FIGURE 2.

Thumb domain interactions affect desensitization. A, topological models for ASIC1a (blue), ASIC2a (red), ASIC1a-F2a, ASIC1a-T2a, ASIC1a-FT2a, ASIC2a-T1a, and ASIC2a-FT1a. Constructs were generated by swapping the finger and thumb domains between ASIC1a and ASIC2a (sequences in Fig. 1B). B, dose-response activation curves for ASIC1a, ASIC2a, and finger-thumb chimeras. 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 of ASIC1a, ASIC2a, and finger-thumb chimeras were compared. Statistically significant differences in apparent proton affinity were found between ASIC2a, ASIC2a-T1a, ASIC2a-FT1a, ASIC1a-F2a, and ASIC1a-FT2a with ASIC1a and between ASIC1a, ASIC1a-F2a, ASIC1a-T2a, ASIC1a-FT2a, and ASIC2a-FT1a with ASIC2a (n = 10–31, p < 0.05). C, representative recordings of experiments conducted with oocytes expressing ASIC1a and ASIC2a. The whole-cell current decay after extracellular acidification was fitted to a single exponential function as described under “Experimental Procedures” (red lines). D, time constant (τ) of desensitization of ASIC1a and ASIC2a as a function of pH of activation (n = 10–53). E and F, time constants of desensitization of ASIC1a, ASIC2, and finger-thumb chimeras. Time constants of desensitization were determined as described above. E, the time constants of desensitization of ASIC1a, ASIC1a-F2a, ASIC1a-T2a, and ASIC1a-FT2a at pH 5.0 and of ASIC2 at pH 4.0 (n = 13–46). F, the time constants of desensitization of ASIC1a, ASIC2a-T1a, ASIC2a-FT1a, and ASIC2a at pH 4.0 (n = 12–28). Statistically significant differences in the time constants of desensitization between ASIC1a, ASIC2a, and chimeric channels are indicated. **, p < 0.01; +, p < 0.001 (analysis of variance followed by Dunn's multiple comparisons test).