Cryo-EM structure of a human spliceosome activated for step 2 of splicing.

Recent years have seen substantial progress in understanding the structure of various intermediates of the splicing process. Two groups, led by Reinhard Lhrmann and Kiyoshi Nagai, now describe the cryo-electron microscopy structures (from human and yeast cells, respectively) of the splicing intermediate known as the C complex. The notable feature observed in this complex, relative to the preceding catalytic intermediate (the C complex), is a remodelling that positions the branch-site adenosine and the branched intron out of the catalytic core, opening up space for the 3 exon to dock in preparation for exon ligation.