Structure of a yeast catalytic step I spliceosome at 3.4 A resolution
How spliceosomes make the first cut In eukaryotes, transcribed precursor mRNA includes noncoding sequences that must be spliced out. This is done by the spliceosome, a dynamic complex in which five small nuclear RNAs and several proteins go through a series of ordered interactions and conformational rearrangements to achieve splicing. Two protein structures provide a look at the first catalytic step in the pathway. Yan et al. report the structure of the activated spliceosome (the Bact complex) at 3.5 A resolution, revealing how latency is maintained even though the complex is mostly primed for catalysis. Wan et al. report the structure of the catalytic step 1 spliceosome (the C complex) at 3.4 A resolution; this complex forms after the first step of the splicing reaction. Science, this issue pp. 904 and 895 Two structures give insight into the molecular basis of splicing. Each cycle of pre messenger RNA splicing, carried out by the spliceosome, comprises two sequential transesterification reactions, which result in the removal of an intron and the joining of two exons. Here we report an atomic structure of a catalytic step I spliceosome (known as the C complex) from Saccharomyces cerevisiae, as determined by cryo electron microscopy at an average resolution of 3.4 angstroms. In the structure, the 2' OH of the invariant adenine nucleotide in the branch point sequence (BPS) is covalently joined to the phosphate at the 5' end of the 5' splice site (5'SS) forming an intron lariat. The freed 5' exon remains anchored to loop I of U5 small nuclear RNA (snRNA) and the 5'SS and BPS of the intron form duplexes with conserved U6 and U2 snRNA sequences, respectively. Specific placement of these RNA elements at the catalytic cavity of Prp8 is stabilized by 15 protein components, including Snu114 and the splicing factors Cwc21, Cwc22, Cwc25, and Yju2. These features, representing the conformation of the spliceosome after the first step reaction, predict structural changes that are needed for the execution of the second step transesterification reaction.
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