AUF1 binding affinity to A+U-rich elements correlates with rapid mRNA degradation

Rapid degradation of many labile mRNAs is regulated in part by an A+U-rich element (ARE) in their 3'-untranslated regions. Extensive mutational analyses of various AREs have identified important components of the ARE, such as the nonamer motif UUAUUUAUU, two copies of which serve as a potent mRNA destabilizer. To investigate the roles of trans-acting factors in ARE-directed mRNA degradation, we previously purified and molecularly cloned the RNA binding protein AUF1 and demonstrated that both cellular and recombinant AUF1 bind specifically to AREs as shown by UV cross-linking assays in vitro. In the present work, we have examined the in vitro RNA-binding properties of AUF1 using gel mobility shift assays with purified recombinant His(6)-AUF1 fusion protein. We find that ARE binding affinities of AUF1 correlate with the potency of an ARE to direct degradation of a heterologous mRNA, These results support a role for AUF1 in ARE directed mRNA decay that is based upon its affinity for different AREs.