title: Loop diuretic and ion-binding residues revealed by scanning mutagenesis of transmembrane helix 3 (TM3) of Na-K-Cl cotransporter (NKCC1).
abstract: The Na-K-Cl cotransporter (NKCC) plays central roles in cellular chloride homeostasis and in epithelial salt transport, but to date little is known about the mechanism by which the transporter moves ions across the membrane. We examined the functional role of transmembrane helix 3 (TM3) in NKCC1 using cysteine- and tryptophan-scanning mutagenesis and analyzed our results in the context of a structural homology model based on an alignment of NKCC1 with other amino acid polyamine organocation superfamily members, AdiC and ApcT. Mutations of residues along one face of TM3 (Tyr-383, Met-382, Ala-379, Asn-376, Ala-375, Phe-372, Gly-369, and Ile-368) had large effects on translocation rate, apparent ion affinities, and loop diuretic affinity, consistent with a proposed role of TM3 in the translocation pathway. The prediction that Met-382 is part of an extracellular gate that closes to form an occluded state is strongly supported by conformational sensitivity of this residue to 2-(trimethylammonium)ethyl methanethiosulfonate, and the bumetanide insensitivity of M382W is consistent with tryptophan blocking entry of bumetanide into the cavity. Substitution effects on residues at the intracellular end of TM3 suggest that this region is also involved in ion coordination and may be part of the translocation pathway in an inward-open conformation. Mutations of predicted pore residues had large effects on binding of bumetanide and furosemide, consistent with the hypothesis that loop diuretic drugs bind within the translocation cavity. The results presented here strongly support predictions of homology models of NKCC1 and demonstrate important roles for TM3 residues in ion translocation and loop diuretic inhibition.
authors.affiliation: Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520. Electronic address: suma.somasekharan@yale.edu.
authors.forename: Suma
authors.initials: S
authors.lastname: Somasekharan
authors.affiliation: Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520.
authors.forename: Jessica
authors.initials: J
authors.lastname: Tanis
authors.affiliation: Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520.
authors.forename: Biff
authors.initials: B
authors.lastname: Forbush
mesh_terms: D001665:Binding Sites; D002034:Bumetanide / Q000493:pharmacokinetics / Q000494:pharmacology*; D004232:Diuretics / Q000493:pharmacokinetics / Q000494:pharmacology*; D057809:HEK293 Cells; D006801:Humans; D017136:Ion Transport / Q000187:drug effects; D008698:Mesylates / Q000494:pharmacology; D016296:Mutagenesis; D020125:Mutation, Missense; D017433:Protein Structure, Secondary; D028021:Sodium-Potassium-Chloride Symporters / Q000235:genetics / Q000378:metabolism*; D064506:Solute Carrier Family 12, Member 2; D013439:Sulfhydryl Reagents / Q000494:pharmacology
chemical_list: D004232:Diuretics; D008698:Mesylates; C578877:SLC12A2 protein, human; D028021:Sodium-Potassium-Chloride Symporters; D064506:Solute Carrier Family 12, Member 2; D013439:Sulfhydryl Reagents; D002034:Bumetanide; C109154:(2-(trimethylammonium)ethyl)methanethiosulfonate