An allosteric modulator activates BK channels by perturbing coupling between Ca 2+ binding and pore opening.

BK type Ca 2+ -activated K + channels activate in response to both voltage and Ca 2+ . The membrane-spanning voltage sensor domain (VSD) activation and Ca 2+ binding to the cytosolic tail domain (CTD) open the pore across the membrane, but the mechanisms that couple VSD activation and Ca 2+ 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 2+ dependent activation mechanism. BC5 activates the channel in the absence of Ca 2+ binding but Ca 2+ binding inhibits BC5 effects. Thus, BC5 perturbs a pathway that couples Ca 2+ 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 2+ dependent activation and is an important site for allosteric agonists to modulate BK channel activation.