Fig. 5. Rearrangements of U2 snRNP during human spliceosome activation.
a, b U2 snRNP, comprising U2 snRNA, SF3a, SF3b and the core, undergoes remarkable positional changes in a stepwise manner during spliceosome activation. In the B-to-pre-Bact transition, U2 snRNP undergoes a large translocation, moving toward the main body of spliceosome. U2 snRNP has a series of minor positional and conformational changes from pre-Bact to Bact-IV. Notably, the SF3b component SF3B6 and the SF3a components SF3A1/SF3A2 are dissociated in the transitions of Bact-I to Bact-II and Bact-III to Bact-IV, respectively. The last component SF3A3 of the SF3a complex is released in the Bact-IV to post-Bact transition. In the B*/C complexes, the SF3b complex is dissociated; U2 snRNP core undergoes prominent movements.