensemble of NMR structures. Our results show that TM VII is a critical transmembrane segment structured as an interrupted helix, with several residues that are essential to both protein function and sensitivity to inhibition.
authors.affiliation: Department of Biochemistry and Protein Engineering Network of Centres of Excellence, University of Alberta, Edmonton, Alberta T6G 2H7, Canada.
authors.forename: Jie
authors.initials: J
authors.lastname: Ding
authors.forename: Jan K
authors.initials: JK
authors.lastname: Rainey
authors.forename: Caroline
authors.initials: C
authors.lastname: Xu
authors.forename: Brian D
authors.initials: BD
authors.lastname: Sykes
authors.forename: Larry
authors.initials: L
authors.lastname: Fliegel
mesh_terms: D000409:Alanine / Q000235:genetics; D000595:Amino Acid Sequence; D027682:Cation Transport Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000235:genetics / Q000502:physiology*; D002460:Cell Line; D018360:Crystallography, X-Ray; D006801:Humans; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000235:genetics / Q000502:physiology*; D020033:Protein Isoforms / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000502:physiology; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary / Q000235:genetics; D000074082:Sodium-Hydrogen Exchanger 1; D017923:Sodium-Hydrogen Exchangers / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000235:genetics / Q000502:physiology*; D013329:Structure-Activity Relationship
chemical_list: D027682:Cation Transport Proteins; D010446:Peptide Fragments; D020033:Protein Isoforms; C490998:SLC9A1 protein, human; D000074082:Sodium-Hydrogen Exchanger 1; D017923:Sodium-Hydrogen Exchangers; D000409:Alanine