Tubulin and Microtubules in the Early Development of the Axolotl and Other Amphibia

Abstract

SYNOPSIS. Unfertilized eggs of the axolotl, Ambystoma mexicanum , contain a pool of soluble tubulin accumulated during oogenesis. After initiation of cleavage the tubulin pool decreases somewhat and then remains constant through early development. Some properties of tubulin alter during development, but at least some of these changes are not due to changes in tubulin perse. However, the tubulin in axolotl oocytes, eggs, and embryos differs in some electrophoretic properties from tubulin in adult axolotl brain and testis. Equivalent differences were observed in Necturus maculosus tubulins. Heterogeneity of axolotl tubulins was confirmed by peptide mapping: Different patterns of peptides were formed by specific limited proteolysis of soluble tubulin from eggs and testis. The heterogeneity was more marked in the a than in the β subunit. Mobilization of soluble tubulin into the mitotic apparatus depends on the functioning of microtubule organizing centers after activation of the egg at fertilization. In eggs of the nc mutant axolotl there is a lesion in some step of activation, one effect of which is that even though the eggs contain an essentially normal pool of tubulin, microtubules fail to assemble, no mitotic apparatus forms, and embryonic development does not begin. These eggs can be partially corrected by injection of heterologous microtubule fragments, which elicit the mobilization of nc tubulin into arrays of microtubules, followed by initiation of cleavage and development to a partial blastula stage. The results of these experiments are discussed in comparison with other reports in the literature about the function of microtubule organizing centers during amphibian egg development.