Research Article

Molecular architecture of the human U4/U6.U5 tri-snRNP

Science  25 Mar 2016:
Vol. 351, Issue 6280, pp. 1416-1420
DOI: 10.1126/science.aad2085

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A human spliceosomal subcomplex

The spliceosome is an RNA and protein molecular machine that cuts out introns from messenger RNAs. Agafonov et al. used cryo-electron microscopy to determine the structure of the largest intermediate subcomplex on the assembly pathway for the human spliceosome (see the Perspective by Cate). The structure shows substantial differences from the equivalent yeast complex. It also reveals how the subcomplex must dock onto the rest of the spliceosome and hints at the structural changes the complex must go through to form the mature spliceosome.

Science, this issue p. 1416; see also p. 1390

Abstract

The U4/U6.U5 triple small nuclear ribonucleoprotein (tri-snRNP) is a major spliceosome building block. We obtained a three-dimensional structure of the 1.8-megadalton human tri-snRNP at a resolution of 7 angstroms using single-particle cryo–electron microscopy (cryo-EM). We fit all known high-resolution structures of tri-snRNP components into the EM density map and validated them by protein cross-linking. Our model reveals how the spatial organization of Brr2 RNA helicase prevents premature U4/U6 RNA unwinding in isolated human tri-snRNPs and how the ubiquitin C-terminal hydrolase–like protein Sad1 likely tethers the helicase Brr2 to its preactivation position. Comparison of our model with cryo-EM three-dimensional structures of the Saccharomyces cerevisiae tri-snRNP and Schizosaccharomyces pombe spliceosome indicates that Brr2 undergoes a marked conformational change during spliceosome activation, and that the scaffolding protein Prp8 is also rearranged to accommodate the spliceosome’s catalytic RNA network.

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