Structure of the SUDcore monomer and comparison with the SARS-CoV X-domain.
(A) Ribbon representation of the SUDcore structure (residues 389–652 of Nsp3). The flexible linker connecting the two macrodomains is indicated by a dotted line. The disulfide bond between cysteines 492 of SUD-N and 623 of SUD-M is shown in orange. (B) Stereo image of the 2Fo–Fc electron-density map (1σ above the mean) around the disulfide bond connecting the SUD-N and SUD-M subdomains. (C) Structure-based sequence alignment of the SUDcore subdomains N (SUD-N) and M (SUD-M), and the SARS-CoV X-domain (SARS-X). α-Helices and β-strands are marked red and blue, respectively. Residues 518–524 form the linker between the two SUD subdomains and have not been included in the alignment. Asterisks mark SARS-X residues involved in binding of ADP-ribose. (D) Superimposition of the structures of the SUD-N (violet) and SUD-M (green) subdomains with the SARS-CoV X-domain (cyan).