Poly(A) polymerase (PAP) is present in multiple forms in mammalian cells and tissues. Here we show that the 90-kDa isoform is the product of the gene PAPOLG, which is distinct from the previously identified genes for poly(A) polymerases. The 90-kDa isoform is referred to as human PAP␥ (hsPAP␥). hsPAP␥ shares 60% identity to human PAPII (hsPAPII) at the amino acid level. hsPAP␥ exhibits fundamental properties of a bona fide poly(A) polymerase, specificity for ATP, and cleavage and polyadenylation specificity factor/hexanucleotidedependent polyadenylation activity. The catalytic parameters indicate similar catalytic efficiency to that of hsPAPII. Mutational analysis and sequence comparison revealed that hsPAP␥ and hsPAPII have similar organization of structural and functional domains. hsPAP␥ contains a U1A protein-interacting region in its C terminus, and PAP␥ activity can be inhibited, as hsPAPII, by the U1A protein. hsPAP␥ is restricted to the nucleus as revealed by in situ staining and by transfection experiments. Based on this and previous studies, it is obvious that multiple isoforms of PAP are generated by three distinct mechanisms: gene duplication, alternative RNA processing, and post-translational modification. The exclusive nuclear localization of hsPAP␥ establishes that multiple forms of PAP are unevenly distributed in the cell, implying specialized roles for the various isoforms.

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