Distinct Ligand-binding Modes for Integrin αvβ 3-Mediated Adhesion to Fibronectin versus Vitronectin

Abstract

Cell surface integrins can adopt distinct conformations in response to ligand binding and intracellular signals. Several integrins including αvβ3 can bind to multiple ligands. The binding of αvβ3 to fibronectin and vitronectin was used as a model to determine whether the same or distinct forms of the receptor were utilized in strong binding to the two different ligands. A spinning-disc device was used to measure the relative strength of the α vβ3-ligand bonds. The initial binding reaction for both ligands occurred in the absence of metabolic energy and resulted in a strong adhesion to fibronectin but a weak adhesion to vitronectin. Increases in the strength of the αvβ3-vitronectin bond required phosphorylation of the β3 cytoplasmic domain, intracellular signals, and the binding of cytoskeletal proteins to cytoplasmic domains of β3 controlled by Tyr-747 and Tyr-759. In contrast, αvβ3-mediated adhesion to fibronectin was unaffected by phorbol 12-myristate 13-acetate, mutations of Tyr-747 and Tyr-759 to phenylalanine, or availability of metabolic energy. This suggests that strong adhesion to fibronectin used the initial binding conformation, whereas strong binding to vitronectin required signaling-induced changes in the conformation of αvβ3.