To ligate exons in pre-messenger RNA (pre-mRNA) splicing, the spliceosome must reposition the substrate after cleaving the 5′ splice site. Because spliceosomal small nuclear RNAs (snRNAs) bind the substrate, snRNA structures may rearrange to reposition the substrate. However, such rearrangements have remained undefined. Although U2 stem IIc inhibits binding of U2 snRNP to pre-mRNA during assembly, we found that weakening U2 stem IIc suppressed a mutation in prp16, a DExD/H box ATPase that promotes splicing after 5′ splice site cleavage. The prp16 mutation was also suppressed by mutations flanking stem IIc, suggesting that Prp16p facilitates a switch from stem IIc to the mutually exclusive U2 stem IIa, which activates binding of U2 to pre-mRNA during assembly. Providing evidence that stem IIa switches back to stem IIc before exon ligation, disrupting stem IIa suppressed 3′ splice site mutations, and disrupting stem IIc impaired exon ligation. Disrupting stem IIc also exacerbated the 5′ splice site cleavage defects of certain substrate mutations, suggesting a parallel role for stem IIc at both catalytic stages. We propose that U2, much like the ribosome, toggles between two conformations—a closed stem IIc conformation that promotes catalysis and an open stem IIa conformation that promotes substrate binding and release.
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机译:为了在信使前RNA(pre-mRNA)剪接中连接外显子,剪接体必须在切割5'剪接位点后重新定位底物。由于剪接小核RNA(snRNA)结合底物,因此snRNA结构可能会重新排列以重新定位底物。但是,这种重排仍然不确定。尽管U2茎IIc在组装过程中抑制了U2 snRNP与pre-mRNA的结合,但我们发现弱化的U2茎IIc抑制了prp16中的突变,这是一种DExD / H box ATPase,可在5'剪接位点切割后促进剪接。 prp16突变也被茎IIc侧翼的突变所抑制,表明Prp16p促进了从茎IIc到互斥的U2茎IIa的转换,这在组装过程中激活了U2与pre-mRNA的结合。提供证据表明,茎IIa在外显子结扎之前转换回茎IIc,破坏茎IIa抑制了3'剪接位点突变,破坏茎IIc损伤了外显子结扎。破坏茎IIc还会加剧某些底物突变的5'剪接位点切割缺陷,这表明茎IIc在两个催化阶段均具有平行作用。我们建议,U2与核糖体很像,在两个构象之间切换-促进催化的闭合茎IIc构象和促进底物结合和释放的开放茎IIa构象。
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