INTERACTIONS BETWEEN PRP9 AND SPP91 SPLICING FACTORS IDENTIFY A PROTEIN COMPLEX REQUIRED IN PRESPLICEOSOME ASSEMBLY

The PRP9 protein is a yeast splicing factor implicated in the early st eps of spliceosome assembly whose sequence contains an amino-terminal putative leucine zipper structure and two carboxy-terminal motifs remi niscent of zinc fingers. Here, we show that the deletion of the second carboxy-terminal motif results in a dominant lethal phenotype. This o bservation, combined with an in vivo-binding assay for protein-protein interactions, reveals the presence of two distinct binding sites on t he PRP9 protein. The carboxy-terminal region contributes to the PRP9 h omodimerization, whereas the amino-terminal region binds the SPP91 spl icing factor. Further experiments suggest that other factors bind to P RP9 and SPP91 proteins. Finally, we demonstrate that the PRP9 protein acts after the formation of the U1 snRNP-pre-mRNA complex. The existen ce of a protein complex including the PRP9 factor is discussed.