Mechanism of β4 subunit modulation of BK channels

Large-conductance (BK-type) Ca2+-activated potassium channels are activated by membrane depolarization and cytoplasmic Ca2+. BK channels are expressed in a broad variety of cells and have a corresponding diversity in properties. Underlying much of the functional diversity is a family of four tissue-specific accessory subunits (beta 1-beta 4). Biophysical characterization has shown that the beta 4 subunit confers properties of the so-called type II BK channel isotypes seen in brain. These properties include slow gating kinetics and resistance to iberiotoxin and charybdotoxin blockade. In addition, the beta 4 subunit reduces the apparent voltage sensitivity of channel activation and has complex effects on apparent Ca2+ sensitivity. Specifically, channel activity at low Ca2+ is inhibited, while at high Ca2+, activity is enhanced. The goal of this study is to understand the mechanism underlying beta 4 subunit action in the context of a dual allosteric model for BK channel gating. We observed that beta 4's most profound effect is a decrease in P-o (at least 11-fold) in the absence of calcium binding and voltage sensor activation. However, beta 4 promotes channel opening by increasing voltage dependence of P-o-V relations at negative membrane potentials. In the context of the dual allosteric model for BK channels, we find these properties are explained by distinct and opposing actions of beta 4 on BK channels. beta 4 reduces channel opening by decreasing the intrinsic gating equilibrium (L-0), and decreasing the allosteric coupling between calcium binding and voltage sensor activation (E). However, beta 4 has a compensatory effect on channel opening following depolarization by shifting open channel voltage sensor activation (Vh(o)) to more negative membrane potentials. The consequence is that beta 4 causes a net positive shift of the G-V relationship (relative to a subunit alone) at low calcium. At higher calcium, the contribution by Vh(o) and an increase in allosteric coupling to Ca2+ binding (C) promotes a negative G-V shift of alpha+beta 4 channels as compared to a subunits alone. This manner of modulation predicts that type II BK channels are downregulated by beta 4 at resting voltages through effects on L-0. However, beta 4 confers a compensatory effect on voltage sensor activation that increases channel opening during depolarization.