This model represents the papain-like protease (PLPro) found in non-structural protein 3 (Nsp3) of gammacoronavirus, including Avian coronavirus, Canada goose coronavirus, and Beluga whale coronavirus SW1. 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. 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 in several CoVs has also been shown to antagonize host innate immune induction of type I interferon by interacting with IRF3 and blocking its activation.
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