Subcellular localization of a low Mr GTP-binding protein, c25KG, in resting and stimulated human platelets.

Subcellular fractions were prepared from human platelets by sucrose density gradient centrifugation and the presence of a GTP-binding protein, c25KG, was analyzed by immunoblotting. c25KG was found to be located in plasma membrane and alpha-granule fractions in resting platelets by immunoblot analysis. Disappearance of the protein by the pronase treatment of intact alpha-granules indicated that c25KG is exposed on the cytoplasmic face of the granules. Activation of platelets by thrombin induced the redistribution of c25KG from alpha-granules to plasma membranes. On the other hand, Gi2, which is the main pertussis toxin-substrate in human platelets, was predominantly present in plasma membranes and thrombin-stimulation did not alter its distribution.