Domains Responsible for Constitutive and Ca2+-Dependent Interactions between Calmodulin and Small Conductance Ca2+-Activated Potassium Channels

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Abstract
Small conductance Ca2+-activated potassium channels (SK channels) are coassembled complexes of pore-forming SK α subunits and calmodulin. We proposed a model for channel activation in which Ca2+ binding to calmodulin induces conformational rearrangements in calmodulin and the α subunits that result in channel gating. We now report fluorescence measurements that indicate conformational changes in the α subunit after calmodulin binding and Ca2+ binding to the α subunit–calmodulin complex. Two-hybrid experiments showed that the Ca2+-independent interaction of calmodulin with the α subunits requires only the C-terminal domain of calmodulin and is mediated by two noncontiguous subregions; the ability of the E-F hands to bind Ca2+ is not required. Although SK α subunits lack a consensus calmodulin-binding motif, mutagenesis experiments identified two positively charged residues required for Ca2+-independent interactions with calmodulin. Electrophysiological recordings of SK2 channels in membrane patches from oocytes coexpressing mutant calmodulins revealed that channel gating is mediated by Ca2+ binding to the first and second E-F hand motifs in the N-terminal domain of calmodulin. Taken together, the results support a calmodulin- and Ca2+-calmodulin-dependent conformational change in the channel α subunits, in which different domains of calmodulin are responsible for Ca2+-dependent and Ca2+-independent interactions. In addition, calmodulin is associated with each α subunit and must bind at least one Ca2+ ion for channel gating. Based on these results, a state model for Ca2+ gating was developed that simulates alterations in SK channel Ca2+sensitivity and cooperativity associated with mutations in CaM.