Evidence For A Separate Localization Of Platelet Glycoprotein II_b And III In The α-Granule Membrane

Characterization of a well defined human platelet α-granule preparation by crossed immunoelectrophoresis revealed the presence of an immunopre- cipitate which previously has been shown to contain GP II_b and GP III_a. The corresponding antigen is known to be surface-located. Surface iodination of whole platelets followed by subcellular fractionation, crossed immunoelectrophoresis of the fractions and autoradiography of the immunplates revealed a considerable labelling of this antigen in the membrane fraction whereas no labelling was detected in the corresponding antigen in the α-granules. The surface membrane form seems to be immunologically identical to the α-granule form, however. Another surface membrane-located glycoprotein, glycocalicin-related protein, was present in the α-granules in only trace amounts indicating little contamination by plasma membranes. The presence of GP II_b and GP III_a in the α-granules was confirmed by SDS-polyacrylamide gel electrophoresis. The antigen containing these glycoproteins was found to be a constituent of the α-granule membrane. We conclude that GP II_b and GP III_a are constituents of the α-granule membrane as well as the plasma membrane in platelets.