Dose response curves for ATP inhibition reveal no effect of tandem linkage on ATP sensitivity and suggest coassembly of S1-WT with S2-ND. (A) Fractional currents remaining in the presence of various concentrations of cytosolic ATP for channels formed by SUR1+Kir6.2, S1-WT, S1-ND (black circles, triangles, and diamonds), SUR2A+Kir6.2, S2-WT and S2-ND (blue circles, triangles, and diamonds). The lines are fits to the Hill equation of the data obtained for the different channel groups. The values of K_i and the Hill coefficient (n) are listed. The K_i and n of SUR1+Kir6.2 and S1-WT, or of SUR2A+Kir6.2 and S2-WT, are indistinguishable. (B) Fractional currents remaining in the presence of various concentrations of cytosolic ATP in patches excised from oocytes injected with an equal molar amount of S1-WT and S2-ND cRNA (red symbols). The data and fit lines for homomeric S1-WT (black) and S2-ND (blue) channels are replotted from A for comparison. The red solid line is the best fit to the S1-WT+S2-ND coexpression data assuming that S1-WT and S2-ND do not coassemble; i.e., a mixture of the two Hill functions obtained for homomeric S1-WT (black dashed line) and S2-ND (blue dotted line) was fitted by leaving the fractional amplitude (a₁) of the S1-WT component as the sole free parameter. This fit returned a₁ = 0.68 ± 0.14, but clearly failed to describe the data. The red dotted line is the fit to the same data with a₁ fixed to 0.5.