FIGURE 8.

Covalent modification of residues in the finger and thumb domains accelerates channel desensitization. A–D, dose-response proton activation curves for channels bearing Cys mutations at selected positions in the finger and thumb domains. Proton-activated currents were elicited by a drop in extracellular pH from 8.0 to solutions of lower pH. To determine statistical significance, the confidence intervals for the pH₅₀ values were compared. Statistically significant differences in apparent proton affinity between untreated and MTSET-treated channels were found for the M154C and D345C mutants (n = 9–32, p < 0.05). E–H, reactivity of F152C, M154C, M325C, and D345C channels toward thiol-reactive reagents of increasing length. Whole-cell currents were evoked by a change in extracellular pH from 8.0 to 5.0 before and after treatment with MTSMT, MTSET, or MTSPT at pH 7.0. Non-treated oocytes expressing mutant channels served as controls. Statistically significant differences are indicated. **, p < 0.01; +, p < 0.001 (n = 10–14, analysis of variance followed by Tukey's multiple comparisons test). I–L, time constants of desensitization for channels bearing Cys mutations at positions 152, 154, 325, and 345 before and after covalent modification with thiol-reactive reagents. +, p < 0.001 (n = 10–14, analysis of variance followed by Tukey's multiple comparisons test).