Characterization of the proteins of isolated human platelet α-granules

Abstract

The protein composition of a well-defined α-granule preparation isolated from human platelets has been studied. Crossed immunoelectrophoresis against polyspecific platelet antibodies revealed more than 20 immunoprecipitates. The glycoprotein II_b-III_a complex represented a major antigen in the Triton X-100-solubilized α-granule preparation and cross-reacted with the corresponding platelet membrane antigen. Furthermore, after lactoperoxidase-catalyzed ¹²⁵I-iodination of whole platelets it was not labelled, in contrast to its membrane-located counterpart. This indicates an intracellular location of glycoproteins II_b and III_a, probably as constituents of the α-granules. Fibrinogen, platelet factor 4, albumin, factor VIII-related antigen and the main granule glycoprotein (thrombin-sensitive protein, thrombospondin) were identified in the α-granule preparation by the crossed immunoelectrophoresis technique. Crossed affinity immunoelectrophoresis using lectins revealed the presence of at least seven glycoproteins, and six sialoglycoproteins were identified by their altered electrophoretic mobility after neuraminidase treatment. Sodium dodecyl sulphate polyacrylamide gel electrophoresis of reduced samples of the α-granules revealed at least 15 Coomassie Brilliant Blue-staining polypeptide bands, one of which comigrated with myosin heavy chain. No prominent band was observed in the actin region. Five glycopolypeptide bands were obsevered after periodic acid-Schiff staining. The dominant three represented the main granule glycoprotein, glycoprotein II_b and glycoprotein III_a, respectively. More glycoproteins seem to be present in the α-granules than was previously recognized.