Differential tyrosine-specific phosphorylation of integrin in Rous sarcoma virus transformed cells with differing transformed phenotypes.

We have investigated the localization and phosphorylation of the fibronectin receptor in chick embryo fibroblasts transformed either by wild-type Prague C strain Rous sarcoma virus, which induces a rounded, less adhesive phenotype with a loss of surface fibronectin and gross cytoskeletal changes, or transformed by rASV2234.3, a variant which induces a flat, adhesive morphology with the retention of surface fibronectin and more normal cytoskeleton. Immunofluorescence showed co-distribution of pp60v-src with integrin and fibronectin fibrils in rASV2234.3-transformed cells. Total levels and surface expression of integrin were unchanged in both transformed cell types compared with untransformed cells. However, whereas integrin band 3 (beta 1 subunit) in Prague C-transformed cells was hyperphosphorylated on tyrosine, this was reduced virtually to normal in rASV2234.3-transformed cells. A similar differential phosphorylation of integrin band 3 could be found in membranes phosphorylated in vitro. rASV2234.3 pp60v-src was less efficient in phosphorylating a synthetic peptide containing the putative integrin tyrosine phosphorylation site, indicating that this variant pp60v-src has an altered substrate specificity compared to Prague C pp60v-src. The correlation between tyrosine-specific phosphorylation of integrin and loss of surface fibronectin suggests that this could play an important role in the induction of the transformed phenotype.