Strategy to test if PUFA facilitates horizontal S4 movement. (A) Schematic illustration of the assumed charge-transfer motion used for distance calculations (Eq. 3). The only change in electric charges between each gating state is that one gating charge is moved from the intracellular side to the extracellular side. Gating charges within the membrane electric field pair up with conserved negative charges. (B) Structure of the Shaker K channel in the open state (based on the Kv1.2/2.1 chimera). One VSD and part of the pore domain are shown. The four most extracellular-positive charges in S4 are colored: red, R362; orange, R365; green, R368; blue, R371. The negative charge denotes the position for the effective PUFA molecule. Arrows denote the distances from the effective PUFA site to the charge of R362 and to the site where the positive charges of R365 and R368 emerge on the surface, respectively. (C) Structural comparison of an arginine, a MTSES⁻-modified cysteine, and a MTSET⁺-modified cysteine.