Mutations in the von Willebrand Factor A1 Domain That Increase the Affinity for Collagen Have Different Effects on the Binding to Glycoprotein Ib.

Abstract

The interaction of plasma von Willebrand factor (VWF) with collagen at the site of vascular injury plays a critical role in the initiation of thrombus formation under high shear stress. It does this by forming a bridge between the fibrils of collagen in the subendothelium and the platelet glycoprotein Ib-IX-V complex (GPIb). The A1 domain of VWF is the binding site for GPIb whereas the collagen-function of VWF is controlled by both A1 and A3 domains. The VWF-A3 domain is important to support binding to fibrils of collagen Types I and III while the A1 domain is involved in the binding to microfibrillar collagen Type VI. It is assumed that the interaction of VWF with fibrillar collagen (via the A3 domain) may regulate the expression of the GPIb-binding site in the A1 domain. However, there is no a definite data to substantiate that hypothesis. Our goal was to demonstrate that a direct interaction between the A1 domain and fibrillar collagen Types I or III exposes the GPIb binding site. Thus, we postulated that platelet GPIb is able to interact with isolated A1 domain that is bound to collagen. We have demonstrated that the VWF-A1 protein binds specifically to human placenta collagen Types I and III with a KD ~ 200 nM by using surface plasmon resonance (SPR). Using plasma-free blood, we have provided strong evidence that isolated VWF-A1 domain bound to either collagen Type I or III is able to support platelet adhesion under high flow conditions. This platelet interaction was effectively blocked with antibodies against either GPIb or A1 domain. These results clearly show the ability of the A1 domain to concurrently interact with both GPIb and collagen fibrils and they also suggest that the collagen-A1 binding may regulate the expression of the GPIb-binding site in the A1 domain. To test this hypothesis, we analyzed three residues that in a previous mutagenesis study they increased the binding of VWF to GPIb, reasoning that they may have an effect on the collagen binding activity as well. The three residues are located in the a7 helix (rear face) of the folded A1 domain and mutagenesis studies of other I(A)-domains have demonstrated that this helix plays a role in regulating the affinity of the ligand-binding. We introduced point mutations into the 3 residues and the recombinant mutant proteins were expressed in bacteria. The three mutants (R687E, D688R, and E689R) were purified as wild type and their structural integrity was confirmed with three conformation-specific antibodies. All the mutants bound to both collagens Type I or III with an affinity much higher than the wild type (WT) (KD~ 9 -1 nM). The mutants were assessed by their ability to mediate platelet adhesion to collagen, and their ability to inhibit both ristocetin-induced platelet agglutination and shear-induced platelet aggregation. Interestingly, in the three assays the R687E mutant had an activity higher than WT while the D688R had a markedly decrease activity. The mutant E689R had an activity similar to that of WT for the three assays. Together our data indicate that a direct association between the VWF-A1 domain and collagen fibrils influences the expression of GPIb binding function in VWF. Further, these data indicate that residue R687 located in the a7 helix plays a novel and important role in modulating the collagen/A1/GPIb binding.