Integrins isolated from Rous sarcoma virus-transformed chicken embryo fibroblasts

Avian integrins are a complex of three integral membrane glycoproteins that are thought to have a role both in anchoring the cytoskeleton to the plasma membrane and establishing linkages to the extracellular matrix. We previously demonstrated that bands 2 and 3 of integrin are phosphorylated on both tyrosine and serine residues in chicken embryo fibroblasts (CEF) transformed with Rous Sarcoma virus (RSV) and other oncogenic retroviruses. The effects of RSV transformation on integrins from chick cells are now further characterized. The major site of tyrosine phosphorylation on band 3 in RSV transformed CEF has been identified as tyrosine 788. We also demonstrate that the product of the RSV oncogene, pp60v-src, can phosphorylate integrin in vitro, at the same residue. Tryptic peptide mapping and tunicamycin treatment indicates that a previously observed 4-8 k increase in the Mr of integrins from RSV-transformed cells can be attributed to an alteration in a post-translational modification such as glycosylation. Equilibrium gel filtration assays were used to test the ability of integrins from RSV-transformed CEF to interact with talin and fibronectin. Tyrosine phosphorylated integrins showed a decreased ability to interact with both these ligands in vitro. Conversely, integrins isolated from RSV-transformed CEF metabolically labeled in the absence of phosphatase inhibitor contained only low levels of phosphotyrosine and showed an almost normal ability to interact with ligands. Competition experiments indicated that the region of integrin containing tyrosine 788 is also important for talin binding to integrins.