The gating charge transfer center (Tao et al., 2010). (A) The membrane topology of one subunit of the tetrameric Shaker K⁺ channel is shown. The approximate positions of conserved charged residues and I287, F290, F324, and A359 in the voltage sensor domain are indicated. Conserved charged residues are labeled using the following generic nomenclature: E0, E247 in S1; E1, E283 in S2; E2, E293 in S2; D3, D316 in S3; R1, R362 in S4; R2, R365 in S4; R3, R368 in S4; R4, R371 in S4; K5, K374 in S4. The S2, S3, and S4 transmembrane segments are shown in yellow, red, and blue, respectively, for comparison to B. (B) The charge transfer center consists of F290 (orange) and E2 in S2 (yellow/red) and D3 in S3 (yellow/red), and is occupied by K5 in S4 (gray/blue) in the Kv1.2/Kv2.1 paddle chimera x-ray structure (Long et al., 2007). Also shown are I287 in S2 and F324 in S3 (green), which correspond to residues that form a naturally occurring binding site for extracellular divalent cations in eag (Silverman et al., 2000; Lin et al., 2010). I287 and F324 are located extracellular to the charge transfer center. Ribbons representing the backbone atoms of S2, S3, and S4 are shown in yellow, red, and blue, respectively. Backbone atoms and the indicated side chains were extracted from the 2r9r x-ray structure and labeled according to the Shaker sequence (Long et al., 2007). In the chimera, the F324 position is occupied by a tyrosine residue, which was mutated in silico using PyMOL (v1.3; The PyMOL Molecular Graphics System; Schrödinger LLC). The figure was made using PyMOL.