The ribosomal translocase homologue Snu114p is involved in unwinding U4/U6 RNA during activation of the spliceosome

Abstract

Snu114p is a yeast U5 snRNP protein homologous to the ribosomal elongation factor EF‐2. Snu114p exhibits the same domain structure as EF‐2, including the G‐domain, but with an additional N‐terminal domain. To test whether Snu114p in the spliceosome is involved in rearranging RNA secondary structures (by analogy to EF‐2 in the ribosome), we created conditionally lethal mutants. Deletion of this N‐terminal domain (snu114ΔN) leads to a temperature‐sensitive phenotype at 37°C and a pre‐mRNA splicing defect in vivo. Heat treatment of snu114ΔN extracts blocked splicing in vitro before the first step. The snu114ΔN still associates with the tri‐snRNP, and the stability of this particle is not significantly impaired by thermal inactivation. Heat treatment of snu114ΔN extracts resulted in accumulation of arrested spliceosomes in which the U4 RNA was not efficiently released, and we show that U4 is still base paired with the U6 RNA. This suggests that Snu114p is involved, directly or indirectly, in the U4/U6 unwinding, an essential step towards spliceosome activation.