Studies on nucleotide and receptor regulation of Gi proteins: effects of pertussis toxin

Molecular Endocrinology
T SunyerLutz Birnbaumer

Abstract

In intact membranes as well as after reconstitution into phospholipid vesicles, pertussis toxin (PT)-mediated ADP-ribosylation of G proteins causes loss of receptor-mediated regulation of effectors and/or G protein-mediated regulation of receptor binding. Studies were carried out to test which of several discrete steps known to constitute the basal and receptor-stimulated regulatory cycles of Gi proteins are affected by PT. Experiments with the Gs-deficient Gi-regulated adenylyl cyclase of cyc- S49 cell membranes indicated that PT blocks Gi activation by GTP without affecting GDP dissociation or GTP binding to a major extent. This suggested that the block lies in the transition of inactive GTP-Gi to active GTP-Gi (G to G* transition). Experiments with purified Gi in solution and after incorporation into phospholipid vesicles showed that PT does not increase or decrease the intrinsic GTPase activity of Gi. Experiments in which Gi was incorporated into phospholipid vesicles with rhodopsin, a receptor that interacts with Gi to stimulate the rate of guanosine 5'-O-(3-thio)triphosphate binding and GTP hydrolysis, indicated that PT does not affect the basal GTPase activity of Gi, but blocks its activation by the photoreceptor. Taken ...Continue Reading

Citations

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