An allosteric modulator activates BK channels by perturbing coupling between Ca binding and pore opening.

BK type Ca-activated K channels activate in response to both voltage and Ca. The membrane-spanning voltage sensor domain (VSD) activation and Ca binding to the cytosolic tail domain (CTD) open the pore across the membrane, but the mechanisms that couple VSD activation and Ca binding to pore opening  are not clear. Here we show that a compound, BC5, identified from in silico screening, interacts with the CTD-VSD interface and specifically modulates the Ca dependent activation mechanism. BC5 activates the channel in the absence of Ca binding but Ca binding inhibits BC5 effects. Thus, BC5 perturbs a pathway that couples Ca binding to pore opening to allosterically affect both, which is further supported by atomistic simulations and mutagenesis. The results suggest that the CTD-VSD interaction makes a major contribution to the mechanism of Ca dependent activation and is an important site for allosteric agonists to modulate BK channel activation.