Mechanism Of Processive And Cap-Stimulated Mrna Poly(A) Tail Degradation

Poly(A)‐specific ribonuclease (PARN) is an oligomeric, processive and cap‐interacting exoribonuclease that efficiently degrades mRNA poly(A) tails. PARN is a key regulator of AU‐rich element (ARE) containing mRNAs encoding proto‐oncogenes and cytokines. Here, we propose a mechanistic framework for PARN action. The model has been experimentally tested and is supported by structural evidence. In the model the RNA recognition motif (RRM) of PARN pushes the poly(A) tail into the active site, which simultaneously pulls the substrate into the hydrolytic site where the movement of a catalytically essential histidine residue is coordinated with the pushing of the RRM. The model provides a mechanistic explanation for how the cap‐structure of the mRNA can stimulate poly(A) tail hydrolysis. We have also investigated the functional significance of divalent metal ions in the active site and revealed that divalent metal ions are required for both hydrolysis and substrate translocation in the active site. Our data imply that three divalent metal ions are required for proper action. Two ions participate in hydrolysis while the third plays a key role during translocation. The generality of this proposal in relationship to other processive enzymes participating in cleavage or formation of phosphodiester bonds will be discussed. Supported by the Swedish Research Council and VR Linnaeus Support.