Specific dephosphorylation of membrane proteins in Rous sarcoma virus-transformed chick embryo fibroblasts

Abstract

Chick embryo fibroblasts (CEF) infected with avian sarcoma virus become rapidly transformed as a result of expression of the viral src gene^1,2 in the form of a single polypeptide of molecular weight 60,000 (pp60^src)³ with protein kinase activity⁴ and suggested preferential association with the plasma membrane^5,6. Studies with normal avian and mammalian cells have revealed the presence of an antigenically related protein^7,8 which seems to have similar kinase activity, but which is present at less than 1% of the levels of virally induced src protein found in transformed cells^8,9. As dynamic protein phosphorylation is important in numerous regulatory processes^10,11, the phenotypic expression of transformation may arise from an imbalance in one or more regulatory mechanisms that are controlled by protein phosphorylation^4,8,12. The cell membrane is affected during transformation¹³, including its phosphotransferase activity^14,15. The latter has been shown using isolated membrane fractions whose properties may be changed during preparation¹⁶. Therefore, we have compared the phosphorylation state of individual membrane proteins found in intact normal and RSV-transformed cells and report here the identification of two heavily phosphorylated, acidic membrane proteins in normal CEF which are specifically dephosphorylated on transformation by wild-type and temperature-sensitive Rous sarcoma viruses.