TY - JOUR T1 - Structures of the fully assembled <em>Saccharomyces cerevisiae</em> spliceosome before activation JF - Science JO - Science SP - 1423 LP - 1429 DO - 10.1126/science.aau0325 VL - 360 IS - 6396 AU - Bai, Rui AU - Wan, Ruixue AU - Yan, Chuangye AU - Lei, Jianlin AU - Shi, Yigong Y1 - 2018/06/29 UR - http://science.sciencemag.org/content/360/6396/1423.abstract N2 - 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. 1423The 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. ER -