ESSENTIAL DOMAINS OF THE PRP21 SPLICING FACTOR ARE IMPLICATED IN THE BINDING TO PRP9 AND PRP11 PROTEINS AND ARE CONSERVED THROUGH EVOLUTION

The yeast Prp9p, Prp11p, Prp21p proteins form a multimolecular complex identified as the SF3a splicing factor in higher eukaryotes, This fac tor is required for the assembly of the prespliceosome. Prp21p interac ts with both Prp9p and Prp11p, but the molecular basis of these intera ctions is unknown. Prp21p, its human homologue, and the so-called SWAP proteins share a tandemly repeated motif, the surp module, Given the evolutionary conservation and the role of SWAP proteins as splicing re gulators, it has been proposed that surp motifs are essential for inte ractions between Prp21p and other splicing factors, In order to charac terize functional domains of Prp21p and to identify potential addition al functions of this protein, we isolated a series of heat-sensitive p rp21 mutants. Our results indicate that prp21 heat-sensitive mutations are associated with defects in the interaction with Prp9p, but not wi th Prp11p, Interestingly, most heat-sensitive point mutants associate a strong splicing defect with a pre-mRNA nuclear export phenotype, as does the prp9-1 heat-sensitive mutant, Deletion analyses led to the de finition of domains required for viability, These domains are responsi ble for the interaction with Prp9p and Prp11p and are conserved throug h evolution, They do not include the most conserved surp1 module, sugg esting that the conservation of this motif in two families of proteins may reflect a still unknown function dispensable in yeast under stand ard conditions.