Acidic residues in the voltage sensor of EAG superfamily channels. (A) A single subunit of a tetrameric EAG superfamily K⁺ channel is diagrammed with transmembrane domains S1–S6 depicted as rectangles. S1–S4 form the voltage sensor, and S5–S6 constitute the conduction pathway. Acidic residues of the voltage sensor are marked in red. Basic gating charges in the S4 helix are depicted with + symbols, and an Elk-specific histidine residue at the outer edge of S4 is marked in green. Acidic residues 1–3 are highly conserved across voltage-gated cation channels, and acidic residues 4–6 are specific to the EAG superfamily (among K⁺ channels). Residue positions for the three S2/S3 acidics that are accessible from the extracellular side (D1, D/E5, and D6) are given for mouse Kv10.2, human Kv11.1, and mouse Kv12.1. (B) Schematic structural drawing of a single subunit of an Elk family K⁺ channel. Side chains of four residues participating in binding of divalent cations are depicted with oxygens and nitrogens highlighted in red and green, respectively. These residues (D1, E5, D6, and the Elk-specific histidine in S4) are predicted to lie in close proximity within an aqueous cleft in the outer voltage sensor. (C) Amino acid alignments of the transmembrane voltage sensor helices S2–S4 are shown for various EAG superfamily K⁺ channels, the olfactory CNG channel α subunit (CNGA2), a sea urchin HCN channel (SPIH), and Drosophila Shaker. Species prefixes m and h in the channel names refer to mouse and human, respectively. Conserved residues are shaded. Acidic residues are labeled in red and marked with the position numbers defined in A. Asterisks mark the EAG-specific acidic residue positions in S2 and S3. Basic residues and Elk-specific histidine in S4 are highlighted in blue and green, respectively.