Structural basis for catalysis and ubiquitin recognition by the Severe acute respiratory syndrome coronavirus papain-like protease

Papain-like protease (PL^pro) is one of two cysteine proteases involved in the proteolytic processing of the polyproteins of Severe acute respiratory syndrome coronavirus (SARS-CoV). PL^pro also shows significant in vitro deubiquitinating and de-ISGylating activities, although the detailed mechanism is still unclear. Here, the crystal structure of SARS-CoV PL^pro C112S mutant in complex with ubiquitin (Ub) is reported at 1.4 Å resolution. The Ub core makes mostly hydrophilic interactions with PL^pro, while the Leu-Arg-Gly-Gly C-terminus of Ub is located in the catalytic cleft of PL^pro, mimicking the P4-P1 residues and providing the first atomic insights into its catalysis. One of the O atoms of the C-­terminal Gly residue of Ub is located in the oxyanion hole consisting of the main-chain amides of residues 112 and 113. Mutations of residues in the PL^pro-Ub interface lead to reduced catalytic activity, confirming their importance for Ub binding and/or catalysis. The structure also revealed an N-­cyclohexyl-2-aminethanesulfonic acid molecule near the catalytic triad, and kinetic studies suggest that this binding site is also used by other PL^pro inhibitors. Overall, the structure provides a foundation for understanding the molecular basis of coronaviral PL^pro catalysis.