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

In this communication, we describe the crystal structures at 2.2 Ã… and 2.8 Ã… resolution (monoclinic and triclinic form, respectively) of the core of the SARS-unique domain (SUD core , Nsp3 residues 389-652). SUD core turns out to consist of two subdomains, SUD-N (Nsp3 residues 389-517) and SUD-M (525-652), each exhibiting the fold of a macrodomain. The two subdomains are connected by a flexible linker (residues 518-524) and a disulfide bond. Even though coronavirus replication occurs in the cytosol, where the environment is reductive, it is unlikely that the formation of this disulfide is an artifact owing to handling of the protein: As the linker between the SUD-N and SUD-M subdomains is very short (seven residues), and the mutual orientation of the subdomains is fixed due to the tight dimerization, cysteine residues no. 492 and 623 will be very close to one another irrespective of the exact conformation of the linker. In fact, disulfide bonds are not uncommon in coronaviral nonstructural proteins (Nsps) involved in RNA replication or transcription. Among others, they have been observed in HCoV-229E Nsp9 [25] and turkey coronavirus Nsp15 [26] , but in these cases, the disulfide bond connects two subunits of the homo-oligomeric proteins, whereas the occurrence in SUD core is the first case of an intramolecular disulfide bond described in a coronavirus Nsp.