Alternative leader sequences in insulin-like growth factor I mRNAs modulate translational efficiency and encode multiple signal peptides.
Alternative leader sequences in insulin-like growth factor I mRNAs modulate translational efficiency and encode multiple signal peptides.
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DOI：
10.1210/mend.9.10.8544846
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发表时间：
1995-10
影响因子：
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通讯作者：
Huan Yang;Martin L. Adamo;A. P. Koval;Michele C. McGuinness;Haya Ben-Hur;Yanzhu Yang;D. Leroith;Charles T. Roberts
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文献类型：
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作者：
Huan Yang;Martin L. Adamo;A. P. Koval;Michele C. McGuinness;Haya Ben-Hur;Yanzhu Yang;D. Leroith;Charles T. Roberts
由于使用了从多个转录起始位点转录的前导外显子以及前导外显子 1 内的选择性剪接，大鼠胰岛素样生长因子 I (IGF-I) mRNA 含有多个 5'-非翻译区。具有对应于外显子 1 转录起始位点使用的 5'-末端的合成 RNA 在体外翻译成前原 IGF-I 肽，起始于外显子 1 中的 Met-48 密码子或 Met-22外显子 3 中的密码子和具有对应于主要外显子 2 转录起始位点的 5' 端的 RNA 被翻译成在外显子 2 中的 Met-32 密码子处起始的前原 IGF-I 肽。所有形式的前原-IGF-I 均由犬胰腺微粒体加工，表明所有这些前肽都充当信号肽。 IGF-I RNA 的翻译效率与 5'-非翻译区的长度成反比。外显子 1 中三个上游 AUG 密码子中第一个的突变可能启动 14 个氨基酸的开放阅读框，但不会影响前原 IGF-I 的翻译。另外两个 AUG 密码子后面紧跟着终止密码子。与全长 RNA 相比，完全剪接的外显子 1 衍生的 RNA 中两个上游 AUG 密码子的缺失增强了该 RNA 的体外和体内翻译能力。下游起始密码子的突变尤其增加了体外和完整细胞中的翻译效率，表明Met-48密码子处的低效重新起始事件导致IGF-I mRNA（其中出现这些上游AUGUGA基序）的翻译较差。我们得出结论，IGF-I mRNA 可能编码多种形式的 preproIGF，并且其 5'-非翻译区的特定差异为 IGF-I 生物合成的翻译控制提供了分子基础。
Rat insulin-like growth factor I (IGF-I) mRNAs contain multiple 5'-untranslated regions due to the use of leader exons transcribed from several transcription initiation sites and to alternative splicing within leader exon 1. Synthetic RNAs with 5'-ends corresponding to the use of exon 1 transcription initiation sites were translated in vitro into prepro-IGF-I peptides initiated at a Met-48 codon in exon 1 or a Met-22 codon in exon 3, and RNAs with a 5'-end corresponding to the major exon 2 transcription start site were translated into a prepro-IGF-I peptide initiated at a Met-32 codon in exon 2. All forms of prepro-IGF-I were processed by canine pancreatic microsomes, suggesting that all these prepeptides function as signal peptides. The translational efficiency of IGF-I RNAs was inversely proportional to the length of the 5'-untranslated region. Mutation of the first of three upstream AUG codons in exon 1, which potentially initiates a 14-amino acid open reading frame, did not affect prepro-IGF-I translation. The other two AUG codons are immediately followed by stop codons. The absence of both upstream AUG codons in a completely spliced exon 1-derived RNA enhanced the in vitro and in vivo translatability of this RNA as compared with the full-length RNA. Mutation of the downstream initiation codon in particular increased translational efficiency in vitro and in intact cells, suggesting that an inefficient reinitiation event at the Met-48 codon contributes to the poorer translation of IGF-I mRNAs in which these upstream AUGUGA motifs occur. We conclude that IGF-I mRNAs potentially encode multiple forms of preproIGF and that specific differences in their 5'-untranslated regions provide a molecular basis for translational control of IGF-I biosynthesis.