Poly A Polymerase Alpha (PAPOLa)

Polyadenylation of the 3-prime ends of eukaryotic mRNAs is a key event that takes place in the nucleus during maturation of mRNA. The reaction occurs in 2 distinct steps: endoribonucleolytic cleavage of the pre-RNA at the poly(A) site, followed by synthesis of the poly(A) tail at the 3-prime end of the upstream cleavage product. The poly(A) polymerase (PAP) is required for the adenosine addition reaction. Western blot analysis of HeLa cell extracts detected heat-resistant 100- and 106-kD proteins in both nuclear and cytoplasmic fractions, and a 90-kD heat-sensitive protein in nuclear fractions. The 106-kD protein is phosphorylated. Functional analysis of mutated peptides showed that the N-terminal region of PAP is required for nonspecific polymerase activity, while both the N and C termini are required for specific polymerase activity.