Charge distribution and voltage dependence of opening of WT and mutated Shaker K channels. (A and B) Gating charges R1–R4 in S4 denoted as blue sticks in a VSD structure (side view [A] and top view [B]) of an open K channel (the Kv1.2/2.1 chimera structure with Shaker side chains; Long et al., 2007; Henrion et al., 2012). The approximate interaction site for DHA is marked with the red encircled negative sign in B (Börjesson and Elinder, 2011). (C and D) R1 (R362) is highlighted in five superimposed structures of S4 (side view [C] from residue 356 and top view [D] from residue 361), for four closed (C1–C4) and one open (O) model states (Henrion et al., 2012). Backbones are color coded according to opening level, from light gray (C4) to black (O). In D, the approximate interaction site for DHA is marked with the red encircled negative sign (Börjesson and Elinder, 2011). The arrow in D denotes the movement of R1 when S4 moves from C1 to O, the step most sensitive to DHA (Börjesson and Elinder, 2011). (E) Amino acid sequences of the extracellular end of S4 (Shaker channel) from the eight single-residue mutations investigated. The positively charged arginines (R) are marked in blue. The mutated region is shown in gray. R1 = R362. (F) The voltage required to reach 50% of the maximum conductance (V_1/2) plotted against the residue number of the positive charge. The dotted line corresponds to V_1/2 for the channel without charges in the studied region (R362Q). Green symbols denote positive midpoint voltages relative the neutral 356–362 segment (open symbol), and the red symbols denote negative midpoint voltages. (G) Residues 356–362 are denoted as sticks on the open Shaker VSD structure with same color coding as in F.