In this paper, we have examined the functional domain of platelet membrane glycoprotein lb (GPIb) by using elastase and a monoclonal antibody against GPIb which specific inhibits both von Willebrand factor (vWF) and thrombin interaction with platelets. Elastase was purified from human granulocytes by using affinity column chromatography according to the method of Okada et al.. A monoclonal antibody against platelet membrane GPIb (56-2) which inhibits both vWF and thrombin-binding to platelets was used for this study. Platelet surface glycoproteins were labelled with 3H by the method of Nurden et al.. Purified GPIb was obtained by a modification of the method of Coller et al. and labelled with 125I using chloramine-T method. Either 3H-labelled platelets or 125I-labelled GPIb was treated with elastase for various time periods. Elastase-treated l25I-GPIb was subjected to immunoaffinity chromatography using 56-2 antibody to determine the functional site of GPIb. Elastase inhibited platelet aggregation or 5-HT release by thrombin, ristocetin-induced platelet agglutination and vWF-binding to platelets in the presence of ristocetin in a dose- and time dependent manner. A fluorogram of SDS-PAGE of 3H-labelled platelets treated with elastase revealed that GPIb band was reduced gradually, and fragments with MW of 97, 70, 60, 47, 44, 37, 25 and 15 KD were released from the platelets. The 47 KD fragment was initially cleaved from the platelets, and subsequently other fragments were digested. Similar results were obtained when purified 125I-GPIb was digested by elastase. When the fragments from purified 125I-GPIb were reacted with 56-2 antibody, only three fragments with MW of 47, 44 and 25 KD were immunoisolated. The electrophoretic mobility of all these three bands was altered under reduced conditions, indicating that all these fragments contain disulfide bonds in their molecules. The 25 KD band showed a much fainter in 3H-labelling than in 125I-labelling.These results suggest that the functional domains of GPIb for both vWF and thrombin-binding may be located in a less glycosylated fragment with a MW of 25 KD on the distal portion of the GPIb molecule, which should contain at least one intramolecular disulfide bond.
Isolation and characterization of the major oligosaccharide of human platelet membrane glycoprotein GPIb.