AU-A, AN RNA-BINDING ACTIVITY DISTINCT FROM HNRNP A1, IS SELECTIVE FOR AUUUA REPEATS AND SHUTTLES BETWEEN THE NUCLEUS AND THE CYTOPLASM

The 3'-untranslated regions of many labile transcripts contain AU-rich sequences that serve as cis determinants of mRNA stability and transl ational efficiency. Using a photocrosslinking technique, our laborator y has previously defined three cytoplasmic RNA-binding activities spec ific for the AUUUA multimers found in the 3'-untranslated regions of l ymphokine mRNAs. One of these activities, AU-A, has an apparent molecu lar mass of 34 kDa, is constitutively expressed in both primary T cell s and the Jurkat T cell leukemia line, and binds to a variety of U-ric h RNA sequences. Previous studies had shown that AU-A is more prevalen t in the nucleus than the cytoplasm, raising the possibility that AU-A is really a nuclear RNA-binding activity that is found in cytoplasmic extracts because of nuclear leakage during cell fractionation. We now show that AU-A shuttles between the cytoplasm and the nucleus. Our re sults indicate that AU-A is a candidate protein component of ribonucle oprotein complexes that participate in nucleocytoplasmic transport of mRNA and cytoplasmic mRNA metabolism. The properties of AUA activity a re similar to those of heterogenous nuclear ribonucleoprotein A1 (hnRN P A1). However, using monoclonal antibodies to hnRNP A1 and protease d igestion patterns, we show that AU-A activity and hnRNP A1 protein are distinct. These studies have also allowed us to define a fourth RNA-b inding activity of apparent molecular mass 41 kDa with specificity for AUUUA multimers. This activity is restricted to the nucleus and conta ins the hnRNP C protein.