ATP sensitivity of Q52-Kir6.2 mutants is charge dependent. (A) Representative traces from inside-out patch voltage-clamp recordings from COSm6 cells transfected with WT-SUR1 and WT-Kir6.2, Q52R-Kir6.2, or Q52E-Kir6.2 cDNA. Patches were exposed to various concentrations of ATP or nucleotide-free Kint/EDTA control solution. Dashed lines represent zero current. (B) Dose–response data illustrating the mean currents in several ATP concentrations relative to maximum in nucleotide-free solution. Best fit curves (solid lines) were generated using the Hill equation for WT and Q52E (IC₅₀: 11 ± 1 µM and 2 ± 0 µM, respectively). Q52R dose–response data were fit manually (dashed line), with an estimated IC₅₀ of ∼1 mM, similar to the value we reported previously (IC₅₀ = 889 µM; Lin et al., 2006a). n = 3–10 for each data point. Error bars represent SEM, and some are smaller than the symbols.