Abstract
A series of proteolytic fragments of human von Willebrand Factor (vWF) was purified to characterize the functional site that supports its interaction with sulfatides. SpIII, an N-terminal homodimer generated by V-8 protease (amino acids [AA] 1 to 1365), bound to sulfatides in a dose-dependent and saturable way. SpIII also totally inhibited the binding of vWF to sulfatides and SpIII binding was completely abolished by vWF. In contrast, SpII, the complementary C-terminal homodimer (AA 1366 to 2050), did not exhibit any binding affinity for sulfatides. Four purified fragments overlapping the sequence of SpIII were also tested for their ability to interact with sulfatides. An N-terminal monomeric 34-Kd fragment (P34, AA 1 to 272) generated by plasmin, a central monomer (SpI, AA 911 to 1365) produced by digestion with V-8 protease, and a tetrameric fragment III-T2 (comprising a pair of the two sequences AA 273 to 511 and AA 674 to 728) produced by secondary digestion of SpIII with trypsin did not interact with sulfatides. In contrast, a monomeric 39/34-Kd fragment produced by dispase (AA 480 to 718) bound specifically and with a high affinity to sulfatides and totally displaced vWF or SpIII binding. Conversely, binding of the 39/34-Kd species was totally abolished by vWF or SpIII. Thus, a functional site responsible for sulfatide binding was localized between AA 480 and 718 and comparison of the binding properties of the 39/34-Kd and III-T2 fragments indicated that the sequence 512 to 673 is necessary for the binding to sulfatides. Further mapping of this new functional domain of vWF, based on experiments of competitive inhibition of binding by either heparin or monoclonal antibodies directed toward vWF, showed that the site interacting with sulfatides is distinct from those involved in binding to platelet glycoprotein Ib, collagen, or heparin. This finding was confirmed by experiments using synthetic peptides which also indicated that the sequence comprising AA 569 to 584 is part of the sulfatide-binding domain or influences its activity.
