This model represents the papain-like protease (PLPro) found in non-structural protein 3 (Nsp3) of betacoronavirus, including highly pathogenic betacoronaviruses such as Severe acute respiratory syndrome-related coronavirus (SARS-CoV), SARS-CoV2 (also called 2019 novel CoV or 2019-nCoV), and Middle East respiratory syndrome-related (MERS) CoV. CoVs utilize a multi-subunit replication/transcription machinery. A set of non-structural proteins (Nsps) generated as cleavage products of the ORF1a and ORF1ab viral polyproteins assemble to facilitate viral replication and transcription. PLPro is a key enzyme in this process, making it a high value target for the development of anti-coronavirus therapeutics. PLPro, which belongs to the MEROPS peptidase C16 family, participates in the proteolytic processing of the N-terminal region of the replicase polyprotein; it can cleave Nsp1|Nsp2, Nsp2|Nsp3, and Nsp3|Nsp4 sites and its activity is dependent on zinc. In SARS-CoV and murine hepatitis virus (MHV), the C-terminal non-structural protein 3 region spanning transmembrane regions TM1 and TM2 with 3Ecto domain in between, are important for the PL2pro domain to process Nsp3-Nsp4 cleavage. Besides cleaving the polyproteins, PLPro also possesses a related enzymatic activity to promote virus replication: deubiquitinating (DUB) and de-ISGylating activities. Both, ubiquitin (Ub) and Ub-like interferon-stimulated gene product 15 (ISG15), are involved in preventing viral infection; coronaviruses utilize Ubl-conjugating pathways to counter the pro-inflammatory properties of Ubl-conjugated host proteins via the action of PLPro, which processes both 'Lys-48'- and 'Lys-63'-linked polyubiquitin chains from cellular substrates. The Nsp3 PLPro domain of many of these CoVs has also been shown to antagonize host innate immune induction of type I interferon by interacting with IRF3 and blocking its activation. Interactions of SARS-CoV and MERS-CoV with antiviral interferon (IFN) responses of human cells are remarkably different; high-dose IFN treatment (type I and type III) shows MERS-CoV was substantially more IFN sensitive than SARS-CoV. This may be due to differences in the architecture of the oxyanion hole and of the S3 as well as the S5 specificity sites, despite the overall structures of SARS-CoV and MERS-CoV PLPro being similar.
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