Research Article

Structure of a yeast step II catalytically activated spliceosome

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Science  15 Dec 2016:
aak9979
DOI: 10.1126/science.aak9979

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Abstract

Each cycle of 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 (cryo-EM) structure of a C* complex from Saccharomyces cerevisiae at an average resolution of 4.0 Å. Compared with the preceding spliceosomal complex (C complex), the lariat junction has been translocated by 15 to 20 Å 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 RNaseH-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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