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

Ca2+- 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 coassemble in the same channels, and (2) segregation 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 β2, β3a, and BK α subunits also reveals that β2:β3a:α ternary complexes form.