Figure 3.

Substrate dose responses for C-C mutant and WT before and after modification with MTSEA. (A) P_i activation for WT (open diamonds) and C-C mutant (filled squares) before modification with MTSEA at V _h = −50 mV, measured in ND100 with P_i as variable substrate. For the C-C mutant, data are also shown after incubation with MTSEA (open squares), by measuring the change in holding current induced by P_i relative to the holding current in 0 mM P_i. Each data point represents mean ± SEM (n = 4). The modified Hill equation was fit to the data to give: K _m ^Pi = 0.06 ± 0.010 (C-C, −MTSEA); K _m ^Pi = 0.06 ± 0.01 (C-C, +MTSEA) and K _m ^Pi = 0.060 ± 0.01 (WT), n _H was constrained to 1.0 for fit. (B) Na⁺ activation for WT (open diamonds) and C-C mutant (filled squares) before modification with MTSEA at V _h = −50 mV measured at 1 mM P_i with Na⁺ as variable substrate. For the C-C mutant, data are also shown after incubation with MTSEA (open squares) by measuring the change in holding current induced by P_i relative to the current in 0 mM P_i. Data were pooled as in A (n = 4); fit of Hill equation gave: K _m ^Na = 52 ± 8 (C-C, −MTSEA), K _m ^Na = 55 ± 6 (C-C, +MTSEA), K _m ^Na = 61 ± 10 (WT), n _H = 2.3, 2.7, and 2.1, respectively. Points without error bars have SEMs smaller than the symbol size.