Point mutations in alpha bENaC regulate channel gating, ion selectivity, and sensitivity to amiloride

alpha-ENaC is a functional element of the hypertonicity-induced cation channel in HepG2 cells and it mediates proliferation

The molecular correlate of hypertonicity-induced cation channels (HICCs) and their role in proliferation vs. apoptosis is a matter of debate. We report in this paper that, in whole-cell patch-clamp recordings, hypertonic stress (340→450 mosM) reversibly increased the Na⁺ conductance of HepG2 cells from 0.8 to 5.8 nS. The effect was dose-dependently inhibited by flufenamate and amiloride, known blockers of HICCs, with some 50% efficiency at 300 μM. In parallel, both drugs decreased HepG2 cell proliferation [in 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays and with automatic cell counting]. Small interfering RNA (siRNA) silencing of the α-subunit of the epithelial Na⁺ channel (ENaC) reduced hypertonicity-induced Na⁺ currents to 60%, whereas the rate of HepG2 cell proliferation was approximately half of that of the control. Moreover, α-ENaC siRNA inhibited the regulatory volume increase of HepG2 cells (measured with scanning acoustic microscopy) by 60%. In florescence-activated cell sorting measurements, silencing of α-ENaC led to a significant decrease in the G1 and an increase in the G2/M phase of the cell cycle, whereas the S phase was not changing. Finally (determined by a caspase 3/7 assay), HICC inhibition by flufenamate and silencing of α-ENaC increased the rate of apoptosis in HepG2 cells. It is concluded that α-ENaC is one functional element of the HICC in HepG2 cells and that the channel is an important mediator of cell proliferation; likewise, HICC blockage shifts the system from a proliferative into a rather apoptotic one. This is the first report of a role of α-ENaC in cell proliferation.

TRPML3 mutations cause impaired mechano-electrical transduction and depolarization by an inward-rectifier cation current in auditory hair cells of varitint-waddler mice

TRPML3 (mucolipin-3) belongs to one of the transient-receptor-potential (TRP) ion channel families. Mutations in the Trpml3 gene cause disorganization of the stereociliary hair bundle, structural aberrations in outer and inner hair cells and stria vascularis defects, leading to deafness in the varitint-waddler (Va) mouse. Here we refined the stereociliary localization of TRPML3 and investigated cochlear hair cell function in varitint-waddler (Va(J)) mice carrying the TRPML3<I362T/A419P> mutations. Using a TRPML3-specific antibody we detected a approximately 68 kDa protein with near-equal expression levels in cochlea and vestibule of wild-type and Va(J) mutants. At postnatal days 3 and 5, we observed abundant localization of TRPML3 at the base of stereocilia near the position of the ankle links. This stereociliary localization domain was absent in Va(J) heterozygotes and homozygotes. Electrophysiological recordings revealed reduced mechano-electrical transducer currents in hair cells from Va(J)/+ and Va(J)/Va(J) mice. Furthermore, FM1-43 uptake and [(3)H]gentamicin accumulation were decreased in hair cells in cultured organs of Corti from Va(J)/+ and Va(J)/Va(J) mice. We propose that TRPML3 plays a critical role at the ankle-link region during hair-bundle growth and that an adverse effect of mutant TRPML3 on bundle development and mechano-electrical transduction is the main cause of hearing loss in Va(J)/+ mutant mice. Outer hair cells of Va(J)/Va(J) mice additionally had depolarized resting potentials due to an inwardly rectifying leak conductance formed by the mutant channels, leading over time to hair-cell degeneration and contributing to their deafness. Our findings argue against TRPML3 being a component of the hair-cell transducer channel.

Drug–protein interaction with Vpu from HIV-1: proposing binding sites for amiloride and one of its derivatives

Vpu is an 81-amino-acid auxiliary protein of the genome of HIV-1. It is proposed that one of its roles is to enhance particle release by self-assembling to form water-filled channels enabling the flux of ions at the site of the plasma membrane of the infected cell. Hexamethylene amiloride has been shown to block Vpu channel activity when the protein is reconstituted into lipid bilayers. In a docking approach with monomeric, pentameric and hexameric bundle models of Vpu corresponding to the transmembrane part of the protein, a putative binding site of hexamethylene amiloride is proposed and is compared with the site for the nonpotent amiloride. The binding mode for both ligands is achieved by optimizing hydrogen bond interactions with serines. Binding energies and binding constants are the lowest for protonated hexamethylene amiloride in the pentameric bundle.

Characterization of an amiloride binding region in the alpha-subunit of ENaC

One of the defining characteristics of the epithelial sodium channel (ENaC) is its block by the diuretic amiloride. This study investigates the role of the extracellular loop of the alpha-subunit of ENaC in amiloride binding and stabilization. Mutations were generated in a region of the extracellular loop, residues 278-283. Deletion of this region, WYRFHY, resulted in a loss of amiloride binding to the channel. Channels formed from wild-type alpha-subunits or alpha-subunits containing point mutations in this region were examined and compared at the single-channel level. The open probabilities (Po) of wild-type channels were distributed into two populations: one with a high Po and one with a low Po. The mean open times of all the mutant channels were shorter than the mean open time of the wild-type (high-Po) channel. Besides mutations Y279A and H282D, which had amiloride binding affinities similar to that of wild-type alpha-ENaC, all other mutations in this region caused changes in the amiloride binding affinity of the channels compared with the wild-type channel. These data provide new insight into the relative position of the extracellular loop with respect to the pore of ENaC and its role in amiloride binding and channel gating.

Multiple epithelial Na+ channel domains participate in subunit assembly

Epithelial sodium channels (ENaCs) are composed of three structurally related subunits that form a tetrameric channel. The Xenopus laevis oocyte expression system was used to identify regions within the ENaC alpha-subunit that confer a dominant negative phenotype on functional expression of alphabetagamma-ENaC to define domains that have a role in subunit-subunit interactions. Coexpression of full-length mouse alphabetagamma-ENaC with either 1) the alpha-subunit first membrane-spanning domain and short downstream hydrophobic domain (alpha-M1H1); 2) alpha-M1H1 and its downstream hydrophilic extracellular loop (alpha-M1H1-ECL); 3) the membrane-spanning domain of a control type 2 transmembrane protein (glutamyl transpeptidase; gamma-GT) fused to the alpha-ECL (gamma-GT-alpha-ECL); 4) the extracellular domain of a control type 1 transmembrane protein (Tac) fused to the alpha-subunit second membrane-spanning domain and short upstream hydrophobic domain (Tac-alpha-H2M2); or 5) the alpha-subunit cytoplasmic COOH terminus (alpha-Ct) significantly reduced amiloride-sensitive Na+ currents in X. laevis oocytes. Functional expression of Na+ channels was not inhibited when full-length alphabetagamma-ENaC was coexpressed with either 1) the alpha-ECL lacking a signal-anchor sequence, 2) alpha-M1H1 and alpha-Ct expressed as a fusion protein, 3) full-length gamma-GT, or 4) full-length Tac. Furthermore, the expression of ROMK channels was not inhibited when full-length ROMK was coexpressed with either alpha-M1H1-ECL or alpha-Ct. Full-length FLAG-tagged alpha-, beta-, or gamma-ENaC coimmunoprecipitated with myc-tagged alpha-M1H1-ECL, whereas wild-type gamma-GT did not. These data suggest that multiple sites within the alpha-subunit participate in subunit-subunit interactions that are required for proper assembly of the heterooligomeric ENaC complex.

External nickel inhibits epithelial sodium channel by binding to histidine residues within the extracellular domains of alpha and gamma subunits and reducing channel open probability

Epithelial sodium channels (ENaC) are regulated by various intracellular and extracellular factors including divalent cations. We studied the inhibitory effect and mechanism of external Ni(2+) on cloned mouse alpha-beta-gamma ENaC expressed in Xenopus oocytes. Ni(2+) reduced amiloride-sensitive Na(+) currents of the wild type mouse ENaC in a dose-dependent manner. The Ni(2+) block was fast and partially reversible at low concentrations and irreversible at high concentrations. ENaC inhibition by Ni(2+) was accompanied by moderate inward rectification at concentrations higher than 0.1 mm. ENaC currents were also blocked by the histidine-reactive reagent diethyl pyrocarbonate. Pretreatment of the oocytes with the reagent reduced Ni(2+) inhibition of the remaining current. Mutations at alphaHis(282) and gammaHis(239) located within the extracellular loops significantly decreased Ni(2+) inhibition of ENaC currents. The mutation alphaH282D or double mutations alphaH282R/gammaH239R eliminated Ni(2+) block. All mutations at gammaHis(239) eliminated Ni(2+)-induced inward current rectification. Ni(2+) block was significantly enhanced by introduction of a histidine at alphaArg(280). Lowering extracellular pH to 5.5 and 4.4 decreased or eliminated Ni(2+) block. Although alphaH282C-beta-gamma channels were partially inhibited by the sulfhydryl-reactive reagent [2-(trimethylammonium)ethyl] methanethiosulfonate bromide (MTSET), alpha-beta-gamma H239C channels were insensitive to MTSET. From patch clamp studies, Ni(2+) did not affect unitary current but decreased open probability when perfused into the recording pipette. Our results suggest that external Ni(2+) reduces ENaC open probability by binding to a site consisting of alphaHis(282) and gammaHis(239) and that these histidine residues may participate in ENaC gating.

Epithelial sodium channel/degenerin family of ion channels: a variety of functions for a shared structure

The recently discovered epithelial sodium channel (ENaC)/degenerin (DEG) gene family encodes sodium channels involved in various cell functions in metazoans. Subfamilies found in invertebrates or mammals are functionally distinct. The degenerins in Caenorhabditis elegans participate in mechanotransduction in neuronal cells, FaNaC in snails is a ligand-gated channel activated by neuropeptides, and the Drosophila subfamily is expressed in gonads and neurons. In mammals, ENaC mediates Na+ transport in epithelia and is essential for sodium homeostasis. The ASIC genes encode proton-gated cation channels in both the central and peripheral nervous system that could be involved in pain transduction. This review summarizes the physiological roles of the different channels belonging to this family, their biophysical and pharmacological characteristics, and the emerging knowledge of their molecular structure. Although functionally different, the ENaC/DEG family members share functional domains that are involved in the control of channel activity and in the formation of the pore. The functional heterogeneity among the members of the ENaC/DEG channel family provides a unique opportunity to address the molecular basis of basic channel functions such as activation by ligands, mechanotransduction, ionic selectivity, or block by pharmacological ligands.

Second transmembrane domains of ENaC subunits contribute to ion permeation and selectivity

Epithelial sodium channels (ENaC) are composed of three structurally related subunits (alpha, beta, and gamma). Each subunit has two transmembrane domains termed M1 and M2, and residues conferring cation selectivity have been shown to reside in a pore region immediately preceding the M2 domains of the three subunits. Negatively charged residues are interspersed within the M2 domains, and substitution of individual acidic residues within human alpha-ENaC with arginine essentially eliminated channel activity in oocytes, suggesting that these residues have a role in ion permeation. We examined the roles of M2 residues in contributing to the permeation pore by individually mutating residues within the M2 domain of mouse alphaENaC to cysteine and systematically characterizing functional properties of mutant channels expressed in Xenopus oocytes by two-electrode voltage clamp. The introduction of cysteine residues at selected sites, including negatively charged residues (alphaGlu(595), alphaGlu(598), and alphaAsp(602)) led to a significant reduction of expressed amiloride-sensitive Na(+) currents. Two mutations (alphaE595C and alphaD602C) resulted in K(+)-permeable channels whereas multiple mutations altered Li(+)/Na(+) current ratios. Channels containing alphaD602K or alphaD602A also conducted K(+) whereas more conservative mutations (alphaD602E and alphaD602N) retained wild type selectivity. Cysteine substitution at the site equivalent to alphaAsp(602) within beta mENaC (betaD544C) did not alter either Li(+)/Na(+) or K(+)/Na(+) current ratios, although mutation of the equivalent site within gamma mENaC (gammaD562C) significantly increased the Li(+)/Na(+) current ratio. Mutants containing introduced cysteine residues at alphaGlu(595), alphaGlu(598), alphaAsp(602), or alphaThr(607) did not respond to externally applied sulfhydryl reagent with significant changes in macroscopic currents. Our results suggest that some residues within the M2 domain of alphaENaC contribute to the channel's conduction pore and that, in addition to the pore region, selected sites within M2 (alphaGlu(595) and alphaAsp(602)) may have a role in conferring ion selectivity.

Regions in the carboxy terminus of alpha-bENaC involved in gating and functional effects of actin

Gating differences occur between the alpha-subunits of the bovine and rat clones of an amiloride-sensitive epithelial Na+ channel (ENaC). Deletion of the carboxy terminus of bovine alpha-ENaC (alpha-bENaC) at R567 converted the gating properties to that of rat alpha-ENaC (alpha-rENaC). The equivalent truncation in alpha-rENaC was without effect on the gating of the rat homologue. The addition of actin to ENaC channels composed of either alpha-rENaC or alpha-bENaC alone produced a twofold reduction in conductance and an increase in open probability. Neither alpha-rENaC (R613X) nor alpha-bENaC (R567X) was responsive to actin. Using a chimera consisting of alpha-rENaC1-615 and alpha-bENaC570-650, we examined several different carboxy-terminal truncation mutants plus and minus actin. When incorporated into planar bilayers, the gating pattern of this construct was identical to wild-type (wt) alpha-bENaC. Premature stop mutations proximal to E685X produced channels with gating patterns like alpha-rENaC. Actin had no effect on the E631X truncation, whereas more distal truncations all interacted with actin, as did wt alpha-bENaC. Key findings were confirmed using channels expressed in Xenopus oocytes and studied by cell-attached patch-clamp recording. Our results suggest that the site of actin regulation at the carboxy terminus of the chimera is located between residues 631 and 644.

Characterization of the selectivity filter of the epithelial sodium channel

The epithelial sodium channel (ENaC) is composed of three homologous subunits termed alpha, beta, and gamma. Previous studies suggest that selected residues within a hydrophobic region immediately preceding the second membrane-spanning domain of each subunit contribute to the conducting pore of ENaC. We probed the pore of mouse ENaC by systematically mutating all 24 amino acids within this putative pore region of the alpha-subunit to cysteine and co-expressing these mutants with wild type beta- and gamma-subunits of mouse ENaC in Xenopus laevis oocytes. Functional characteristics of these mutants were examined by two-electrode voltage clamp and single channel recording techniques. Two distinct domains were identified based on the functional changes associated with point mutations. An amino-terminal domain (alpha-Val(569)-alpha-Gly(579)) showed minimal changes in cation selectivity or amiloride sensitivity following cysteine substitution. In contrast, cysteine substitutions within the carboxyl-terminal domain (alpha-Ser(580)-alpha-Ser(592)) resulted in significant changes in cation selectivity and moderately altered amiloride sensitivity. The mutant channels containing alphaG587C or alphaS589C were permeable to K(+), and mutation of a GSS tract (positions alpha587-alpha589) to GYG resulted in a moderately K(+)-selective channel. Our results suggest that the C-terminal portion of the pore region within the alpha-subunit contributes to the selectivity filter of ENaC.

Peptide inhibition of constitutively activated epithelial Na(+) channels expressed in Xenopus oocytes

The hypothesis that 30-amino acid peptides corresponding to the C-terminal portion of the beta- and/or gamma-rat epithelial sodium channel (rENaC) subunits block constitutively activated ENaC was tested by examining the effects of these peptides on wild-type (wt) rENaC (alphabetagamma-rENaC), truncated Liddle's mutants (alphabeta(T)gamma-, alphabetagamma(T)-, and alphabeta(T)gamma(T)-rENaC), and point mutants (alphabeta(Y)gamma-, alphabetagamma(Y)-rENaC) expressed in Xenopus oocytes. The chord conductances of alphabeta(T)gamma-, alphabetagamma(T)-, and alphabeta(T)gamma(T)-rENaC were 2- or 3-fold greater than for wt alphabetagamma-rENaC. Introduction of peptides into oocytes expressing alphabeta(T)gamma-, alphabetagamma(T)-, and alphabeta(T)gamma(T)-rENaC produced a concentration-dependent inhibition of the amiloride-sensitive Na(+) conductances, with apparent dissociation constants (K(d)) ranging from 1700 to 160 microM, depending upon whether individual peptides or their combination was used. Injection of peptides alone or in combination into oocytes expressing wt alphabetagamma-rENaC or single-point mutants did not affect the amiloride-sensitive whole-cell currents. The single channel conductances of all the mutant ENaCs were the same as that of wild type (alphabetagamma-). The single channel activities (N.P(o)) of the mutants were approximately 2.2-2.6-fold greater than wt alphabetagamma-rENaC (1.08 +/- 0.24, n = 7) and were reduced to 1.09 +/- 0.17 by 100 microM peptide mixture (n = 9). The peptides were without effect on the single channel properties of either wt or single-point mutants of rENaC. Our data demonstrate that the C-terminal peptides blocked the Liddle's truncation mutant (alphabeta(T)gamma(T)) expressed in Xenopus oocytes but not the single-point mutants (alphabeta(Y)gamma or alphabetagamma(Y)). Moreover, the blocking effect of both peptides in combination on alphabeta(T)gamma(T)-rENaC was synergistic.

The second hydrophobic domain contributes to the kinetic properties of epithelial sodium channels

The epithelial sodium channel (ENaC) is the prototype of a new class of ion channels known as the ENaC/Deg family. The hallmarks of ENaC are a high selectivity for Na(+), block by amiloride, small conductance, and slow kinetics that are voltage-independent. We have investigated the contribution of the second hydrophobic domain of each of the homologous subunits alpha, beta, and gamma to the kinetic properties of ENaC. Chimeric subunits were constructed between alpha and beta subunits (alpha-beta) and between gamma and beta subunits (gamma-beta). Chimeric and wild-type subunits were expressed in various combinations in Xenopus oocytes. Analysis of whole-cell and unitary currents made it possible to correlate functional properties with specific sequences in the subunits. Functional channels were generated without the second transmembrane domain from alpha subunits, indicating that it is not essential to form functional pores. The open probability and kinetics varied with the different channels and were influenced by the second hydrophobic domains. Amiloride affinity, Li(+)/Na(+) selectivity, and single channel conductance were also affected by this segment.

Functional domains within the degenerin/epithelial sodium channel (Deg/ENaC) superfamily of ion channels

Application of recombinant DNA technology and electrophysiology to the study of amiloride-sensitive Na+ channels has resulted in an enormous increase in the understanding of the structure-function relationships of these channels. Moreover, this knowledge has permitted the elucidation of the physiological roles of these ion channels in cellular processes as diverse as transepithelial salt and water movement, taste perception, volume regulation, nociception, neuronal function, mechanosensation, and even defaecation. Although members of this ever-growing superfamily of ion channels (the Deg/ENaC superfamily) share little amino acid identity, they are all organized similarly, namely, two short N- and C-termini, two short membrane-spanning segments, and a very large extracellular loop domain. In this brief Topical Review, we discuss the structural features of each domain of this Deg/ENaC superfamily and, using ENaC as a model, show how each domain relates to overall channel function.

DEG/ENaC channels: a touchy superfamily that watches its salt

To the surprise of many, studies of molecular mechanisms of touch transduction and analyses of epithelial Na+ transport have converged to define a new class of ion channel subunits. Based on the names of the first two identified subfamilies, the Caenorhabditis elegans degenerins and the vertebrate epithelial amiloride-sensitive Na+ channel, this ion channel class is called the DEG/ENaC superfamily. Members of the DEG/ENaC superfamily have been found in nematodes, flies, snails, and vertebrates. Family members share common topology, such that they span the membrane twice and have intracellular N- and C-termini; a large extracellular loop includes a conserved cysteine-rich region. DEG/ENaC channels have been implicated a broad spectrum of cellular functions, including mechanosensation, proprioception, pain sensation, gametogenesis, and epithelial Na+ transport. These channels exhibit diverse gating properties, ranging from near constitutive opening to rapid inactivation. We discuss working understanding of DEG/ENaC functions, channel properties, structure/activity correlations and possible evolutionary relationship to other channel classes.

On the molecular basis of ion permeation in the epithelial Na+ channel

The epithelial Na+ channel (ENaC) is highly selective for Na+ and Li+ over K+ and is blocked by the diuretic amiloride. ENaC is a heterotetramer made of two alpha, one beta, and one gamma homologous subunits, each subunit comprising two transmembrane segments. Amino acid residues involved in binding of the pore blocker amiloride are located in the pre-M2 segment of beta and gamma subunits, which precedes the second putative transmembrane alpha helix (M2). A residue in the alpha subunit (alphaS589) at the NH2 terminus of M2 is critical for the molecular sieving properties of ENaC. ENaC is more permeable to Li+ than Na+ ions. The concentration of half-maximal unitary conductance is 38 mM for Na+ and 118 mM for Li+, a kinetic property that can account for the differences in Li+ and Na+ permeability. We show here that mutation of amino acid residues at homologous positions in the pre-M2 segment of alpha, beta, and gamma subunits (alphaG587, betaG529, gammaS541) decreases the Li+/Na+ selectivity by changing the apparent channel affinity for Li+ and Na+. Fitting single-channel data of the Li+ permeation to a discrete-state model including three barriers and two binding sites revealed that these mutations increased the energy needed for the translocation of Li+ from an outer ion binding site through the selectivity filter. Mutation of betaG529 to Ser, Cys, or Asp made ENaC partially permeable to K+ and larger ions, similar to the previously reported alphaS589 mutations. We conclude that the residues alphaG587 to alphaS589 and homologous residues in the beta and gamma subunits form the selectivity filter, which tightly accommodates Na+ and Li+ ions and excludes larger ions like K+.

Paradoxical stimulation of a DEG/ENaC channel by amiloride

Extracellular amiloride inhibits all known DEG/ENaC ion channels, including BNC1, a proton-activated human neuronal cation channel. Earlier studies showed that protons cause a conformational change that activates BNC1 and exposes residue 430 to the extracellular solution. Here we demonstrate that, in addition to blocking BNC1, amiloride also exposes residue 430. This result suggested that, like protons, amiloride might be capable of activating the channel. To test this hypothesis, we introduced a mutation in the BNC1 pore that reduces amiloride block, and found that amiloride stimulated these channels. Amiloride inhibition was voltage-dependent, suggesting block within the pore, whereas stimulation was not, suggesting binding to an extracellular site. These data show that amiloride can have two distinct effects on BNC1, and they suggest two different interaction sites. The results suggest that extracellular amiloride binding may have a stimulatory effect similar to that of protons in BNC1 or extracellular ligands in other DEG/ENaC channels.

Na+-driven flagellar motor resistant to phenamil, an amiloride analog, caused by mutations in putative channel components

The rotation of the Na+-driven flagellar motor is specifically and strongly inhibited by phenamil, an amiloride analog. Here, we provide the first evidence that phenamil interacts directly with the Na+-channel components (PomA and PomB) of the motor. The alterations in Mpar (motility resistant to phenamil) strains were mapped to the pomA and/or pomB genes. We cloned and sequenced pomA and pomB from two Mpar strains, NMB205 and NMB201, and found a substitution in pomA (Asp148 to Tyr; NMB205) and in pomB (Pro16 to Ser; NMB201). Both residues are predicted to be near the cytoplasmic ends of the putative transmembrane segments. Mutational analyses at PomA-Asp148 and PomB-Pro16 suggest that a certain structural change around these residues affects the sensitivity of the motor to phenamil. Co-expression of the PomA D148Y and PomB P16S proteins resulted in an Mpar phenotype which seemed to be less sensitive to phenamil than either of the single mutants, although motility was more severely impaired in the absence of inhibitors. These results support the idea that PomA and PomB interact with each other and suggest that multiple residues, including Asp148 of PomA and Pro16 of PomB, constitute a high-affinity phenamil-binding site at the inner face of the PomA/PomB channel complex.

Physical origin of selectivity in ionic channels of biological membranes

This paper shows that the selectivity properties of monovalent cation channels found in biological membranes can originate simply from geometrical properties of the inner core of the channel without any critical contribution from electrostatic interactions between the permeating ions and charged or polar groups. By using well-known techniques of statistical mechanics, such as the Langevin equations and Kramer theory of reaction rates, a theoretical equation is provided relating the permeability ratio PB/PA between ions A and B to simple physical properties, such as channel geometry, thermodynamics of ion hydration, and electrostatic interactions between the ion and charged (or polar) groups. Diffusive corrections and recrossing rates are also considered and evaluated. It is shown that the selectivity found in usual K+, gramicidin, Na+, cyclic nucleotide gated, and end plate channels can be explained also in the absence of any charged or polar group. If these groups are present, they significantly change the permeability ratio only if the ion at the selectivity filter is in van der Waals contact with them, otherwise these groups simply affect the channel conductance, lowering the free energy barrier of the same amount for the two ions, thus explaining why single channel conductance, as it is experimentally observed, can be very different in channels sharing the same selectivity sequence. The proposed theory also provides an estimate of channel minimum radius for K+, gramicidin, Na+, and cyclic nucleotide gated channels.

Identification of the pH sensor and activation by chemical modification of the ClC-2G Cl- channel

Rabbit and human ClC-2G Cl- channels are voltage sensitive and activated by protein kinase A and low extracellular pH. The objective of the present study was to investigate the mechanism involved in acid activation of the ClC-2G Cl- channel and to determine which amino acid residues play a role in this acid activation. Channel open probability (Po) at +/-80 mV holding potentials increased fourfold in a concentration-dependent manner with extracellular H+ concentration (that is, extracellular pH, pHtrans), with an apparent acidic dissociation constant of pH 4.95 +/- 0.27. 1-Ethyl-3(3-dimethylaminopropyl)carbodiimide-catalyzed amidation of the channel with glycine methyl ester increased Po threefold at pHtrans 7.4, at which the channel normally exhibits low Po. With extracellular pH reduction (protonation) or amidation, increased Po was due to a significant increase in open time constants and a significant decrease in closed time constants of the channel gating, and this effect was insensitive to applied voltage. With the use of site-directed mutagenesis, the extracellular region EELE (amino acids 416-419) was identified as the pH sensor and amino acid Glu-419 was found to play the key or predominant role in activation of the ClC-2G Cl- channel by extracellular acid.

Identification of an amiloride binding domain within the alpha-subunit of the epithelial Na+ channel

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.