Figure 5.

Stathmin/Op18 hyperphosphorylation results from MT assembly and becomes maximal after a significant lag. (A) SH-induced tubulin polymerization at 22°C in high-speed mitotic Xenopus egg extract results in the immediate phosphorylation of stathmin/Op18 on Ser 16 (stathmin 16P), and hyperphosphorylation becomes maximal between 15 and 30 min (stathmin and stathmin 16P). Repeated experiments show this lag to be closer to 15 min. After 45 min, addition of NZ results in rapid MT depolymerization, as judged by the decrease in the amount of tubulin in the pellet (P), and the slow disappearance of the hyperphosphorylated isoforms of stathmin/Op18 after ∼15 min in the supernatant (S, see arrows). (B) Graph showing the intensity of the bands corresponding to phosphorylation on stathmin/Op18 Ser 16 in A (stathmin 16P). Results are expressed for each lane as a ratio of phosphorylation on Ser 16 (stathmin 16P) to total stathmin/Op18 (stathmin) that remains constant throughout the experiment and thus represents an internal control.