The size of the movement and the molecular identity of the moving parts of the voltage sensor of a voltage-gated ion channel are debated. In the helical-screw model, the positively charged fourth transmembrane segment S4 slides and rotates along negative counter charges in S2 and S3, while in the paddle model, S4 carries the extracellular part of S3 (S3b) as a cargo. Here, we show that S4 slides 16-26 Å along S3b. We introduced pairs of cysteines in S4 and S3b of the Shaker K channel to make disulfide bonds. Residue 325 in S3b makes close and state-dependent contacts with a long stretch of residues in S4. A disulfide bond between 325 and 360 was formed in the closed state, while a bond between 325 and 366 was formed in the open state. These data are not compatible with the voltage-sensor paddle model, but support the helical-screw model. © 2008 Elsevier Inc. All rights reserved.