The SARS-Unique Domain (SUD) of SARS Coronavirus Contains Two Macrodomains That Bind G-Quadruplexes

Tan, Jinzhi; Vonrhein, Clemens; Smart, Oliver S.; Bricogne, Gerard; Bollati, Michela; Kusov, Yuri; Hansen, Guido; Mesters, Jeroen R.; Schmidt, Christian L.; Hilgenfeld, Rolf

Overall structure SUD core exhibits a two-domain architecture (Figure 2A ). The N-terminal subdomain (SUD-N) comprises Nsp3 residues 389-517, and the C-terminal subdomain of SUD core contains residues 525-652. We call the latter the ''middle SUD subdomain'', or SUD-M, because full-length SUD has a C-terminal extension of 74 residues compared to SUD core . The SUD-N and SUD-M subdomains have a similar fold and can be superimposed with an r.m.s.d. of 3.3-3.4 Ã… (based on Ca positions); they share 11% sequence identity (see Figure 2C for a structural alignment). Of the 14 amino-acid residues identical between the two subdomains, four form a conserved Leu-Glu-Glu-Ala motif at the N-terminus of helix a4. The linker between the two subdomains (residues 518-524) has no visible electron density. This is due to elevated mobility of the linker, rather than proteolytic cleavage, since we showed by SDS-PAGE of dissolved crystals that the SUD core polypeptide (in the presence of b-mercaptoethanol) has the apparent molecular mass to be expected (,29 kDa; not shown). In addition to the linker, SUD-N and SUD-M are connected by a disulfide bond between cysteines 492 and 623 ( Figure 2B ). Disulfide bonds are rare in cytosolic proteins, but in coronaviral Nsps, examples of such bonds have been reported [25, 26] .