Over the past few years, it has become clear that an important mechanism by which large-conductance Ca2+-activated K+ channel (BKCa) activity is regulated is the tissue-specific expression of auxiliary β subunits. The first of these to be identified, β1, is expressed predominately in smooth muscle and causes dramatic effects, increasing the apparent affinity of the channel for Ca2+ 10-fold at 0 mV, and shifting the range of voltages over which the channel activates −80 mV at 9.1 μM Ca2+. With this study, we address the question: which aspects of BKCa gating are altered by β1 to bring about these effects: Ca2+ binding, voltage sensing, or the intrinsic energetics of channel opening? The approach we have taken is to express the β1 subunit together with the BKCa α subunit in Xenopus oocytes, and then to compare β1's steady state effects over a wide range of Ca2+ concentrations and membrane voltages to those predicted by allosteric models whose parameters have been altered to mimic changes in the aspects of gating listed above. The results of our analysis suggest that much of β1's steady state effects can be accounted for by a reduction in the intrinsic energy the channel must overcome to open and a decrease in its voltage sensitivity, with little change in the affinity of the channel for Ca2+ when it is either open or closed. Interestingly, however, the small changes in Ca2+ binding affinity suggested by our analysis (Kc 7.4 μM → 9.6 μM; Ko = 0.80 μM → 0.65 μM) do appear to be functionally important. We also show that β1 affects the mSlo conductance–voltage relation in the essential absence of Ca2+, shifting it +20 mV and reducing its apparent gating charge 38%, and we develop methods for distinguishing between alterations in Ca2+ binding and other aspects of BKCa channel gating that may be of general use.
Role of the β1 Subunit in Large-Conductance Ca2+-Activated K+ Channel Gating Energetics: Mechanisms of Enhanced Ca2+ Sensitivity
Abbreviations used in this paper: BKCa, large-conductance Ca2+-activated K+ channel; MWC, Monod-Wyman-Changeux; VD-MWC, voltage-dependent MWC.
The parameter L(0) is defined differently in Schemes I and III (see Fig. 3 and Fig. 10 B, respectively). In Scheme I, L(0) represents the equilibrium constant between open and closed at 0 mV in the absence of [Ca2+]; whereas, in Scheme III, L(0) represents the equilibrium constant between open and closed at 0 mV in the absence of [Ca2+] if no voltage sensors are active. For Scheme III, the open-to-closed equilibrium constant in the absence of [Ca2+] at 0 mV is given by:
which thus depends on Z, Vhc, Vho, and L(0).
D.H. Cox, R.W. Aldrich; Role of the β1 Subunit in Large-Conductance Ca2+-Activated K+ Channel Gating Energetics: Mechanisms of Enhanced Ca2+ Sensitivity. J Gen Physiol 1 September 2000; 116 (3): 411–432. doi: https://doi.org/10.1085/jgp.116.3.411
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