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Structural analysis shows spliceosome-induced closure of an α-helical super-helix in the proto-oncogenic splicing factor SF3b1

The SF3b1 splicing factor is a subunit of a multiprotein SF3b complex that is a checkpoint protein for spliceosome assembly. SF3b1 comprises a super-helix of repeating -helical units. In the context of the spliceosome, the SF3b1 -helical repeats bind a duplex between the spliceosomal U2 snRNA and the pre-mRNA branch point sequence, which ultimately offers an adenosine nucleophile for the splicing reaction. Several SF3b structures now are available, including human homologues in the isolated state, pre-B, B, or Bact spliceosome assemblies1, and yeast homologues in the A, pre-B, B, or Bact spliceosome assemblies2. Acquired mutations of SF3b1 commonly recur among hematologic malignancies and cancers. Active “opening” of the SF3b1 torus by ATP-dependent RNA unwindases has been proposed to load SF3b1 on the U2 – BPS duplex and alter branchpoint choice in the presence of cancer-associated mutations. Yet, whether the SF3b1 superhelix opens or closes, and the underlying structural changes, have yet to be rigorously analyzed.