External Cell Surface Protein Phosphorylation in Normal and Rous Sarcoma Virus Transformed Chick Embryo Fibroblasts

Abstract

<p>Endogenous protein kinase activity was detected on the external cell surface of both normal and Rous sarcoma virus (RSV)-transformed chick embryo fibroblasts (CEF). Cells growing in plastic dishes were incubated with [Υ-³²P]ATP for 20 minutes. Under the conditions employed, only proteins located on the external cell surface were labeled, as the radioactivity could be removed by mild trypsin treatment. In addition, exogenous histones were phosphorylated when added to the reaction mixture. The addition of cAMP and cGMP to the reaction had virtually no effect on ³²P incorporation, suggesting there is little or no cyclic nucleotide-dependent protein kinase activity present on the external cell surface. Cell surface protein kinase activity was higher in RSV-transformed CEF than in normal CEF, and, using a temperature-sensitive src mutant, this difference was found to be transformation-specific. Several differences were observed in the cell surface proteins phosphorylated in normal and transformed cells, and at least two of these were transformation-specific. These data suggest that changes in external cell surface protein phosphorylation are associated with RSV transformation and thus could play a role in the formation of the transformed cell phenotype.</p>