Affinity isolation and characterization of immunoglobulin G Fc fragment-binding glycoprotein from human blood platelets.

Immune complexes bind to several eukaryotic cell types including human blood platelets through the Fc fragment of immunoglobulin (Ig) G. Utilizing immobilized Fc fragment of IgG enabled us to isolate from human blood platelets a glycoprotein of an apparent Mr = 255,000 which, upon reduction, dissociated into sub-units of an apparent Mr = 50,000. This Fc fragment-binding glycoprotein has an isoelectric point between pH 6.3 and 6.9 and is composed of 34% hydrophobic, 25% acidic, and 14% basic amino acids. The Fc fragment-binding glycoprotein was also isolated from human platelet membrane preparations and was unaffected by prior treatment of platelets with thrombin. Isolated Fc fragment-binding glycoprotein formed an in vitro complex with aggregated immunoglobulin G. These results suggest that the isolated Fc fragment-binding component may prove useful in studies concerning the functional role of glycoproteins as cellular receptors for the Fc fragment of IgG.