von Willebrand factor but not alpha-thrombin binding to platelet glycoprotein Ibalpha is influenced by the HPA-2 polymorphism.
OBJECTIVE
Glycoprotein (GP) Ibalpha is the functionally dominant subunit of the platelet GPIb-IX-V receptor complex. The N-terminal domain of the GPIbalpha chain contains binding sites for alpha-thrombin and von Willebrand factor (VWF). The human platelet alloantigen (HPA)-2 polymorphism of the GPIbalpha gene is associated with a C/T transition at nucleotide 1018, resulting in a Thr/Met dimorphism at residue 145 of GPIbalpha. To study the structural and functional effects of this dimorphism, N-terminal fragments (AA1-289) of the HPA-2a and HPA-2b alloform of GPIbalpha expressed in CHO cells were used.


RESULTS
Of 74 moAbs directed against human GPIbalpha, 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 alpha-thrombin to the recombinant alloform fragments or of antibodies directed against the alpha-thrombin binding anionic sulfated tyrosine sequence (AA269-282).


CONCLUSIONS
Whereas the Thr145Met dimorphism does not affect alpha-thrombin binding, it does influence the conformation of the N-terminal flanking region and first leucine-rich repeat of GPIbalpha and by this has an effect on VWF binding.
Ulrichts
Hans
H
Laboratory for Thrombosis Research, IRC, KULeuven Campus Kortrijk, E. Sabbelaan 53, 8500 Kortrijk, Belgium.
Vanhoorelbeke
Karen
K
Cauwenberghs
Sandra
S
Vauterin
Stephan
S
Kroll
Hartmut
H
Santoso
Sentot
S
Deckmyn
Hans
H
D000818:Animals; D016824:Antigens, Human Platelet / Q000235:genetics* / Q000378:metabolism*; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism; D016466:CHO Cells; D006224:Cricetinae; D003435:Crotalid Venoms / Q000494:pharmacology; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000378:metabolism*; D011110:Polymorphism, Genetic; D011994:Recombinant Proteins / Q000235:genetics; D012310:Ristocetin / Q000494:pharmacology; D013917:Thrombin / Q000378:metabolism*; D014841:von Willebrand Factor / Q000378:metabolism*
C115212:2a alloantigen, human; C071221:2b alloantigen, human; D016824:Antigens, Human Platelet; D003435:Crotalid Venoms; D019038:Platelet Glycoprotein GPIb-IX Complex; D011994:Recombinant Proteins; D014841:von Willebrand Factor; D012310:Ristocetin; C030961:botrocetin; D013917:Thrombin