Studies on protein and nucleic acid metabolism in virus-infected mammalian cells. The formation of a virus-specific antigen in Krebs II ascites-tumour cells infected with encephalomyocarditis virus

1. Krebs II mouse ascites-tumour cells infected with encephalomyocarditis virus were found to contain, in addition to mature virus, a virus-specific protein antigen. An assay, based on the ability of this antigen to block the neutralization of purified virus by its specific antiserum, was developed. 2. This antigen was present both in the culture fluid 17 hr. after the infection of cells with virus and intracellularly, where its titre increased at a time when viral capsid protein was being synthesized. Within the cell, it was mostly localized in the soluble cell sap. 3. In contrast with virus, the antigen did not agglutinate sheep erythrocytes, and its immunological properties were destroyed by digestion with trypsin. Ribonucleic acid was not detected in concentrated preparations of the antigen, nor was the titre of antigen affected by ribonuclease. 4. The antigen had a sedimentation coefficient (20°) of approx. 14s, and its diffusion coefficient, determined by the method of Allison & Humphrey (1960), was 3·2×10⁻⁷ cm.²sec.⁻¹. The particle weight of the antigen was hence 420000±40000. 5. The capsid protein from purified encephalomyocarditis virus could be degraded by treatment with ethanolamine into a protein of sedimentation coefficient (20°) of approx. 4s. The 14s antigen, when similarly treated, yielded a protein of similar size. However, no such smaller antigen was detected in virus-infected cells. 6. It is concluded that the non-haemagglutinating antigen represents a polymeric form of the basic viral capsid-protein molecule and that it is synthesized in the cytoplasm of infected cells. It may be either an intermediate or a by-product in the process of viral capsid-protein synthesis.