B complex (S. cerevisiae)
The pre-catalytic spliceosome (B complex) is preceded by the pre-B complex. We determined the cryo-EM structures of the Saccharomyces cerevisiae pre-B and B complexes at average resolutions of 3.3-4.6 and 3.9 Å, respectively. In the pre-B complex, the duplex between the 5′-splice site (5′SS) and U1 snRNA is recognized by Yhc1, Luc7, and the Sm ring. In the B complex, U1 snRNP is dissociated, the 5′-exon-5′SS sequences are translocated near U6 snRNA, and three B-specific proteins may orient the pre-mRNA. 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 elucidation of the B complex fills an important void in mechanistic understanding of pre-mRNA splicing by the spliceosome. Structural analysis reveals the mechanism of assembly and activation for the yeast spliceosome.
Bai, R., Wan, R., Yan, C., Lei, J., & Shi, Y. (2018). Structures of the fully assembled Saccharomyces cerevisiae spliceosome before activation. Science, 360(6396), 1423-1429.
PDB code: 5ZWO
Pymol session file