Polyamine Sharing between Tubulin Dimers Favours Microtubule Nucleation and Elongation via Facilitated Diffusion
Figure 4
Effect of polyamines on the maximum slope of microtubule assembly.
The tubulin concentration (12 µM) was purposely fixed to a value near the critical concentration in buffer M. (A) Both spermidine and spermine promote microtubule assembly at submilimolar concentrations whereas putrescine is unable to improve tubulin polymerization even at large concentrations (>1 mM). However, at high concentrations of spermidine and spermine (>200 µM), tubulin aggregates are formed which lowers the apparent assembly slope leading to a bell-shape profile. (B) SDS page analyses reveal that spermidine gradually increases the pellet mass. As observed by AFM, the increase of the pellet mass is due to microtubule formation at moderate spermidine concentrations (25–200 µM) but, at higher spermidine concentrations (0.5–2 mM), oligomers and (or) tubulin aggregates are also observed and participate to the increase of the pellet mass. The dashed line represents the transition between the zones of microtubule assembly and tubulin aggregation.