Isolation and partial characterization of platelet alpha-granule membranes

Porcine alpha-granules, prepared by a modification of pre-existing methods, were found to be essentially homogeneous by transmission electron microscopy. Freeze-fractured samples of isolated granules revealed numerous intramembranous particles on the EF (exoplasmic fracture) surface and to a lesser extent on the PF (protoplasmic fracture) surface whereas the PS (protoplasmic) surface was relatively smooth. The granules appear to be sealed, as evidenced by: a) the retention of their electron dense core material; b) the inability of impermeant labels to react with the granule contents, and c) the finding that the intragranular proteins are refractory to mild hydrolysis by externally added proteases. Membranes were isolated by alkali extraction of the granules and used for biochemical characterization. Approximately 87% of the protein, but only insignificant amounts of phospholipid were removed by this procedure, which yielded membrane vesicles devoid of the dense core. The membranes contain one major and several minor polypeptides of molecular weights ranging from 28,000 to 230,000, as determined by polyacrylamide gel electrophoresis. The major polypeptide contains carbohydrate residues. The exposure of specific proteins on the cytoplasmic surface of the granule membrane was determined by a combination of surface-specific labeling and proteolysis of intact granules, followed by membrane isolation and analysis. In sealed granules, only a limited number of bands are modified by the reagents whereas most of them are affected following granule lysis, indicating asymmetry in their transmembrane disposition. The fraction eluted by alkali extraction was also analyzed and found to contain nine major polypeptides of molecular weights ranging from 230,000 to 43,000. These are compared to the weights of the macromolecules believed to be secreted from alpha-granules, as determined by radioimmunological techniques.