Thrombospondin binds to amino-terminal fragments of plasma fibronectin

Thrombospondin is a 420-kD trimeric glycoprotein that can bind type V collagen, heparin, fibrinogen and certain cells and may be one of the lectins responsible for platelet aggregation. Thrombospondin binds another glycoprotein, fibronectin, that is also released during platelet aggregation and also binds similar ligands. This work shows that the amino-terminal 29-kD segment of fibronectin binds thrombospondin, the interaction occurs within minutes, and one 29-kD molecule binds per thrombospondin subunit. The interaction was not inhibited by fibrinogen, type V collagen, or heparin. Two subfragments of the 29-kD fragment, an amino-terminal 20-kD and a carboxyl-terminal 8-kD subfragment, the latter containing a single disulfide-rigidified type I loop of the five homologous loops in the 29-kD fragment, reacted with thrombospondin while the reduced counterparts did not.