Distribution of nonmuscle actin during Xenopus laevis development

Using antisera specific for the N-terminal peptides of nonmuscle isoforms of actin we studied these proteins in the developing egg and embryo of Xenopus laevis. The antisera did not crossreact with muscle actin and did not react with the other nonmuscle isoform. By immunofluorescence we found a large maternal actin store in the oocyte, in the Balbiani body and the nucleoli, respectively. In the blastula, nonmuscle actin was preferentially expressed in the mitotic apparatus of the individual blastomeres. A gradual decrease in the amount of these isoforms in muscle tissue could be observed during development, apparently corresponding to an increase in the concentration of muscle specific isoforms. No differences between the distribution of the two isoforms could be found. The results would seem to indicate that development of the embryo is accompanied by a decline in the concentration of cytoskeletal actin isoforms down to an almost background level, accompanied by an increase in the concentration of the muscle specific isoforms. This points to potential function of nonmuscle actins in the early embryo.