Identification of an avian oncovirus polyprotein in uninfected chick cells.

Eisenman, R and Shaikh, R and Mason, W S (1978) Identification of an avian oncovirus polyprotein in uninfected chick cells. Cell, 14 (1). pp. 89-104. ISSN 0092-8674

Abstract

We have identified an avian oncovirus-specific polypeptide of 120,000 dalton molecular weight (P120) in extracts of uninfected chick embryo fibroblast cells (CEF). P120 can be detected in CEF positive for the expression of avian oncovirus group-specific antigen (gs+), but its presence cannot be correlated with phenotypic expression of endogenous oncovirus envelope glycoprotein (chf). Immune precipitation experiments indicate that P120 contains antigenic determinants of the virion core proteins (products of the gag gene) and reverse transcriptase, but not of the envelope glycoprotein gp85. Double-label ion-exchange chromatography of tryptic digests of P120 reveals that this polypeptide contains amino acid sequences related to the virion core proteins p270, p19 and p12 of the chicken endogenous virus RAV-0, but lacks the tryptic peptides of p15. In addition, P120 contains three tryptic peptides which appear to be present in both the α and β subunits of reverse transcriptase. The order of the tryptic peptides on P120 was determined using the pactamycin procedure. The data demonstrate that the P120 tryptic peptides which are presumed to be related to reverse transcriptase lie between the gag region and the carboxy terminus of the protein. Our data are consistent with the idea that P120 is equivalent to a gag-pol polyprotein that is missing both p15 and significant portions of reverse transcriptase. In pulse-chase experiments, P120 does not appear to be specifically cleaved to generate virion structural proteins. Furthermore, we have been unable to detect P120 in RAV-0 producing line 7 and line 15 CEF. Instead, only Pr76gag (the direct precusor to virion core proteins in productively infected cells) is observed in these cells. Thus P120 is unlikely to act as a functional intermediate in virus maturation. We have also observed that infection of gs+ CEF with exogenous avian sarcoma and leukemia viruses results in a significant but variable decrease in the level of P120.

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