A protein related to splicing factor U2AF(35) that interacts with U2AF(65) and SR proteins in splicing of pre-mRNA

Recognition of a functional 3' splice site in pre-mRNA splicing requires a heterodimer of the proteins U2AF(65)/U2AF(35). U2AF(65) binds to RNA at the polypyrimidine tract(1,2), whereas U2AF(35) is thought to interact through its arginine/serine-rich (RS) domain with other RS-domain-containing factors;bound at the 5' splice site, assembled in splicing enhancer complexes, or associated with the U4/U6 . U5 small nuclear ribonucleoprotein complex(3-7). It is unclear, however, how such network interactions can all be established through the small RS domain in U2AF(35). Here we describe the function of a U2AF(35)-related protein (Urp), which is the human homologue of a mouse imprinted gene. Nuclear extracts depleted of Urp are defective in splicing, but activity can be restored by addition of recombinant Urp. U2AF(35) could not replace Urp in complementation, indicating that their functions do not overlap. Co-immunodepletion showed that Urp is associated with the U2AF(65)/U2AF(35) heterodimer, Binding studies revealed that Urp specifically interacts with U2AF(65) through a U2AF(35)-homologous region and with SR proteins (a large family of RS-domain-containing proteins) through its RS domain. Therefore, Urp and U2AF(35) may independently position RS-domain-containing factors within spliceosomes.