Calcium-gated potassium channel. Homotetramer. The channel is composed of 4 repeated units, each containing a transmembrane pore part and a gating ring part. The gating ring is composed of eight identical RCK (Regulators of K conductance) domains, in an alternating arrangement of one domain from each of the four subunits and four from the intracellular solution. Two protein interfaces between dimers of RCK domains from the pore-forming subunit and from the intracellular solution hold the ring together. One is called the fixed interface and the other the flexible interface. The flexible interface forms a cleft where calcium binds. Upon calcium binding the gating ring undergoes a conformational change that enables it to pull open the inner helices of the pore, allowing ion conduction. It is not known whether calcium is the physiological ligand. Inhibited by charybdotoxin (CTX), a protein from scorpion venom.