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C Nanoff

Publications and source records attributed to C Nanoff.

At least 19 recordsLinked to original sources
PubMed2001

Mapping of calmodulin and Gbetagamma binding domains within the C-terminal region of the metabotropic glutamate receptor 7A.

Ca(2+)/calmodulin (Ca(2+)/CaM) and the betagamma subunits of heterotrimeric G-proteins (Gbetagamma) have recently been shown to interact in a mutually exclusive fashion with the intracellular C terminus of the presynaptic metabotropic glutamate receptor 7 (mGluR 7). Here, we further characterized the core CaM and Gbetagamma binding sequences. In contrast to a previous report, we find that the CaM binding motif localized in the N-terminal region of the cytoplasmic tail domain of mGluR 7 is conserved in the related group III mGluRs 4A and 8 and allows these receptors to also bind Ca(2+)/CaM. Mutational analysis of the Ca(2+)/CaM binding motif is consistent with group III receptors containing a conventional CaM binding site formed by an amphipathic alpha-helix. Substitutions adjacent to the core CaM target sequence selectively prevent Gbetagamma binding, suggesting that the CaM-dependent regulation of signal transduction involves determinants that overlap with but are different from those mediating Gbetagamma recruitment. In addition, we present evidence that Gbetagamma uses distinct nonoverlapping interfaces for interaction with the mGluR 7 C-terminal tail and the effector enzyme adenylyl cyclase II, respectively. Although Gbetagamma-mediated signaling is abolished in receptors lacking the core CaM binding sequence, alpha subunit activation, as assayed by agonist-dependent GTPgammaS binding, was not affected. This suggests that Ca(2+)/CaM may alter the mode of group III mGluR signaling from mono- (alpha) to bidirectional (alpha and betagamma) activation of downstream effector cascades.

Amino Acid Sequence↗
PubMed2001

Suramin and the suramin analogue NF307 discriminate among calmodulin-binding sites.

Calmodulin-binding sites on target proteins show considerable variation in primary sequence; hence compounds that block the access of calmodulin to these binding sites may be more selective than compounds that inactivate calmodulin. Suramin and its analogue NF307 inhibit the interaction of calmodulin with the ryanodine receptor. We have investigated whether inhibition of calmodulin binding to target proteins is a general property of these compounds. Suramin inhibited binding of [(125)I]calmodulin to porcine brain membranes and to sarcoplasmic reticulum from skeletal muscle (IC(50)=4.9+/-1.2 microM and 19.9+/-1.8 microM, respectively) and blocked the cross-linking of [(125)I]calmodulin to some, but not all, target proteins in brain membranes by [(125)I]calmodulin. Four calmodulin-binding proteins were purified [ryanodine receptor-1 (RyR1) from rabbit skeletal muscle, neuronal NO synthase (nNOS) from Sf9 cells, G-protein betagamma dimers (Gbetagamma) from porcine brain and a glutathione S-transferase-fusion protein comprising the C-terminal calmodulin-binding domain of the metabotropic glutamate receptor 7A (GST-CmGluR7A) from bacterial lysates]. Three of the proteins employed (Gbetagamma, GST-CmGluR7A and RyR1) display a comparable affinity for calmodulin (in the range of 50-70 nM). Nevertheless, suramin and NF307 only blocked the binding of Gbetagamma and RyR1 to calmodulin-Sepharose. In contrast, the association of GST-CmGluR7A and nNOS was not impaired, whereas excess calmodulin uniformly displaced all proteins from the matrix. Thus suramin and NF307 are prototypes of a new class of calmodulin antagonists that do not interact directly with calmodulin but with calmodulin-recognition sites. In addition, these compounds discriminate among calmodulin-binding motifs.

Allosteric Regulation↗
PubMed2000

Inhibition of adenylyl and guanylyl cyclase isoforms by the antiviral drug foscarnet.

The pyrophosphate (PP(i)) analog foscarnet inhibits viral DNA-polymerases and is used to treat cytomegalovirus and human immunodeficiency vius infections. Nucleotide cyclases and DNA-polymerases catalyze analogous reactions, i.e. a phosphodiester bond formation, and have similar topologies in their active sites. Inhibition by foscarnet of adenylyl cyclase isoforms was therefore tested with (i) purified catalytic domains C1 and C2 of types I and VII (IC1 and VIIC1) and of type II (IIC2) and (ii) membrane-bound holoenzymes (from mammalian tissues and types I, II, and V heterologously expressed in Sf9 cell membranes). Foscarnet was more potent than PP(i) in suppressing forskolin-stimulated catalysis by both, IC1/IIC2 and VIIC1/IIC2. Stimulation of VIIC1/IIC2 by Galpha(s) relieved the inhibition by foscarnet but not that by PP(i). The IC(50) of foscarnet on membrane-bound adenylyl cyclases also depended on their mode of regulation. These findings predict that receptor-dependent cAMP formation is sensitive to inhibition by foscarnet in some, but not all, cells. This was verified with two cell lines; foscarnet blocked cAMP accumulation after A(2A)-adenosine receptor stimulation in PC12 but not in HEK-A(2A) cells. Foscarnet also inhibited soluble and, to a lesser extent, particulate guanylyl cylase. Thus, foscarnet interferes with the generation of cyclic nucleotides, an effect which may give rise to clinical side effects. The extent of inhibition varies with the enzyme isoform and with the regulatory input.

Adenylyl Cyclase Inhibitors↗
PubMed2000

Binding of calmodulin to the D2-dopamine receptor reduces receptor signaling by arresting the G protein activation switch.

Signaling by D(2)-dopamine receptors in neurons likely proceeds in the presence of Ca(2+) oscillations. We describe here the biochemical basis for a cross-talk between intracellular Ca(2+) and the D(2) receptor. By activation of calmodulin (CaM), Ca(2+) directly inhibits the D(2) receptor; this conclusion is based on the following observations: (i) The receptor contains a CaM-binding motif in the NH(2)-terminal end of the third loop, a domain involved in activating G(i/o). A peptide fragment encompassing this domain (D2N) bound dansylated CaM in a Ca(2+)-dependent manner (K(D) approximately 0.1 micrometer). (ii) Activation of purified Galpha(i1) by D2N, and D(2) receptor-promoted GTPgammaS (guanosine 5'-(3-O-thio)triphosphate) binding in membranes was suppressed by Ca(2+)/CaM (IC(50) approximately 0.1 micrometer). (iii) If Ca(2+) influx was elicited in D(2) receptor-expressing HEK293 cells, agonist-dependent inhibition of cAMP formation decreased. This effect was not seen with other G(i)-coupled receptors (A(1)-adenosine and Mel(1A)-melatonin receptor). (iv) The D(2) receptor was retained by immobilized CaM and radiolabeled CaM was co-immunoprecipitated with the receptor. Specifically, inhibition by CaM does not result from uncoupling the D(2) receptor from its cognate G protein(s); rather, CaM directly targets the D(2) receptor to block the receptor-operated G protein activation switch.

Amino Acid Sequence↗
PubMed2000

Two different signaling mechanisms involved in the excitation of rat sympathetic neurons by uridine nucleotides.

UTP stimulates transmitter release and inhibits M-type K(+) channels in rat superior cervical ganglion neurons via G protein-coupled P2Y receptors. To investigate the underlying signaling mechanisms, we treated the neurons with either pertussis or cholera toxin; neither treatment altered the inhibition of M-type K(+) channels by 10 microM UTP. However, pertussis toxin reduced UTP-evoked [(3)H]noradrenaline release by 66%. UTP, UDP, ATP, and ADP caused accumulation of inositol trisphosphate in a pertussis toxin-insensitive manner. Pharmacological inhibition of inositol trisphosphate-induced Ca(2+) release (by inhibition of phospholipase C, of inositol trisphosphate receptors, and of the endoplasmic Ca(2+)-ATPase) prevented the UTP-dependent inhibition of M currents but failed to alter UTP-evoked [(3)H]noradrenaline release. Chelation of intracellular Ca(2+) by 1,2-bis(2-aminophenoxy)ethane-N, N,N',N'-tetraacetic acid also reduced the inhibition of M currents by UTP. In addition, all these manipulations attenuated the inhibition of M currents by bradykinin, but hardly affected the inhibitory action of oxotremorine M. These results demonstrate that UTP inhibits M-type K(+) channels via an inositol trisphosphate-dependent signaling cascade that is also used by bradykinin but not by muscarinic acetylcholine receptors. In contrast, the secretagogue action of UTP is largely independent of this signaling cascade but involves pertussis toxin-sensitive G proteins. Thus, UTP-sensitive P2Y receptors excite sympathetic neurons via at least two different signal transduction mechanisms.

Animals↗
PubMed1999

Calmodulin dependence of presynaptic metabotropic glutamate receptor signaling.

Glutamatergic neurotransmission is controlled by presynaptic metabotropic glutamate receptors (mGluRs). A subdomain in the intracellular carboxyl-terminal tail of group III mGluRs binds calmodulin and heterotrimeric guanosine triphosphate-binding protein (G protein) betagamma subunits in a mutually exclusive manner. Mutations interfering with calmodulin binding and calmodulin antagonists inhibit G protein-mediated modulation of ionic currents by mGluR 7. Calmodulin antagonists also prevent inhibition of excitatory neurotransmission via presynaptic mGluRs. These results reveal a novel mechanism of presynaptic modulation in which Ca(2+)-calmodulin is required to release G protein betagamma subunits from the C-tail of group III mGluRs in order to mediate glutamatergic autoinhibition.

Amino Acid Sequence↗
PubMed1999

Kinetics of ternary complex formation with fusion proteins composed of the A(1)-adenosine receptor and G protein alpha-subunits.

High affinity agonist binding to G protein-coupled receptors depends on the formation of a ternary complex between agonist, receptor, and G protein. This process is too slow to be accounted for by a simple diffusion-controlled mechanism. We have tested if the interaction between activated receptor and G protein is rate-limiting by fusing the coding sequence of the human A(1)-adenosine receptor to that of Galpha(i-1) (A(1)/Galpha(i-1)) and of Galpha(o) (A(1)/Galpha(o)). Fusion proteins of the expected molecular mass were detected following transfection of HEK293 cells. Ternary complex formation was monitored by determining the kinetics for binding of the high affinity agonist (-)-N(6)-3[(125)I](iodo-4-hydroxyphenylisopropyl)adenosine; these were similar in the wild-type receptor and the fusion proteins over the temperature range of 10 to 30 degrees C. Agonist dissociation may be limited by the stability of the ternary complex. This assumption was tested by creating fusion proteins in which the Cys(351) of Galpha(i-1) was replaced with glycine (A(1)/Galpha(i-1)C351G) or isoleucine (A(1)/Galpha(i-1)C351I) to lower the affinity of the receptor for the G protein. In these mutated fusion proteins, the dissociation rate of the ternary complex was accelerated; in contrast, the rate of the forward reaction was not affected. We therefore conclude that (i) receptor activation per se rather than its interaction with the G protein is rate-limiting in ternary complex formation; (ii) the stability of the ternary complex is determined by the dissociation rate of the G protein. These features provide for a kinetic proofreading mechanism that sustains the fidelity of receptor-G protein coupling.

Cell Line↗
PubMed1999

Metal-dependent nucleotide binding to the Escherichia coli rotamase SlyD.

Upon expression and purification of the first catalytic domain of mammalian adenylate cyclase type 1 (IC1), a 27 kDa contaminant was observed, which was labelled by three radioactive ATP analogues (8-azido-ATP, 3'-O-(4-benzoyl)benzoyl-ATP and 2',3'-dialdehyde-ATP); the protein was purified separately and identified as Escherichia coli SlyD by N-terminal amino acid sequence determination. SlyD is the host protein required for lysis of E. coli upon infection with bacteriophage PhiX174 and has recently been shown to display rotamase (peptidylproline cis-trans-isomerase) activity. The covalent incorporation of ATP analogues into SlyD was promoted by bivalent transition metal ions (Zn(2+)>/=Ni(2+)>Co(2+)>Cu(2+)) but not by Mg(2+) or Ca(2+); this is consistent with the known metal ion specificity of SlyD. ATP, ADP, GTP and UTP suppressed labelling of SlyD with comparable potencies. Similarly, SlyD bound 2',3'-O-(-2,4, 6-trinitrophenyl)-ATP with an affinity in the range of 10 microM, as determined by fluorescence enhancement. This interaction was further augmented in the presence of Zn(2+) (K(d)= approximately 2 microM at saturating Zn(2+)) but not of Mg(2+). Irrespective of the assay conditions, hydrolysis of nucleotides by SlyD was not detected. Upon gel filtration on a Superose HR12 column, SlyD (predicted molecular mass=21 kDa) migrated with an apparent molecular mass of 44 kDa, indicating that the protein was a dimer. However, the migration of SlyD was not affected by the presence of Zn(2+) or of Zn(2+) and ATP. Thus we concluded that SlyD binds nucleotides in the presence of metal ions. These findings suggest that SlyD serves a physiological role that goes beyond that accounted for by its intrinsic rotamase activity, which is observed in the absence of metal ions.

Adenosine Diphosphate↗
PubMed1999

G proteins as drug targets.

The structure and function of heterotrimeric G protein subunits is known in considerable detail. Upon stimulation of a heptahelical receptor by the appropriate agonists, the cognate G proteins undergo a cycle of activation and deactivation; the alpha-subunits and the beta gamma-dimers interact sequentially with several reaction partners (receptor, guanine nucleotides and effectors as well as regulatory proteins) by exposing appropriate binding sites. For most of these domains, low molecular weight ligands have been identified that either activate or inhibit signal transduction. These ligands include short peptides derived from receptors, G protein subunits and effectors, mastoparan and related insect venoms, modified guanine nucleotides, suramin analogues and amphiphilic cations. Because compounds that act on G proteins may be endowed with new forms of selectivity, we propose that G protein subunits may therefore be considered as potential drug targets.

Animals↗
PubMed1999

G protein antagonists.

Heterotrimeric G proteins couple membrane-bound heptahelical receptors to their cellular effector systems (ion channels or enzymes generating a second messenger). In current pharmacotherapy, the input to G protein-regulated signalling is typically manipulated by targeting the receptor with appropriate agonists or antagonists and, to a lesser extent, by altering second messenger levels, most notably by inhibiting phosphodiesterases that hydrolyse cyclic nucleotides. When stimulated, G proteins undergo a cycle of activation and deactivation in which the alpha-subunits and the betagamma-dimers sequentially expose binding sites for their reaction partners (receptors, guanine nucleotides and effectors, as well as regulatory proteins). These domains can be blocked by inhibitors and this produces effects that cannot be achieved by receptor antagonists. Here, the structural and mechanistic information on G protein antagonists is summarized and an outline of the arguments supporting the hypothesis that G proteins per se are also potential drug targets is provided.

Animals↗
PubMed1999

Tight association of the human Mel(1a)-melatonin receptor and G(i): precoupling and constitutive activity.

If stably expressed in human embryonic kidney (HEK)293 cells, the human Mel(1a)-melatonin receptor activates G(i)-dependent, pertussis toxin-sensitive signaling pathways, i.e., inhibition of adenylyl cyclase and stimulation of phospholipase Cbeta; the latter on condition that G(q) is coactivated. The antagonist luzindole blocks the effects of melatonin and acts as an inverse agonist at the Mel(1a) receptor in both intact cells and isolated membranes. This suggests that the Mel(1a) receptor is endowed with constitutive activity, a finding confirmed on reconstitution of the Mel(1a) receptor with G(i). Because the receptor density is in the physiological range, constitutive activity is not an artifact arising from overexpression of the receptor. In addition, the following findings indicate that the Mel(1a) receptor forms a very tight complex with G(i) which can be observed both in the presence and absence of an agonist. 1) In intact cells and in membranes, high-affinity agonist binding is resistant to the destabilizing effect of guanine nucleotides. 2) The ability to bind an agonist with high affinity is preserved even after exposure of the cells to pertussis toxin, because a fraction of G(i) is inaccessible to the toxin in cells expressing Mel(1a) receptors (but not the A(1)-adenosine receptor, another G(i)-coupled receptor). 3) An antiserum directed against the Mel(1a) receptor coprecipitates G(i) even in the absence of an agonist. We therefore conclude that the Mel(1a) receptor is tightly precoupled and that its constitutive activity may play a role in pacing the biological clock, an action known to involve the melatonin receptors in the suprachiasmatic nucleus.

Adenylate Cyclase Toxin↗
PubMed1999

Dual signaling of human Mel1a melatonin receptors via G(i2), G(i3), and G(q/11) proteins.

Mel 1a melatonin receptors belong to the super-family of guanine nucleotide-binding regulatory protein (G protein)-coupled receptors. So far, interest in Mel 1a receptor signaling has focused mainly on the modulation of the adenylyl cyclase pathway via pertussis toxin (PTX)-sensitive G proteins. To further investigate signaling of the human Mel 1a receptor, we have developed an antibody directed against the C terminus of this receptor. This antibody detected the Mel 1a receptor as a protein with an apparent molecular mass of approximately 60 kDa in immunoblots after separation by SDS-PAGE. It also specifically precipitated the 2-[125I]iodomelatonin (125I-Mel)-labeled receptor from Mel 1a-transfected HEK 293 cells. Coprecipitation experiments showed that G(i2), G(i3), and G(q/11) proteins couple to the Mel 1a receptor in an agonist-dependent and guanine nucleotide-sensitive manner. Coupling was selective since other G proteins present in HEK 293 cells, (G(i1), G(o), G(s), G(z), and G12) were not detected in receptor complexes. Coupling of the Mel 1a receptor to G(i) and G(q) was confirmed by inhibition of high-affinity 125I-Mel binding to receptors with subtype-selective G protein alpha-subunit antibodies. G(i2) and/or G(i3) mediated adenylyl cyclase inhibition while G(q/11) induced a transient elevation in cytosolic calcium concentrations in HEK 293 cells stably expressing Mel 1a receptors. Melatonin-induced cytosolic calcium mobilization via PTX-insensitive G proteins was confirmed in primary cultures of ovine pars tuberalis cells endogenously expressing Mel 1a receptors. In conclusion, we report the development of the first antibody recognizing the cloned human Mel 1a melatonin receptor protein. We show that Mel 1a receptors functionally couple to both PTX-sensitive and PTX-insensitive G proteins. The previously unknown signaling of Mel 1a receptors through G(q/11) widens the spectrum of potential targets for melatonin.

Adenylate Cyclase Toxin↗
PubMed1998

The C2 catalytic domain of adenylyl cyclase contains the second metal ion (Mn2+) binding site.

Membrane-bound mammalian adenylyl cyclase isoforms contain two internally homologous cytoplasmic domains (C1 and C2). When expressed separately, C1 and C2 are catalytically inactive, but conversion of ATP to cAMP is observed if C1 and C2 are combined. By analogy with DNA polymerases, adenylyl cyclases are thought to require two divalent metal ions for nucleotide binding and phosphodiester formation; however, only one Mg2+ ion (liganded to C1) has been visualized in the recently solved crystal structure of a C1-C2 complex [Tesmer, J. J. G., Sunahara, R. K., Gilman, A. G., and Sprang, S. R. (1997) Science 278, 1907-1916]. Here, we have studied the binding of ATP to IIC2 (from type II adenylyl cyclase) using ATP analogues [2',3'-dialdehyde ATP (oATP), a quasi-irreversible inhibitor that is covalently incorporated via reduction of a Schiff base, the photoaffinity ligand 8-azido-ATP (8N3-ATP), and trinitrophenyl-ATP (TNP-ATP), a fluorescent analogue] and fluorescein isothiocyanate (FITC). [alpha-32P]oATP and 8N-[alpha-32P]ATP are specifically incorporated into IIC2. Labeling of IIC2 by [alpha-32P]oATP and by FITC is greatly enhanced by Mn2+ and to a much lesser extent by Mg2+. Similarly, TNP-ATP binds to IIC2 as determined by fluorescence enhancement, and this binding is promoted by Mn2+. Thus, a second metal ion binding site (preferring Mn2+) is contained within the C2 domain, and this finding highlights the analogy in the reaction catalyzed by DNA polymerases and adenylyl cyclases.

Adenosine Triphosphate↗
PubMed1998

Gsalpha-selective G protein antagonists.

Suramin acts as a G protein inhibitor because it inhibits the rate-limiting step in activation of the Galpha subunit, i.e., the exchange of GDP for GTP. Here, we have searched for analogues that are selective for Gsalpha. Two compounds have been identified: NF449 (4,4',4",4'"-[carbonyl-bis[imino-5,1,3-benzenetriyl bis-(carbonylimino)]]tetrakis-(benzene-1,3-disulfonate) and NF503 (4, 4'-[carbonylbis[imino-3,1-phenylene-(2, 5-benzimidazolylene)carbonylimino]]bis-benzenesulfonate). These compounds (i) suppress the association rate of guanosine 5'-[gamma-thio]triphosphate ([35S]GTP[gammaS]) binding to Gsalpha-s but not to Gialpha-1, (ii) inhibit stimulation of adenylyl cyclase activity in S49 cyc- membranes (deficient in endogenous Gsalpha) by exogenously added Gsalpha-s, and (iii) block the coupling of beta-adrenergic receptors to Gs with half-maximum effects in the low micromolar range. In contrast to suramin, which is not selective, NF503 and NF449 disrupt the interaction of the A1-adenosine receptor with its cognate G proteins (Gi/Go) at concentrations that are >30-fold higher than those required for uncoupling of beta-adrenergic receptor/Gs tandems; similarly, the angiotensin II type-1 receptor (a prototypical Gq-coupled receptor) is barely affected by the compounds. Thus, NF503 and NF449 fulfill essential criteria for Gsalpha-selective antagonists. The observations demonstrate the feasibility of subtype-selective G protein inhibition.

Adenylyl Cyclases↗
PubMed1998

Differential uncoupling of A1 adenosine and D2 dopamine receptors by suramin and didemethylated suramin (NF037).

Suramin analogues uncouple two Gi/Go-coupled receptors, the D2 dopamine receptor in rat striatum and the A1 adenosine receptor in human cerebral cortex, with distinct structure-activity relations. This discrepancy may reflect true differences in the affinity of the analogues for specific receptor/G protein complexes or may be attributable to differences in species or in the tissue source used. We addressed this question by using human embryonic kidney 293 cells that stably express the human A1 and rat A1 receptor and the human D2 receptor. Suramin is 10-fold more potent than its didemethylated analogue NF037 in inhibiting the interaction between G proteins and the rat A1 or human A1 receptor; in contrast, both compounds are equipotent in uncoupling the D2 receptor. These differences are observed regardless of whether (1) inhibition of high affinity agonist binding to the receptors or (2) agonist-stimulated GTPgammaS binding is used as readout, (3) the receptors are allowed to interact with the G protein complement in human embryonic kidney 293 cell membranes, or (4) the receptors are forced to interact with a defined G protein alpha subunit (i.e., after reconstituting pertussis toxin-treated membranes with exogenous rGi alpha-1). The apparent affinity of suramin depends in a linear manner on receptor occupancy, which shows that suramin and the receptor compete for the G protein. Finally, the affinity of the receptors for rGi alpha-1 (human A1 > rat A1 > human D2) is inversely correlated with the potency of suramin in uncoupling ternary complexes formed by these receptors and thus determines the selectivity of the suramin analogues for specific receptor/G protein tandems.

Animals↗
PubMed1997

G protein coupling of the rat A1-adenosine receptor--partial purification of a protein which stabilizes the receptor-G protein association.

A membrane protein identified in cortical brain membranes and termed 'coupling cofactor', modulates G protein-coupling of the A1-adenosine receptor by reducing the catalytic efficiency of the receptor. Coupling cofactor traps the A1-adenosine receptor in the high affinity complex and, thus, is responsible for the resistance of high affinity A1-agonist binding to modulation by guanine nucleotides. In the present work, this effect was used for assaying the activity of coupling cofactor by reconstituting guanine-nucleotide resistant agonist binding to rat A1-adenosine receptors in detergent extracted brain membranes or in membranes from 293 cells after stable transfection with receptor cDNA. Coupling cofactor was partially purified from porcine brain membranes. The specific activity was modestly enriched (approximately 5-fold) after three chromatographic steps (DEAE-Sephacel, AcA34, MonoQ pH 8). Rechromatography of coupling cofactor over MonoQ at pH 7 resulted in a loss in specific activity if membranes of 293 cells but not if brain membranes were used as acceptor membranes. In addition, the molecular mass estimated by gel filtration decreased from > 150 kDa in the initial stage of purification to 40-30 kDa after this fourth chromatographic step. These two observations suggest that coupling cofactor requires an additional component that is present in brain membranes and is lost in later stages of purification. The activity of partially purified preparations of coupling cofactor activity relied also on the abundance of G protein alpha-subunits in the membrane. The activity on reconstitution with brain membranes or pertussis toxin pretreated 293 membranes was supported by addition of Gi alpha (rank order of protency: alpha i1 > alpha i3 > alpha i2) but not of G(o alpha). The selectivity for G protein alpha-subunits suggests that coupling cofactor may provide for an additional level of specificity in organizing receptor-G protein coupling.

Animals↗