Removal of an intron from a pre-mRNA by the spliceosome results in the ligation of two exons in the post-catalytic spliceosome (known as the P complex). Here, we present a cryo-EM structure of the P complex from Saccharomyces cerevisiae at an average resolution of 3.6 Å. The ligated exon is held in the active site through RNA-RNA contacts. Three bases at the 3' end of the 5' exon remain anchored to loop I of U5 small nuclear RNA, and the conserved AG nucleotides of the 3'-splice site (3'SS) are specifically recognized by the invariant adenine of the branch point sequence, the guanine base at the 5' end of the 5'SS, and an adenine base of U6 snRNA. The 3'SS is stabilized through an interaction with the 1585-loop of Prp8. The P complex structure provides a view on splice junction formation critical for understanding the complete splicing cycle.

中文翻译：

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酿酒酵母的催化后剪接体的结构。

剪接体从前mRNA除去内含子导致催化后剪接体（称为P复合体）中两个外显子的连接。在这里，我们介绍了来自酵母菌的P复合物的低温-EM结构，其平均分辨率为3.6。连接的外显子通过RNA-RNA接触保持在活性位点。5'外显子的3'末端的三个碱基仍锚定在U5小核RNA的环I，并且3'-剪接位点（3'SS）的保守AG核苷酸被分支的恒定腺嘌呤特异性识别点序列，5'SS的5'末端的鸟嘌呤碱基和U6 snRNA的腺嘌呤碱基。3'SS通过与Prp8的1585环相互作用而稳定。