Role of the β1 Subunit in Large-Conductance Ca ²⁺-Activated K ⁺ Channel Gating Energetics : Mechanisms of Enhanced Ca ²⁺ Sensitivity

Abstract

Over the past few years, it has become clear that an important mechanism by which large-conductance Ca ²⁺-activated K ⁺ channel (BK _Ca) 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 Ca ²⁺ 10-fold at 0 mV, and shifting the range of voltages over which the channel activates −80 mV at 9.1 μM Ca ²⁺. With this study, we address the question: which aspects of BK _Ca gating are altered by β1 to bring about these effects: Ca ²⁺ binding, voltage sensing, or the intrinsic energetics of channel opening? The approach we have taken is to express the β1 subunit together with the BK _Ca α subunit in Xenopus oocytes, and then to compare β1's steady state effects over a wide range of Ca ²⁺ 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 Ca ²⁺ when it is either open or closed. Interestingly, however, the small changes in Ca ²⁺ binding affinity suggested by our analysis (K _c 7.4 μM → 9.6 μM; K _o = 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 Ca ²⁺, shifting it +20 mV and reducing its apparent gating charge 38%, and we develop methods for distinguishing between alterations in Ca ²⁺ binding and other aspects of BK _Ca channel gating that may be of general use.