Gillian-Daniel D L, Gray N K, Aström J, Barkoff A, Wickens M
Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706, USA.
Mol Cell Biol. 1998 Oct;18(10):6152-63. doi: 10.1128/MCB.18.10.6152.
The translation of specific maternal mRNAs is regulated during early development. For some mRNAs, an increase in translational activity is correlated with cytoplasmic extension of their poly(A) tails; for others, translational inactivation is correlated with removal of their poly(A) tails. Recent results in several systems suggest that events at the 3' end of the mRNA can affect the state of the 5' cap structure, m7G(5')ppp(5')G. We focus here on the potential role of cap modifications on translation during early development and on the question of whether any such modifications are dependent on cytoplasmic poly(A) addition or removal. To do so, we injected synthetic RNAs into Xenopus oocytes and examined their cap structures and translational activities during meiotic maturation. We draw four main conclusions. First, the activity of a cytoplasmic guanine-7-methyltransferase increases during oocyte maturation and stimulates translation of an injected mRNA bearing a nonmethylated GpppG cap. The importance of the cap for translation in oocytes is corroborated by the sensitivity of protein synthesis to cap analogs and by the inefficient translation of mRNAs bearing nonphysiologically capped 5' termini. Second, deadenylation during oocyte maturation does not cause decapping, in contrast to deadenylation-triggered decapping in Saccharomyces cerevisiae. Third, the poly(A) tail and the N-7 methyl group of the cap stimulate translation synergistically during oocyte maturation. Fourth, cap ribose methylation of certain mRNAs is very inefficient and is not required for their translational recruitment by poly(A). These results demonstrate that polyadenylation can cause translational recruitment independent of ribose methylation. We propose that polyadenylation enhances translation through at least two mechanisms that are distinguished by their dependence on ribose modification.
特定母体mRNA的翻译在早期发育过程中受到调控。对于一些mRNA，翻译活性的增加与其多聚腺苷酸（poly(A)）尾巴的细胞质延伸相关；对于另一些mRNA，翻译失活与其多聚腺苷酸尾巴的去除相关。几个系统最近的研究结果表明，mRNA 3'端的事件可以影响5'帽结构m7G(5')ppp(5')G的状态。我们在此关注帽修饰在早期发育过程中对翻译的潜在作用，以及是否存在任何此类修饰依赖于细胞质多聚腺苷酸化或去除的问题。为此，我们将合成RNA注射到非洲爪蟾卵母细胞中，并在减数分裂成熟过程中检查它们的帽结构和翻译活性。我们得出四个主要结论。第一，细胞质鸟嘌呤-7-甲基转移酶的活性在卵母细胞成熟过程中增加，并刺激带有未甲基化GpppG帽的注射mRNA的翻译。蛋白质合成对帽类似物的敏感性以及带有非生理性帽化5'末端的mRNA翻译效率低下，证实了帽对卵母细胞翻译的重要性。第二，与酿酒酵母中去腺苷酸化引发的脱帽不同，卵母细胞成熟过程中的去腺苷酸化不会导致脱帽。第三，在卵母细胞成熟过程中，多聚腺苷酸尾巴和帽的N-7甲基协同刺激翻译。第四，某些mRNA的帽核糖甲基化效率非常低，并且多聚腺苷酸对其翻译募集不是必需的。这些结果表明，多聚腺苷酸化可以导致独立于核糖甲基化的翻译募集。我们提出，多聚腺苷酸化通过至少两种机制增强翻译，这两种机制的区别在于它们对核糖修饰的依赖性。
