Proteolytic maturation of replicase polyprotein pp1a by the nsp4 main proteinase is essential for equine arteritis virus replication and includes internal cleavage of nsp7.

The study delves into the role of a specific proteinase, nsp4, in the replication process of the Equine arteritis virus (EAV), a type of arterivirus. It was found that the proteolytic maturation process driven by nsp4 is critical for the virus replication and includes the internal cleavage of a protein known as nsp7.

Theoretical Background

• The research focuses on Equine arteritis virus (EAV), a type of arterivirus, which has a positive-stranded RNA genome. This genome encodes two polyproteins, pp1a and pp1ab, which are slightly overlapping.
• These polyproteins undergo cleavage by three viral proteinases into 12 non-structural proteins (nsps), with the nsp4 proteinase being responsible for eight of these breakdowns.
• Processing of the C-terminal half of pp1a (including the nsp3-8 region) was theorized to proceed through one of two proteolytic pathways, labeled as ‘major’ and ‘minor’.

Experiment and Findings

• Research was conducted on the relevance of these two proteolytic pathways by utilizing a reverse-genetics system and deactivating each of the cleavage points through site-directed mutagenesis.
• The outcome displayed that for all pp1a cleavage locations, mutations that hindered cleavage by the nsp4 proteinase resulted in either a complete block or severe inhibition of EAV RNA synthesis.
• More so, a new nsp4 cleavage site was distinguished within nsp7, revealing itself to be conserved among arteriviruses. The N-terminal nsp7 fragment, or nsp7alpha, is derived from this cleavage and was discoverable in cells infected with EAV or those transiently expressing pp1a.
• Introducing a mutation to this newly-found cleavage site in the context of an EAV full-length cDNA clone was proven lethal. This result emphasizes the importance of the highly controlled nsp4-mediated processing of the C-terminal half of pp1a in the lifecycle of arteriviruses.

Implication

• The study underscores the vital role of the nsp4 proteinase in the lifecycle of EAV and arterivirus replication, and identification of its novel cleavage site expands our current understanding of these mechanisms.
• This work could open doors to further research and potential therapeutic strategies to combat diseases caused by arteriviruses.