An extracellular domain of the accessory β1 subunit is required for modulating BK channel voltage sensor and gate

A family of tissue-specific auxiliary beta subunits modulates large conductance voltage- and calcium-activated potassium (BK) channel gating properties to suit their diverse functions. Paradoxically, beta subunits both promote BK channel activation through a stabilization of voltage sensor activation and reduce BK channel openings through an increased energetic barrier of the closed-to-open transition. The molecular determinants underlying beta subunit function, including the dual gating effects, remain unknown. In this study, we report the first identification of a beta 1 functional domain consisting of Y74, S104, Y105, and I106 residues located in the extracellular loop of beta 1. These amino acids reside within two regions of highest conservation among related beta 1, beta 2, and beta 4 subunits. Analysis in the context of the Horrigan-Aldrich gating model revealed that this domain functions to both promote voltage sensor activation and also reduce intrinsic gating. Free energy calculations suggest that the dual effects of the beta 1 Y74 and S104-I106 domains can be largely accounted for by a relative destabilization of channels in open states that have few voltage sensors activated. These results suggest a unique and novel mechanism for beta subunit modulation of voltage-gated potassium channels wherein interactions between extracellular beta subunit residues with the external portions of the gate and voltage sensor regulate channel opening.