InterPro domain: IPR014492

Members of this group are poly(A) polymerases (polynucleotide adenylyltransferases, PAP, 2.7.7.19 ). In eukaryotes, polyadenylation of pre-mRNA plays an essential role in the initiation step of protein synthesis, as well as in the export and stability of mRNAs. Poly(A) polymerase, the central enzyme of the polyadenylation machinery, is a template-independent RNA polymerase that specifically incorporates ATP at the 3' end of mRNA [ 1 , 2 ].

The catalytic domain of poly(A) polymerase shares substantial structural homology with other nucleotidyl transferases such as DNA polymerase beta and kanamycin transferase [ 3 ]. The three invariant aspartates of the catalytic triad ligate two of the three active site metals. One of these metals also contacts the adenine ring. Other conserved, catalytically important residues contact the nucleotide. These contacts, taken together with metal coordination of the adenine base, provide a structural basis for ATP selection by poly(A) polymerase [ 3 ].

The central domain of poly(A) polymerase shares structural similarity with the allosteric activity domain of ribonucleotide reductase R1, which comprises a four-helix bundle and a three-stranded mixed beta-sheet. Even though the two enzymes bind ATP, the ATP-recognition motifs are different [ 3 ]. The C-terminal domain is predicted to be an RNA-binding domain because it folds into a compact domain reminiscent of the RNA-recognition motif fold [ 3 ].

The C-terminal region beyond the predicted RNA-binding domain is only conserved in vertebrates and is dispensable for catalytic activity in vitro . The extended C-terminal domain of vertebrate PAPs is rich in serines and threonines, and enzyme activity can be down regulated by phosphorylation at multiple sites [ 3 , 4 ]. The extreme C terminus of PAP is also the target for another type of regulation. The U1A protein, a component of the U1 snRNP which functions in 5 splice site recognition, is known to inhibit polyadenylation of its own mRNA by binding to PAP [ 4 ]. The C terminus of PAP is also involved in protein-protein interactions with the splicing factor U2AF65 [ 5 ] and the snRNP protein U1-70K [ 5 ].

1. Crystal structure of mammalian poly(A) polymerase in complex with an analog of ATP. EMBO J. 19, 4193-203
2. Formation of mRNA 3' ends in eukaryotes: mechanism, regulation, and interrelationships with other steps in mRNA synthesis. Microbiol. Mol. Biol. Rev. 63, 405-45
3. 3'-End processing of pre-mRNA in eukaryotes. FEMS Microbiol. Rev. 23, 277-95
4. The challenge of oral cancer in Tennessee. null 55, 195
5. U1 snRNP inhibits pre-mRNA polyadenylation through a direct interaction between U1 70K and poly(A) polymerase. Mol. Cell 1, 255-64