Purification of glycoproteins IIb and III from human platelet plasma membranes and characterization of a calcium-dependent glycoprotein IIb-III complex

Human platelet membrane glycoproteins IIb and III are two major integral membrane components that have been identified as sites mediating thrombin-induced aggregation. For purposes of our study, glycoproteins IIb and III were solubilized by extracting platelet plasma membranes with a buffer containing 0.1% Triton X-100 and were separated by gel filtration chromatography on Sephacryl S-300, employing Triton X-100-containing column buffers with or without urea or guanidine hydrochloride. The physical properties of the purified glycoproteins were: for glycoprotein IIb, Rs = 61 A, s20.w = 4.7, f/f0 = 1.7, Mr = 125,000 (hydrodynamic values), Mr = 136,000 (sodium dodecyl sulfate gels); for glycoprotein III, Rs = 67 A, s20,w = 3.2 f/f0 = 2.1, Mr = 93,000 (hydrodynamic values), Mr = 95,000 (sodium dodecyl sulfate gels). Although the amino acid compositions of the two glycoproteins were similar, antibodies raised against glycoprotein IIb did not crossreact with glycoprotein III. If divalent cations were not chelated in the Triton extract, glycoproteins IIb and III coeluted during gel filtration chromatography (apparent Stokes radius of 71 A) and co-sedimented on sucrose gradients (apparent s20.w of 8.6), from which Mr = 265,000 was calculated. Glycoproteins IIb and III were coprecipitated by an antibody monospecific for glycoprotein IIb. The two glycoproteins dissociated into monomers when EDTA was added to Triton lysates. Readdition of Ca2+ caused them to reassociate into a complex with physical properties similar to those of the complex in the original Triton lysate. The data show that glycoproteins IIb and III are a heterodimer complex, that complex formation depends upon the presence of Ca2+, and that chelation of Ca2+ causes dissociation into monomeric glycoproteins.