Localization of von Willebrand factor binding domains to endothelial extracellular matrix and to type VI collagen

We have recently shown that von Willebrand factor (vWF) binds to endothelial and fibroblastic extracellular matrixes (ECM) in a dose-dependent, specific, and saturable way. To localize the domain on the vWF subunit responsible for this interaction, purified proteolytic fragments of vWF were compared for their ability to inhibit 125I-vWF binding to ECM. A tryptic dimeric fragment of 116 kD (T116), extending from amino acid (aa) residues 449 to 728, produced a significant inhibition of 125I-vWF binding to the ECM. In contrast, P34 (aa 1-272), SpI (aa 911-1,365), and SpII (aa 1,366-2,050) had no significant effect on 125I-vWF binding to the ECM. Using an immunofluorescence technique, we identified type VI collagen and heparan sulfate in the endothelial ECM. 125I-vWF was found to bind specifically to purified type VI collagen. Unlabeled vWF and SpIII were able to completely inhibit 125I-vWF binding to type VI collagen. T116 and SpI appeared as competitors of this interaction, whereas P34 and SpII were not. Our data suggest that vWF binds to the endothelial ECM through the T116 fragment and that T116 and SpI each contain a binding site for type VI collagen. Heparin is known to be a vWF ligand, but did not appear as a competitor of vWF binding to the ECM, nor did heparan sulfate.