Purification and characterization of a poly(A)-specific exoribonuclease from calf thymus

An exoribonuclease from calf thymus which specifically cleaves poly(A) in the single- or in the double-stranded form has been isolated and purified to homogeneity. The enzyme has a molecular weight of about 80,000 as estimated by gel filtration, and consists of two subunits with molecular weights of 58,000 and 31,000 as analyzed by sodium dodecyl sulfate-gel electrophoresis. For optimal activity, the poly(A)-specific exoribonuclease requires divalent cations, alkaline pH, and 39 degrees C. The enzyme is inhibited at 0.2 ionic strength and is sensitive to reagents for thiol groups. The only product formed by the action of the enzyme is 5'-AMP. It is suggested that this enzyme plays a role in the degradation of the poly(A) sequence of mRNA in the nucleus.