Polyamine Sharing between Tubulin Dimers Favours Microtubule Nucleation and Elongation via Facilitated Diffusion
Figure 7
Facilitated nucleation mediated by polyamines.
(A) Microtubules were assembled at various concentrations of tubulin with or without 100 µM spermidine at 37°C. (25 mM MES-KOH pH 6.8, 1 mM EGTA, 30 µM tubulin, 2 mM MgCl2, 20% glycerol). The samples were placed in a pre-warmed cuvette to initiate polymerization. (B) Log-log plot of tenth time versus tubulin concentrations extracted from (A). The experimental data are properly fitted by a straight line with a slope of −2.9 in the absence of spermidine. In contrast, in the presence of spermidine, the tenth time is nearly constant over a large range of tubulin concentration (20–40 µM), as represented by the horizontal line. Indeed the shortest nucleation duration is already reached at tubulin concentration larger than CL (∼20 µM) because of facilitated nucleation. For lower concentrations than CL, the tenth time increases sharply when the tubulin concentration decreases, as represented by the linear increase in the log–log plot.