title: Dissociation of guanosine 5'-[gamma-thio]triphosphate from guanine-nucleotide-binding regulatory proteins in native cardiac membranes. Regulation by nucleotides and muscarinic acetylcholine receptors.
abstract: Binding of the poorly hydrolyzable GTP analog, guanosine 5'-[gamma-thio]triphosphate (GTP[S]), to purified guanine-nucleotide-binding regulatory proteins (G proteins) has been shown to be nonreversible in the presence of millimolar concentrations of Mg2+. In porcine atrial membranes, binding of [35S]GTP[S] to G proteins was stable in the presence of 1 mM Mg2+. However, either large dilution or, even more strongly, addition of unlabelled guanine nucleotides, in the potency order, GTP[S] greater than GTP greater than or equal to guanosine 5'-[beta,gamma-imino]triphosphate greater than GDP greater than or equal to guanosine 5'-[beta-thio]diphosphate greater than GMP, markedly enhanced the observed dissociation, with 20-30% of bound [35S]GTP[S] being released by unlabelled guanine nucleotide within 20 min at 25 degrees C. Most interestingly, dissociation of [35S]GTP[S] was rapidly and markedly stimulated by agonist (carbachol) activation of cardiac muscarinic acetylcholine receptors. Carbachol-stimulated release of [35S]GTP[S] was strictly dependent on the presence of Mg2+ and an unlabelled guanine nucleotide. Although having different potency and efficiency in releasing [35S]GTP[S] from the membranes by themselves, the guanine nucleoside triphosphates and diphosphates studied, at maximally effective concentrations, promoted the carbachol-induced dissociation to the same extent, while GMP and ATP were ineffective. GTP[S]-binding-saturation experiments indicated that one agonist-activated muscarinic acetylcholine receptor can cause release of bound GTP[S] from three to four G proteins. The data presented indicate that binding of GTP[S] to G proteins in intact membranes, in contrast to purified G proteins, is reversible, and that agonist-activated receptors can even, either directly or indirectly, interact with GTP[S]-bound G proteins, resulting in release of bound guanine nucleoside triphosphate.
authors.affiliation: Pharmakologisches Institut, Universität Heidelberg, Federal Republic of Germany.
authors.forename: G
authors.initials: G
authors.lastname: Hilf
authors.forename: C
authors.initials: C
authors.lastname: Kupprion
authors.forename: T
authors.initials: T
authors.lastname: Wieland
authors.forename: K H
authors.initials: KH
authors.lastname: Jakobs
mesh_terms: D000818:Animals; D001345:Autoradiography; D002217:Carbachol / Q000494:pharmacology; D002413:Cations, Divalent; D002855:Chromatography, Thin Layer; D019204:GTP-Binding Proteins / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism*; D009711:Nucleotides / Q000502:physiology*; D011976:Receptors, Muscarinic / Q000502:physiology*; D013552:Swine
chemical_list: D002413:Cations, Divalent; D009711:Nucleotides; D011976:Receptors, Muscarinic; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D002217:Carbachol; D019204:GTP-Binding Proteins; D008274:Magnesium