Identification of a Viral Protein Involved in Post-translational Maturation of the Encephalomyocarditis Virus Capsid Precursor

Translation of encephalomyocarditis virus RNA in a cell-free system from uninfected Krebs ascites cells results in the synthesis of a major polypeptide product with a molecular weight of approximately 112,000. In contrast, when the viral RNA is translated in a cell-free system from virus-infected cells, this polypeptide is absent and the largest polypeptide produced has a molecular weight of about 100,000. This latter polypeptide comigrates on sodium dodecyl sulfate-gels with in vivo virus capsid precursor A, and the two have identical patterns of CNBr-generated peptides. A polypeptide having a molecular weight of 12,500 is also a major translation product in the system from infected cells (but not from uninfected cells). This polypeptide appears to be generated by cleavage of the NH-2-terminal portion of the viral RNA-dependent polypeptides by a proteolytic activity present in the infected cell-free system. This proteolytic activity copurifies with the 23,000-molecular weight viral capsid protein gamma, found in infected cells, through chromatography on DEAE-cellulose and cellulose phosphate. This suggests that gamma is itself a proteolytic enzyme involved in maturation of the viral capsid precursor.