Soluble Cytokeratins in Xenopus Laevis Oocytes and Eggs

Xenopus oocytes contain a radial network of cytokeratins which seems to fragment during meiosis reinitiation (maturation). The mature egg contains only a cortical network of cytokeratins. We have looked for the presence of soluble cytokeratins in oocytes and unfertilized eggs and have found them in both cases. However, the proportion of soluble to insoluble cytokeratins is slightly higher in the egg than in the oocyte. Soluble cytokeratins incorporate 35S-methionine at a high rate in the oocyte but to a lesser extent in the egg. This suggests that they are biosynthetic intermediates in the oocyte. In the egg, at least a fraction of the soluble cytokeratins may arise from the fragmentation of the polymer which seems to occur during the maturation process. Insoluble cytokeratins are strongly labeled with 32P both in oocytes and eggs. On the other hand only the soluble keratins of the egg incorporate 32P. Since the isoelectric point of soluble and insoluble cytokeratins is the same in oocytes and eggs, their absolute level of phosphorylation probably remains relatively constant. This suggests that: i) phosphate turnover is very slow in oocyte soluble cytokeratins, ii) phosphorylation is not a major way of changing the structural state of cytokeratins in amphibian oocytes and eggs.