BINDING OF THE VON-WILLEBRAND-FACTOR A1 DOMAIN TO HISTONE

Activation of the von Willebrand Factor (vWF) Al domain is a critical factor in regulating the interaction of vWF with its platelet membrane receptor, the glycoprotein (GP) Ib-IX-V complex. This activation cont rols vWF-dependent platelet adhesion at high shear. The vWF-GP Ib-IX-V interaction is induced in vivo by exposure of platelet-rich plasma to high shear force, or by association of VWF with one or more unidentif ied components of the subendothelial matrix. In vitro, soluble vWF is activated to bind to platelets by nonphysiological modulators, such as the bacterial glycopeptide, ristocetin, or the snake venom protein, b otrocetin, or by removal of negatively-charged sialic acid residues. A nalysis of vWF modulators and the very marked charge asymmetry of amin o acid sequences within the Al domain has led to an electrostatic mode l for vWF modulation. Endothelial membrane/matrix and detergent-solubl e fractions of human placenta were screened for the ability to bind vW F by electrophoresis of extracts on SDS-polyacrylamide gels, electrotr ansferring to nitrocellulose and probing with fluid-phase I-125-labele d vWF or a 39/34-kDa vWF fragment (Leu-480-Gly-718) that encompasses t he Al domain. In the course of these studies, it was found that both v WF and the 39/34-kDa vWF fragment bound strongly to histone. Purified soluble histone also bound vWF since, like ristocetin, it induced vWF flocculation. Histone binding to vWF did not activate or inhibit vWF b inding to platelets. While the vWF-histone interaction has no conceiva ble physiological role, it suggests that binding to the Al domain of v WF alone is insufficient to modulate vWF adhesive activity. This impli es that specific interactions of the vWF Al domain with either ristoce tin or botrocetin are required for GP Ib-M-V recognition to occur. (C) 1997 Elsevier Science Ltd.