Characterization of immediate-early and early proteins of murine cytomegalovirus synthesized in permissive and nonpermissive cells

The gene products produced by murine cytomegalovirus (MCMV) in infected cells prior to viral DNA synthesis are believed to control the interaction of the virus with the cells, determining whether a permissive infection results, with virus replication, or whether further virus gene expression is inhibited, resulting in a latent or abortive infection. The aim of this study was to characterize the early viral gene products that are produced in permissive and nonpermissive cells. The proteins produced in 3T3-L1 cells, permissively infected with MCMV, during the first six hours of infection (the period prior to viral DNA replication) were characterized by polyacrylamide gel electrophoresis. Ten of the proteins were classified as immediate-early (IE) and seven as early according to their time of synthesis and also according to their synthesis in the presence of actinomycin D following the reversal of a cycloheximide mediated block in protein synthesis. The estimated molecular weights ranged from 28K - 100K. The synthesis of a dominant IE protein of 100K was significantly increased, after the reversal of a cycloheximide block, compared to unenhanced conditions. The synthesis of two other major IE proteins of 96K and 89K were also significantly enhanced by this treatment. The 100K and 89K proteins partitioned with the nuclear, cytoplasmic and cytoskeletal fractions, while the 96K protein partitioned more strongly with the nuclei. These proteins were phosphorylated. The other IE proteins were synthesized in lesser amounts. The major early proteins, which had molecular weights of 39K and 36K, were also phosphorylated and were exclusively nucleus-associated. A number of the IE and early proteins had affinity for native and denatured DNA-cellulose. The same major IE and early proteins were identified in nonpermissively infected J774A.1 macrophage cells. Although 0.6% of these cells became permissively infected with MCMV and the rest appeared to be nonpermissively infected, viral DNA and late protein synthesis was not detected. The major difference between the proteins produced in 3T3-L1 cells and J774A.1 cells was the affinity of the 96K protein for denatured DNA-cellulose, which was only observed when the protein was synthesized in J774A.1 cells. The main IE and early MCMV induced proteins were also synthesized in nonpermissively infected human fibroblast cells. The only difference between the proteins produced in these cells and 3T3-L1 cells was that the 100K IE protein appeared to have a greater nuclear-affinity, when produced in the human fibroblasts, than was found when synthesized in infected 3T3-L1 cells. In conclusion, a larger number of IE and early MCMV-induced proteins were identified in infected cells than had been previously characterized. There was no evidence of restricted MCMV gene expression occurring in two different cell types that were nonpermissively infected. This appeared to indicate that, in the nonpermissive experiments described, MCMV replication was inhibited at the stage of viral DNA synthesis.Medicine, Faculty ofPathology and Laboratory Medicine, Department ofGraduat