POSSIBLE INVOLVEMENT OF PERTUSSIS-TOXIN-SENSITIVE GTP-BINDING PROTEININ THE ALPHA(2)-ADRENOCEPTOR-MEDIATED MELANOSOME-AGGREGATION RESPONSEOF GOLDFISH MELANOPHORES

The signal-transduction system involving alpha-adrenoceptors, which co ntrols the aggregation of melanosomes, was investigated in the melanop hores of the black-moor goldfish, Carassius auratus. The order of effe ctiveness of alpha-subtype-specific agonists and antagonists indicated that the major subtype of alpha-adrenoceptor in these Cells is alpha2 . After a 12-hr treatment, pertussis toxin markedly attenuated the mel anosome-aggregation response to norepinephrine (NE), but it failed to inhibit the Li+-induced aggregation of melanosomes. Pertussis toxin ca talyzed the P-32!-ADP-ribosylation of a 43-kDa protein in a crude pre paration of membranes from the goldfish melanophores. The radioactivit y incorporation into the protein was much reduced by the presence of a n analog of GTP, guanosine-5'-(3-O-thio)-triphosphate, in the presence of NE. When the membranes were prepared from cells that had been pret reated with pertussis toxin, P-32!-ADP-ribosylation of the protein wa s not observed. These observations indicate that the 43-kDa protein is the alpha subunit of a regulatory GTP-binding protein, and that pertu ssis toxin inhibits the action of NE on the cells via ADP-ribosylation of the protein. From these results, it is concluded that stimulation Of alpha2-adrenoceptors of goldfish melanophores induces the aggregati on of melanosomes in a process mediated by a pertussis toxin-sensitive GTP-binding protein.