mRNA maturation in giant viruses: variation on a theme.
Bottom Line: Unexpectedly, the vPAPs are homodimeric and uniquely self-processive.The vPAP backbone structures exhibit a symmetrical architecture with two subdomains sharing a nucleotidyltransferase topology, suggesting that vPAPs originate from an ancestral duplication.A Poxvirus processivity factor homologue encoded by Megavirus chilensis displays a conserved 5'-GpppA 2'O methyltransferase activity but is also able to internally methylate the mRNAs' polyA tails.
Affiliation: Architecture et Fonction des Macromolécules Biologiques, CNRS UMR 7257, Aix-Marseille Université, 163 Avenue de Luminy, Case 932, 13288 Marseille cedex 9, France.Show MeSH
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Mentions: The second domain (D2) exhibits the typical topology of a nucleotidyltransferase (NT) domain made of a mixed five-stranded β sheet and two connecting helices and, as for the VP55 structure, the two helices pack against one side of the β sheet parallel to the β strands (Figures 1B and 3). The canonical acidic residues D94-E96-D143 are properly positioned and ordered in the crystal structure as evidenced by the 2Fo-Fc electron density maps contoured at 1σ on the of M. chilensis and Mimivirus PAP active sites (Supplementary Figure S6A). A water molecule replacing the Mg2+ ion usually found in structures in complex with ATP is used to help locate the active sites in the figures (Figures 3 and 5B, Supplementary Figure S4). The helix-turn motif within the β sheet presents a 10-residues longer loop relative to the Vaccinia virus PAP structure. This loop stabilizes the α1 helix of the same monomer in an orientation perpendicular to D2. This domain thus appears equivalent to the catalytic domain of VP55 made of an NT domain extended by one helix (Mg561 α5, VP55 α12), two antiparallel strands (β6-β7) and terminated by two parallel helices (Mg561 α6-α7, VP55 α13-α15). This topology thus defines the catalytic domain of viral PAPs (Figure 1B).
Affiliation: Architecture et Fonction des Macromolécules Biologiques, CNRS UMR 7257, Aix-Marseille Université, 163 Avenue de Luminy, Case 932, 13288 Marseille cedex 9, France.