Hemostasis Mediated by the Platelet Glycoprotein (GP)Ib alpha-Ib beta-IX Complex

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The platelet membrane glycoprotein (GP)Ib-V-IX complex is the receptor for von Willebrand factor and is composed of four polypeptides: GPIb alpha, GPIb beta, GPIX, and GPV, all of which feature leucine-rich repeat motifs. A qualitative or quantitative deficiency in this complex causes the rare human bleeding diathesis Bernard-Soulier syndrome (BSS). BSS is an autosomal recessive trait presenting in infancy with thrombocytopenia, circulating "giant" platelets and bleeding tendency in infancy. Bleeding in BSS is disproportionately more severe than predicted by platelet count, and is explained by a defect in primary hemostasis. We identified a novel mutation (P96S) at the GPIb beta locus in an infant haploinsufficient for this gene, due to heterozygous deletion of chromosome 22q11 (Velocardiofacial syndrome). Flow cytometry and confocal imaging of transfected Chinese hamster ovary cells that stably surface-express human GPIb alpha and GPIX (CHO alpha-IX) when transfected with wild-type GPIb beta demonstrated that P96S GPIb beta abrogates surface assembly of the platelet vWF receptor complex. Based on amino acid homology to the nogo-66 neuronal receptor (also a leucine-rich repeat protein, the crystal structure for which has been characterized), we proposed a model of GPIb beta protein structure that supports the importance of P96 and other residues previously reported as missense mutations in the conformation of GPIb beta, and its interaction with GPIX. GPIb beta represents the most critical component of this recently characterized platelet adhesion complex. Further study of GPIb beta and its critical role in platelet adhesion and hemostasis is progress to illuminate more precisely its interaction with GPIX and GPIb alpha and in hope of novel therapeutic approaches for BSS patients. In the past year, we successfully expressed the a FLAG-tagged GPIb beta cDNA in HEK-293 cells and performed partial amino acid sequencing that confirmed its identity. Our efforts to purify the native protein for crystallographic analysis as well as to generate specific antibodies against it for future studies of the (GP)Ib-V-IX complex are continuing.