Camguk/CASK enhances Ether-á-go-go potassium current by a phosphorylation-dependent mechanism

Signaling complexes are essential for the modulation of excitability within restricted neuronal compartments. Adaptor proteins are the scaffold around which signaling complexes are organized. Here, we demonstrate that the Camguk (CMG)/CASK adaptor protein functionally modulates Drosophila Ether-á-go-go (EAG) potassium channels. Coexpression of CMG with EAG in Xenopus oocytes results in a more than twofold average increase in EAG whole-cell conductance. This effect depends on EAG-T787, the residue phosphorylated by calcium- and calmodulin-dependent protein kinase II (Wang et al., 2002). CMG coimmunoprecipitates with wild-type and EAG-T787A channels, indicating that T787, although necessary for the effect of CMG on EAG current, is not required for the formation of the EAG-CMG complex. Both CMG and phosphorylation of T787 increase the surface expression of EAG channels, and in COS-7 cells, EAG recruits CMG to the plasma membrane. The interaction of EAG with CMG requires a noncanonical Src homology 3-binding site beginning at position R1037 of the EAG sequence. Mutation of basic residues, but not neighboring prolines, prevents binding and prevents the increase in EAG conductance. Our findings demonstrate that membrane-associated guanylate kinase adaptor proteins can modulate ion channel function; in the case of CMG, this occurs via an increase in the surface expression and phosphorylation of the EAG channel.