Poly(A) polymerase (EC:2.7.7.19) catalyses template-independent extension of the 3'-end of a DNA or RNA strand by one nucleotide at a time. The Poxvirus enzyme creates the 3'(poly)A tail of mRNAs, and is a heterodimer composed of a catalytic (VP55) and a regulatory subunit (VP39).

VP55 comprises three domains: the N-terminal or N domain, the central or catalytic domain, and the C-terminal or C domain, all three domains having distinct topologies. The core comprises a mixed nine-stranded twisted beta sheet, with the first seven strands being folded from one long (118 residue) polypeptide segment that follows the N domain and the last two strands originating from the extreme C-terminal-most 15 residues of the entire polypeptide chain. Of the four alpha-helices in the catalytic domain, the two longest (J and L) near the surface pack against one side of the beta-sheet in a nearly parallel orientation to the beta-strands.

This superfamily entry represents the nucleotidyltransferase (central) domain of the catalytic subunit VP55 PMID:16678106.

INTERPRO:IPR024231, PFAM:PF03296,DOI:10.1016/j.molcel.2006.03.015