Von Willebrand factor but not α-thrombin binding to platelet glycoprotein Ibα is influenced by the HPA-2 polymorphism

Abstract

Objective - Glycoprotein (GP) Ibα is the functionally dominant subunit of the platelet GPIb-IX-V receptor complex. The N-terminal domain of the GPIbβ chain contains binding sites for a-thrombin and von Willebrand factor (VWF). The human platelet alloantigen (HPA)-2 polymorphism of the GPIbα gene is associated with a C/T transition at nucleotide 1018, resulting in a Thr/Met dimorphism at residue 145 of GPIbα. To study the structural and functional effects of this dimorphism, N-terminal fragments (AA1-289) of the HPA-2a and HPA-2b alloform of GPIbα expressed in CHO cells were used. Methods and Results -Of 74 moAbs directed against human GPIbα, 2 antibodies with epitope between AA1-59 could differentiate between both alloforms. In addition, VWF bound with a higher affinity to the recombinant HPA-2a fragment or to homozygous HPA-2a platelets. In contrast, no difference was found in the binding of α-thrombin to the recombinant alloform fragments or of antibodies directed against the a-thrombin binding anionic sulfated tyrosine sequence (AA269-282). Conclusions - Whereas the Thr145Met dimorphism does not affect α-thrombin binding, it does influence the conformation of the N-terminal flanking region and first leucine-rich repeat of GPIbα and by this has an effect on VWF binding.