The platelet GPIIb/IIIa complex is a receptor for several adhesive glycoproteins and has been found to be a prototypic member of a family of Arg-Gly-Asp (RGD)-specific adhesion receptors. Relatively little is known of the other members of this family but it is apparent that they differ in molecular weight, cell distribution, and specificity for RGD-containing ligands, all of which probably underlie functional differences. For two years, we have been investigating a set of cell surface glycoproteins (the VLA antigens) which are present on a wide variety of cells including platelets, where we have shown they are identical with platelet GPI/a, GPI/c, and GPII/a. Very recently, it was found that these VLA antigens are a member of the family of RGD-adhesion receptors. Utilizing approaches similar to those we have used to study GPIIb/IIIa, we aim to use our experience in the study of VLA antigens to determine their structure and functional role on platelets, and determine how this function is related to that of platelet GPIIb/IIIa. Our studies of GPIIb/IIIa have focused on elucidation of the mechanism by which the ligand-binding activity of GPIIb/IIIa is regulated. Our studies suggest that platelets contain a large pool of GPIIb/IIIa, perhaps located within the surface connected canalicular system (SCCS), and that in resting platelets the SCCS can be entered by some but not all extra cellular proteins. This compartment may not interact with extracellular adhesive glycoproteins until accessibility constraints are overcome by platelet activation. We will test this hypothesis utilizing immunochemical and electron microscopic techniques. If this model is supported, we will further study the mechanisms which regulate accessibility to the SCCS and determine the role of this compartment in ligand binding. If this hypothesis is proven incorrect, we will test alternative models which propose that the induction of GPIIb/IIIa-ligand binding activity is due to either microenvironmental or conformational changes. This will be done by studying the binding activity of fragments of PAC-1, a large IgM monoclonal antibody which binds to GPIIb/IIIa only on activated platelets. We will also determine if the distances between different monoclonal antibody-defined subregions of GPIIb/IIIa molecules change subsequent to platelet activation. Utilizing similar approaches, we will then determine the mechanisms which regulate the ligand-binding activity of platelet VLA-antigens.
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