Synthesis of Proteins and Glycoproteins in Cells Infected with Human Cytomegalovirus

Abstract

In cytomegalovirus-infected [human fibroblast] cells, the rate of protein synthesis was detected as 2 peaks. One occurred during the early phase of infection, 0-36 h postinfection, and the other occurred during the late phase, after the initiation of viral DNA synthesis. Double-isotopic-label difference analysis demonstrated that host and viral proteins were synthesized simultaneously during both phases. In the early phase, about 70-90% of the total proteins synthesized were host proteins, but about 10-30% were viral, even at a multiplicity of infection of 20 PFU[plaque forming units]/cell. Virus-related proteins or glycoproteins were referred to as infected-cell specific (ICS). Two ICS glycoproteins (gp145 and 100) were clearly detectable and were synthesized preferentially in the early phase of infection. Their synthesis was concomitant with stimulation of the protein synthesis rate. In the late phase of infection, about 50-60% of the total protein synthesis was viral and about 40-50% was host. The ICS proteins and glycoproteins detected during the late phase of infection were viral structural proteins. Infectious virus was not detectable until 48-72 h postinfection. An inhibitor of viral DNA synthesis, phosphonoacetic acid, prevented the appearance of the late-phase ICS proteins and glycoproteins, but there was little or no effect on early ICS glycoprotein synthesis. Radiolabeled ICS proteins and glycoproteins were identified by their relative rates of synthesis, by their different electrophoretic mobilities compared with those of host proteins and host glycoproteins and by their similar electrophoretic mobilities compared to those of proteins and glycoproteins associated with virions and dense bodies of cytomegalovirus. Structural viral antigens in the infected-cell extracts were removed by immunoprecipitation, using F(ab'')2 fragments of cytomegalovirus-specific antibodies, and identified as described above. The last 2 criteria were used to identify viral structural ICS proteins and glycoproteins. Although about 35 structural proteins were associated with purified virions and dense bodies, the continued synthesis of host cell proteins complicated their identification in infected cells. Nevertheless, 7 of the 9 structural glycoproteins were identified as ICS glycoproteins.