The U2 small nuclear ribonucleoprotein auxiliary factor (U2AF) is a heterodimeric splicing factor composed of a large and a small subunit []. The large U2AF subunit recognises 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. The small subunit interacts with the 3' splice site dinucleotide AG and is essential for regulated splicing. The subunits shuttle continuously between the nucleus and the cytoplasm via a mechanism that involves carrier receptors and is independent of binding to mRNA. Both subunits contain an arginine/serine-rich (RS) domain, which acts as a nuclear localisation signal.Furthermore, the presence of an RS domain on either subunit is sufficient to trigger the nucleocytoplasmic import of the heterodimeric complex [, , ].The human form of the U2 auxiliary factor small subunit, hU2AF35, contains a degenerate RNA recognition motif (RRM) and a C-terminal RS domain. Mutations in this protein alters U2AF1 function affects the alternative splicing of target genes associated with myelodysplastic syndrome (MDS) []. The murine form has been shown to be genomically imprinted with monoallelic expression from the paternal allele. However, this is not the case in humans [].
