Voltage- and Ca2+-Activation of BK Channels Studied with Highly Constrained Allosteric Gating Mechanisms

Abstract

BK channels are activated synergistically by depolarization and micromolar intracellular Ca2+. An allosteric gating scheme for a simplified BK channel with one voltage sensor and one Ca2+ site on each of the four subunits would lead to a minimal 50 state two-tiered model with 25 closed states (upper tier) and 25 open states (lower tier). It has been previously shown that allosteric models of this type with few constrained rate constants can approximate the gating of BK channels. We now explore to what extent such models with idealized gating (imposed by highly constrained rate constants) can account for the single-channel gating. Single-channel data were collected over wide ranges of voltage and Ca2+, and successive interval durations were measured and binned into 2-D dwell-time distributions. The idealized models were then globally fitted to the distributions using maximum likelihood methods to estimate the rate constants and allosteric gating parameters. An idealized model with independent voltage and Ca2+ sensors modulating the opening and closing rates could approximate the gating, but with some obvious differences between predicted and experimental data. The most likely parameters in this model indicated that each of the activated voltage and Ca2+ sensors increased the opening rates an additional ∼10-40 fold, with little effect on the closing rates. Adding a tier of flicker closed states and/or allowing specified cooperativity among and between the voltage and Ca2+ sensors improved the description of the data. Such highly constrained models provide a means to include the large numbers of states entered during gating of BK channels with multiple sensors per subunit, while limiting the number of gating parameters sufficiently to allow insight into gating mechanism. Supported by AHA grant 10POST4490012 and NIH grant AR32805.