THE YEAST PAN2 PROTEIN IS REQUIRED FOR POLY(A)-BINDING PROTEIN-STIMULATED POLY(A)-NUCLEASE ACTIVITY

The removal of the mRNA poly(A) tail in the yeast Saccharomyces cerevi siae is stimulated by the poly(A)binding protein (Pab1p), A large scal e purification of the Pab1p-stimulated poly(A) ribonuclease (PAN) iden tifies a 76-kDa and two 135-Da polypeptides as candidate enzyme subuni ts, Antibodies against the Pan1p protein, which is the minor 135-kDa p rotein in the preparation, can immunodeplete Pan1p but not PAN activit y, The protein sequence of the major 135-kDa protein, Pan2p, reveals a novel protein that was also found in the previously reported PAN puri fication (Sachs, A. B., and Deardorff, J. A, (1992) Cell 70, 961-973). Deletion of the non-essential PAN2 gene results in an increase of the average length of mRNA poly(A) tails in vivo, and a loss of Pab1p-sti mulated PAN activity in crude extracts, These data confirm that Pan2p and not Pan1p is required for PAN activity, and they suggest that ribo nucleases other than the Pab1p-stimulated PAN are capable of shortenin g poly(A) tails in vivo.