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

Ca ^2+- and voltage-activated BK-type K ⁺ channels are influenced profoundly by associated regulatory subunits, including β subunits ( Kcnmb1-4 ; β1-β4). Although overlap in expression of different BK β subunits occurs in native tissues, whether they can coassemble in the same channel complex is not known. We coexpress β2- and β3a subunits together with BK α 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 β2 and β3a subunits co-assemble in the same channels, and 2) segregated in which β2 and β3a are found in separate complexes. Our results support the view that, for β2 and β3a, BK currents arise from the random, independent assembly of both subunits in the same channels. Single channel recordings directly confirm coassembly of β2 and β3a subunits in the same channels. Quantitative biochemical analysis of coexpression of tagged BK subunits also reveals that ternary complexes form.