In contrast to other members of the Eag family of voltage-gated, outwardly rectifying potassium channels, the human eag-related gene (HERG) has now been shown to encode an inwardly rectifying potassium channel. The properties of HERG channels are consistent with the gating properties of Eag-related and other outwardly rectifying, S4-containing potassium channels, but with the addition of an inactivation mechanism that attenuates potassium efflux during depolarization. Because mutations in HERG cause a form of long-QT syndrome, these properties of HERG channel function may be critical to the maintenance of normal cardiac rhythmicity.

We have identified a conserved family of genes related to Drosophila eag, which encodes a distinct type of voltage-activated K channel. Three related genes were recovered in screens of cDNA libraries from Drosophila, mouse, and human tissues. One gene is the mouse counterpart of eag; the other two represent additional subfamilles. The human gene maps to chromosome 7. Family members share at least 47% amino acid identity in their hydrophobic cores and all contain a segment homologous to a cyclic nucleotide-bindig domain. Sequence comparisons indicate that members of this family are most closely related to vertebrate cyclic nudleotidegated cation channels and plant inward-rectifying K+ channels. The existence of another family of K+ channel structural genes further extends the known diversity of Ki channels and has important implications for the structure, function, and evolution of the superfamily of voltage-sensitive ion channels.Voltage-activated ion channels are members of evolutionarily conserved multigene families (1, 2). For example, the Shaker (Sh) family of K+ channels comprises four related genes in Drosophila, each of which has one or more mammalian homologs (3). Together these genes define at least four subfamilies of voltage-activated K+ channels within the Sh family. Most of our present understanding of the structure and function of K+ channels is based on studies of the polypeptides encoded by these genes (4).Analysis ofother K+ channels that are not members ofthe Sh family will expand our understanding of these channels. Genes encoding additional types of K+ channels can be identified in Drosophila via molecular analysis of mutations affecting membrane excitability (5). Mutations of eag, identified by their leg-shaking phenotype, cause repetitive firing and enhanced transmitter release in motor neurons, suggesting a possible defect in K+ channels (6,7). Molecular studies revealed that eag encodes a polypeptide structurally related both to K+ channels in the Sh family and to cyclic nucleotide-gated cation channels (8-10). Expression in Xenopus oocytes confirms that the eag polypeptide assembles into channels conducting a voltageactivated K+-selective outward current (11,12

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Many of the signaling properties of neurons and other electrically excitable cells are determined by a diverse family of potassium channels. A number of genes that encode potassium channel polypeptides have been cloned from various organisms on the basis of their sequence similarity to the Drosophila Shaker (Sh) locus. As an alternative strategy, a molecular analysis of other Drosophila genes that were defined by mutations that perturb potassium channel function was undertaken. Sequence analysis of complementary DNA from the ether à go-go (eag) locus revealed that it encodes a structural component of potassium channels that is related to but is distinct from all identified potassium channel polypeptides.

The fruit fly Drosophila melanogaster was used to examine the mode of action of the novel insecticide and acaricide nodulisporic acid. Flies resistant to nodulisporic acid were selected by stepwise increasing the dose of drug in the culture media. The resistant strain, glc 1 , is at least 20-fold resistant to nodulisporic acid and 3-fold cross-resistant to the parasiticide ivermectin, and exhibited decreased brood size, decreased locomotion, and bang sensitivity. Binding assays using glc 1 head membranes showed a marked decrease in the affinity for nodulisporic acid and ivermectin. A combination of genetics and sequencing identified a proline to serine mutation (P299S) in the gene coding for the glutamategated chloride channel subunit DmGluCl␣. To examine the effect of this mutation on the biophysical properties of DmGluCl␣ channels, it was introduced into a recombinant DmGluCl␣, and RNA encoding wild-type and mutant subunits was injected into Xenopus oocytes. Nodulisporic acid directly activated wild-type and mutant DmGluCl␣ channels. However, mutant channels were Ϸ10-fold less sensitive to activation by nodulisporic acid, as well as ivermectin and the endogenous ligand glutamate, providing direct evidence that nodulisporic acid and ivermectin act on DmGluCl␣ channels.