Research Article

A human postcatalytic spliceosome structure reveals essential roles of metazoan factors for exon ligation

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Science  15 Feb 2019:
Vol. 363, Issue 6428, pp. 710-714
DOI: 10.1126/science.aaw5569

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A human P spliceosome structure

Splicing of some pre–messenger RNAs could be regulated by cell type–specific splicing factors. Fica et al. describe the cryo–electron microscopy structure of the human postcatalytic (P) spliceosome. Surprisingly, it lacks the splicing factor Prp18, which plays an essential role in exon ligation in the yeast spliceosome. Instead, a metazoan-specific splicing factor, FAM32A, compensates for Prp18 and promotes exon ligation by penetrating the active sites and directly stapling the 5′ exon and the 3′ splice site. These findings suggest a way to control tissue-specific alternative splicing.

Science, this issue p. 710

Abstract

During exon ligation, the Saccharomyces cerevisiae spliceosome recognizes the 3′-splice site (3′SS) of precursor messenger RNA (pre-mRNA) through non–Watson-Crick pairing with the 5′SS and the branch adenosine, in a conformation stabilized by Prp18 and Prp8. Here we present the 3.3-angstrom cryo–electron microscopy structure of a human postcatalytic spliceosome just after exon ligation. The 3′SS docks at the active site through conserved RNA interactions in the absence of Prp18. Unexpectedly, the metazoan-specific FAM32A directly bridges the 5′-exon and intron 3′SS of pre-mRNA and promotes exon ligation, as shown by functional assays. CACTIN, SDE2, and NKAP—factors implicated in alternative splicing—further stabilize the catalytic conformation of the spliceosome during exon ligation. Together these four proteins act as exon ligation factors. Our study reveals how the human spliceosome has co-opted additional proteins to modulate a conserved RNA-based mechanism for 3′SS selection and to potentially fine-tune alternative splicing at the exon ligation stage.

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