Biophysical characterizations of the recognition of the AAUAAA polyadenylation signal

Most eukaryotic messenger RNA precursors must undergo 39-end cleavage and polyadenylation for maturation. We and others recently reported the structure of the AAUAAA polyadenylation signal (PAS) in complex with the protein factors CPSF-30, WDR33 and CPSF-160, revealing the molecular mechanism for this recognition. Here we have characterized in detail the interactions between the PAS RNA and the protein factors using fluorescence polarization experiments. Our studies show that AAUAAA is recognized with ~1 nM affinity by the CPSF-160-WDR33-CPSF-30 ternary complex. Variations in the RNA sequence can greatly reduce the affinity. Similarly, mutations of residues that have van der Waals interactions with the bases of AAUAAA also lead to substantial reductions in affinity. Finally, our studies confirm that both CPSF-30 and WDR33 are required for binding the PAS RNA, and determine a ~7 nM affinity between CPSF-30 and the CPSF-160-WDR33 binary complex.