Auxiliary protein of the large-conductance, voltage and calcium-activated potassium channel (BK alpha). Modulates gating properties by producing a marked shift in the BK channel's voltage dependence of activation in the hyperpolarizing direction, and in the absence of calcium. Interacts with KCNMA1. Mainly expressed in brain. The transmembrane domain is necessary for interaction with KCNMA1. Extended N-terminus. Extended N-terminus. Extended N-terminus. Extended N-terminus.