EFFECT OF DELETION OF THE A1-DOMAIN OF VONWILLEBRAND-FACTOR ON ITS BINDING TO HEPARIN, COLLAGEN AND PLATELETS IN THE PRESENCE OF RISTOCETIN

In order to study the functional importance of the collagen, heparin and glycoprotein-Ib-binding domain, we deleted the A1 domain of von Willebrand factor (vWF), corresponding to residues 478 - 716, by oligonucleotide-directed mutagenesis. The resulting DELTA-A1-vWF cDNA was expressed in COS-1 monkey kidney cells and compared to wild-type vWF. The higher-molecular-mass multimers were decreased in DELTA-A1 recombinant von Willebrand factor (DELTA-A1-rvWF) compared to plasma vWF and rvWF. The reactivity of DELTA-A1-rvWF and rvWF with monoclonal antibodies directed against the collagen-binding domain (residues 969-992), the vessel-wall-binding domain, and the binding site for glycoprotein IIb-III a on platelets was identical. The interaction with vWF of the monoclonal antibody directed against the glycoprotein Ib binding domain was abolished for DELTA-A1-rvWF, and similar to plasma vWF for rvWF. The binding of factor VIII to DELTA-A1-rvWF and rvWF was similar. DELTA-A1-rvWF and rvWF bound similarly to collagen, but the binding of DELTA-A1-rvWF to heparin and to platelets in the presence of ristocetin were abolished. These data indicate that the heparin-binding site in the A1 domain is essential. There is no second binding domain for glycoprotein Ib outside the A1 domain. The collagen-binding domain in the A1 domain is either not active or its action can be compensated by the second collagen-binding domain.