p70^S6K Controls Selective mRNA Translation during Oocyte Maturation and Early Embryogenesis in Xenopus laevis

Abstract

In mammalian cells, p70^S6K plays a key role in translational control of cell proliferation in response to growth factors. Because of the reliance on translational control in early vertebrate development, we cloned a Xenopus homolog of p70^S6K and investigated the activity profile of p70^S6K during Xenopus oocyte maturation and early embryogenesis. p70^S6K activity is high in resting oocytes and decreases to background levels upon stimulation of maturation with progesterone. During embryonic development, three peaks of activity were observed: immediately after fertilization, shortly before the midblastula transition, and during gastrulation. Rapamycin, an inhibitor of p70^S6K activation, caused oocytes to undergo germinal vesicle breakdown earlier than control oocytes, and sensitivity to progesterone was increased. Injection of a rapamycin-insensitive, constitutively active mutant of p70^S6K reversed the effects of rapamycin. However, increases in S6 phosphorylation were not significantly affected by rapamycin during maturation. mosmRNA, which does not contain a 5′-terminal oligopyrimidine tract (5′-TOP), was translated earlier, and a larger amount of Mos protein was produced in rapamycin-treated oocytes. In fertilized eggs rapamycin treatment increased the translation of the Cdc25A phosphatase, which lacks a 5′-TOP. Translation assays in vivo using both DNA and RNA reporter constructs with the 5′-TOP from elongation factor 2 showed decreased translational activity with rapamycin, whereas constructs without a 5′-TOP or with an internal ribosome entry site were translated more efficiently upon rapamycin treatment. These results suggest that changes in p70^S6K activity during oocyte maturation and early embryogenesis selectively alter the translational capacity available for mRNAs lacking a 5′-TOP region.

ACKNOWLEDGMENTS

We are grateful to Brad Lattes, Jan Kyes, and Andrea Lewellyn for excellent technical assistance and to Jo Erikson and C. Finkielstein for critical reading of the manuscript. We are also thankful to Thomas Radimerski for preparing two-dimensional gels.

This work was supported in part by a grant from the NIH to J.L.M. (DK28353-17) and grants to S.C.K. and G.T. from the EEC and HFSPO. M.S.S. is an Associate and J.L.M. is an Investigator of the Howard Hughes Medical Institute.