Cap ribose methylation of c-mos mRNA stimulates translation and oocyte maturation in Xenopus laevis

In Xenopus oocytes, progesterone stimulates the cytoplasmic polyadenylation and resulting translational activation of c-mos mRNA, which is necessary for the induction of oocyte maturation. Although details of the biochemistry of polyadenylation are beginning to emerge, the mechanism by which 3' poly(A) addition stimulates translation initiation is enigmatic. A previous report showed that polyadenylation induced cap-specific 2'-O-methylation, and suggested that this 5' end modification was important for translational activation. Here, we demonstrate that injected c-mos RNA undergoes polyadenylation and cap ribose methylation. Inhibition of this methylation by S-isobutylthioadenosine (SIBA), a methyltransferase inhibitor, has little effect on progesterone-induced c-mos mRNA polyadenylation or general protein synthesis, but prevents the synthesis of Mos protein as well as oocyte maturation. Maturation can be rescued, however, by the injection of factors that act downstream of Mos, such as cyclin A and B mRNAs. Most importantly, we show that the translational efficiency of injected mRNAs containing cap-specific 2'-O-methylation (cap I) is significantly enhanced compared to RNAs that do not contain the methylated ribose (cap 0). These results suggest that cap ribose methylation of c-mos mRNA is important for translational recruitment and for the progression of oocytes through meiosis.