Mutations in the small subunit of the Drosophila U2AF splicing factor cause lethality and developmental defects

The essential eukaryotic pre-mRNA splicing factor U2AF (U2 small nuclear ribonucleoprotein auxiliary factor) is required to specify the 3' splice site at an early step in spliceosome assembly. U2AF binds site-specifically to the intron polypyrimidine tract and recruits U2 small nuclear ribonucleoprotein to the branch site. Human U2AF (hU2AF) is a heterodimer composed of a large (hU2AF(65)) and small (hU2AF(35)) subunit. Although these proteins associate in a tight complex, the biochemical requirement for U2AF activity can be satisfied solely by the large subunit, The requirement for the small subunit in splicing has remained enigmatic, No biochemical activity has been found for hU2AF(35) and it has been implicated in splicing only indirectly by its interaction with known splicing factors, In the absence of a biochemical assay, we have taken a genetic approach to investigate the function of the small subunit in the fruit fly Drosophila melanogaster. A cDNA clone encoding the small subunit of Drosophila U2AF (dU2AF(38)) has been isolated and sequenced, The dU2AF(38) protein is highly homologous to hU2AF(35) containing a conserved central arginine- and serine-rich (RS) domain, A recessive P-element insertion mutation affecting dU2AF(38) causes a reduction in viability and fertility and morphological bristle defects. Consistent with a general role in splicing, a null allele of dU2AF(38) is fully penetrant recessive lethal, like null alleles of the Drosophila U2AF large subunit.