SELECTIVE AND RAPID NUCLEAR TRANSLOCATION OF A C-MYC-CONTAINING COMPLEX AFTER FERTILIZATION OF XENOPUS-LAEVIS EGGS

We report here unusual features of c-Myc specific to early embryonic d evelopment in Xenopus laevis, a period characterized by generalized tr anscriptional quiescence and rapid biphasic cell cycles, Two c-Myc pro tein forms, p61 and p64, are present in large amounts in the oocyte as well as during early development, In contrast, only p64 c-Myc is pres ent in Xenopus somatic cells, p61 c-Myc is the direct translation prod uct from both endogenous c-myc mRNAs and c-myc recombinant DNA, It is converted to the p64 c-Myc form after introduction into an egg extract , in the presence of phosphatase inhibitors, p61 and p64 belong to two distinct complexes localized in the cytoplasm of the oocyte, A 15S co mplex contains p64 c-Myc, and a 17.4S complex contains p61 c-Myc. Fert ilization triggers the selective and total entry of only p64 c-Myc int o the nucleus, This translocation occurs in a nonprogressive manner an d is completed during the first cell cycles, This phenomenon results i n an exceptionally high level of c-Myc in the nucleus, which returns t o a somatic cell-like level only at the end of the blastulation period . During early development, when the entire embryonic genome is transc riptionally inactive, c-Myc does not exhibit a DNA binding activity wi th Max, Moreover, embryonic nuclei not only prevent the formation of c -Myc/Max complexes but also dissociate such preformed complexes, These peculiar aspects of c-Myc behavior suggest a function that could be l inked to the rapid DNA replication cycles occurring during the early c ell cycles rather than a function involving transcriptional activity.