Cwc2 and its human homologue RBM22 promote an active conformation of the spliceosome catalytic centre

RNA‐structural elements play key roles in pre‐mRNA splicing catalysis; yet, the formation of catalytically competent RNA structures requires the assistance of spliceosomal proteins. We show that the S. cerevisiae Cwc2 protein functions prior to step 1 of splicing, and it is not required for the Prp2‐mediated spliceosome remodelling that generates the catalytically active B * complex, suggesting that Cwc2 plays a more sophisticated role in the generation of a functional catalytic centre. In active spliceosomes, Cwc2 contacts catalytically important RNA elements, including the U6 internal stem‐loop (ISL), and regions of U6 and the pre‐mRNA intron near the 5′ splice site, placing Cwc2 at/near the spliceosome's catalytic centre. These interactions are evolutionarily conserved, as shown by studies with Cwc2's human counterpart RBM22, indicating that Cwc2/RBM22–RNA contacts are functionally important. We propose that Cwc2 induces an active conformation of the spliceosome's catalytic RNA elements. Thus, the function of RNA–RNA tertiary interactions within group II introns, namely to induce an active conformation of domain V, may be fulfilled by proteins that contact the functionally analogous U6‐ISL, within the spliceosome. The nineteen complex plays an important role in regulating in regulating spliceosome conformation and fidelity. Cwc2, a constituent of the complex, is located at the vicinity of the catalytic centre of the spliceosome and is required for the first step of pre‐mRNA splicing, but not spliceosome assembly, by positioning the pre‐mRNA and spliceosomal RNA.