Diffusion-limited currents of Shaker K channels. (A, C, and E) Single-channel current traces of Shaker variants in 100 mM of symmetrical KCl (top) and with 100 mM KCl plus 2 M sucrose in the bath (bottom). (A) Shaker-WT, (C) P475Q, and (E) P475D. (B, D, and F) Open-channel I-V relations of at least five traces from two to four independent experiments. (B) Shaker-WT, (D) P475Q, and (F) P475D. Open and closed symbols correspond to control and sucrose data points, respectively. The solid lines are fits of a third-degree polynomial function with no theoretical meaning. The last four points taken at the most positive voltage of each curve were averaged to obtain the diffusion-limited current value for each data series. Shaker-WT, i_DL = 1.25 ± 0.032 pA; P475Q, i_DL = 1.22 ± 0.045 pA; P475D, i_DL = 2.40 ± 0.27 pA.