Tubulin assembly in the presence of JAI-51 and binding of JAI-51 on tubulin. (A) Effect of various concentrations of JAI-51 (1: 0 μM; 2: 5 μM; 3: 7.5 μM; 4: 10 μM; 5: 12.5 μM; 6: 15 μM) on 15 μM tubulin, in polymerization buffer. Assembly was started by warming the samples at 37°C (arrow). (B) Represents the fraction of the reduction of the plateau absorbance values (inhibition) as a function of total JAI-51 concentrations. (C) Percentage of polymerization inhibition as a function of the mole ratio of the total JAI-51 to total tubulin in the solution (R). (D) Study of the binding of JAI-51 on tubulin by analytical ultracentrifugation. Circles are the experimental points and the solid line is the fit obtained as described in the Methods section. In these experiments one binding site was found (n = 1.3 ± 0.6) with an affinity constant of (1.9 ± 0.6) 105 M-1.