Human patient membrane glycoproteins GPII-llla and GPIb-IX represent heterodimer complexes that are known to serve as receptors for platelet cohesion and adhesion to the vessel wall, respectively. GPIIb-IIIa also functions as an activation-dependent fibronectin (Fn) receptor. We have used a polyclonal antibody specific for the beta chain of the fibroblast fibronectin receptor (FR) to immunopurify an activation-independent FR from human platelets. Together with GPIIb-IIIa, this human platelet FR is shown to contribute to normal platelet-adhesion to Fn-coated surfaces. It alone can support the adhesion of thrombasthenic platelets which lack GPIIb-IIIa to the same surfaces. Immunopurified human platelet FR is a heterodimer like the fibroblast FR, composed of platelet glycoproteins previously designated GPIc (alpha chain) and GPIIa (beta chain). The first aim of this proposal is to undertake a comprehensive investigation of the relative role of GPIc-IIa and GPIIb-IIIa in platelet adhesion to fibronectin and other physiologic surfaces. Accumulated evidence suggests that specific conformational rearrangements within and between glycoprotein complexes are probably required for receptor activation and that GP complexes interact specifically with components of the cytoskeleton. We hypothesize that alterations in the association of these glycoproteins with each other and with the cytoskeleton not only modulate receptor function but are likely operative in the mechanisms whereby these receptors are turned on and off. To address this hypothesis, the second aim of this proposal is to analyze specific glycoprotein interactions using methods that can detect changes in glycoprotein-glycoprotein or glycoprotein- cytoskeleton associations in the intact platelet, and to pursue the development and application of protein-specific electron spin resonance (ESR) probes produced by a method developed in our laboratory. These studies will lead to a better understanding of the mechanisms whereby membrane glycoprotein receptor function is maintained and controlled in human platelets, with general appli- cability to all cell types, and to a better appreciation of qualitative defects in these platelet glycoprotein receptors that lead to impaired hemostasis or uncontrolled thrombosis in vivo.