Regulation of BK Channels by Beta and Gamma Subunits

Abstract

Ca²⁺- and voltage-gated K⁺ channels of large conductance (BK channels) are expressed in a diverse variety of both excitable and inexcitable cells, with functional properties presumably uniquely calibrated for the cells in which they are found. Although some diversity in BK channel function, localization, and regulation apparently arises from cell-specific alternative splice variants of the single pore-forming α subunit ( KCa1.1, Kcnma1, Slo1) gene, two families of regulatory subunits, β and γ, define BK channels that span a diverse range of functional properties. We are just beginning to unravel the cell-specific, physiological roles served by BK channels of different subunit composition.

Keywords: BK channels; Ca- and voltage-dependent K channels; auxiliary subunits; beta subunits; gamma subunits.

Figure 1

Two families of regulatory subunits generate BK channel functional diversity. (a) General topology of a single BK α subunit, showing the voltage-sensor domain (VSD) formed by transmembrane segments S0–S4, a pore-gate domain (PGD) arising from S5–S6 bracketing the selectivity filter, and a cytosolic domain (CTD) involving regulator of conductance for potassium ligand–sensing modules. Panel adapted from Reference under the terms of the Creative Commons Attribution 4.0 International License, http://creativecommons.org/licenses/by/4.0. (b) General transmembrane arrangement of β and γ subunits, with basic residues in red, acidic residues in blue, and extracellular β subunit cysteines in orange. Residues correspond to β3a and γ1. Model of γ1 extracellular structure follows an LRRC domain in a hagfish lymphocyte receptor (99). Panel adapted from Reference (left) and Reference (right). (c) BK channels are tetramers, with up to four β subunits per channel. Positions of β subunits were inferred from cross-linking experiments (164). Panel adapted from Reference . (d) Both β and γ subunits can assemble with 1–4 subunits per BK channel. Each β subunit in a BK channel incrementally shifts gating (bottom left), while a single γ1 subunit is sufficient to produce a gating shift similar to a set of four γ1 subunits. The red curve indicates equimolar α and either β or γ subunit. Panel adapted from Reference . (e–g) Idealized GV curves for (e) BK α-only subunits (32), (f ) β1-containing BK channels (33), and (g) γ1-containing BK channels (34), with a gray bar highlighting the range from −80 to +20 mV. (h) GV curvevs for various Kv channels encoded by distinct genes.

Figure 2

Functional signatures of various BK regulatory subunits. (a–f ) Activation at 0 (left) and 10 μM Ca²⁺ is shown for the indicated BK channel composition and voltage protocol (maximum voltage in each case indicated on the right-hand family of traces). Traces for β1 and β2 are on a slower time base. Note inward current for β1, β2, and γ1 indicative of leftward-gating shifts and the prominent and kinetically distinct inactivation for β2, β3a, and β3b. (g) Instantaneous tail currents are shown at +100, 0, and −100 mV after depolarization to +180 mV at 10 μM Ca²⁺, highlighting outward rectification of β2 and β3 variants. Both β2 and β3 constructs had their N-terminal inactivation domain removed. (h) Full instantaneous IV curves for different β subunits. (i) Normalized deactivation for different α + β currents following repolarization from peak current activation. Note slowing of deactivation by β1, β2, and β4 (all at 10 μM Ca²⁺) but not β3. ( j) Normalized deactivation following repolarization from steady-state current at +180 mV. Note absence of appreciable tail current for β2 and pronounced slowing with β3a, while β3b is similar to α alone. (k) Single-channel traces showing the absence of tail reopenings for β2-containing channels, but the unusual reduced current burst for β3a that accounts for its tail prolongation. (l) Steady-state conductance for β2 and β3b at 10 μM Ca²⁺, indicating that steady-state activity reflects strong inactivation for β2 channels and rapid voltage-dependent block for β3b. (m) Magnitude of outward current for β3a and β3b is dominated by inactivation, which can be readily removed by brief cytosolic trypsin application. Panels a, d, e, and j are modified from Reference ; panels c and f from Reference under the terms of the Creative Commons Attribution 4.0 International License, http://creativecommons.org/licenses/by/4.0; and panels g, i, and j from Reference .

Figure 3

Potential impact of subunit stoichiometry on BK pharmacology. (a) Model of toxin sensitivity in which channels lacking a β4 subunit have four potential toxin-binding orientations, while the addition of 1–4 β4 subunits reduces the available binding orientations, resulting in different forward rates of block and Kds. (b) Calculated toxin inhibition for channel populations, each of a given stoichiometry shown in panel a. (c) Calculated onset and recovery of inhibition by 10 nM of nominal toxin, showing that even channels (all of an identical stoichiometry) with up to three β4 subunits may exhibit appreciable inhibition. The single-site forward rate was assumed as 2 × 10⁶ M⁻¹s⁻¹, with a 0.02 s⁻¹ unblock rate. (d) Toxin inhibition for a population of channels with regulatory subunits distributed in a binomial fashion for β4 subunit mole fractions of 0 to 1 in steps of 0.1. (e) Calculated onset and recovery based on populations of channels with different mole fractions of β4 subunits.