Poly(A)-Specific Ribonuclease (Parn): Molecular Mechanisms Of Mrna Cap And Poly(A) Tail Recognition

PARN is a homodimeric, processive, and cap‐interacting 3′ exoribonuclease that degrades mRNA poly(A) tails. To reveal mechanisms by which PARN recognizes cap and poly(A) we have performed a structural and functional study of PARN. A crystal structure and subsequent mutational analysis revealed that amino acid residues involved in cap recognition were crucial for cap‐stimulated deadenylation activity and poly(A) hydrolysis. A detailed kinetic analysis showed that PARN harbors specificity for adenosine recognition in its active site. We propose that two binding pockets, which interact with the nucleotides surrounding the scissile bond, play a pivotal role in providing specificity towards adenosine recognition. Interestingly, the binding pocket required for recognition of the penultimate adenosine residue is also required for cap binding and efficient hydrolysis. We conclude that the cap‐binding site partially overlaps with the active site of PARN. This work was supported by the Swedish Research Council, the Linneus Support from the Swedish Research Council to the Uppsala RNA Research Centre and the Lennanders Foundation at Uppsala University. H.S. acknowledges financial support from the Biomedical Research Council of A*STAR (Agency for Science, Technology, and Research) in Singapore.