Identification of a 33-Kd protein associated with the alpha-granule membrane (GMP-33) that is expressed on the surface of activated platelets.

To identify antigens on the platelet plasma membrane that are exposed after activation, we developed a monoclonal antibody (MoAb) designated RUU-SP 1.77. The RUU-SP 1.77 antigen is present on the membrane of resting platelets at a basal level and is strongly expressed on the plasma membrane after thrombin activation. Freshly fixed platelets bound 4,150 +/- 1,935 (mean +/- SD) RUU-SP 1.77 molecules per platelet; on fixed thrombin-stimulated platelets the number of binding sites was upregulated to 19,050 +/- 5,120 (kd 4.5 +/- 0.8 nmol/L). MoAb RUU-SP 1.77 recognized a major protein of 33 Kd and a minor 28-Kd protein, both under nonreduced and reduced conditions. Immunoelectron microscopic studies showed the presence of the protein associated with the membrane of alpha-granules. Due to the localization associated with the alpha-granule membrane, we have designated it GMP-33 (granule membrane protein with a molecular weight of 33 Kd). Based on structural properties, we conclude that GMP-33 is a protein associated with the alpha-granule membrane that has not been described before.