Drosophila SPF45: a bifunctional protein with roles in both splicing and DNA repair.

The sequence of the SPF45 protein is significantly conserved, yet functional studies have identified it as a splicing factor in animal cells and as a DNA-repair protein in plants. Using a combined genetic and biochemical approach to investigate this apparent functional discrepancy, we unify and validate both of these studies by demonstrating that the Drosophila melanogaster protein is bifunctional, with independent functions in DNA repair and splicing. We find that SPF45 associates with the U2 snRNP and that mutations that remove the C-terminal end of the protein disrupt this interaction. Although animals carrying this mutation are viable, they are nevertheless compromised in their ability to regulate Sex-lethal splicing, demonstrating that Sex-lethal is an important physiological target of SPF45. Furthermore, these mutant animals exhibit phenotypes diagnostic of difficulties in recovering from exogenously induced DNA damage. The conclusion that SPF45 functions in the DNA-repair pathway is strengthened by finding both genetic and physical interactions between SPF45 and RAD201, a previously uncharacterized member of the RecA/Rad51 protein family. Together with our finding that the fly SPF45 protein increases the survival rate of mutagen-treated bacteria lacking the RecG helicase, these studies provide the tantalizing suggestion that SPF45 has an ancient and evolutionarily conserved role in DNA repair.