Research Article

Structure of a yeast step II catalytically activated spliceosome

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Science  13 Jan 2017:
Vol. 355, Issue 6321, pp. 149-155
DOI: 10.1126/science.aak9979

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Poised for the second step of splicing

In eukaryotes, noncoding sequences in transcribed precursor mRNA are cut out by a dynamic macromolecular machine, the spliceosome. This involves two sequential reactions. The first cuts one end of the noncoding intron and loops it back on itself to form an intron lariat, and the next excises the intron and ligates the coding mRNA. Insights into the first step of splicing have come from the structures of two intermediates: the Bact complex, which is primed for catalysis, and the C complex, which is formed after the first splicing reaction. Yan et al. now report a high-resolution structure of the step II catalytically activated spliceosome (the C* complex). This structure shows conformational changes that position catalytic motifs to accomplish the second splicing reaction.

Science, this issue p. 149

Abstract

Each cycle of precursor messenger RNA (pre-mRNA) splicing comprises two sequential reactions, first freeing the 5′ exon and generating an intron lariat–3′ exon and then ligating the two exons and releasing the intron lariat. The second reaction is executed by the step II catalytically activated spliceosome (known as the C* complex). Here, we present the cryo–electron microscopy structure of a C* complex from Saccharomyces cerevisiae at an average resolution of 4.0 angstroms. Compared with the preceding spliceosomal complex (C complex), the lariat junction has been translocated by 15 to 20 angstroms to vacate space for the incoming 3′-exon sequences. The step I splicing factors Cwc25 and Yju2 have been dissociated from the active site. Two catalytic motifs from Prp8 (the 1585 loop and the β finger of the ribonuclease H–like domain), along with the step II splicing factors Prp17 and Prp18 and other surrounding proteins, are poised to assist the second transesterification. These structural features, together with those reported for other spliceosomal complexes, yield a near-complete mechanistic picture on the splicing cycle.

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