CHANGES IN LEVELS OF POLYMERIC TUBULIN ASSOCIATED WITH ACTIVATION AND DORSOVENTRAL POLARIZATION OF THE FROG EGG

The level of polymeric tubulin was measured during the 1st cell cycle of the electrically activated and the fertilized egg of Xenopus laevis. Eggs were homogenized in a microtubule-stabilizing medium, and the amount of tubulin pelleted by centrifugation was determined by quantitative Western blots. The pelleted tubulin (polymer) was in the form of microtubules, based on the presence of microtubules in the pellet and on the effects of cold, nocodazole and D2O. Unactivated eggs had a high level of polymer (> 0.01 .mu.g/egg) which disappeared within minutes of activation. The level of polymer stayed low (< 0.02 .mu.g/egg) until halfway through the cell cycle (0.5 on a normalized time scale), when the level rose to the preactivation value. There was a decrease associated with metaphase (0.85 normalized time) and a return to a high level at 1st cleavage (1.0 normalized time). Fertilized eggs showed a similar pattern, although the amount of polymer increased earlier (0.3-0.5 normalized time), presumbably due to the spermaster. The depolymerization of microtubules at activation indicates that there is a dramatic change of the cytoskeleton at this time. The polymerization at 0.5 normalized time coincides with the start of the cytoplasmic shift leading to dorsoventral polarity. Evidently, microtubules are involved in dorsoventral polarization of the embryo.