β-2 and γ-1 Auxiliary Subunits Coassemble in the Same BK Channel and Independently Contribute to Regulation of Channel Gating

Abstract

Large conductance, Ca2+-and-voltage activated K+ (BK) channels are oligomeric proteins whose minimal functional unit is a homotetramer of the pore-forming α subunit. The α subunit is almost ubiquitously expressed in mammalian tissues, but native BK channel complexes can also contain associated auxiliary subunits which are tissue-specifically expressed. Members of two structurally distinct families of auxiliary subunits, β1- β4 and γ1-γ3, have each been shown to separately assemble with BK-α subunit. Each auxiliary subunit defines important gating properties of the BK channel complex as well as its pharmacology. However, the possibility of coassembly of the two types of auxiliary subunits in the same BK channel has not been closely examined. The only antecedent reports that overexpression of β1 subunit precludes the ability of γ1 to affect BK gating. Here, using single channel and macroscopic patch-clamp recordings we show that β2 and γ1 can coassemble in the same channel. When α, β2 and γ1 subunits are coexpressed at typical RNA ratios, the resulting BK-current shows a complete inactivation, indicating the presence of β2 in all channels, but also exhibits a gating shift consistent with the presence of γ1 in all channels. The effects produced by β2 and γ1 subunits on BK channel gating are approximately additive, consistent with independent mechanistic effects. Furthermore, coassembly of both subunits in the same channel produces a constitutive inactivation of BK current at physiological conditions even at 0 [Ca2+]. This possibility that β and γ subunits may also coassemble in the same BK channels in native tissues adds significantly to the potential functional diversity of BK channels.This work was supported by National Institutes of Health Grant GM-081748 (to C.J.L.) and National Research Service Award GM103138 (to V.G.-P.).