Cyclin E and its associated cdk activity do not cycle during early embryogenesis of the sea urchin

Female sea urchins store their gametes as haploid eggs. The zygote enters S -phase 1 h after fertilization, initiating a series of cell cycles that lac k gap phases. We have cloned cyclin E from the sea urchin Strongylocentrotu s purpuratus. Cyclin E is synthesized during oogenesis, is present in the g erminal vesicle, and is released into the egg cytoplasm at oocyte maturatio n. Cyclin E synthesis is activated at fertilization, although there is no i ncrease in cyclin E protein levels due to continuous turnover of the protei n. Cyclin E protein levels decline in morula embryos, while cyclin E mRNA l evels remain high. After the blastula stage, cyclin E mRNA and protein leve ls are very low, and cyclin E expression is predominant only in cells that are actively dividing. These include cells in the left coelomic pouch, whic h forms the adult rudiment in the embryo. The cyclin E present in the egg i s complexed with a protein kinase. Activity of the cyclin E/cdk2 changes li ttle during the initial cell cycles. In particular, cyclin E-cdk2 levels re main high during both S-phase and mitosis. Our results suggest that progres sion through the early embryonic cell cycles in the sea urchin does not req uire fluctuations in cyclin E kinase activity. (C) 2001 Academic Press.