Ion sensing in the RCK1 domain of BK channels

BK-type K+ channels are activated by voltage and intracellular Ca2+, which is important in modulating muscle contraction, neural transmission, and circadian pacemaker output. Previous studies suggest that the cytosolic domain of BK channels contains two different Ca2+ binding sites, but the molecular composition of one of the sites is not completely known. Here we report, by systematic mutagenesis studies, the identification of E535 as part of this Ca2+ binding site. This site is specific for binding to Ca2+ but not Cd2+. Experimental results and molecular modeling based on the X-ray crystallographic structures of the BK channel cytosolic domain suggest that the binding of Ca2+ by the side chains of E535 and the previously identified D367 changes the conformation around the binding site and turns the side chain of M513 into a hydrophobic core, providing a basis to understand how Ca2+ binding at this site opens the activation gate of the channel that is remotely located in the membrane.