Conformational Regulation of the Fibronectin Binding and α3β1 Integrin-mediated Adhesive Activities of Thrombospondin-1

• Published in: The Journal of biological chemistry Vol. 276; no. 30; p. 27913
• Main Authors: Rui G. Rodrigues, Neng-hua Guo, Longen Zhou, John M. Sipes, Sybil B. Williams, Nancy Smyth Templeton, Harvey R. Gralnick, David D. Roberts
• Format: Journal Article
• Language: English
• Published: American Society for Biochemistry and Molecular Biology 27-07-2001
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M009518200

The recognition of extracellular matrix components can be regulated by conformational changes that alter the activity of cell
surface integrins. We now demonstrate that conformational regulation of the matrix glycoprotein thrombospondin-1 (TSP1) can
also modulate its binding to an integrin receptor. F18 1G8 is a conformation-sensitive TSP1 antibody that binds weakly to
soluble TSP1 in the presence of divalent cations. However, binding of the antibody to melanoma cells was strongly stimulated
by adding exogenous TSP1 in the presence of calcium, suggesting that TSP1 undergoes a conformational change following its
binding to the cell surface. This conformation was not induced by known cell surface TSP1 receptors, whereas binding of F18
was stimulated when TSP1 bound to fibronectin but not to heparin or fibrinogen. Conversely, binding of F18 to TSP1 enhanced
TSP1 binding to fibronectin. Exogenous fibronectin also stimulated TSP1-dependent binding of F18 to melanoma cells. Binding
of the fibronectin-TSP1 complex to melanoma cells was mediated by α 4 β 1 and α 5 β 1 integrins. Furthermore, binding to F18 or fibronectin strongly enhanced the adhesive activity of immobilized TSP1 for some
cell types. This enhancement of adhesion was mediated by α 3 β 1 integrin and required that the α 3 β 1 integrin be in an active state. Fibronectin also enhanced TSP1 binding to purified α 3 β 1 integrin. Therefore, both fibronectin and the F18 antibody induce conformational changes in TSP1 that enhance the ability
of TSP1 to be recognized by α 3 β 1 integrin. The conformational and functional regulation of TSP1 activity by fibronectin represents a novel mechanism for extracellular
signal transduction.