SARS-CoV 3C-like protease (3CLpro) is an attractive target for anti- severe acute respiratory syndrome (SARS) drug discovery, and its dimerization has been extensively proved to be indispensable for enzymatic activity. However, the reason why the dissociated monomer is inactive still remains unclear due to the absence of the monomer structure. In this study, we showed that mutation of the dimer-interface residue Gly-11 to alanine entirely abolished the activity of SARS-CoV 3CLpro. Subsequently, we determined the crystal structure of this mutant and discovered a complete crystallographic dimer dissociation of SARS-CoV 3CLpro. The mutation might shorten the ฮฑ-helix Aโ€ฒ of domain I and cause a mis- oriented N-terminal finger that could not correctly squeeze into the pocket of another monomer during dimerization, thus destabilizing the dimer structure. Several structural features essential for catalysis and substrate recognition are severely impaired in the G11A monomer. Moreover, domain III rotates dramatically against the chymotrypsin fold compared with the dimer, from which we proposed a putative dimerization model for SARS-CoV 3CLpro. As the first reported monomer structure for SARS-CoV 3CLpro, the crystal structure of G11A mutant might provide insight into the dimerization mechanism of the protease and supply direct structural evidence for the incompetence of the dissociated monomer. ยฉ 2008 by The American Society for Biochemistry and Molecular Biology, Inc.