Loss-of-function K Ca 2.2 mutations abolish channel activity.

Small-conductance Ca 2+ -activated potassium channels subtype 2 (K Ca 2.2, also called SK2) are operated exclusively by a Ca 2+ -calmodulin gating mechanism. Heterozygous genetic mutations of K Ca 2.2 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 K Ca 2.2 channel. No measurable current was detected from HEK293 cells heterologously expressing these pathogenic K Ca 2.2 mutants. When coexpressed with the K Ca 2.2_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 K Ca 2.2_WT channel. Coexpression of the K Ca 2.2_I289N and the K Ca 2.2_WT channels reduced the apparent Ca 2+ sensitivity compared with the K Ca 2.2_WT channel, which was rescued by a K Ca 2.2 positive modulator.