Possible involvement of pertussis toxin‐sensitive GTP‐binding protein in the α2‐adrenoceptor‐mediated melanosome‐aggregation response of goldfish melanophores

Abstract

AbstractThe signal‐transduction system involving α‐adrenoceptors, which controls the aggregation of melanosomes, was investigated in the melanophores of the black‐moor goldfish, Carassius auratus.The order of effectiveness of α‐subtype‐specific agonists and antagonists indicated that the major subtype of α‐adrenoceptor in these cells is α2. After a 12‐hr treatment, pertussis toxin markedly attenuated the melanosome‐aggregation response to norepinephrine (NE), but it failed to inhibit the Li+ ‐induced aggregation of melanosomes.Pertussis toxin catalyzed the [32P]‐ADP‐ribosylation of a 43‐kDa protein in a crude preparation of membranes from the goldfish melanophores. The radioactivity incorporation into the protein was much reduced by the presence of an analog of GTP, guanosine‐5′‐(3‐O‐thio)‐triphosphate, in the presence of NE. When the membranes were prepared from cells that had been pretreated with pertussis toxin, [32P]‐ADP‐ribosylation of the protein was not observed. These observations indicate that the 43‐kDa protein is the α subunit of a regulatory GTP‐binding protein, and that pertussis toxin inhibits the action of NE on the cells via ADP‐ribosylation of the protein.From these results, it is concluded that stimulation of α2‐adrenoceptors of goldfish melanophores induces the aggregation of melanosomes in a process mediated by a pertussis toxin‐sensitive GTP‐binding protein. © 1993 Wiley‐Liss, Inc.