Characterization of Ether-à-go-go Channels Present in Photoreceptors Reveals Similarity to I_Kx, a K⁺ Current in Rod Inner Segments

In this study, we describe two splice variants of an ether-à-go-go (EAG) K⁺ channel cloned from bovine retina: bEAG1 and bEAG2. The bEAG2 polypeptide contains an additional insertion of 27 amino acids in the extracellular linker between transmembrane segments S3 and S4. The heterologously expressed splice variants differ in their activation kinetics and are differently modulated by extracellular Mg²⁺. Cooperativity of modulation by Mg²⁺ suggests that each subunit of the putative tetrameric channel binds a Mg²⁺ ion. The channels are neither permeable to Ca²⁺ ions nor modulated by cyclic nucleotides. In situ hybridization localizes channel transcripts to photoreceptors and retinal ganglion cells. Comparison of EAG currents with I_Kx, a noninactivating K⁺ current in the inner segment of rod photoreceptors, reveals an intriguing similarity, suggesting that EAG polypeptides are involved in the formation of K_x channels.