STUDIES ON VIRAL CYSTEINE PROTEASES AS POTENTIAL ANTIVIRAL TARGETS

Abstract

Pathogenic RNA viruses threaten global public health due to their rapid rate of adaptive evolution and their biological diversity. They are the leading agents of transmission of zoonotic diseases and have the potential to cause epidemics and pandemics. On 11 March 2022, COVID 19 was declared a pandemic and a situation of global emergency by the World Health Organization (WHO). The viral causative agent of COVID-19 has been designated as SARS CoV-2. The genomic RNA of SARS-CoV-2 gets translated into structural and non-structural proteins. Viral polyproteins pp1a and pp1ab are translated from ORF1a and ORF1b and get proteolytically cleaved by virus-encoded main protease (Mpro or 3CL pro) and papain-like protease (PLpro), releasing individual mature non-structural proteins (nsp). SARS-CoV-2 PLpro is responsible for cleaving viral polyproteins pp1a and pp1ab to release nsp1, nsp2, and nsp3. Like SARS-CoV-2, Chikungunya Fever (CHIKF) epidemic is caused by another enveloped single-stranded positive-sense RNA virus, Chikungunya virus (CHIKV). Notably, it led to an epidemic in India in 2006. Of the two ORFs in the genome, the one at the 5′ end encodes the non-structural polyprotein nsP1234. nsP2 protease (nsP2pro) cleaves the polyprotein at nsP3/nsP4 site followed by nsP1/nsP2 and nsP2/nsP3 to release individual nsPs. Both SARS-CoV-2 PLpro and CHIKV nsP2pro are cysteine proteases involved in processing non-structural polyproteins. The present study targeted PLpro of SARS-CoV-2 to identify potential drug molecules by expression and purification of PLpro protein, followed by biochemical and biophysical characterization studies and co-crystallization. Antiviral activity of a few inhibitors found to be active against PLpro of SARS-CoV-2 was investigated biochemically against CHIKV nsP2pro.