Nucleocytoplasmic shuttling of heterodimeric splicing factor U2AF

The U2 small nuclear ribonucleoprotein auxiliary factor (U2AF) is a heterod imeric splicing factor composed of 65-kDa (U2AF(65)) and 35-kDa (U2AF(35)) subunits. The large subunit of USAF recognizes the intronic polypyrimidine tract, a sequence located adjacent to the 3 ' splice site that serves as an important signal for both constitutive and regulated pre-mRNA splicing. Th e small subunit U2AF35 interacts with the 3 ' splice site dinucleotide AG a nd is essential for regulated splicing, Like several other proteins involve d in constitutive and regulated splicing, both U2AF65 and U2AF35 contain an arginine/ serine-rich (RS) domain. In the present study we determined the role of RS domains in the subcellular localization of U2AF. Both U2AF65 and U2AF35 are shown to shuttle continuously between the nucleus and the cytop lasm by a mechanism that involves carrier receptors and is independent from binding to mRNA. The RS domain on either U2AF65 or U2AF35 acts as a nuclea r localization signal and is sufficient to target a heterologous protein to the nuclear speckles. Furthermore, the results suggest that the presence o f an RS domain in either U2AF subunit is sufficient to trigger the nucleocy toplasmic import of the heterodimeric complex. Shuttling of U2AF between nu cleus and cytoplasm possibly represents a means to control the availability of this factor to initiate spliceosome assembly and therefore contribute t o regulate splicing.