In vitro synthesis of a functional avian sarcoma virus transforming-gene product

Abstract

A SINGLE avian sarcoma virus (ASV) gene (src) is responsible for the induction and maintenance of cell transformation in vitro and tumour production in infected animals1. The product of the ASV src gene is a phosphoprotein with an apparent molecular weight of 60,000 and has been designated p60src (refs 2–4). This src protein seems to act as a protein kinase when assayed by the transfer of 32P from γ-labelled ATP to IgG in specific immune complexes5. As this protein kinase activity is growth and temperature dependent in chick cells transformed with an ASV temperature-sensitive transformation mutant5, these results suggest that ASV may transform cells by aberrant phosphorylation of cellular proteins. Radiolabelled p60src is synthesised in cell-free extracts programmed by the 3′ third of the viral RNA, the region of the genome which contains the src gene, but not by similar RNA from a mutant which has a deletion of the src gene3,4. Because of the structural similarity between the in vitro and in vivo products4, we tested the in vitro translation product for phosphotransferase activity. We have found that it exhibits protein kinase activity, providing direct evidence that p60src is the functional product of the avian sarcoma virus transformation gene.