Modeling and functional analysis of the interaction between von Willebrandfactor A1 domain and glycoprotein Ib alpha

Binding of the von Willebrand factor (vWF) A1 domain to the glycoprotein (G P) Ib-IX-V complex mediates platelet adhesion to reactive substrates under high shear stress conditions, a key event in hemostasis and thrombosis. We have now used the known three-dimensional structure of the A1 domain to mod el the interaction with the GP Ib alpha sequence 271-279, which has previou sly been implicated in ligand binding: Docking procedures suggested that A1 domain residues in strand beta 3 and preceding loop (residues 559-566) as well as in helix alpha 3 (residues 594-603) interact with Asp residues 272, 274, 277 and sulfated Tyr residues 278 and 279 in GP Ib alpha. To verify t his model, 14 mutant A1 domain fragments containing single or multiple side chain substitutions were tested for their ability to mediate platelet adhe sion under flow. Each of the vWF residues Tyr(565), GlU(596), and Lys(599) proved to be strictly required for A1 domain function, which, in agreement with previous findings, was also dependent on Gly(561). Moreover, an access ory functional role was apparent for a group of positively charged residues , including Arg at positions 629, 632, 636 and Lye at positions 643 and 645 , possibly acting in concert. There was, however, no evidence from the mode l that these residues directly participate in forming the complex with GP I b alpha. These results provide a partial model of the vWF-GP Ib alpha inter action linked to the manifestation of functional activity in platelet adhes ion.