Research Article

Structure of a yeast catalytically activated spliceosome at 3.5 Å resolution

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Science  21 Jul 2016:
DOI: 10.1126/science.aag0291

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Pre-mRNA splicing is carried out by the spliceosome, which undergoes an intricate assembly and activation process. Here we report an atomic structure of a catalytically activated spliceosome (known as the Bact complex) from Saccharomyces cerevisiae, determined by cryo–electron microscopy (cryo-EM) at an average resolution of 3.52 angstroms. The final refined model contains U2 and U5 small nuclear ribonucleoprotein particles (snRNPs), U6 small nuclear RNA (snRNA), nineteen complex (NTC), NTC-related (NTR), and a 71-nucleotide pre-mRNA molecule, which amount to 13,505 amino acids from 38 proteins and a combined molecular mass of approximately 1.6 megadaltons. The 5ʹ-exon is anchored by loop I of U5 snRNA, whereas the 5ʹ-splice site (5ʹSS) and the branch point sequence (BPS) of the intron are specifically recognized by U6 and U2 snRNA, respectively. Except coordination of the catalytic metal ions, the RNA elements at the catalytic cavity of Prp8 are mostly primed for catalysis. The catalytic latency is maintained by the SF3b complex, which encircles the BPS, and the splicing factors Cwc24 and Prp11, which shield the 5ʹ-exon-5ʹSS junction. This structure, together with those determined earlier, outline a molecular framework for the pre-mRNA splicing reaction.

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