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 steps of spliceosome assembly whose sequence contains an amino-terminal putative leucine zipper structure and two carboxy-terminal motifs reminiscent of zinc fingers. Here, we show that the deletion of the second carboxy-terminal motif results in a dominant lethal phenotype. This observation, combined with an in vivo-binding assay for protein-protein interactions, reveals the presence of two distinct binding sites on the PRP9 protein. The carboxy-terminal region contributes to the PRP9 homodimerization, whereas the amino-terminal region binds the SPP91 splicing factor. Further experiments suggest that other factors bind to PRP9 and SPP91 proteins. Finally, we demonstrate that the PRP9 protein acts after the formation of the U1 snRNP - pre-mRNA complex. The existence of a protein complex including the PRP9 factor is discussed.