Structure of a yeast activated spliceosome at 3.5 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. Pre messenger RNA (pre mRNA) splicing is carried out by the spliceosome, which undergoes an intricate assembly and activation process. Here, we report an atomic structure of an activated spliceosome (known as the Bact complex) from Saccharomyces cerevisiae, determined by cryo electron microscopy at an average resolution of 3.52 angstroms. The final refined model contains U2 and U5 small nuclear ribonucleoprotein particles (snRNPs) U6 small nuclear RNA (snRNA) nineteen complex (NTC) NTC related (NTR) protein, and a 71 nucleotide pre mRNA molecule, which amount to 13,505 amino acids from 38 proteins and a combined molecular mass of about 1.6 megadaltons. The 5' exon is anchored by loop I of U5 snRNA, whereas the 5' splice site (5'SS) and the branch point sequence (BPS) of the intron are specifically recognized by U6 and U2 snRNA, respectively. Except for coordination of the catalytic metal ions, the RNA elements at the catalytic cavity of Prp8 are mostly primed for catalysis. The catalytic latency is maintained by the SF3b complex, which encircles the BPS, and the splicing factors Cwc24 and Prp11, which shield the 5' exon 5'SS junction. This structure, together with those determined earlier, outlines a molecular framework for the pre mRNA splicing reaction.
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