Research Article

Structures of the fully assembled Saccharomyces cerevisiae spliceosome before activation

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Science  29 Jun 2018:
Vol. 360, Issue 6396, pp. 1423-1429
DOI: 10.1126/science.aau0325

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Structural basis for spliceosome assembly

The spliceosome removes noncoding sequences from precursor RNA and ligates coding sequences into useful mRNA. The pre-spliceosome (A complex) associates with a small nuclear ribonucleoprotein (snRNP) complex called U4/U6.U5 tri-snRNP to form the pre-B complex, which is converted into the precatalytic B complex. Bai et al. solved the cryo–electron microscopy structures of the pre-B and B complexes isolated from yeast. These structures show the U1 and U2 snRNPs and allow modeling of the A complex to reveal the early steps of spliceosome assembly and activation.

Science, this issue p. 1423


The precatalytic spliceosome (B complex) is preceded by the pre-B complex. Here we report the cryo–electron microscopy structures of the Saccharomyces cerevisiae pre-B and B complexes at average resolutions of 3.3 to 4.6 and 3.9 angstroms, respectively. In the pre-B complex, the duplex between the 5′ splice site (5′SS) and U1 small nuclear RNA (snRNA) is recognized by Yhc1, Luc7, and the Sm ring. In the B complex, U1 small nuclear ribonucleoprotein is dissociated, the 5′-exon–5′SS sequences are translocated near U6 snRNA, and three B-specific proteins may orient the precursor messenger RNA. In both complexes, U6 snRNA is anchored to loop I of U5 snRNA, and the duplex between the branch point sequence and U2 snRNA is recognized by the SF3b complex. Structural analysis reveals the mechanism of assembly and activation for the yeast spliceosome.

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