Four distinct B* complexes (S. cerevisiae)
Pre-mRNA splicing is executed by the spliceosome. Structural characterization of the catalytically activated complex (B*) is pivotal for understanding the branching reaction. In this study, we assembled the B* complexes on two different pre-mRNAs from Saccharomyces cerevisiae and determined the cryo-EM structures of four distinct B* complexes at overall resolutions of 2.9-3.8 Å. The duplex between U2 snRNA and the branch point sequence (BPS) is discretely away from the 5’-splice site (5’SS) in the three B* complexes that are devoid of the step I splicing factors Yju2 and Cwc25. Recruitment of Yju2 into the active site brings the U2/BPS duplex into the vicinity of 5’SS, with the BPS nucleophile positioned 4 Å away from the catalytic metal M2. This analysis reveals the functional mechanism of Yju2 and Cwc25 in branching. These structures on different pre-mRNAs reveal substrate-specific conformations of the spliceosome in a major functional state.
Wan, R., Bai, R., Yan, C., Lei, J., & Shi, Y. (2019). Structures of the catalytically activated yeast spliceosome reveal the mechanism of branching. Cell. 177(2):339-351.
PDB code: 6J6G
6J6H
6J6Q
6J6N
Pymol session file