Infectious bronchitis virus 3C-like proteinase (3CLpro)

Infectious bronchitis virus (IBV) 3C-like proteinase (3CLpro), also known as the main protease (Mpro) or nsp5, is a vital enzyme for the replication of IBV, a gammacoronavirus that causes highly contagious respiratory, renal, and reproductive diseases in poultry (UniProt P0C6Y1; Frontiers in Microbiology, 2021). It is responsible for the proteolytic processing of the viral polyproteins (pp1a and pp1ab) at eleven conserved sites, releasing functional non-structural proteins (nsps) that form the viral replication-transcription complex (Journal of Virology, 2000; Wikipedia). Because of its essential role in the viral life cycle and the lack of closely related homologs in host cells, 3CLpro is considered a primary therapeutic target for antiviral development (NIH, 2023). Inhibitors such as the broad-spectrum coronavirus protease inhibitor GC376 and various peptidomimetics have demonstrated the ability to block enzymatic activity and reduce viral replication in experimental settings (MDPI, 2025; NIH, 2020). Effective targeting of this protease could provide a crucial tool for managing IBV outbreaks, which currently cause significant economic losses to the global poultry industry due to the limited cross-protection offered by existing vaccines (ResearchGate, 2025). In addition to 3CLpro, IBV also encodes a papain-like protease (PLpro) that contributes to polyprotein processing and host immune evasion, further expanding the potential for protease-targeted therapies (Frontiers in Cellular and Infection Microbiology, 2021).

Other names

Main proteaseMpronsp5C30 endopeptidaseInfectious bronchitis virus main proteaseIBV 3CLpro3C-like proteaseInfectious bronchitis virus protease

Inhibition of viral polyprotein cleavage by binding to the catalytic site of the 3C-like proteinase, thereby preventing the release of functional non-structural proteins required for viral replication.