Visual signal transduction: the cycle of transducin shuttling between rhodopsin and cGMP phosphodiesterase.
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3',5'-Cyclic-GMP Phosphodiesterases↗
Biomedical subjects
P Deterre
Publications and source records attributed to P Deterre.
Antibodies directed against transducin beta subunits interfere with the regulation of adenylate cyclase activity in brain membranes.
In an attempt to study the mechanisms of action of membrane-bound adenylate cyclase, we have applied to rat brain synaptosomal membranes antibodies raised against purified bovine transducin (T) beta gamma subunits. The antibodies recognized one 36-kDa protein in Western blots of the membranes. Adenylate cyclase activation by GTP non-hydrolyzable analogues was greatly decreased in immune, as compared to preimmune, antibody-treated membranes, whereas the enzyme basal activity was unaffected by both types of antibodies. The inhibition of forskolin-stimulated adenylate cyclase by guanine 5'-(beta, gamma-imino)triphosphate (Gpp-(NH)p) was decreased in membranes preincubated with immune, but not preimmune, antibodies. Anti-T beta antibodies moderately decreased the extent of subsequent adenylate cyclase activation by forskolin, while not affecting activation by Al3+/F-. The enzyme activation by Gpp(NH)p in untreated membranes remained the same upon further incubation in the presence of either type of antibodies. Such results were consistent with the decreased exchange of guanine nucleotides which occurred in membrane treated with immune, but not preimmune antibodies, upon addition of GTP. The blockade of the regulation of adenylate cyclase by Gpp(NH)p observed in membranes pretreated by anti-T beta antibodies thus appears to be caused by the impairment of the guanine nucleotide exchange occurring on Gs alpha subunits. The G beta subunits in the adenylate cyclase complex seem to be instrumental in the guanine nucleotide exchange on G alpha subunits, just as T beta subunits are in the transducin complex.
Adenylyl Cyclases↗
Fluoride complexes of aluminium or beryllium act on G-proteins as reversibly bound analogues of the gamma phosphate of GTP.
Fluoride activation of G proteins requires the presence of aluminium or beryllium and it has been suggested that AIF4- acts as an analogue of the gamma-phosphate of GTP in the nucleotide site. We have investigated the action of AIF4- or of BeF3- on transducin (T), the G protein of the retinal rods, either indirectly through the activation of cGMP phosphodiesterase, or more directly through their effects on the conformation of transducin itself. In the presence of AIF4- or BeF3-, purified T alpha subunit of transducin activates purified cyclic GMP phosphodiesterase (PDE) in the absence of photoactivated rhodopsin. Activation is totally reversed by elution of fluoride or partially reversed by addition of excess T beta gamma. Activation requires that GDP or a suitable analogue be bound to T alpha: T alpha-GDP and T alpha-GDP alpha S are activable by fluorides, but not T alpha-GDP beta S, nor T alpha that has released its nucleotide upon binding to photoexcited rhodopsin. Analysis of previous works on other G proteins and with other nucleotide analogues confirm that in all cases fluoride activation requires that a GDP unsubstituted at its beta phosphate be bound in T alpha. By contrast with alumino-fluoride complexes, which can adopt various coordination geometries, all beryllium fluoride complexes are tetracoordinated, with a Be-F bond length of 1.55 A, and strictly isomorphous to a phosphate group. Our study confirms that fluoride activation of transducin results from a reversible binding of the metal-fluoride complex in the nucleotide site of T alpha, next to the beta phosphate of GDP, as an analogue of the gamma phosphate.(ABSTRACT TRUNCATED AT 250 WORDS)
3',5'-Cyclic-GMP Phosphodiesterases↗
The retinal phototransduction process: enzymatic cascade and regulation.
Among cellular systems performing the transduction of an external stimulus, phototransduction in vertebrate rod cells is a unique case which allows convergent approaches to electrophysiological, biophysical and biochemical analyses. The framework of the molecular processes involved in the corresponding enzymatic cascade is now elucidated and can be considered as a model for G protein mediated transductions. We present here the main features of this cascade, its amplification and regulation properties. The mode of stimulation by the aluminofluoride ion is particularly addressed.
Membrane Proteins↗
Activation of retinal rod cyclic GMP-phosphodiesterase by transducin: characterization of the complex formed by phosphodiesterase inhibitor and transducin alpha-subunit.
The GTP-binding subunit of transducin (T alpha) activates the cGMP phosphodiesterase (PDE) of bovine retinal rods by relieving the constraint imposed by the inhibitory subunit PDE gamma. We have isolated and characterized the complex T alpha.GTP gamma S-PDE gamma formed when T alpha is activated by the nonhydrolyzable analog GTP gamma S. Sedimentation and light-scattering techniques demonstrate that, in contrast to free T alpha.GTP gamma S, which is soluble, the T alpha.GTP gamma S-PDE gamma complex, as well as T alpha.GTP-PDE gamma, is membrane bound at cytosolic ionic strength. It is eluted from the membrane at low ionic strength as a monomeric and 1:1 stoichiometric complex. The relative affinities of PDE gamma for PDE alpha beta and for T alpha.GTP are discussed.
3',5'-Cyclic-GMP Phosphodiesterases↗
Serotonin- and dopamine-sensitive adenylate cyclase in molluscan nervous system. Biochemical and electrophysiological analysis of the pharmacological properties and the GTP-dependence.
Helix aspersa neuronal cell membranes contain distinct serotonin (5-HT) and dopamine (DA) sensitive adenylate cyclases. We have taken advantage of the fact that in this system, both in vitro (enzymatic assays) and in vivo (electrophysiological measurements) experiments can be used to explore the GTP dependence and the pharmacological properties of this neurotransmitter-sensitive enzyme system. The first property was studied using non-hydrolysable GTP analogs (guanosine 5'-O-(3-thio-triphosphate) or GTP gamma S, and guanosine 5'-imido diphosphate or Gpp(NH)p). In vitro, these two components stimulate the enzyme activity but with different potencies (Kapparent = 10(-8) to 5 X 10(-8) M for GTP gamma S, and 10(-5) M for Gpp(NH)p). Intracellular injections of GTP gamma S, but not of Gpp(NH)p, produced an electrophysiological response similar to the one elicited by 5-HT and DA. These results imply that, even in the presence of the high endogenous GTP concentration normally present in the cell (10(-3) M), GTP gamma S may bind to the GTP-binding protein. Such an interpretation is consistent with the in vitro competition experiments between GTP and GTP gamma S for adenylate cyclase activation. The pharmacology of 5-HT and DA receptors involved in adenylate cyclase stimulation and electrophysiological responses was studied. Serotoninergic antagonists and neuroleptics inhibited the 5-HT-sensitive adenylate cyclase in a stereospecific manner. However, their inhibition was not simply competitive. Our results suggest that they irreversibly bind a component localized on the cytoplasmic side of the membrane. Unexpectedly, the DA receptor coupled with adenylate cyclase was insensitive to any of the several antagonists tested.
Adenylyl Cyclases↗
Fluoroaluminates activate transducin-GDP by mimicking the gamma-phosphate of GTP in its binding site.
Fluoride activation of the cGMP cascade of vision requires the presence of aluminum, and is shown to be mediated by the binding of one A1F-4 to the GDP/GTP-binding subunit of transducin. The presence of GDP in the site is required: A1F-4 is ineffective when the site is empty or when GDP beta S is substituted for GDP. This sensitivity to the sulfur of GDP beta S suggests that A1F-4 is in contact with the GDP. Striking structural similarities between A1F-4 and PO3-4 lead us to propose that A1F-4 mimics the role of the gamma-phosphate of GTP.
Aluminum↗
Tissue expression and phylogenetic appearance of the beta and gamma subunits of GTP binding proteins.
Antibodies raised against the T-beta gamma dimer of bovine retinal transducin specifically bind to the beta and gamma subunits of transducin in calf retina. Tissues from different vertebrates, but not from invertebrates, contained a band comigrating with the beta subunit of transducin (T-beta) which was immunostained. This protein most likely corresponds to the beta subunit of GTP binding proteins of hormonal systems (G-beta). In non-retinal vertebrate membranes, the antibodies did not recognize the gamma subunits of G proteins whereas a band comigrating with bovine T-gamma was detected in frog or rat retina. Although T-beta was precipitated by the T-beta gamma antiserum, we failed to immunoprecipitate the G-beta from calf brain.
Animals↗
Inhibition of hormonally regulated adenylate cyclase by the beta gamma subunit of transducin.
Transducin (T), the GTP-binding protein of the retina activates the cGMP phosphodiesterase system, and presents analogies with the proteins GS and Gi which respectively mediate adenylate cyclase activation and inhibition by hormone receptors. These proteins are all comprised of an alpha subunit carrying the GTP-binding site and a beta gamma subunit made of two peptides. The beta peptide (35 kd) appears similar in the three proteins. We demonstrate here that purified T beta gamma inhibits adenylate cyclase from human platelet membranes. This inhibition was observed when adenylate cyclase was stimulated by GTP, prostaglandin E1 (PGE1), NaF and forskolin, but not when stimulated by GTP(gamma)S. In the presence of GTP and forskolin, the T beta gamma-induced maximal inhibition was not additive with the alpha 2-receptor-induced adenylate cyclase inhibition mediated by Gi. Both inhibitions were suppressed at high Mg2+ concentrations, which as also known to dissociate T beta gamma from T alpha-GDP. This suggests that these adenylate cyclase inhibitions are due to the formation of inactive complexes of GS alpha-GDP with T beta gamma or Gi beta gamma. T beta gamma-induced inhibition did not require detergent and could be suppressed by simple washing. T beta gamma effects are dependent on its concentration rather than on its total amount. This suggests that T beta gamma can operate in solution with no integration into the membrane. Similar inhibitory effects of T beta gamma are observed on adenylate cyclase from anterior pituitary and lymphoma S49 cell lines.
Adenylyl Cyclase Inhibitors↗
Two different mechanisms of calcium spike modulation by dopamine.
Dopamine (10 to 50 microM) modulates in two different ways the duration of the Ca2+-dependent action potential recorded in the cell body of identified neurons of the snail Helix aspersa. In some neurons (cells E13 and F1) dopamine increases the amplitude of their Ca2+-dependent spike plateau by decreasing the S-current (Klein, M., J.S. Camardo, and E. R. Kandel (1982) Proc. Natl. Acad. Sci. U.S.A. 79: 5713-5717), a K+ current controlled by cyclic AMP. In another neuron (cell D2), dopamine decreases the Ca2+-dependent plateau of the somatic action potential by evoking a decrease in Ca2+-current resulting from a decrease in Ca2+ conductance. Both modulatory effects could be observed in the same single neuron in which dopamine induces decreases of both the Ca2+ conductance and cyclic AMP-dependent K+ conductance. Nevertheless, in these cells (such as cell F5) dopamine only evokes a decrease of the amplitude of the Ca2+ spike plateau. Since the modulation of the duration of the Ca2+ action potential recorded in the neuronal soma has been shown to constitute a good model of events taking place at synaptic endings, it is suggested that these modulatory mechanisms evoked by dopamine may be involved in processes of presynaptic facilitation and inhibition.
Action Potentials↗
Guanine nucleotides and magnesium dependence of the association states of the subunits of transducin.
When GTP gamma S is bound to transducin (T), the two subunits T alpha X GTP gamma S and T beta gamma dissociate, independently of the ionic environment. When GDP is bound, these subunits are associated as a monomeric T alpha X GDP-T beta gamma complex of 75 kDa when the ionic environment is comparable to that of the cytoplasm, but they dissociate in the presence of 10-100 mM Mg2+ or Ca2+. Using this property, the subunits could be separated and purified by a rapid one-step procedure on an ion-exchange column (FPLC), and their molecular masses were verified by neutron small angle scattering. The physiological relevances of the dissociating effect of Mg2+ are discussed.
Animals↗
[Identification of the so-called 48 K protein that interacts with illuminated rhodopsin in retinal rods, and the retinal S antigen, inductor of experimental autoimmune uveoretinitis].
In Vertebrate retinal rod outer segments, a soluble "48 K" protein binds to disk membranes upon illumination in presence of ATP or GTP (H. Kühn, Biochemistry, 17, 1978, p. 4389). Its binding to photoexcited rhodopsin implies a probable role of the "48 K" protein in the ATP dependent regulation of the photoinduced enzymatic cascade which controls the hydrolysis of cGMP. The "retinal S antigen" is also a soluble protein located in photoreceptor cells which is known to be an organ-specific auto-antigen inducing experimental autoimmune uveoretinitis. Using extracts of purified cattle and frog rod outer segments, purified bovine S antigen, and monoclonal antibodies against S antigen, we found that both proteins exhibit identical characteristics with respect to: their migration in SD S-gel electrophoresis; their binding to rod disc membranes upon illumination in presence of ATP or GTP; their immunological reactivity with monoclonal antibodies.
Animals↗
GTP-dependent anion-sensitive adenylate cyclase in snail ganglia potentiation of neurotransmitter effects.
Snail ganglia possess an anion-sensitive adenylate cyclase. This enzyme was stimulated 100% by chloride in a strictly GTP-dependent manner. The apparent affinity of chloride for adenylate cyclase was 2 X 10(-4) M. Halogens were found to be the most active anions. Some inorganic anions such as SO4(2-) and H2PO4- were inactive, as were all the organic anions tested. Stimulation was not cumulative for any maximal concentration of the active anions except fluoride. Chloride potentiated the effect of fluoride, indicating that the anion effect is not fluoride-like. Another striking result is that chloride enhanced adenylate cyclase sensitivity to the neurotransmitters serotonin and dopamine. The absence of chloride stimulation when Mg2+ was replaced by Mn2+ further indicates a role of the GTP-binding protein (the G/F unit). Chloride could reversibly stimulate the adenylate cyclase activity already maximally stimulated by guanyl 5'-imidodiphosphate. We therefore suggest that, in snail ganglia, chloride raises the activity of the G/F unit-catalytic unit complex at some stage after its formation. The same specific anion-sensitive adenylate cyclase was also found in some of the rat tissues tested.
Adenylyl Cyclases↗
cAMP-mediated decrease in K+ conductance evoked by serotonin and dopamine in the same neuron: a biochemical and physiological single-cell study.
The extracellular application of either serotonin or dopamine and the intracellular injection of cAMP all evoke in the same identified neurons of the snail Helix aspersa inward currents associated with a decrease in K+ conductance. The serotonin-, dopamine-, and cAMP-induced inward currents all show the same maximal amplitude. When the response to one transmitter is maximal, the response to the other is blocked. Using a single-cell microassay, we found that both serotonin and dopamine stimulate the adenylate cyclase [adenosine triphosphate pyrophosphate-lyase (cyclizing), EC 4.6.6.1] activity of the neurons giving the inward-current responses; on the other hand, the adenylate cyclase activity of a neuron that does not show the serotonin- and dopamine-induced currents was not stimulated by the transmitters. In contrast with the nonsummation of the maximal inward-current responses, the maximal stimulating effects of the transmitters on the enzyme activity are additive. The diterpene forskolin, which stimulates the adenylate cyclase activity of the single cells 9-fold, also evokes an inward current. We conclude that single snail neurons are endowed with independent serotonin and dopamine receptors linked to the adenylate cyclase. Activation of each of these receptors evokes a cAMP-mediated decrease in K+ conductance. The physiological interaction between the transmitters probably takes place at a late step in the chain of events leading from the increase in cAMP to the closing of the K+ channels.
Adenylyl Cyclases↗
Relationship between two voltage-dependent serotonin responses of molluscan neurones.
A serotonin (5-HT)-induced slow inward current was reanalyzed in identified snail neurones and found to result from a decrease in a voltage-dependent K+-conductance, sensitive to [Ca2+]0 changes. 5-HT evoked in the same neurones an increase in the spike plateau known to be associated to a K+-conductance decrease. Both 5-HT responses appear to reflect the same decrease in K+-conductance.
Animals↗
Role of cyclic AMP in a serotonin-evoked slow inward current in snail neurones.
One model of synaptic transmission suggests that transmitters modify postsynaptic permeability through the intermediary of cyclic AMP. Thus, serotonin (5-hydroxytryptamine) evokes in molluscan neurones a decrease in a voltage-dependent K+ conductance which in turn generates a slow inward current when studied in steady voltage-clamp conditions. The serotonin-induced increase of the plateau phase of the spike of an Aplysia sensory neurone can be mimicked by both intracellularly injected cyclic AMP and extracellularly applied phosphodiesterase inhibitors, suggesting that cyclic AMP mediates the effect. We have tested whether a similar mechanism could account for the serotonin slow inward current in identified snail neurones and have found that the intracellular injection of cyclic AMP, but not of cyclic GMP or 5'-AMP, evokes a slow inward current showing similar voltage dependence, inversion potential and ionic properties to the serotonin slow inward current. Phosphodiesterase inhibitors at low concentrations (1-20 microM) potentiate the serotonin slow inward current and at higher concentrations evoke by themselves an inward current, partially or totally occluding the serotonin and cyclic AMP currents. Finally, we have found that in homogenates of pooled identified snail neurones serotonin stimulates the adenylate cyclase, increasing its activity by 50-100%.
Animals↗
Report on the 2nd International CD38 workshop.
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ADP-ribosyl Cyclase↗