Mutations across murine hepatitis virus nsp4 alter virus fitness and membrane modifications.

A common feature of infection by positive-sense RNA virus is the modification of host cell cytoplasmic membranes that serve as sites of viral RNA synthesis. Coronaviruses induce double-membrane vesicles (DMVs), but the role of DMVs in replication and virus fitness remains unclear. Coronaviruses encode 16 nonstructural proteins (nsps), three of which, nsp3, nsp4, and nsp6, are necessary and sufficient for DMV formation. It has been shown previously that mutations in murine hepatitis virus (MHV) nsp4 loop 1 that alter nsp4 glycosylation are associated with disrupted DMV formation and result in changes in virus replication and RNA synthesis. However, it is not known whether DMV morphology or another function of nsp4 glycosylation is responsible for effects on virus replication. In this study, we tested whether mutations across nsp4, both alone and in combination with mutations that abolish nsp4 glycosylation, affected DMV formation, replication, and fitness. Residues in nsp4 distinct from glycosylation sites, particularly in the endoplasmic reticulum (ER) luminal loop 1, independently disrupted both the number and morphology of DMVs and exacerbated DMV changes associated with loss of glycosylation. Mutations that altered DMV morphology but not glycosylation did not affect virus fitness while viruses lacking nsp4 glycosylation exhibited a loss in fitness. The results support the hypothesis that DMV morphology and numbers are not key determinants of virus fitness. The results also suggest that nsp4 glycosylation serves roles in replication in addition to the organization and stability of MHV-induced double-membrane vesicles. IMPORTANCE All positive-sense RNA viruses modify host cytoplasmic membranes for viral replication complex formation. Thus, defining the mechanisms of virus-induced membrane modifications is essential for both understanding virus replication and development of novel approaches to virus inhibition. Coronavirus-induced membrane changes include double-membrane vesicles (DMVs) and convoluted membranes. Three viral nonstructural proteins (nsps), nsp3, nsp4, and nsp6, are known to be required for DMV formation. It is unknown how these proteins induce membrane modification or which regions of the proteins are involved in DMV formation and stability. In this study, we show that mutations across nsp4 delay virus replication and disrupt DMV formation and that loss of nsp4 glycosylation is associated with a substantial fitness cost. These results support a critical role for nsp4 in DMV formation and virus fitness.