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

Small-conductance (K_CA2.2) and intermediate-conductance (K_CA3.1) CA²⁺-activated K⁺ channels are gated by a CA²⁺-calmodulin dependent mechanism. NS309 potentiates the activity of both K_CA2.2 and K_CA3.1, while rimtuzalcap selectively activates K_CA2.2. Rimtuzalcap has been used in clinical trials for the treatment of spinocerebellar ataxia and essential tremor. We report cryo-electron microscopy structures of K_CA2.2 channels bound with NS309 and rimtuzalcap, in addition to K_CA3.1 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_CA2.2. Calmodulin's N-lobes in the K_CA2.2 structure are far apart and undergo conformational changes to accommodate either NS309 or rimtuzalcap. Calmodulin's Nlobes in the K_CA3.1 structure are closer to each Other and are constrained by the HC helices of K_CA3.1, which allows binding of NS309 but not of the bulkier rimtuzalcap. These structures provide a framework for structure-based drug design targeting K_CA2.2 channels.