PMID: 20843797
Tzeng J, Lee BL, Sykes BD, Fliegel L
Structural and functional analysis of transmembrane VI of the NHE1 isoform of the Na+/H+ exchanger.
J Biol Chem. 2010 Sep 15.
The Na+/H+ exchanger isoform 1 is a ubiquitously expressed integral membrane protein. It resides on the plasma membrane of cells and regulates intracellular pH in mammals by extruding an intracellular H+ in exchange for one extracellular Na+. We characterized structural and functional aspects of the transmembrane (TM) segment VI (residues 227-249) by using cysteine scanning mutagenesis and high resolution NMR. Each residue of TM VI was mutated to cysteine in the background of the cysteineless NHE1 protein and the sensitivity to water soluble sulfhydryl reactive compounds MTSET ([2- (Trimethylammonium) Ethyl]Methanethiosulfonate) and MTSES ([2-Sulfonatoethyl]Methanethiosulfonate) was determined for those residues with significant activity remaining. Three residues were essentially inactive when mutated to Cys, Asp238, Pro239 and Glu247. Of the remaining residues, proteins with the mutations N227C, I233C, and L243C were strongly inhibited by MTSET while amino acids Phe230, Gly231, Aal236, Val237, Ala244, Val245 and Glu248 were partially inhibited by MTSET. MTSES did not affect activity of the mutant NHE1 proteins. The structure of a peptide representing TM VI was determined using high resolution NMR spectroscopy in dodecylphosphocholine (DPC) micelles. TM VI contains two helical regions oriented at an approximate right angle to each other (residues 229-236 and 239-250) surrounding a central unwound region. This structure bares a resemblance to TM IV of the E. coli protein NhaA. The results demonstrate that TM VI of NHE1 is a discontinuous pore lining helix with residues Asn227, Ile233 and Leu243 lining the translocation pore.