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.
Keywords: 3′SS recognition; DEAH-box ATPase/helicase; P complex; Prp22; atomic structure; cryo-EM; exon ligation; mRNA release; post-catalytic splicesome; pre-mRNA splicing.
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