An avian sarcoma virus mutant that is temperature sensitive for virion assembly

The temperature-sensitive defect in replication of LA334, a double mutant of Rous sarcoma virus, has been characterized both biologically and biochemically. This mutant is complemented for replication at the nonpermissive temperature by both leukosis virus and by RSV(−). Both experiments indicate that LA334 synthesizes functional glycoproteins at the restrictive temperature. This conclusion was confirmed by interference experiments which showed that LA334 induced specific cellular resistance at 41° against superinfection with viruses of subgroup C. Noninfectious virus particles are synthesized at 41° and possess a higher density in sucrose gradients (1.175 g/ml) when compared to those produced at 35° (1.15 g/ml). In addition to the major viral structural proteins these noninfectious virions contain four novel polypeptides, at least three of which appear to be viral in origin. An analysis of viral polypeptide processing indicates that the rate of cleavage of the polyprotein precursor to nonglycosylated structural polypeptides is reduced significantly at the restrictive temperature. Addition of cycloheximide to infected cultures does not prevent the rapid production of infectious virus, seen on shifting cells from the restrictive to the permissive temerature, which suggests that proteins synthesized at 41° can be processed correctly at 35°. Large budding structures that lack the characteristic morphology of C-type particles are visible in electron micrographs of cells infected with the mutant at the nonpermissive temperature. Large virions corresponding in size to some of the budding structures are detectable in negatively stained preparations of noninfectious virus.