Session 7 : Structural Studies of Large Assemblies Lectures L 7 . 1 Structure of the spliceosome

The spliceosome is a dynamic RNA-protein complex involved in the removal of non-coding segments (introns) from the precursors of mRNAs (pre-mRNAs). Spliceosome assembles on pre-mRNA substrates by the sequential binding of five canonical subunits – small nuclear ribonucleoprotein particles (U1, U2, U4/U6 and U5 snRNPs) and numerous non-snRNP factors. Hierarchical assembly of the complex is initiated by the recognition of the 5’-splice site and branch site by U1 and U2 snRNPs. Subsequent recruitment of U4/U6.U5 tri-snRNP results in a fully assembled but catalytically inactive complex B. A series of complex structural and compositional rearrangements lead to the formation of a group II intron-like RNA catalytic core required for the two trans-esterification steps of splicing. The enormous complexity and highly dynamic nature of spliceosomes have limited high-resolution structural analysis by x-ray crystallography to individual proteins and smaller sub-complexes. Recent developments in the field of cryo-electron microscopy (cryo-EM) provided solutions to some of these limitations, as illustrated by numerous high-resolution reconstructions of spliceosomal complexes reported in the last three years. I will present our recent progress in the structural analysis of the spliceosomal complexes by cryo-EM. L7.2