The RCK1 domain of the human BK_(Ca) channel transduces Ca^(2+) binding into structural rearrangements

Large-conductance voltage- and Ca^(2+)-activated K^+ (BK_(Ca)) channels play a fundamental role in cellular function by integrating information from their voltage and Ca2+ sensors to control membrane potential and Ca^(2+) homeostasis. The molecular mechanism of Ca^(2+)-dependent regulation of BKCa channels is unknown, but likely relies on the operation of two cytosolic domains, regulator of K^+ conductance (RCK)1 and RCK2. Using solution-based investigations, we demonstrate that the purified BK_(Ca) RCK1 domain adopts an α/β fold, binds Ca^(2+), and assembles into an octameric superstructure similar to prokaryotic RCK domains. Results from steady-state and time-resolved spectroscopy reveal Ca^(2+)-induced conformational changes in physiologically relevant [Ca^(2+)]. The neutralization of residues known to be involved in high-affinity Ca^(2+) sensing (D362 and D367) prevented Ca^(2+)-induced structural transitions in RCK1 but did not abolish Ca^(2+) binding. We provide evidence that the RCK1 domain is a high-affinity Ca^(2+) sensor that transduces Ca^(2+) binding into structural rearrangements, likely representing elementary steps in the Ca^(2+)-dependent activation of human BK_(Ca) channels