Amino acid sequence of the von Willebrand factor-binding domain of platelet membrane glycoprotein Ib.

SummaryWe have evidence that ristocetin and botrocetin mediate binding of von Willebrand Factor (vWF) to platelet glycoprotein lb (GPIb) through two distinct domains on the vWF molecule. This was established by using monoclonal antibodies (MAbs) to vWF and synthetic peptides derived from the sequence of vWF. MAb 322 and MAb NMC/vW 4 both recognize native vWF as well as fragments containing the GPIb-binding domain of vWF, obtained with the following enzymes: trypsin (116 kDa), V-8 pro tease (Spill, 320 kDa) and V-8 protease plus subtilisin (33-28 kDa). Nevertheless, the lack of reciprocal displacement between the two MAbs in experiments of competitive inhibition for binding to vWF demonstrate that their respective epitopes are separate. Both MAbs inhibit 125I-vWF binding to platelet membrane GPIb and vWF-dependent platelet agglutination induced by ristocetin. However, only MAb NMC/vW4 inhibits these functions in the presence of botrocetin and when ristocetin-induced platelet agglutination is inhibited by MAb 322, botrocetin is still able to restore the agglutination. The involvement of two distinct domains of vWF for binding to GPIb in the presence of ristocetin or botrocetin was confirmed in experiments of binding of 125I-vWF to platelets using as competitor synthetic peptides corresponding to the GPIb binding domain of vWF (Cys 474 to Pro 488 and Ser 692 to Pro 708). At a final concentration of 2.5 mM both peptides inhibit more than 90% of the binding of vWF to ristocetin-treated platelets but are unable to modify this binding in the presence of botrocetin. In conclusion our data suggest that botrocetin and ristocetin involve distinct sites on vWF for binding to GPIb.

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CHARACTERIZATION OF THE VON WILLEBRAND FACTOR-BINDING DOMAIN OF PLATELET MEMBRANE GLYCOPROTEIN Ib

10.1055/s-0038-1642925 ◽

1987 ◽

Cited By ~ 1

Author(s):

M Handa ◽

K Titani ◽

K Takio ◽

Z M Ruggeri

Keyword(s):

Von Willebrand Factor ◽

Binding Domain ◽

Platelet Membrane ◽

Membrane Glycoprotein ◽

Tryptic Fragment ◽

Amino Terminal ◽

Platelet Membrane Glycoprotein ◽

Limited Digestion ◽

Von Willebrand ◽

Willebrand Factor

We have previously obtained immunochemical evidence that the von Willebrand factor (vWF)-binding domain of the platelet membrane glycoprotein (GP) Ib is located near the amino terminus of the a subunit (Journal of Biological Chemistry 261: 12579-12585, 1986). We have now determined the complete amino acid sequence of the 45 kDa tryptic fragment of glycocalicin that contains this domain. Purified glycocalicin was subjected to limited digestion with trypsin and the proteolytic fragments were separated by size-exclusion high-pressure liquid chromatography. Two fragments of 45 kDa and 84 kDa, respectively, were obtained under nonreducing conditions. After reduction and S-carboxymethylation, the 84 kDa fragment was unchanged, while the 45 kDa fragment yielded two new fragments, one of 35 kDa and the other of 7 kDa. This finding proves the existence of a trypsin cleavage site within a disulfide loop. Two primary sets of overlapping fragments were obtained by cleavage of the carboxymethylated protein at methionyl and lysyl bonds following treatment with cyanogen bromide and Achromobacter protease I, respectively. Additional fragments were obtained by treatment of glycocalicin with Staphylococcus aureus V8 protease and Serratia marcescens protease. Analysis of all these fragments provided data that allowed determination of the sequence of the amino terminal 299 residues of the GP Ib a-chain. This includes the 45 kDa tryptic fragment containing the vWF-binding domain. This 299-residue sequence, corresponding approximately to two thirds of the α-chain polypeptide, is largely hydrophobic and contains only two N-linked and one O-linked carbohydrate chains. A hydrophilic region exists between residues 215-299, with a cluster of ten negatively charged residues at 269-287. This area is likely to attract positively charged molecules. The hydrophilic, highly glycosylated (at Ser/Thr residues) region corresponding to the previously described "macroglycopeptide" begins at residue 292. The determined sequence of glycocalicin contains a region with seven repeats, indicative of gene duplication, and is highly homologous to human leucine-rich α2-glycoprotein.

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