Cryo-EM Structure of a Pre-catalytic Human Spliceosome Primed for Activation

Cell. 2017 Aug 10;170(4):701-713.e11. doi: 10.1016/j.cell.2017.07.011. Epub 2017 Aug 3.


Little is known about the spliceosome's structure before its extensive remodeling into a catalytically active complex. Here, we report a 3D cryo-EM structure of a pre-catalytic human spliceosomal B complex. The U2 snRNP-containing head domain is connected to the B complex main body via three main bridges. U4/U6.U5 tri-snRNP proteins, which are located in the main body, undergo significant rearrangements during tri-snRNP integration into the B complex. These include formation of a partially closed Prp8 conformation that creates, together with Dim1, a 5' splice site (ss) binding pocket, displacement of Sad1, and rearrangement of Brr2 such that it contacts its U4/U6 substrate and is poised for the subsequent spliceosome activation step. The molecular organization of several B-specific proteins suggests that they are involved in negatively regulating Brr2, positioning the U6/5'ss helix, and stabilizing the B complex structure. Our results indicate significant differences between the early activation phase of human and yeast spliceosomes.

Keywords: B complex spliceosome; B-specific proteins; cryo-EM; pre-catalytic spliceosome; pre-mRNA splicing; spliceosome; spliceosome structure.

MeSH terms

  • Cell Nucleus / chemistry
  • Cryoelectron Microscopy
  • HeLa Cells
  • Humans
  • Models, Molecular
  • RNA-Binding Proteins / chemistry
  • Ribonucleoproteins, Small Nuclear / chemistry
  • Saccharomyces cerevisiae / chemistry
  • Spliceosomes / chemistry*
  • Spliceosomes / ultrastructure


  • RNA-Binding Proteins
  • Ribonucleoproteins, Small Nuclear