PARN – The Tail End : Function and mechanisms of specificity and processivity

Poly(A)-specific ribonuclease (PARN) is an exoribonuclease that is processive, poly(A) specific and cap-binding. PARN deadenylates the poly(A) tails present on a subset of mRNAs and non-coding RNAs, including among others certain snoRNAs, miRNAs and precursor rRNAs. Here, we have investigated molecular mechanisms behind PARN’s specificity for adenine and ribose, essential properties for PARN’s ability to degrade poly(A) on RNAs. We have applied enzyme kinetics and used divalent metal ions as mechanistic probes to show that PARN’s poly(A) specificity is tightly linked to a translocation event during the hydrolytic cycle of PARN action. To further understand the mechanism of PARN’s processive mode of action we are developing a kinetic model that allow us to measure the probability of processive action for each round of the hydrolytic cycle. Our kinetic model will be general and applicable to the processive action of any processive enzymatic activity. In conclusion, our study has so far established a mechanistic link between PARN’s processive mode of action, hydrolytic activity and preference for degrading poly(A). Human patients with genetic lesions in PARN suffer from a spectrum of syndromes called telomere biology disorders (TBD), which are associated with short telomeres. PARN is involved in the maturation of the snoRNA telomerase RNA component (TERC) that is used as template during the elongation of the telomer by the telomerase. Point mutations in the gene for PARN have been identified in patients. We show that point mutations in PARN that perturb its deadenylation activity correlated with TBDs and developmental disorders. Our mechanistic studies of PARN action will provide a framework for our understanding of PARN’s physiological role and in extension the molecular basis for human diseases caused by perturbed PARN action. Our studies will also be of general interest for our detailed and mechanistic understanding of basic and essential mechanisms of gene expression and RNA biology