The NH₂ Terminus of RCK1 Domain Regulates Ca²⁺-dependent BK_Ca Channel Gating

Large conductance, voltage- and Ca²⁺-activated K⁺ (BK_Ca) channels regulate blood vessel tone, synaptic transmission, and hearing owing to dual activation by membrane depolarization and intracellular Ca²⁺. Similar to an archeon Ca²⁺-activated K⁺ channel, MthK, each of four α subunits of BK_Ca may contain two cytosolic RCK domains and eight of which may form a gating ring. The structure of the MthK channel suggests that the RCK domains reorient with one another upon Ca²⁺ binding to change the gating ring conformation and open the activation gate. Here we report that the conformational changes of the NH₂ terminus of RCK1 (AC region) modulate BK_Ca gating. Such modulation depends on Ca²⁺ occupancy and activation states, but is not directly related to the Ca²⁺ binding sites. These results demonstrate that AC region is important in the allosteric coupling between Ca²⁺ binding and channel opening. Thus, the conformational changes of the AC region within each RCK domain is likely to be an important step in addition to the reorientation of RCK domains leading to the opening of the BK_Ca activation gate. Our observations are consistent with a mechanism for Ca²⁺-dependent activation of BK_Ca channels such that the AC region inhibits channel activation when the channel is at the closed state in the absence of Ca²⁺; Ca²⁺ binding and depolarization relieve this inhibition.