β2 and β3a regulatory subunits can coassemble in the same BK channels

Abstract

Ca2+- and voltage-activated BK-type K+ channels are influenced profoundly by associated regulatory subunits, including {beta} subunits (Kcnmb1-4; {beta}1-{beta}4). Although overlap in expression of different BK {beta} subunits occurs in native tissues, whether they can coassemble in the same channel complex is not known. We coexpress {beta}2- and {beta}3a subunits together with BK and, through a combination of macroscopic and single channel recordings, along with quantitative pull-down of tagged subunits, test whether coassembly can occur. We evaluate two models: 1) random mixing in which {beta}2 and {beta}3a subunits co-assemble in the same channels, and 2) segregated in which {beta}2 and {beta}3a are found in separate complexes. Our results support the view that, for {beta}2 and {beta}3a, BK currents arise from the random, independent assembly of both subunits in the same channels. Single channel recordings directly confirm coassembly of {beta}2 and {beta}3a subunits in the same channels. Quantitative biochemical analysis of coexpression of tagged BK subunits also reveals that ternary complexes form.