Loss-of-function K2.2 mutations abolish channel activity.

Small-conductance Ca-activated potassium channels subtype 2 (K2.2, also called SK2) are operated exclusively by a Ca-calmodulin gating mechanism. Heterozygous genetic mutations of K2.2 channels have been associated with autosomal dominant neurodevelopmental disorders including cerebellar ataxia and tremor in humans and rodents. Taking advantage of these pathogenic mutations, we performed structure-function studies of the rat K2.2 channel. No measurable current was detected from HEK293 cells heterologously expressing these pathogenic K2.2 mutants. When coexpressed with the K2.2_WT channel, mutations of the pore-lining amino acid residues (I360M, Y362C, G363S, and I389V) and two proline substitutions (L174P and L433P) dominant negatively suppressed and completely abolished the activity of the coexpressed K2.2_WT channel. Coexpression of the K2.2_I289N and the K2.2_WT channels reduced the apparent Ca sensitivity compared with the K2.2_WT channel, which was rescued by a K2.2 positive modulator.