Formation of the arterivirus replication/transcription complex: a key role for nonstructural protein 3 in the remodeling of intracellular membranes.

The research focuses on the Arterivirus Equine Arteritis Virus (EAV) and emphasizes the critical role of the nonstructural protein 3 (nsp3) in the formation of the virus’s replication/transcription complex, which is crucial for the virus’s life cycle. The study uses site-directed mutagenesis to investigate nsp3’s role in detail.

Arterivirus Equine Arteritis Virus (EAV)

• This virus has a replication/transcription complex associated with paired membranes or double-membrane vesicles (DMVs) believed to originate from the endoplasmic reticulum, an organelle found in eukaryotic cells.
• Previous studies had shown that coexpression of two transmembrane nonstructural proteins (nsp2 and nsp3) was enough to induce these DMVs, which are typical of arterivirus infection.

Role of Nonstructural Protein 3 (nsp3)

• In this study, the researchers used site-directed mutagenesis, a method for making specific and intentional changes to the DNA sequence of a gene, to study the role of nsp3 in more detail.
• The release of the hydrophobic N terminus of nsp3, usually achieved by the cleavage of the nsp2/3 junction by the nsp2 protease, was found to be nonessential for the formation of DMVs.
• However, the substitution of each of four conserved cysteine residues found in a predicted luminal loop of nsp3 blocked DMV formation. Some of these mutant nsp3 proteins were found to be highly cytotoxic and affected the endoplasmic reticulum significantly.
• The researchers verified the presence of the cysteine-containing loop in the lumen and confirmed the transmembrane nature of nsp3 by engineering an N glycosylation site in the loop. This mutant displayed an interesting intermediate phenotype with respect to DMV formation. Curved and paired membranes were formed, but DMV formation was impaired.

Importance in the Viral Life Cycle

• The researchers also studied the effect of nsp3 mutations on the processing of the replicase polyprotein, a sequence of linked proteins required for virus replication. Several mutations were found to influence the processing of the region downstream of nsp3 by the nsp4 main protease.
• The researchers tested these nsp3 mutations in an EAV reverse genetics system. None of these mutations were tolerated, reinforcing the importance of nsp3 in the virus’s life cycle.

Conclusion

• This research provides key insights into the character of the arterivirus equine arteritis virus and the critical role of its nonstructural protein 3 in the virus’s formation and life cycle. This may inform future research and treatments for infections related to this virus.