Intra- and Intersubunit Cooperativity in Activation of BK Channels by Ca²⁺

The activation of BK channels by Ca²⁺ is highly cooperative, with small changes in intracellular Ca²⁺ concentration having large effects on open probability (Po). Here we examine the mechanism of cooperative activation of BK channels by Ca²⁺. Each of the four subunits of BK channels has a large intracellular COOH terminus with two different high-affinity Ca²⁺ sensors: an RCK1 sensor (D362/D367) located on the RCK1 (regulator of conductance of K⁺) domain and a Ca-bowl sensor located on or after the RCK2 domain. To determine interactions among these Ca²⁺ sensors, we examine channels with eight different configurations of functional high-affinity Ca²⁺ sensors on the four subunits. We find that the RCK1 sensor and Ca bowl contribute about equally to Ca²⁺ activation of the channel when there is only one high-affinity Ca²⁺ sensor per subunit. We also find that an RCK1 sensor and a Ca bowl on the same subunit are much more effective in increasing Po than when they are on different subunits, indicating positive intrasubunit cooperativity. If it is assumed that BK channels have a gating ring similar to MthK channels with alternating RCK1 and RCK2 domains and that the Ca²⁺ sensors act at the flexible (rather than fixed) interfaces between RCK domains, then a comparison of the distribution of Ca²⁺ sensors with the observed responses suggest that the interface between RCK1 and RCK2 domains on the same subunit is flexible. On this basis, intrasubunit cooperativity arises because two high-affinity Ca²⁺ sensors acting across a flexible interface are more effective in opening the channel than when acting at separate interfaces. An allosteric model incorporating intrasubunit cooperativity nested within intersubunit cooperativity could approximate the Po vs. Ca²⁺ response for eight possible subunit configurations of the high-affinity Ca²⁺ sensors as well as for three additional configurations from a previous study.