PDF(Pubmed)
Abstract：
Spliceosome is often assembled across an exon and undergoes rearrangement to span a neighboring intron. Most states of the intron-defined spliceosome have been structurally characterized. However, the structure of a fully assembled exon-defined spliceosome remains at large. During spliceosome assembly, the pre-catalytic state (B complex) is converted from its precursor (pre-B complex). Here we report atomic structures of the exon-defined human spliceosome in four sequential states: mature pre-B, late pre-B, early B, and mature B. In the previously unknown late pre-B state, U1 snRNP is already released but the remaining proteins are still in the pre-B state; unexpectedly, the RNAs are in the B state, with U6 snRNA forming a duplex with 5\'-splice site and U5 snRNA recognizing the 3\'-end of the exon. In the early and mature B complexes, the B-specific factors are stepwise recruited and specifically recognize the exon 3\'-region. Our study reveals key insights into the assembly of the exon-defined spliceosomes and identifies mechanistic steps of the pre-B-to-B transition.
摘要：
剪接体通常在外显子上组装，并经过重排以跨越相邻的内含子。内含子定义的剪接体的大多数状态已在结构上进行了表征。然而,完全组装的外显子定义的剪接体的结构仍然很大。在剪接体组装期间，预催化状态(B络合物)从其前体(pre-B络合物)转化。在这里，我们以四种顺序状态报告了外显子定义的人类剪接体的原子结构：成熟的pre-B，后期pre-B，早期B,和成熟的B。在以前未知的晚期B状态下，U1snRNP已经释放，但剩余的蛋白质仍然处于前B状态；出乎意料的是，RNA处于B状态，U6snRNA形成具有5'-剪接位点的双链体，U5snRNA识别外显子的3'末端。在早期和成熟的B复合物中，B特异性因子是逐步招募的，并特异性识别外显子3'区。我们的研究揭示了外显子定义的剪接体组装的关键见解，并确定了B到B转换前的机制步骤。
