Figure 1.

Competitive binding of Mg²⁺ and Ca²⁺ to PIP2-containing monolayers. (A) Langmuir adsorption isotherms of calcium binding to L-α-PI(4,5)P2 under different Mg²⁺ concentration. (B) Same isotherms normalized to their ΔП _Max. (C) Langmuir plot of calcium adsorption isotherms. (D) Time course of surface pressure change when 1 mM Ca²⁺, 10 mM Mg²⁺, and 9 mM Ca²⁺ were added sequentially to a monolayer. (Surface pressures were reset to 20 mN/m using the motorized barriers each time before adding divalent cations.) For all surface pressure experiments, 25 mol% PIP2 in SOPC monolayers were used on 10 mM HEPES buffered subphase with 1 μM EDTA and 5 mM DTT, at pH 7.4, room temperature.