Structures of the Catalytically Activated Yeast Spliceosome Reveal the Mechanism of Branching

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 small nuclear RNA (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.Graphical Abstract

Source: Cell - March 15, 2019 Category: Cytology Source Type: research