Interferon redundancy counteracts proteolytic inactivation by SARS-CoV-2 3CL main protease

Interferons (IFNs) are protein molecules released by cells in response to a viral infection, which trigger antiviral defenses through phosphorylation of signal transducer and activator of transcription (STAT) proteins. Previous research discovered that the SARS-CoV-2 main protease, 3CLpro, was expelled from infected cells via gasdermin D/E pores and retained proteolytic activity in human serum against extracellular substrates.

In this study, the researchers found that 3CLpro selectively cleaves and inactivates glycosylated interferons L1, L2, and a rare variant of interferon gamma (Arg160Gln), but does not affect wild-type interferon gamma, L3, L4, alpha or beta. The study identified the importance of O-linked glycosylation at specific sites of IFN-L1 and N-linked glycosylation at Asn65 for signaling.

Surprisingly, the researchers observed that O-glycosylation was essential for the cleavage and inactivation of IFN-L1 by 3CLpro at two sites, leading to reduced STAT1 phosphorylation and impaired induction of IFN-stimulated proteins MX1, OAS2, and IFIT1. However, cleavage of IFN-L2 did not disrupt signaling or antiviral activity against SARS-CoV-2 and vesicular stomatitis virus.

Additionally, the study revealed that matrix metalloproteinases (MMPs) 2, 7, 8, and 12 degrade 3CLpro, while 3CLpro does not affect these MMPs.