The content discusses the structural insights into the transition from exon-defined to intron-defined spliceosome assembly. It explains that early spliceosome assembly can occur through an intron-defined pathway or an exon-defined pathway. In the exon-defined pathway, U2 binds the branch site upstream of the defined exon, and U1 snRNP interacts with the 5' splice site downstream of it. The U4/U6.U5 tri-snRNP then binds to produce a cross-exon (CE) pre-B complex, which is then converted to the spliceosomal B complex.
The content states that exon definition promotes the splicing of upstream introns and plays a key role in alternative splicing regulation. However, the three-dimensional structure of exon-defined spliceosomal complexes and the molecular mechanism of the conversion from a CE-organized to a cross-intron (CI)-organized spliceosome, a pre-requisite for splicing catalysis, remain poorly understood.
The content then describes cryo-electron microscopy analyses of human CE pre-B complex and B-like complexes, which reveal extensive structural similarities with their CI counterparts. This indicates that the CE and CI spliceosome assembly pathways converge already at the pre-B stage. Add-back experiments using purified CE pre-B complexes, coupled with cryo-electron microscopy, elucidate the order of the extensive remodelling events that accompany the formation of B complexes and B-like complexes. The molecular triggers and roles of B-specific proteins in these rearrangements are also identified.
Finally, the content shows that CE pre-B complexes can productively bind in trans to a U1 snRNP-bound 5' splice site, providing new mechanistic insights into the CE to CI switch during spliceosome assembly and its effect on pre-mRNA splice site pairing at this stage.
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by Zhen... um www.nature.com 05-22-2024
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