U5 small nuclear ribonucleoprotein 200 kDa helicase
U2-type precatalytic spliceosome
Structure and Conformational Dynamics of the Human Spliceosomal B<sup>act</sup> Complex.
The spliceosome is a highly dynamic macromolecular complex that precisely excises introns from pre-mRNA. Here we report the cryo-EM 3D structure of the human B<sup>act</sup> spliceosome at 3.4 Å resolution. In the B<sup>act</sup> state, the spliceosome is activated but not catalytically primed, so that it is functionally blocked prior to the first catalytic step of splicing. The spliceosomal core is similar to the yeast B<sup>act</sup> spliceosome; important differences include the presence of the RNA helicase aquarius and peptidyl prolyl isomerases. To examine the overall dynamic behavior of the purified spliceosome, we developed a principal component analysis-based approach. Calculating the energy landscape revealed eight major conformational states, which we refined to higher resolution. Conformational differences of the highly flexible structural components between these eight states reveal how spliceosomal components contribute to the assembly of the spliceosome, allowing it to generate a dynamic interaction network required for its subsequent catalytic activation.