PAN3 ENCODES A SUBUNIT OF THE PAB1P-DEPENDENT POLY(A) NUCLEASE IN SACCHAROMYCES-CEREVISIAE

The Pab1p-dependent poly(A) nuclease (PAN) from Saccharomyces cerevisi ae copurifies with polypeptides of approximately 127 and 76 kDa, Previ ously, it was demonstrated that the 127-kDa Pan2 protein is required f or PAN activity (R. Boeck, S, Tarun, M. Reiger, J, Deardorff, S. Mulle r-Auer, and A. B, Sachs, J. Biol, Chem, 271:432-438, 1996). Here we de monstrate that the 76-kDa protein, encoded by the nonessential PAN3 ge ne, is also required for enzymatic activity, Deletion of PAN3 resulted in the loss of PAN activity in yeast extracts, and immunodepletion of Pan3p from purified PAN fractions abolished enzymatic activity. We sh ow by coimmunoprecipitation and directed two-hybrid studies that the P an2 and Pan3 proteins physically interact, In addition, we demonstrate that a deletion of PAN2, PAN3, or both resulted in similar increases in mRNA poly(A) tail lengths in vivo. These data strongly suggest that both Pan2p and Pan3p are required subunits of the PAN enzyme and that PAN functions in vivo to shorten mRNA poly(A) tails.