Molecular mechanism of the catalysis for U12-type splicing by the human minor spliceosome
Abstract
Precursor messenger RNA (pre-mRNA) splicing is a central process in eukaryotic gene expression. Among the introns removed, the rare but essential U12-type introns are excised by the minor spliceosome, which in most multicellular eukaryotes constitutes a rate-limiting step with distinct regulatory potential. Despite its critical role, the catalytic mechanism of this distinctive spliceosome remains largely unknown. Here we report the isolation and structure determination of two catalytic states of the human minor spliceosome: the branching-completed C complex and the exon-ligation-ready C* complex. These two cryo-EM structures, at average resolutions of 2.9-3.0 Å, reveal the configuration of key RNA elements and the positions of catalytic metals at the splicing active site. The U12-type catalysis is safeguarded by splicing factors that are unique to the minor spliceosome: MMTAG2 facilitates the stabilization of the branch point sequence (BPS)/U12 duplex to promote branching reaction (step-I); WDR25 and FAM204A orient the BPS/U12 duplex to dock the 3′ splice site (3′SS) for exon-ligation (step-II); RBM41 assists spliceosome binding of the helicase PRP22 to prepare for the mRNA release. These findings reveal catalytic principles of the U12-type splicing and establish new molecular links to development and diseases.
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