title: Identification of an amiloride binding domain within the alpha-subunit of the epithelial Na+ channel.
abstract: Limited information is available regarding domains within the epithelial Na+ channel (ENaC) which participate in amiloride binding. We previously utilized the anti-amiloride antibody (BA7.1) as a surrogate amiloride receptor to delineate amino acid residues that contact amiloride, and identified a putative amiloride binding domain WYRFHY (residues 278-283) within the extracellular domain of alpharENaC. Mutations were generated to examine the role of this sequence in amiloride binding. Functional analyses of wild type (wt) and mutant alpharENaCs were performed by cRNA expression in Xenopus oocytes and by reconstitution into planar lipid bilayers. Wild type alpharENaC was inhibited by amiloride with a Ki of 169 nM. Deletion of the entire WYRFHY tract (alpharENaC Delta278-283) resulted in a loss of sensitivity of the channel to submicromolar concentrations of amiloride (Ki = 26.5 microM). Similar results were obtained when either alpharENaC or alpharENaC Delta278-283 were co-expressed with wt beta- and gammarENaC (Ki values of 155 nM and 22.8 microM, respectively). Moreover, alpharENaC H282D was insensitive to submicromolar concentrations of amiloride (Ki = 6.52 microM), whereas alpharENaC H282R was inhibited by amiloride with a Ki of 29 nM. These mutations do not alter ENaC Na+:K+ selectivity nor single-channel conductance. These data suggest that residues within the tract WYRFHY participate in amiloride binding. Our results, in conjunction with recent studies demonstrating that mutations within the membrane-spanning domains of alpharENaC and mutations preceding the second membrane-spanning domains of alpha-, beta-, and gammarENaC alters amiloride's Ki, suggest that selected regions of the extracellular loop of alpharENaC may be in close proximity to residues within the channel pore.
authors.affiliation: Department of Physiology and Biophysics, University of Alabama at Birmingham, Birmingham, Alabama 35294, USA.
authors.forename: I I
authors.initials: II
authors.lastname: Ismailov
authors.forename: T
authors.initials: T
authors.lastname: Kieber-Emmons
authors.forename: C
authors.initials: C
authors.lastname: Lin
authors.forename: B K
authors.initials: BK
authors.lastname: Berdiev
authors.forename: V G
authors.initials: VG
authors.lastname: Shlyonsky
authors.forename: H K
authors.initials: HK
authors.lastname: Patton
authors.forename: C M
authors.initials: CM
authors.lastname: Fuller
authors.forename: R
authors.initials: R
authors.lastname: Worrell
authors.forename: J B
authors.initials: JB
authors.lastname: Zuckerman
authors.forename: W
authors.initials: W
authors.lastname: Sun
authors.forename: D C
authors.initials: DC
authors.lastname: Eaton
authors.forename: D J
authors.initials: DJ
authors.lastname: Benos
authors.forename: T R
authors.initials: TR
authors.lastname: Kleyman
mesh_terms: D000199:Actins / Q000494:pharmacology; D000584:Amiloride / Q000737:chemistry*; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D004553:Electric Conductivity; D004848:Epithelium; D006639:Histidine / Q000737:chemistry; D007158:Immunologic Techniques; D015640:Ion Channel Gating / Q000187:drug effects; D008051:Lipid Bilayers; D008564:Membrane Potentials; D009865:Oocytes; D018408:Patch-Clamp Techniques; D011994:Recombinant Proteins; D017384:Sequence Deletion; D026941:Sodium Channel Blockers; D015222:Sodium Channels / Q000737:chemistry*; D013329:Structure-Activity Relationship; D014982:Xenopus laevis
chemical_list: D000199:Actins; D008051:Lipid Bilayers; D011994:Recombinant Proteins; D026941:Sodium Channel Blockers; D015222:Sodium Channels; D006639:Histidine; D000584:Amiloride