Characterization of IP7 binding to Pho80-Pho85-Pho81-MD. (a) Native gel EMSA. Pho80-Pho85 (8 μM), Pho81-MD (8 μM) and divalent metal cations (6 mM each; E, EDTA; Mg, MgCl2; Ca, CaCl2; Mn, MnCl2; Zn, ZnSO4) were mixed and incubated with 0.2 μM 4/6-[β-32P]IP7 (5 μCi mmol−1) for 30 min at 30 °C, followed by electrophoresis at 4 °C. All subsequent binding assays were performed in the presence of 6 mM MgCl2 unless noted otherwise. (b) Isomer selectivity. Pho80-Pho85-Pho81-MD (8 μM each) was incubated with 0.2 μM 5-[β-32P]IP7 or 4/6-[β-32P]IP7 (30 °C, 30 min) and analyzed as in a. (c) Specificity of the binding. Binding of 4/6-[β-32P]IP7 (0.2 μM) to Pho80-Pho85-Pho81-MD (8 μM each) was monitored as in a in the presence of 400 μM competitors. (d) Binding reversibility. After formation of the complex between Pho80-Pho85-Pho81-MD and 4/6-IP7 as in a, reversibility of the binding was tested by the addition of 20 mM EDTA (E), or by heating (B; 90 °C, 1 min). N, not treated. (e) Dissociation kinetics. After formation of the complex, samples were diluted in 400 μM unlabeled 4/6-IP7 at 4 or 30 °C, and the dissociation of the complex was monitored by EMSA as a function of the time following the dilution. (f) Association kinetics. After adding 4/6-[β-32P]IP7 to Pho80-Pho85-Pho81-MD and incubating at 4 or 30 °C, a portion of the sample was diluted into ice-cold (4 °C) unlabeled 4/6-IP7 (400 μM) and analyzed by EMSA. For d and e, data were fit to single exponential equations.