Evidence for phosphatidylinositol-linked forms of human and avian fibronectin receptors

Phosphatidylinositol (PI)-linked forms of surface molecules have been hypothesized to mediate the initial stages of cell adhesion or signal transduction. We report evidence for the occurrence of a functional PI-linked subset of cell surface fibronectin receptors (FNR). Treatment of human MG63 osteosarcoma cells or primary chicken embryo fibroblasts (CEF) with PI-specific phospholipase C (PI-PLC) reduced cell surface FNR expression by 30% as detected by immunofluorescence. PI-PLC treatment of cell membranes purified from [35S]methionine-labeled CEF or MG63 cells led to a similar loss of membrane-associated immunoprecipitable FNR from the pelleted membranes, while such treatment led to the appearance of FNR in the supernatant of treated MG63 membranes. Biosynthetic labeling of CEF FNR with [3H]palmitate and [3H]ethanolamine demonstrated the acylation and putative PI linkage of avian FNR subunits. PI-PLC treatment of CEF and MG63 cells also reduced fibronectin-specific adhesion in a short-term in vitro assay, suggesting that the avian and human FNR occur in PI-linked isoforms which appear to contribute to cell adhesion to fibronectin.