A glutaredoxin, encoded by the G4L gene of vaccinia virus, is essential for virion morphogenesis

Vaccinia virus encodes two glutaredoxins, O2L and G4L, both of which exhibi t thioltransferase and dehydroascorbate reductase activities in vitro, Alth ough O2L was previously found to be dispensable for virus replication, we n ow show that G4L is necessary for virion morphogenesis. RNase protection an d Western blotting assays indicated that G4L was expressed at late times af ter infection and was incorporated into mature virus particles, Attempts to isolate a mutant virus with a deleted G4L gene were unsuccessful, suggesti ng that the protein was required for virus replication. This interpretation was confirmed by the construction and characterization of a conditional le thal recombinant virus with an inducible copy of the G4L gene replacing the original one. Expression of G4L was proportional to the concentration of i nducer, and the amount of glutaredoxin could be varied from barely detectab le to greater than normal amounts of protein, Immunogold labeling revealed that the induced G4L protein was associated with immature and mature virion s and adjacent cytoplasmic depots. In the absence of inducer, the productio n of infectious virus was severely inhibited, though viral late protein syn thesis appeared unaffected except for decreased maturation-dependent proteo lytic processing of certain core components. Electron microscopy of cells i nfected under nonpermissive conditions revealed an accumulation of crescent membranes on the periphery of electron-dense globular masses but few matur e particles. We concluded that the two glutaredoxin homologs encoded by vac cinia virus have different functions and that G4L has a role in virion morp hogenesis, perhaps by acting as a redox protein.