Crystal structure of mammalian poly(A) polymerase in complex with an analog of ATP

In eukaryotes, polyadenylation of pre-mRNA plays an essential role in the i nitiation step of protein synthesis, as well as in the export and stability of mRNAs, Poly(A) polymerase, the enzyme at the heart of the polyadenylati on machinery, is a template-independent RNA polymerase which specifically i ncorporates ATP at the 3' end of mRNA, We have solved the crystal structure of bovine poly(A) polymerase bound to an ATP analog at 2.5 Angstrom resolu tion. The structure revealed expected and unexpected similarities to other proteins. As expected, the catalytic domain of poly(A) polymerase shares su bstantial structural homology with other nucleotidyl transferases such as D NA polymerase beta and kanamycin transferase, The C-terminal domain unexpec tedly folds into a compact domain reminiscent of the RNA-recognition motif fold. The three invariant aspartates of the catalytic triad ligate two of t he three active site metals, One of these metals also contacts the adenine ring. Furthermore, conserved, catalytically important residues contact the nucleotide, These contacts, taken together with metal coordination of the a denine base, provide a structural basis for ATP selection by poly(A) polyme rase.