Epinephrine induces changes in the subcellular distribution of the inhibitory GTP-binding protein Gi alpha-2 and a 38-kDa phosphorylated protein in the human platelet.

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Abstract
By using antibodies specific for .alpha. subunits of inhibitory GTP-binding proteins (Gi.alpha. polypeptides) to probe Western blots of whole platelet protein, we detected Gi.alpha.-2 as the predominant Gi.alpha. species present in platelets. The subcellular compartmentalization of distinct Gi.alpha.-2-immunoreactive polypeptides coupled to thrombin an .alpha.2-adrenergic receptors was examined in Triton X-100 platelet lysates prepared by high-speed centrifugation. This treatment permitted separation of the Triton-insoluble membrane skeleton from Triton-soluble cell components. In cells treated with either .alpha.-thrombin or epinephrine, we observed that a greater proportion of Gi.alpha.-2 was localized in the Triton-soluable fraction than in the Triton-insoluble fraction. Pertussis toxin was found to catalyze ADP-ribosylation of Gi.alpha.-2 in whole platelets. In thrombin-stimulated cells, this activity was confined to the Triton-soluble fraction and was markedly lower than that of unstimulated cells. Epinephrine, on the other hand, promoted translocation of a portion of the pertussis toxin-sensitive Gi.alpha.-2 from the Triton-soluble fraction to the Triton-insoluble fraction. In addition, epinephrine stimulated translocation of a phosphorylated protein of .apprxeq. 38 kDa that was not ADP-ribosylated by pertussis toxin. This protein expressed immunoreactivity with the general Gi.alpha. antiserum AS/7 but not with Gi.alpha.-2 antiserum LE/3. These findings suggest a role for specific localization of Gi.alpha. proteins in epinephrine-induced platelet responses.

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