PRP28, a 'DEAD-box' protein, is required for the first step of mRNA splicing in vitro.

We previously reported the isolation of PRP28, a gene in Saccharomyces cerevisiae whose activity is required for the first step of nuclear mRNA splicing in vivo. Sequence analysis revealed that PRP28 is included in the 'DEAD-box' gene family, members of which are thought to function as ATP-dependent RNA helicases. Genetic interactions led us to suggest that PRP28 is functionally associated with the U4/U5/U6 snRNP. We have now purified the PRP28 protein from S. cerevisiae and demonstrated that it is required for the first step of splicing in vitro. Interestingly, PRP28 is not a stably associated snRNP protein. Strand displacement assays indicate that PRP28 does not exhibit RNA helicase activity, suggesting that an additional factor or factors may be required for its activation.