Fig. 1.

Structure of nsp15, the endoribonuclease from SARS-CoV. (a and b) Cartoon representation (a) and topology diagram (b) of nsp15 monomer. The structure consists of three domains: N-terminal domain (α1–α2), central domain (α3–β8), and C-terminal domain (α6–β14). Secondary structures are colored as follows: red, α-helices; yellow, β-sheets; and green, loops. (c) A view of the nsp15 hexamer. In the trimer shown in the foreground, the subdomains of one of the monomers are colored as follows: green, N-terminal domain; yellow, central domain; and red, C-terminal domain. The other two molecules are shown in blue and magenta. The molecules belonging to the trimer in the background are shown in gray. (d) 90° rotation of the nsp15 hexamer as shown in c. (e) Surface representation of the nsp15 hexamer. Solid arrows indicate the active sites of the trimer in the foreground; dashed arrows show the active sites of the trimer in the background. (f) The hexamer shown in e is rotated by 90°. In this orientation, two active sites are visible, and they are highlighted by solid arrows and colored in white.