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 His6-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.