Cbv1 is sufficient to support PIP₂ action, which is amplified by β₁ (but not β₄) subunits. (A) BK channel dimer made of channel-forming (cbv1) and auxiliary β_(1–4) subunits. The RKK to AAA mutation in the cbv1 S6–S7 linker is shown in bold. (B) Unitary currents from an I/O patch expressing cbv1 in the absence (top) and presence (bottom) of PIP₂. Arrowheads, baseline; upward deflections, channel openings. (C) Averaged G-voltage macroscopic current data fitted to Boltzmann functions from wt cbv1, RKKcbv1AAA, and K239cbv1A in the absence and presence of PIP₂; the lipid causes a parallel leftward shift in wt and K239cbv1A but not in the RKKcbv1AAA mutant; n = 4–6. (D) PIP₂-induced increase in NP_o is significantly reduced in the RKKcbv1AAA mutant when compared with wt cbv1 or the K239cbv1A mutant; ***, P < 0.001; n = 4. (E). Unitary currents from I/O patches coexpressing cbv1+β₁ (top) or cbv1+β₄ subunits (bottom) in the absence and presence of PIP₂. Arrowheads, baseline; upward deflections, channel openings. (F) Averaged PIP₂ responses of cbv1, cbv1+β₁, and cbv1+β₄; n = 4–6. For B and E, arrows, baseline. For A, B, and D–F, V = 40 mV; Ca²⁺_i = 0.3 μM.