Cells appear to interact with structural and regulatory molecules in the extracellular matrix by means of specific receptors. We have analyzed putative receptors for two major glycoproteins, fibronectin and collagen. A membrane glycoprotein complex consisting of three components of approximately 140,000 daltons each was shown to co-localize partially with extracellular fibronectin and with intracellular Alpha-actinin in microfilament bundles. Monoclonal antibodies against this complex blocked cell adhesion to fibronectin, and substrate-attached antibody alone could mimic fibronectin-mediated spreading. The components of this putative fibronectin receptor complex were isolated and shown to be three distinct acidic sialoglycoproteins associated noncovalently into an olgomeric complex. Other studies implicated gangliosides in cellular organization of extracellular fibronectin fibrils, and showed a localization of these lipids at sites of their attachment to the cell surface. Possible modulation of fibronectin receptor function by occupancy of the collagen receptor was explored. Incubation of cells with collagen or its Alpha1(I) chain was found to inhibit a specific subset of fibronectin receptor functions in non-competitive fashion. Cell spreading and phagycytosis were inhibited, while direct binding and cell attachment to fibronectin-coated substrates were unaffected. Our future objectives will be to analyze the structures and functions of the receptors for fibronectin and collagen. Monoclonal and polyclonal antibodies will be used with controlled proteolytic cleavage to define structural and functional domains of each. Alterations in their distribution and phosphorylation after transformation and heat shock will be established. Reconstitution experiments and analyses of other molecules needed for biological function should provide a further understanding of their mechanisms of action.
