Voltage-gated channels maintain cellular resting potentials and generate neuronal action potentials by regulating ion flux. Here, we show that Ether-à -go-go (EAG) K ؉ channels also regulate intracellular signaling pathways by a mechanism that is independent of ion flux and depends on the position of the voltage sensor. Regulation of intracellular signaling was initially inferred from changes in proliferation. Specifically, transfection of NIH 3T3 fibroblasts or C2C12 myoblasts with either wild-type or nonconducting (F456A) eag resulted in dramatic increases in cell density and BrdUrd incorporation over vector-and Shaker-transfected controls. The effect of EAG was independent of serum and unaffected by changes in extracellular calcium. Inhibitors of p38 mitogen-activated protein (MAP) kinases, but not p44͞42 MAP kinases (extracellular signal-regulated kinases), blocked the proliferation induced by nonconducting EAG in serum-free media, and EAG increased p38 MAP kinase activity. Importantly, mutations that increased the proportion of channels in the open state inhibited EAG-induced proliferation, and this effect could not be explained by changes in the surface expression of EAG. These results indicate that channel conformation is a switch for the signaling activity of EAG and suggest an alternative mechanism for linking channel activity to the activity of intracellular messengers, a role that previously has been ascribed only to channels that regulate calcium influx.intracellular messenger ͉ mitogen-activated protein kinase ͉ neuromodulation ͉ proliferation ͉ gating V oltage-gated ion channels generate neuronal action potentials, the primary units of information transfer in the brain, by regulating ion f lux (1). Effects of ion channels on synaptic connectivity, transmitter release, plasticity, and other cellular processes are generally assumed to be a secondary consequence of ion f lux. Specifically, changes in membrane potential and action potentials alter Ca 2ϩ inf lux, and Ca 2ϩ regulates multiple intracellular signaling pathways (2-7). Several recent studies, however, have indicated that some voltagegated ion channels are bifunctional proteins (5, 8 -11). These studies show that voltage-gated channels can contribute to transcriptional regulation, protein scaffolding, cell adhesion, and intracellular signaling, and the effects appear largely independent of ion conduction. Recent studies of Ether-á-go-go [EAG (KCNH1)] voltagedependent K ϩ channels suggest that EAG may also be bifunctional. First, a region of Drosophila EAG with similarity to the autoinhibitory domain of Ca 2ϩ ͞calmodulin-dependent protein kinase II can associate with activated, Ca 2ϩ ͞calmodulin-bound Ca 2ϩ ͞calmodulin-dependent protein kinase II. In vitro assays indicate that, once Ca 2ϩ levels decline, EAG-bound kinase retains 5-10% of its maximum Ca 2ϩ -stimulated activity (12). Second, human EAG has been implicated in cell-cycle regulation and cancer: transfection can induce oncogenic transformation, EAG is present in some cancer cell line...

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.