Structural Basis for Calcium and Magnesium Regulation of a Large Conductance Calcium-Activated Potassium Channel with β1 Subunits

Large conductance Ca2+- and voltage-activated potassium (BK) channels, composed of pore-forming α subunits and auxiliary β subunits, play important roles in diverse physiological activities. The β1 is predominately expressed in smooth muscle cells, where it greatly enhances the Ca2+ sensitivity of BK channels for proper regulation of smooth muscle tone. However, the structural basis underlying dynamic interaction between BK mSlo1 α and β1 remains elusive. Using macroscopic ionic current recordings in various Ca2+ and Mg2+ concentrations, we identified two binding sites on the cytosolic N terminus of β1, namely the electrostatic enhancing site (mSlo1(K392,R393)-β1(E13,T14)), increasing the calcium sensitivity of BK channels, and the hydrophobic site (mSlo1(L906,L908)-β1(L5,V6,M7)), passing the physical force from the Ca2+ bowl onto the enhancing site and S6 C-linker. Dynamic binding of these sites affects the interaction between the cytosolic domain and voltage-sensing domain, leading to the reduction of Mg2+ sensitivity. A comprehensive structural model of the BK(mSlo1 α-β1) complex was reconstructed based on these functional studies, which provides structural and mechanistic insights for understanding BK gating.