ABSTRACT
Human coronaviruses are enveloped, positive-strand RNA viruses which cause respiratory diseases ranging in severity from the seasonal common cold to SARS and COVID-19. Of the 7 human coronaviruses discovered to date, 3 emergent and severe human coronavirus strains (SARS-CoV, MERS-CoV, and SARS-CoV-2) have recently jumped to humans in the last 20 years. The COVID-19 pandemic spawned by the emergence of SARS-CoV-2 in late 2019 has highlighted the importance for development of effective therapeutics to target emerging coronaviruses. Upon entry, the replicase genes of coronaviruses are translated and subsequently proteolytically processed by virus-encoded proteases. Of these proteases, nonstructural protein 5 (nsp5, Mpro, or 3CLpro), mediates the majority of these cleavages and remains a key drug target for therapeutic inhibitors. Efforts to develop nsp5 active-site inhibitors for human coronaviruses have thus far been unsuccessful, establishing the need for identification of other critical and conserved non-active-site regions of the protease. In this study, we describe the identification of an essential, conserved horseshoe-shaped region in the nsp5 interdomain loop (IDL) of mouse hepatitis virus (MHV), a common coronavirus replication model. Using site-directed mutagenesis and replication studies, we show that several residues comprising this horseshoe-shaped region either fail to tolerate mutagenesis or were associated with viral temperature-sensitivity. Structural modeling and sequence analysis of these sites in other coronaviruses, including all 7 human coronaviruses, suggests that the identified structure and sequence of this horseshoe regions is highly conserved and may represent a new, non-active-site regulatory region of the nsp5 (3CLpro) protease to target with coronavirus inhibitors. ImportanceIn December 2019, a novel coronavirus (SARS-CoV-2) emerged in humans and triggered a pandemic which has to date resulted in over 8 million confirmed cases of COVID-19 across more than 180 countries and territories (June 2020). SARS-CoV-2 represents the third emergent coronavirus in the past 20 years and the future emergence of new coronaviruses in humans remains certain. Critically, there remains no vaccine nor established therapeutics to treat cases of COVID-19. The coronavirus nsp5 protease is a conserved and indispensable virus-encoded enzyme which remains a key target for therapeutic design. However, past attempts to target the active site of nsp5 with inhibitors have failed stressing the need to identify new conserved non-active-site targets for therapeutic development. This study describes the discovery of a novel conserved structural region of the nsp5 protease of coronavirus mouse hepatitis virus (MHV) which may provide a new target for coronavirus drug development.
