Regulation of ISG15-mediated innate immunity by coronaviral papain-like proteases

The emergence and continuous evolution of SARS-CoV-2 have led to over seven million deaths globally and countless illnesses. Although the rapid and successful vaccine rollout has facilitated early pandemic control, the post-pandemic era remains burdened by breakthrough infections and chronic conditions that require better virologic and immunologic understanding. Coronaviruses dysregulate host immune responses through an arsenal of viral proteins. The papain-like protease (PLpro), a critical target for antiviral drug discovery, is at the core of the viral life cycle and meanwhile profoundly disrupts host antiviral signalling via its enzymatic activities targeting key regulatory mechanisms mediated by protein post-translational modifications, ubiquitylation and ISGylation (ISG15 conjugation). Compared to ubiquitylation, the roles of ISGylation in regulating host cellular processes are far less understood. The global landscape of protein ISGylation in specific disease contexts remains scarcely investigated. The ability of coronaviral PLpro to remove ISG15 from conjugated substrate proteins (called de-ISGylation) offers a unique opportunity to understand ISGylation in the context of a host-virus arms race. The focus of the proposed research is uniquely situated on characterizing the first-of-its-kind SARS-CoV-2 PLpro mutant virus that is specifically deficient in de-ISGylation. The findings will provide mechanistic insights into the role of de-ISGylation in coronavirus pathogenesis and uncover new host factors that may be harnessed for antiviral therapies. The research will also assess whether targeting PLpro-mediated de-ISGylation is a viable strategy for virus attenuation, which will guide the development of new live vaccines with enhanced immunogenicity and safety.