A Big Bang in spliceosome structural biology

Look at a protein-coding gene in the genome of any eukaryote—be it animal, plant, fungus, or protist—and you will likely find the coding region fragmented by intervening sequences known as introns. When the gene is transcribed, these introns have to be removed from the pre-messenger RNA (pre-mRNA) before a protein can be made. How these introns are removed has been studied intensively for decades without the aid of a three-dimensional map of the highly dynamic machine at the heart of the process: the spliceosome. On page 1416 of this issue, Agafonov et al. report the first molecular-resolution reconstruction of a central assembly of the human spliceosome, the U4/U6.U5 triple small nuclear ribonucleoprotein (tri-snRNP) complex, using cryo-electron microscopy (cryo-EM) (1). Together with high-resolution cryo-EM reconstructions of spliceosome assemblies from fungi (2-5) and the x-ray crystal structure of the U1 snRNP (6), these structural models of the splicing machinery launch a new era in understanding eukaryotic gene regulation.