Divalent ion inhibition in mutants of the nonregulatory sites S_pore and S_in.

Dose–response relationships of the ELIC point mutant R91A (A) activated by cysteamine at different concentrations of Ca²⁺. (B) Anomalous difference electron density (calculated at 5 Å and contoured at 5 σ) from data of the mutant N251A in complex with Ba²⁺ superimposed on a model of ELIC in ribbon representation. Ion-binding sites are indicated. Dose–response relationships of the ELIC point mutants N251A (C), S84A (D), and D86A (E) activated by cysteamine at different concentrations of Ca²⁺. The data presented in panels (A), (B), (D), and (E) are averages from at least 5 oocytes; errors are SD. The solid lines show fits to a Hill equation. Currents were recorded at −40 mV. A dose–response curve of WT in the absence of Ca²⁺ (dashed line) is shown for comparison. (F) Schild plots quantifying the inhibition of ELIC mutants by Ca²⁺. EC₅₀ values were obtained from fits to data shown in panels (A), (B), (D), and (E). Potencies of the antagonists (pA values) were obtained by linear regression; the intersection with the x-axis is indicated (•). WT is shown for comparison.