A Novel Nuclear Human Poly(A) Polymerase (PAP), PAPγ

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 PAPγ (hsPAPγ). hsPAPγ shares 60% identity to human PAPII (hsPAPII) at the amino acid level. hsPAPγ exhibits fundamental properties of a bona fidepoly(A) polymerase, specificity for ATP, and cleavage and polyadenylation specificity factor/hexanucleotide-dependent polyadenylation activity. The catalytic parameters indicate similar catalytic efficiency to that of hsPAPII. Mutational analysis and sequence comparison revealed that hsPAPγ and hsPAPII have similar organization of structural and functional domains. hsPAPγ contains a U1A protein-interacting region in its C terminus, and PAPγ activity can be inhibited, as hsPAPII, by the U1A protein. hsPAPγ 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 hsPAPγ establishes that multiple forms of PAP are unevenly distributed in the cell, implying specialized roles for the various isoforms.