Structural Basis of Nuclear pre-mRNA Splicing: Lessons from Yeast

Noncoding introns are removed from nuclear precursor messenger RNA (pre-mRNA) in a twostep phosphoryl transfer reaction by the spliceosome, a dynamic multimegadalton enzyme. Cryo-electron microscopy (cryo-EM) structures of the Saccharomyces cerevisiae spliceosome were recently determined in eight key states. Combined with the wealth of available genetic and biochemical data, these structures have revealed new insights into the mechanisms of spliceosome assembly, activation, catalysis, and disassembly. The structures show how a single RNA catalytic center forms during activation and accomplishes both steps of the splicing reaction. The structures reveal how spliceosomal helicases remodel the spliceosome for active site formation, substrate docking, reaction product undocking, and spliceosome disassembly and how they facilitate splice site proofreading. Although human spliceosomes contain additional proteins, their cryo-EM structures suggest that the underlying mechanism is conserved across all eukaryotes. In this review, we summarize the current structural understanding of pre-mRNA splicing. Outline 1 Introduction 2 Splice site recognition by the U1 and U2 snRNPs and prespliceosome formation 3 U4/U6.U5 tri-snRNP 4 Spliceosome assembly 5 Spliceosome activation 6 Step I (lariat formation) 7 Spliceosome remodeling 8 Step II (exon ligation) 9 Spliceosome disassembly and recycling 10 Conclusion