Structural studies on the von Willebrand factor A1 and A3 domains

Von Willebrand factor (VWF), a multimeric plasma glycoprotein, mediates platelet adhesion to sites of vascular damage. In this proces, the VWF-A3 domain binds to collagen in the vessel wall. This is followed by binding of the VWF-A1 domain to platelet receptor glycoprotein (Gp) Ibalpha. The investigations, using protein crystallography, site-directed mutagenesis and surface-plasmon-resonance, identified the collagen-binding site on the VWF- A3 domain. Based on the surface characteristics of the collagen-binding site, a hypothesis for the binding mode of a collagen triple helix on the VWF-A3 domain has been made. The identification of specific collagen sequences that interact with A3 is subject for future research. To investigate the binding of VWF to GpIbalpha, crystal structures of GpIbalpha, A1GpIbalpha, wt- and gain-of-function mutant R543Q-A1 have been solved. Based on these structures we hypothesized that the N- and C-terminal flanking peptides of the VWF-A1 domain shield one of the two GpIbalpha binding sites in A1 and thereby prevent GpIbalpha binding. Activation of the A1-domain involves a conformational change in these flanking peptides. The exact number of residues in the N- and C-terminal flanking peptides that shield A1 completely is subject for further research. In conclusion, we have gained insight at a molecular level in the interactions of VWF with collagen from the vascular matrix and platelet receptor GpIbalpha. It has not escaped our notice that these results are of significant value for the development of new drugs against arterial thrombosis