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Gating of acid-sensitive ion channel-1: release of Ca2+ block vs. allosteric mechanism.

ABSTRACT: The acid-sensitive ion channels (ASICs) are a family of voltage-insensitive sodium channels activated by external protons. A previous study proposed that the mechanism underlying activation of ASIC consists of the removal of a Ca2+ ion from the channel pore (Immke and McCleskey, 2003). In this work we have revisited this issue by examining single channel recordings of ASIC1 from toadfish (fASIC1). We demonstrate that increases in the concentration of external protons or decreases in the concentration of external Ca2+ activate fASIC1 by progressively opening more channels and by increasing the rate of channel opening. Both maneuvers produced similar effects in channel kinetics, consistent with the former notion that protons displace a Ca2+ ion from a high-affinity binding site. However, we did not observe any of the predictions expected from the release of an open-channel blocker: decrease in the amplitude of the unitary currents, shortening of the mean open time, or a constant delay for the first opening when the concentration of external Ca2+ was decreased. Together, the results favor changes in allosteric conformations rather than unblocking of the pore as the mechanism gating fASIC1. At high concentrations, Ca2+ has an additional effect that consists of voltage-dependent decrease in the amplitude of unitary currents (EC50 of 10 mM at -60 mV and pH 6.0). This phenomenon is consistent with voltage-dependent block of the pore but it occurs at concentrations much higher than those required for gating.

SUBMITTER: Zhang P

PROVIDER: S-EPMC2151491 | biostudies-literature | 2006 Feb

REPOSITORIES: biostudies-literature

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Gating of acid-sensitive ion channel-1: release of Ca2+ block vs. allosteric mechanism.

Zhang Ping P Sigworth Fred J FJ Canessa Cecilia M CM

The Journal of general physiology 20060117 2

The acid-sensitive ion channels (ASICs) are a family of voltage-insensitive sodium channels activated by external protons. A previous study proposed that the mechanism underlying activation of ASIC consists of the removal of a Ca2+ ion from the channel pore (Immke and McCleskey, 2003). In this work we have revisited this issue by examining single channel recordings of ASIC1 from toadfish (fASIC1). We demonstrate that increases in the concentration of external protons or decreases in the concentr ...[more]

PMID: 16418400

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Project description:The Ca2+-dependent gating mechanism of large-conductance calcium-activated K+ (BK) channels from cultured rat skeletal muscle was examined from low (4 microM) to high (1,024 microM) intracellular concentrations of calcium (Ca2+i) using single-channel recording. Open probability (Po) increased with increasing Ca2+i (K0. 5 11.2 +/- 0.3 microM at +30 mV, Hill coefficient of 3.5 +/- 0.3), reaching a maximum of approximately 0.97 for Ca2+i approximately 100 microM. Increasing Ca2+i further to 1,024 microM had little additional effect on either Po or the single-channel kinetics. The channels gated among at least three to four open and four to five closed states at high levels of Ca2+i (>100 microM), compared with three to four open and five to seven closed states at lower Ca2+i. The ability of kinetic schemes to account for the single-channel kinetics was examined with simultaneous maximum likelihood fitting of two-dimensional (2-D) dwell-time distributions obtained from low to high Ca2+i. Kinetic schemes drawn from the 10-state Monod-Wyman-Changeux model could not describe the dwell-time distributions from low to high Ca2+i. Kinetic schemes drawn from Eigen's general model for a ligand-activated tetrameric protein could approximate the dwell-time distributions but not the dependency (correlations) between adjacent intervals at high Ca2+i. However, models drawn from a general 50 state two-tiered scheme, in which there were 25 closed states on the upper tier and 25 open states on the lower tier, could approximate both the dwell-time distributions and the dependency from low to high Ca2+i. In the two-tiered model, the BK channel can open directly from each closed state, and a minimum of five open and five closed states are available for gating at any given Ca2+i. A model that assumed that the apparent Ca2+-binding steps can reach a maximum rate at high Ca2+i could also approximate the gating from low to high Ca2+i. The considered models can serve as working hypotheses for the gating of BK channels.

Project description:Orai1, the pore subunit of Ca(2+) release-activated Ca(2+) channels, has four transmembrane segments (TMs). The first segment, TMI, lines the pore and plays an important role in channel activation and ion permeation. TMIII, on the other hand, does not line the pore but still regulates channel gating and permeation properties. To understand the role of TMIII, we have mutated and characterized several residues in this domain. Mutation of Trp-176 to Cys (W176C) and Gly-183 to Ala (G183A) had dramatic effects. Unlike wild-type channels, which exhibit little outward current and are activated by STIM1, W176C mutant channels exhibited a large outward current at positive potentials and were constitutively active in the absence of STIM1. G183A mutant channels also exhibited substantial outward currents but were active only in the presence of 2-aminoethoxydiphenyl borate (2-APB), irrespective of STIM1. With W176C mutant channels inward, monovalent currents were blocked by Ca(2+) with a high affinity similar to the wild type, but the Ca(2+)-dependent blocking of outward currents differed in the two cases. Although a 50% block of the WT outward current required 250 ?m Ca(2+), more than 6 mm was necessary to have the same effect on W176C mutant channels. In the presence of extracellular Ca(2+), W176C and G183A outward currents developed slowly in a voltage-dependent manner, whereas they developed almost instantaneously in the absence of Ca(2+). These changes in permeation and gating properties mimic the changes induced by mutations of Glu-190 in TMIII and Asp-110/Asp-112 in the TMI/TMII loop. On the basis of these data, we propose that TMIII maintains negatively charged residues at or near the selectivity filter in a conformation that facilitates Ca(2+) inward currents and prevents outward currents of monovalent cations. In addition, to controlling selectivity, TMIII may also stabilize channel gating in a closed state in the absence of STIM1 in a Trp-176-dependent manner.

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Gating of acid-sensitive ion channel-1: release of Ca2+ block vs. allosteric mechanism.

Publications

Gating of acid-sensitive ion channel-1: release of Ca2+ block vs. allosteric mechanism.

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