Studies on rat cytomegalovirus induced structural and non-structural proteins present at (immediate-)early and late times of infection.

ABSTRACT

Radioactive-labelled virions and nucleocapsids of rat cytomegalovirus (RCMV) were purified from the supernatant and subcellular fractions of infected rat embryo fibroblasts (REF) and analyzed by SDS-polyacrylamide gel electrophoresis. Nuclear nucleocapsids contain one major protein of 138 kD, which is considered to be the basic invariant structural element of RCMV. Enveloped virions consist of 28 protein species, five of which were clearly identified as glycoproteins (58, 64, 76, 112 and 118 kD). Using pulse labelling procedures on RCMV-infected REF cells, after removal of a previously established translation block by cycloheximide, two RCMV-induced immediate-early (IE) protein species (71 and 85 kD) were both detected in the nuclear and cytoplasmic cell fractions. When pulse-labelling was performed for 16 hours in presence of Actinomycin D, only the 85 kD IE protein was detected in the nucleus. The results indicate that the 85 kD IE polypeptide is of importance in early transcriptional events of viral genes. Protein metabolism studies revealed that late in the course of RCMV infection (at 3 days p.i.) protein synthesis has been dramatically changed. Some cellular proteins are greatly suppressed while other cellular proteins are clearly enhanced. Moreover, active synthesis of 8 new cytoplasmic proteins and 9 new nuclear proteins occurs. Most of these proteins were identified as structural constituents of virions.