Background Coronaviruses (CoVs) can be classified into alphacoronavirus (group 1), betacoronavirus (group 2), and gammacoronavirus (group 3) based on diversity of the protein sequences. Their 3C-like protease (3CLpro), which catalyzes the proteolytic processing of the polyproteins for viral replication, is a potential target for anti-coronaviral infection. Methodology/Principal Findings Here, we profiled the substrate specificities of 3CLpro from human CoV NL63 (group 1), human CoV OC43 (group 2a), severe acute respiratory syndrome coronavirus (SARS-CoV) (group 2b) and infectious bronchitis virus (IBV) (group 3), by measuring their activity against a substrate library of 19×8 of variants with single substitutions at P5 to P3' positions. The results were correlated with structural properties like side chain volume, hydrophobicity, and secondary structure propensities of substituting residues. All 3CLpro prefer Gln at P1 position, Leu at P2 position, basic residues at P3 position, small hydrophobic residues at P4 position, and small residues at P1' and P2' positions. Despite 3CLpro from different groups of CoVs share many similarities in substrate specificities, differences in substrate specificities were observed at P4 positions, with IBV 3CLpro prefers P4-Pro and SARS-CoV 3CLpro prefers P4-Val. By combining the most favorable residues at P3 to P5 positions, we identified super-active substrate sequences ‘VARLQ↓SGF’ that can be cleaved efficiently by all 3CLpro with relative activity of 1.7 to 3.2, and ‘VPRLQ↓SGF’ that can be cleaved specifically by IBV 3CLpro with relative activity of 4.3. Conclusions/Significance The comprehensive substrate specificities of 3CLpro from each of the group 1, 2a, 2b, and 3 CoVs have been profiled in this study, which may provide insights into a rational design of broad-spectrum peptidomimetic inhibitors targeting the proteases.
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