Prp22, a DExH-box RNA helicase, plays two distinct roles in yeast pre-mRNA splicing.

In order to assess the role of Prp22 in yeast pre-mRNA splicing, we have purified the 130 kDa Prp22 protein and developed an in vitro depletion/reconstitution assay. We show that Prp22 is required for the second step of actin pre-mRNA splicing. Prp22 can act on pre-assembled spliceosomes that are arrested after step 1 in an ATP-independent fashion. The requirement for Prp22 during step 2 depends on the distance between the branchpoint and the 3' splice site, suggesting a previously unrecognized role for Prp22 in splice site selection. We characterize the biochemical activities of Prp22, a member of the DExH-box family of proteins, and we show that purified recombinant Prp22 protein is an RNA-dependent ATPase and an ATP-dependent RNA helicase. Prp22 uses the energy of ATP hydrolysis to effect the release of mRNA from the spliceosome. Thus, Prp22 has two distinct functions in yeast pre-mRNA splicing: an ATP-independent role during the second catalytic step and an ATP-requiring function in disassembly of the spliceosome.