Structural Basis of Voltage-Dependent Gating in BK Channels and Its Coupling to the Calcium Sensor

The allosteric communication between pore domain, voltage sensors, and Ca2+ binding sites in the Ca2+-activated K+ channel (BK) shapes its multiple physiological roles as the preeminent signal integrator in excitable systems. BK displays shallow voltage sensitivity with very fast gating charge kinetics, yet little is known about the molecular underpinnings of this distinctive behavior. Here, we explore the mechanistic basis of coupling between voltage-sensing domains (VSDs) and calcium sensors in Aplysia BK by locking the VSDs in their resting (R196Q and R199Q) and activated (R202Q) states, with or without calcium. Cryo-EM structures of these mutants reveal unique tilts at the S4 C-terminal end, together with large side-chain rotameric excursions of the gating charges. Importantly, the VSD resting structure (R202Q) also revealed BK in its elusive fully closed state, highlighting the reciprocal relation between calcium and voltage sensors. These structures provide a plausible mechanism where voltage and Ca2+ binding converge physically and couple energetically to define the conformation of the pore domain and thus, BK full functional range.