Structural basis for the subtype-selectivity of KCa2.2 channel activators

Small-conductance (K _Ca 2.2) and intermediate-conductance (K _Ca 3.1) Ca ²⁺ -activated K ⁺ channels are gated by a Ca ²⁺ -calmodulin dependent mechanism. NS309 potentiates the activity of both K _Ca 2.2 and K _Ca 3.1, while rimtuzalcap selectively activates K _Ca 2.2. Rimtuzalcap has been used in clinical trials for the treatment of spinocerebellar ataxia and essential tremor. We report cryo-electron microscopy structures of K _Ca 2.2 channels bound with NS309 and rimtuzalcap, in addition to K _Ca 3.1 channels with NS309. The different conformations of calmodulin and the cytoplasmic HC helices in the two channels underlie the subtype-selectivity of rimtuzalcap for K _Ca 2.2. Calmodulin’s N-lobes in the K _Ca 2.2 structure are far apart and undergo conformational changes to accommodate either NS309 or rimtuzalcap. Calmodulin’s N-lobes in the K _Ca 3.1 structure are closer to each other and are constrained by the HC helices of K _Ca 3.1, which allows binding of NS309 but not of the bulkier rimtuzalcap. These structures provide a framework for structure-based drug design targeting K _Ca 2.2 channels.