Characterization of the Ligand Binding Activities of Vitronectin: Interaction of Vitronectin with Lipids and Identification of the Binding Domains for Various Ligands Using Recombinant Domains

Vitronectin is a multifunctional plasma glycoprotein which may regulate the systems related to protease cascades such as the coagulation, fibrinolysis, and complement systems as well as cell adhesion. Solid-phase assays and affinity chromatography on immobilized glycolipids indicated that vitronectin purified under denaturing conditions bound to sulfatide (Gal(3-SO4)β1-1ceramide), cholesterol 3-sulfate, and various phospholipids, but not gangliosides. Only the unfolded or multimeric form of vitronectin bound to sulfatide, suggesting a conformational dependency of the binding activity, while vitronectin bound to cholesterol 3-sulfate regardless of its conformational state. The recombinant domains of human vitronectin and mutants with certain domains deleted were separately expressed in E. coli as fusion proteins. Using the recombinants, sulfatide-, phosphatidylserine-, cholesterol 3-sulfate-, Type I collagen-, heparin-, and β-endorphin-binding activities were found to be attributable to hemopexin domain 2 and hemopexin domain 1. The possibility was suggested that the presence of a somatomedin domain and/or connecting region flanking hemopexin domain 1 inactivated its heparin binding. De-N-glycosylation of plasma vitronectin significantly affected the cholesterol sulfate- and collagen-binding activities, although its effects were opposite. These findings suggest that diverse ligand-binding activities could be attributed to pexin family motifs but that the interdomain interactions and glycosylations modulate the ligand binding activities of vitronectin.