A Mutation in the Intrinsivally Disordered Fragment of SK2 Channel Confers Ca2+ Hypersensitivity

Abstract

Small-conductance Ca2+-activated potassium (SK) channels play an essential role in the regulation of neuronal excitability. There are three subtypes of SK channels expressed in the neurons, with SK2 and SK3 predominant in the motor neurons. Elevated intracellular Ca2+ levels exclusively activate the SK channels. The Ca2+-binding protein calmodulin (CaM) is constitutively associated with SK channels at the CaM binding domain and serves as the Ca2+ sensor of the channels. An intrinsically disordered fragment connects the CaM binding domain and the transmembrane domain S6. Here, we report that a phenylalanine mutation in the intrinsically disordered fragment caused a ∼6-fold increase in the Ca2+ sensitivity of SK2-a channels. Also, equivalent phenylalanine mutations in SK1 or SK3 channels conferred Ca2+ hypersensitivity. Additionally, an equivalent phenylalanine substitution in the Caenorhabditis elegans SK2 ortholog kcnl-2 partially rescued locomotion defects in an existing C. elegans ALS model, in which human SOD1G85R is expressed at high levels in neurons. Combined, these results provide the most direct functional and behavioral evidence that activation of SK channels ameliorates defects in ALS models.