A cellular activator of catecholamine-sensitive adenylate cyclase in rat reticulocytes and erythrocytes: changes during reticulocyte development and effects on the beta receptor

Binding of beta-adrenergic antagonists 3H-DHA and 3H-CGP 12177 to intact rainbow trout (Salmo gairdneri) and carp (Cyprinus carpio) red blood cells

beta-Adrenergic ligand binding to trout (Salmo gairdneri) and carp (Cyprinus carpio) red cells using a hydrophobic (3H-DHA) and a hydrophilic (3H-CGP) ligand was studied. Conditions which lead to the appearance (in vivo and in vitro hypoxia in carp cells) or disappearance ("winter" acclimation history in trout red cells) of beta-adrenergic responses were associated with either an appearance or marked increase of isoproterenol-displaceable binding (carp), or a disappearance of isoproterenol-displaceable binding (trout), respectively. The apparent number of binding sites was markedly greater for the hydrophobic than for the hydrophilic ligand, suggesting that binding of the hydrophobic ligand to internalized receptors and to nonreceptor sites may affect the results obtained with intact cells.

Enhancement of parathyroid hormone-responsive renal cortical adenylate cyclase activity by a cytosol protein activator from rat reticulocytes

The effects of the cytosol activator protein obtained from rat reticulocytes (RCAP) were investigated in a heterologous membrane system--partially purified cell membranes from dog renal cortex. RCAP enhanced the response of dog renal cortical adenylate cyclase to bovine parathyroid hormone (1-34) [bPTH (1-34)] from two- to three-fold. RCAP also enhanced the response to 5 microM arginine vasopressin, 10 microM glucagon, and 10 microM isoproterenol. Analysis of double-reciprocal plots of substrate concentration and enzyme activity indicated that bPTH (1-34) alone and together with RCAP increased the Vmax of the adenylate cyclase enzyme and did not alter the apparent Km of the enzyme for MgATP. Membranes from dog renal cortex contain 42K and 39K proteins that are ADP-ribosylated by cholera toxin and pertussis toxin, respectively, and appear to be the stimulatory (Ns) and inhibitory (Ni) guanine nucleotide binding proteins described in many other hormone-responsive membrane preparations. Similar to its effects in rat reticulocytes, RCAP inhibited ADP-ribosylation of Ns and enhanced ADP-ribosylation of Ni. The muscarinic agonist, carbachol, inhibited PTH-responsive adenylate cyclase activity in dog renal cortical membranes and this inhibition was reversed by RCAP. These results indicate that RCAP enhances stimulation of adenylate cyclase by a variety of hormones in a heterologous membrane preparation and supports the hypothesis that RCAP's site of action is common to all adenylate cyclase systems. RCAP may facilitate coupling between Ns and the catalytic unit of adenylate cyclase by a pertussis toxin-like effect to inactivate Ni. The dual effects of RCAP upon ADP-ribosylation of Ni and Ns alpha subunits suggest that a binding site for RCAP may exist at a site of homology between Ns alpha and Ni alpha.

Reticulocyte cytosol activator protein: its actions upon the beta adrenergic receptor and the N-proteins of adenylate cyclase

Rat reticulocytes contain a cytosol activator protein (RCAP) that augments catecholamine-sensitive adenylate cyclase activity in reticulocyte membranes. A partially purified preparation of RCAP was obtained by Sephacryl S-200 chromatography and used to elucidate further its mechanism of action. The specific activity of the S-200 fraction to augment isoproterenol responsiveness is increased approximately 1100-fold over the starting material from 1.2 nmoles to 1300 nmoles cyclic AMP formed per milligram of RCAP. The molecular weight is approximately 20,000. In addition to its effects on catecholamine-responsive adenylate cyclase, RCAP is associated with significant increases in basal (0.9 +/- 0.2 to 1.5 +/- 0.4 nmol/mg; p less than 0.02), guanyl-5'-yl imidodiphosphate [Gpp(NH)p]; (3.9 +/- 0.9 to 4.4. +/- 1.1 nmol/mg; p less than 0.005) and fluoride (4.1 +/- 0.6 to 4.8 +/- 0.6 nmol/mg; p less than 0.005) associated activities. RCAP stimulates isoproterenol responsiveness in wild type S49 cell membranes but is inactive in the mutant line, cyc-. RCAP alters the characteristics of agonist binding to the beta-adrenergic receptor of reticulocyte and wild S49 cell membranes, causing a significant increase in the IC50 for isoproterenol. Direct assessment of Ns and Ni components of the adenylate cyclase complex demonstrates that RCAP inhibits cholera toxin-specific ADP-ribosylation of the 42K subunit of Ns and stimulates pertussis toxin-specific ADP ribosylation of the 39K subunit of Ni.

The molecular basis for adrenergic desensitization in hamster brown adipose tissue: uncoupling of adenylate cyclase activation

A crude membrane fraction was prepared from hamster brown adipose tissue. Extensive washing of the crude membranes was crucial for the appearance of specific beta-adrenergic receptor binding as assessed by (-)-[3H]dihydroalprenolol. Adrenergic agents competed for the specific binding sites with beta 1-specificity. Binding characteristics were very similar to those earlier found in intact cells, supporting our previous finding that a single (non-tumour) mammalian cell may contain as many as 60,000 beta-adrenergic receptors. Desensitization in situ (i.e. chronic norepinephrine stimulation due to cold acclimation) only marginally affected the number of beta 1-receptors and their affinity (Ki) for norepinephrine. Total (fluoride-stimulated) adenylate cyclase increased somewhat, but the Kact for norepinephrine slightly decreased. Thus the ratio Ki/Kact was rather unaffected by cold acclimation. However, the fraction of adenylate cyclase which could be stimulated by norepinephrine decreased drastically. GTP introduced a low-affinity form (for agonist) of the receptor. The form observed in isolated cells must primarily be the high-affinity form. The basis for desensitization must reside in a diminished ability to transfer the signal from the receptor to the cyclase. This change may be molecularly located in the N-protein or in its interaction with the receptor.

Cytosolic activator of adenylate cyclase: Reconstitution, characterization, and mechanism of action

Rat liver and isolated hepatocytes contain high levels of a soluble adenylate cyclase stimulator, whereas rat erythrocytes lack this activity. Accordingly, a reconstitution system was developed with adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] from erythrocyte ghosts and the soluble activator from liver cytosol. Pretreatment of erythrocyte ghosts with the cytosolic factor resulted in a 5- to 15-fold activation of adenylate cyclase in the presence or absence of NaF, 5'-guanylyl imidodiphosphate, or isoproterenol and GTP. The sequence of addition of the cytosolic component and the other activators was critical in determining the maximal activity of the enzyme. The cytosolic factor appears to be a heat-labile Mr 105,000 protein, which activates adenylate cyclase in a saturable reaction involving binding of the protein to the erythrocyte ghosts. This molecular interaction was accompanied by stabilization of a labile thiol group that was essential for catalytic activity. The cytosolic component also unmasks latent adenylate cyclase activity in human erythrocyte ghosts and in cytoskeletal preparations from rat erythrocyte ghosts. These observations suggest that the cytosolic activator may also occur as a native, peripheral membrane component of adenylate cyclase systems and may be required for the expression and stabilization of catalytic activity.