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 (snu114 Delta N) leads to a temperature-sensitive phenotype at 37 degrees C and a pre-mRNA splicing defect in vivo. Heat treatment of snu114 Delta N extracts blocked splicing in vitro before the first step. The snu114 Delta N still associates with the tri-snRNP, and the stability of this particle is not significantly impaired by thermal inactivation. Heat treatment of snu114 Delta N 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.