Summaryvon Willebrand factor (vWF) mediates platelet adhesion at sites of vascular damage. It acts as a bridge between receptors on platelets and collagens present in the connective tissue. Two collagen binding sites have been identified on the A1 and A3 domain of the vWF subunit. To study the functional importance of these binding sites, we have made two deletion mutants that lack the A1 domain (residues 478-716; ΔA1-vWF; Sixma et al. Eur. J. Biochem. 196,369,1991 [1]) or the A3 domain (residues 910-1113; ΔA3-vWF). After transfection in baby hamster kidney cells overexpressing furin, the mutants were processed and secreted efficiently. Ristocetin or botrocetin induced platelet binding was normal for ΔA3-vWF as was binding to heparin and factor VIII. As reported by Sixma et al. (1) ΔAl-vWF still binds to collagen type III, indicating that the A3 domain is sufficient for the interaction. In the current study, we investigated the binding of ΔA3-vWF to collagen type III. When preincubated on collagen type III it did not support platelet adhesion under flow conditions, whereas it was able to support platelet adhesion when coated directly to a glass surface. The binding of 125I-ΔA3-vWF to collagen was specific but maximal binding was about 40 times less compared to 125I-vWF. When added at 25 times excess, ΔA3-vWF did not compete with 125I-vWF for binding to collagen type III, whereas ΔAl-vWF did. The binding of 125I-ΔA3-vWF could be blocked by excess unlabeled vWF but not by ΔA1-vWF. In conclusion, we demonstrate that the A3 domain in vWF contains the major collagen binding site. The major binding site present on the A3 domain and the minor site present on A1 bind to different sites on collagen.

The interaction of factor VIII with von Willebrand factor (vWF) was investigated on a quantitative and qualitative level. Binding characteristics were determined using a solid phase binding assay and protection of factor VIII by vWF from inactivation by activated protein C (aPC) was studied using three different assays. Deletion mutants of vWF, a 31-kD N-terminal monomeric tryptic fragment of vWF that contained the factor VIII binding site (T31) and multimers of vWF of different size were compared with vWF purified from plasma. We found that deletion of the A1, A2, or A3 domain of vWF had neither an effect on the binding characteristics nor on the protective effect of vWF on factor VIII. Furthermore, no differences in binding of factor VIII were found between multimers of vWF with different size. Also, the protective effect on factor VIII of vWF was not related to the size of the multimers of vWF. A 20-fold lower binding affinity was observed for the interaction of T31 with factor VIII, and T31 did not protect factor VIII from inactivation by aPC in a fluid-phase assay. Comparable results were found for a mutant of vWF that is monomeric at the N- terminus (vWF-dPRO). The lack of multimerization at the N-terminus may explain the decreased affinity of T31 and vWF-dPRO for factor VIII. Because of this decreased affinity, only a small fraction of factor VIII was bound to T31 and to vWF-dPRO. We hypothesized that this fraction was protected from inactivation by aPC but that this protection was not observed due to the presence of an excess of unbound factor VIII in the fluid phase. Therefore, vWF, T31, and vWF-dPRO were immobilized to separate bound factor VIII from unbound factor VIII in the fluid phase. Subsequently, the protective effect of these forms of vWF on bound factor VIII was studied. In this approach, all forms of vWF were able to protect factor VIII against inactivation by aPC completely. We conclude, in contrast with earlier work, that there is no discrepancy between binding of factor VIII to vWF and protection of factor VIII by vWF from inactivation by aPC. The protective effect of T31 was not recognized in previous studies due to its low affinity for factor VIII. The absence of multimerization observed for T31 and vWF- dPRO may explain the low affinity for factor VIII. No other domains than the binding site located at the D′ domain were found to be involved in the protection of factor VIII from inactivation by aPC.