A protein essential for pre-mRNA splicing, the U2 auxiliary factor (U2AF), is composed of a large and small subunit. Previously we cloned and characterized both subunits, spU2AF59 and spU2AF23, from fission yeast. We now report a novel U2AF-associated-protein, spUAP2, which interacts with both subunits. SpUAP2 contains a classical and a degenerate RNA recognition motif (RRM), both of which are required for interaction with spU2AF59. Interaction also requires the arginine/serine-rich region and the first RRM of spU2AF59. A null allele of the gene for spUAP2 is lethal.

The prp2 gene of fission yeast has previously been shown to encode the large subunit of the splicing factor spU2AF. SpU2AF59 is an evolutionarily conserved protein that has an arginine/serine‐rich region and three RNA recognition motifs (RRMs). We have sequenced three temperature‐sensitive alleles of prp2 and determined that the mutations result in single amino acid changes within one of the RRMs or between RRMs. All mutant alleles of prp2 have pre‐mRNA splicing defects at the non‐permissive temperature. Although the mutant strains are growth‐arrested at 37°C, they do not elongate like typical fission yeast cell cycle mutants. The DNA of the prp2− strains stains more intensely than a wild‐type strain, suggesting that the chromatin may be condensed. Ultrastructural studies show differences in the mutant nuclei including a prominent distinction between the chromatin‐ and non‐chromatin‐enriched regions compared to the more homogenous wild‐type nucleus. Two‐hybrid assays indicate that some of the wild‐type protein interactions are altered in the mutant strains. These results suggest that normal functioning of spU2AF59 may be essential not only for pre‐mRNA splicing but also for the maintenance of proper nuclear structure and normal cell cycle progression. Copyright © 2000 John Wiley & Sons, Ltd.