The main protease (Mpro) of severe acute respiratory syndrome coronavirus (SARS-CoV) plays an essential role in the extensive proteolytic processing of the viral polyproteins (pp1a and pp1ab), and it is an important target for anti-SARS drug development. It was found that SARS-CoV M pro exists in solution as an equilibrium of both monomeric and dimeric forms, and the dimeric form is the enzymatically active form. However, the mechanism of SARS-CoV Mpro dimerization, especially the roles of its N-terminal seven residues (N-finger) and its unique C-terminal domain in the dimerization, remain unclear. Here we report that the SARS-CoV Mpro C-terminal domain alone (residues 187 to 306; Mpro-C) is produced in Escherichia coli in both monomeric and dimeric forms, and no exchange could be observed between them at room temperature. The Mpro-C dimer has a novel dimerization interface. Meanwhile, the N-finger deletion mutant of SARS-CoV Mpro also exists as both a stable monomer and a stable dimer, and the dimer is formed through the same C-terminal-domain interaction as that in the Mpro-C dimer. However, no C-terminal domain-mediated dimerization form can be detected for wild-type SARS-CoV Mpro. Our study results help to clarify previously published controversial claims about the role of the N-finger in SARS-CoV Mpro dimerization. Apparently, without the N-finger, SARS-CoV Mpro can no longer retain the active dimer structure; instead, it can form a new type of dimer which is inactive. Therefore, the N-finger of SARS-CoV Mpro is not only critical for its dimerization but also essential for the enzyme to form the enzymatically active dimer. Copyright © 2008, American Society for Microbiology.
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