Biochemical analysis of the transducin-phosphodiesterase interaction

Taste Receptor Signaling

All organisms have the ability to detect chemicals in the environment, which likely evolved out of organisms' needs to detect food sources and avoid potentially harmful compounds. The taste system detects chemicals and is used to determine whether potential food items will be ingested or rejected. The sense of taste detects five known taste qualities: bitter, sweet, salty, sour, and umami, which is the detection of amino acids, specifically glutamate. These different taste qualities encompass a wide variety of chemicals that differ in their structure and as a result, the peripheral taste utilizes numerous and diverse mechanisms to detect these stimuli. In this chapter, we will summarize what is currently known about the signaling mechanisms used by taste cells to transduce stimulus signals.

Gain-of-function screen of α-transducin identifies an essential phenylalanine residue necessary for full effector activation

Two regions on the α subunits of heterotrimeric GTP-binding proteins (G-proteins), the Switch II/α2 helix (which changes conformation upon GDP-GTP exchange) and the α3 helix, have been shown to contain the binding sites for their effector proteins. However, how the binding of Gα subunits to their effector proteins is translated into the stimulation of effector activity is still poorly understood. Here, we took advantage of a reconstituted rhodopsin-coupled phototransduction system to address this question and identified a distinct surface and an essential residue on the α subunit of the G-protein transducin (α_T) that is necessary to fully activate its effector enzyme, the cGMP phosphodiesterase (PDE). We started with a chimeric G-protein α subunit (α_T*) comprising residues mainly from α_T and a short stretch of residues from the G_i1 α subunit (α_i1), which only weakly stimulates PDE activity. We then reinstated the α_T residues by systematically replacing the corresponding α_i1 residues within α_T* with the aim of fully restoring PDE stimulatory activity. These experiments revealed that the αG/α4 loop and a phenylalanine residue at position 283 are essential for conferring the α_T* subunit with full PDE stimulatory capability. We further demonstrated that this same region and amino acid within the α subunit of the G_s protein (α_s) are necessary for full adenylyl cyclase activation. These findings highlight the importance of the αG/α4 loop and of an essential phenylalanine residue within this region on Gα subunits α_T and α_s as being pivotal for their selective and optimal stimulation of effector activity.

Involvement of NADPH-dependent and cAMP-PKA sensitive H+ channels in the chorda tympani nerve responses to strong acids

To investigate if chorda tympani (CT) taste nerve responses to strong (HCl) and weak (CO(2) and acetic acid) acidic stimuli are dependent upon NADPH oxidase-linked and cAMP-sensitive proton conductances in taste cell membranes, CT responses were monitored in rats, wild-type (WT) mice, and gp91(phox) knockout (KO) mice in the absence and presence of blockers (Zn(2+) and diethyl pyrocarbonate [DEPC]) or activators (8-(4-chlorophenylthio)-cAMP; 8-CPT-cAMP) of proton channels and activators of the NADPH oxidase enzyme (phorbol 12-myristate 13-acetate [PMA], H(2)O(2), and nitrazepam). Zn(2+) and DEPC inhibited and 8-CPT-cAMP, PMA, H(2)O(2), and nitrazepam enhanced the tonic CT responses to HCl without altering responses to CO(2) and acetic acid. In KO mice, the tonic HCl CT response was reduced by 64% relative to WT mice. The residual CT response was insensitive to H(2)O(2) but was blocked by Zn(2+). Its magnitude was further enhanced by 8-CPT-cAMP treatment, and the enhancement was blocked by 8-CPT-adenosine-3'-5'-cyclic monophospho-rothioate, a protein kinase A (PKA) inhibitor. Under voltage-clamp conditions, before cAMP treatment, rat tonic HCl CT responses demonstrated voltage-dependence only at ±90 mV, suggesting the presence of H(+) channels with voltage-dependent conductances. After cAMP treatment, the tonic HCl CT response had a quasi-linear dependence on voltage, suggesting that the cAMP-dependent part of the HCl CT response has a quasi-linear voltage dependence between +60 and -60 mV, only becoming sigmoidal when approaching +90 and -90 mV. The results suggest that CT responses to HCl involve 2 proton entry pathways, an NADPH oxidase-dependent proton channel, and a cAMP-PKA sensitive proton channel.

Interaction between the second messengers cAMP and Ca2+ in mouse presynaptic taste cells

The second messenger, 3',5'-cyclic adenosine monophosphate (cAMP), is known to be modulated in taste buds following exposure to gustatory and other stimuli. Which taste cell type(s) (Type I/glial-like cells, Type II/receptor cells, or Type III/presynaptic cells) undergo taste-evoked changes of cAMP and what the functional consequences of such changes are remain unknown. Using Fura-2 imaging of isolated mouse vallate taste cells, we explored how elevating cAMP alters Ca(2+) levels in identified taste cells. Stimulating taste buds with forskolin (Fsk; 1 microm) + isobutylmethylxanthine (IBMX; 100 microm), which elevates cellular cAMP, triggered Ca(2+) transients in 38% of presynaptic cells (n = 128). We used transgenic GAD-GFP mice to show that cAMP-triggered Ca(2+) responses occur only in the subset of presynaptic cells that lack glutamic acid decarboxylase 67 (GAD). We never observed cAMP-stimulated responses in receptor cells, glial-like cells or GAD-expressing presynaptic cells. The response to cAMP was blocked by the protein kinase A inhibitor H89 and by removing extracellular Ca(2+). Thus, the response to elevated cAMP is a PKA-dependent influx of Ca(2+). This Ca(2+) influx was blocked by nifedipine (an inhibitor of L-type voltage-gated Ca(2+) channels) but was unperturbed by omega-agatoxin IVA and omega-conotoxin GVIA (P/Q-type and N-type channel inhibitors, respectively). Single-cell RT-PCR on functionally identified presynaptic cells from GAD-GFP mice confirmed the pharmacological analyses: Ca(v)1.2 (an L-type subunit) is expressed in cells that display cAMP-triggered Ca(2+) influx, while Ca(v)2.1 (a P/Q subunit) is expressed in all presynaptic cells, and underlies depolarization-triggered Ca(2+) influx. Collectively, these data demonstrate cross-talk between cAMP and Ca(2+) signalling in a subclass of taste cells that form synapses with gustatory fibres and may integrate tastant-evoked signals.

Receptors and transduction in taste

Taste is the sensory system devoted primarily to a quality check of food to be ingested. Although aided by smell and visual inspection, the final recognition and selection relies on chemoreceptive events in the mouth. Emotional states of acute pleasure or displeasure guide the selection and contribute much to our quality of life. Membrane proteins that serve as receptors for the transduction of taste have for a long time remained elusive. But screening the mass of genome sequence data that have recently become available has provided a new means to identify key receptors for bitter and sweet taste. Molecular biology has also identified receptors for salty, sour and umami taste.

The amino terminus of Galpha(z) is required for receptor recognition, whereas its alpha4/beta6 loop is essential for inhibition of adenylyl cyclase

G(z) couples to most of the known G(i)-linked receptors and its alpha subunit (Galpha(z)) inhibits adenylyl cyclases as efficiently as Galpha(i) subtypes. A series of chimeric Galpha subunits with different portions of Galpha(z) and Galpha(t1) (a regulator of cGMP phosphodiesterase) were constructed to study the essential structural elements of Galpha(z) that determine receptor coupling and effector interaction. The receptor-mediated functions of the chimeras were assessed in two aspects: 1) stimulation of type 2 adenylyl cyclase through the release of betagamma subunits from the chimeras, and 2) inhibition of isoproterenol-stimulated adenylyl cyclase by the chimeric Galpha subunits. The results suggested that the presence of both termini of Galpha(z) were critical for coupling to delta-opioid receptor, with the N-terminal region being more important. Moreover, a stretch of amino acids (295-319) corresponding to the alpha4/beta6 loop was identified as one of the adenylyl cyclase inhibitory domains of Galpha(z).

Cyclic nucleotide phosphodiesterases: relating structure and function

Cyclic nucleotide phosphodiesterases (PDEs) comprise a superfamily of metallophosphohydrolases that specifically cleave the 3',5'-cyclic phosphate moiety of cAMP and/or cGMP to produce the corresponding 5'-nucleotide. PDEs are critical determinants for modulation of cellular levels of cAMP and/or cGMP by many stimuli. Eleven families of PDEs with varying selectivities for cAMP or cGMP have been identified in mammalian tissues. Within these families, multiple isoforms are expressed either as products of different genes or as products of the same gene through alternative splicing. Regulation of PDEs is important for controlling myriad physiological functions, including the visual response, smooth muscle relaxation, platelet aggregation, fluid homeostasis, immune responses, and cardiac contractility. PDEs are critically involved in feedback control of cellular cAMP and cGMP levels. Activities of the various PDEs are highly regulated by a panoply of processes, including phosphorylation events, interaction with small molecules such as cGMP or phosphatidic acid, subcellular localization, and association with specific protein partners. The PDE superfamily continues to be a major target for pharmacological intervention in a number of medically important maladies.

Roles of the transducin alpha-subunit alpha4-helix/alpha4-beta6 loop in the receptor and effector interactions

The visual GTP-binding protein, transducin, couples light-activated rhodopsin (R*) with the effector enzyme, cGMP phosphodiesterase in vertebrate photoreceptor cells. The region corresponding to the alpha4-helix and alpha4-beta6 loop of the transducin alpha-subunit (Gtalpha) has been implicated in interactions with the receptor and the effector. Ala-scanning mutagenesis of the alpha4-beta6 region has been carried out to elucidate residues critical for the functions of transducin. The mutational analysis supports the role of the alpha4-beta6 loop in the R*-Gtalpha interface and suggests that the Gtalpha residues Arg310 and Asp311 are involved in the interaction with R*. These residues are likely to contribute to the specificity of the R* recognition. Contrary to the evidence previously obtained with synthetic peptides of Gtalpha, our data indicate that none of the alpha4-beta6 residues directly or significantly participate in the interaction with and activation of phosphodiesterase. However, Ile299, Phe303, and Leu306 form a network of interactions with the alpha3-helix of Gtalpha, which is critical for the ability of Gtalpha to undergo an activational conformational change. Thereby, Ile299, Phe303, and Leu306 play only an indirect role in the effector function of Gtalpha.

Mechanism of allosteric regulation of the rod cGMP phosphodiesterase activity by the helical domain of transducin alpha subunit

The G protein alpha subunit (Galpha) is composed of two distinct folding domains: a GTP-binding Ras-like domain and an alpha helical domain (HD). We have recently reported that the helical domain (HDt) of the vertebrate visual transducin alpha subunit (Galphat) synergizes activation of retinal cyclic GMP phosphodiesterase (PDE) by activated Galphat (Liu, W., and Northup, J. K., (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 12878-12883). Here, we examine the molecular basis for this HD-based signaling regulation, and we provide a new model for the activation of the target effector. The HD proteins derived from visual transducin or taste gustducin alpha subunits, but no other Galpha HD proteins, each attenuate the PDE catalytic core (Palphabeta) and synergize Galphat stimulation of the holoPDE (Palphabetagamma2) with similar apparent affinities. The data from studies of both HDt-mediated attenuation and stimulation indicate that the HDt and the PDE inhibitory subunit (Pgamma) interact with PDE at independent sites and that Palphabeta contains the binding sites for HD. The saturation of both processes by HDt displays positive cooperativity with Hill coefficients of 1.5 for the attenuation of Palphabeta activity and 2.1 for synergism of holoPDE activation. Our data suggest the that Galphat-HDt regulates PDE by allosterically decreasing the affinity of Palphabeta for Pgamma and thus simultaneously facilitating the interaction of the activated Galphat-Ras-like domain with Pgamma. Thus, we propose a new model for the high efficiency of PDE activation as well as deactivation, and, overall, a novel mechanism for controlling fidelity, sensitivity, and efficacy of G protein signaling.

Identification of effector residues on photoreceptor G protein, transducin

Transducin is a photoreceptor-specific heterotrimeric GTP-binding protein that plays a key role in the vertebrate visual transduction cascade. Here, using scanning site-directed mutagenesis of the chimeric Galphat/Galphai1 alpha-subunit (Galphat/i), we identified Galphat residues critical for interaction with the effector enzyme, rod cGMP phosphodiesterase (PDE). Our evidence suggests that residue Ile208 in the switch II region directly interacts with the effector in the active GTP-bound conformation of Galphat. Residues Arg201, Arg204, and Trp207 are essential for the conformation-dependent Galphat/effector interaction either via direct contacts with the inhibitory PDE gamma-subunit or by forming an effector-competent conformation through the communication network between switch II and the switch III/alpha3-helix domain of Galphat. Residues His244 and Asn247 in the alpha3 helix of Galphat are responsible for the conformation-independent effector-specific interaction. Insertion of these residues rendered the Galphat/i chimera with the ability to bind PDE gamma-subunit and stimulate PDE activity approaching that of native Galphat. Comparative analysis of the interactions of Galphat/i mutants with PDE and RGS16 revealed two adjacent but distinct interfaces on transducin. This indicates a possibility for a functional trimeric complex, RGS/Galpha/effector, that may play a central role in turn-off mechanisms of G protein signaling systems, particularly in phototransduction.

Identification of common and distinct residues involved in the interaction of alphai2 and alphas with adenylyl cyclase

The G protein alpha subunits, alphas and alphai2, have stimulatory and inhibitory effects, respectively, on a common effector protein, adenylyl cyclase. These effects require a GTP-dependent conformational change that involves three alpha subunit regions (Switches I-III). alphas residues in three adjacent loops, including Switch II, specify activation of adenylyl cyclase. The adenylyl cyclase-specifying region of alphai2 is located within a 78-residue segment that includes two of these loops but none of the conformational switch regions. We have used an alanine-scanning mutagenesis approach within Switches I-III and the 78-residue segment of alphai2 to identify residues required for inhibition of adenylyl cyclase. We found a cluster of conserved residues in Switch II in which substitutions cause major losses in the abilities of both alphai2 and alphas to modulate adenylyl cyclase activity but do not affect alpha subunit expression or the GTP-induced conformational change. We also found two regions within the 78-residue segment of alphai2 in which substitutions reduce the ability of alphai2 to inhibit adenylyl cyclase, one of which corresponds to an effector-activating region of alphas. Thus, both alphai2 and alphas interact with adenylyl cyclase using: 1) conserved Switch II residues that communicate the conformational state of the alpha subunit and 2) divergent residues that specify particular effectors and the nature of their modulation.

Binding of transducin to light-activated rhodopsin prevents transducin interaction with the rod cGMP phosphodiesterase gamma-subunit

In photoreceptor cells of vertebrates, the GTP-bound alpha-subunit of rod G-protein, transducin (G(t alpha)), interacts with the cGMP phosphodiesterase inhibitory gamma-subunit (Pgamma) to activate the effector enzyme. The GDP-bound G(t alpha) can also bind the Pgamma subunit, albeit with a lower affinity than G(t alpha)GTP. In this work, interactions between G(t alpha)GDP and Pgamma or Pgamma-24-45Cys labeled with the fluorescent probe 3-(bromoacetyl)-7-(diethylamino)coumarin (PgammaBC, Pgamma-24-45BC) have been investigated. Addition of G(t alpha)GDP to PgammaBC produced approximately a 6-fold maximal increase in the probe fluorescence, while the fluorescence of Pgamma-24-45BC was enhanced by 2.3-fold. The Kd's for the G(t alpha)GDP binding to PgammaBC and Pgamma-24-45BC were 75 +/- 8 nM and 400 +/- 110 nM, respectively. The G(t betagamma) subunits had no notable effect on the binding of G(t alpha)GDP to PgammaBC or Pgamma-24-45BC, suggesting that Pgamma and G(t betagamma) bind to G(t alpha)GDP noncompetitively. The G(t alpha betagamma) interaction with the fluorescently labeled Pgamma was effectively blocked in the light-activated rhodopsin (R*)-G(t alpha betagamma) complex. Furthermore, addition of excess Pgamma or Pgamma-24-45 prevented binding of G(t alpha betagamma) to R*, indicating that the R* and Pgamma binding surfaces on G(t alpha betagamma) may overlap. It is likely that R* has a binding site within the alpha3-beta5 region of G(t alpha), which is a proposed site of G(t alpha)GDP binding to Pgamma-24-45. Alternatively, R* may induce conformational changes of the G(t alpha) alpha3-beta5 region such that the resulting structural changes alter the adjacent consensus sequence for the guanine ring binding of GDP/GTP(NKXD), and lead to a reduction in the affinity of G-protein for guanine nucleotides.

Localization of the effector-specifying regions of Gi2alpha and Gqalpha

Heterotrimeric G proteins transmit hormonal and sensory signals received by cell surface receptors to effector proteins that regulate cellular processes. Members of the highly conserved family of alpha subunits specifically modulate the activities of a diverse array of effector proteins. To investigate the determinants of alpha subunit-effector specificity, we localized the effector-specifying regions of alphai2, which inhibits adenylyl cyclase, and alphaq, which stimulates phosphoinositide phospholipase C using chimeric alpha subunits. The chimeras were generated using an in vivo recombination method in Escherichia coli. The effector-specifying regions of both alphai2 and alphaq were localized within the GTPase domain. An alphaq/alphai2/alphaq chimera containing only 78 alphai2 residues within the GTPase domain robustly inhibited adenylyl cyclase. This alphai2 segment includes regions corresponding to two of the three regions of alphas that activate adenylyl cyclase, but does not include any of the alpha subunit regions that switch conformation upon binding GTP. Replacement of the alphaq residues that comprise the helical domain with the homologous alphai2 residues resulted in a chimeric alpha subunit that activated phospholipase C. Combined with previous studies of the effector-specifying residues of alphas and alphat, our results suggest that the effector specificity of alpha subunits is generally determined by the GTPase and not the helical domain.

Heterotrimeric G proteins

Over the past year, the thrust of work in the field of heterotrimeric G proteins has been primarily in the following areas: first, resolution of their three-dimensional structures by X-ray crystallography; second, elucidation of the effect of lipid modifications on the Galpha and Ggamma subunits; third, understanding the role of the Gbetagamma dimer in stimulation of a variety of effectors following receptor activation; and fourth, identification of the points of contact among the Galpha, Gbeta, and Ggamma subunits, and between these subunits and their cognate receptor or effector molecules.

Mapping of effector binding sites of transducin alpha-subunit using G alpha t/G alpha i1 chimeras

The G protein transducin has been an often-used model for biochemical, structural, and mechanistic studies of G protein function. Experimental studies have been limited, however, by the inability to express quantities of mutants in heterologous systems with ease. In this study we have made a series of G alpha t/G alpha i1 chimeras differing at as few as 11 positions from native G alpha t. Ten chimeras are properly folded, contain GDP, can assume an A1F4(-)-induced activated conformation, and interact with beta gamma t and light-activated rhodopsin. They differ dramatically in their affinity for GDP, from Gi-like (initial rates 225 mumol/mol s) to Gt-like (initial rates 4.9 mumol/mol s). We have used these chimeras to define contact sites on G alpha t with the effector enzyme cGMP phosphodiesterase. G alpha t GTP but not G alpha t GDP activates it by removing the phosphodiesterase (PDE) gamma inhibitory subunit. In solution, G alpha t GTP interacts with PDE gamma (Kd 12 nM), while G alpha t GDP binds PDE gamma more weakly (Kd 0.88 microM). The interaction of G alpha i GDP with PDE gamma is undetectable, but G alpha i GDP-A1F4- interacts weakly with PDE gamma (Kd 2.4 microM). Using defined G alpha t/G alpha i chimeras, we have individuated the regions on G alpha t most important for interaction with PDE gamma in the basal and activated states. The G alpha t sequence encompassing alpha helix 3 and the alpha 3/beta 5 loop contributes most binding energy to interaction with PDE gamma. Another composite P gamma interaction site is the conserved switch, through which the GTP-bound G alpha t as well as G alpha i1 interact with P gamma. Competition studies between PDE gamma and truncated regions of PDE gamma provide evidence for the point-to-point interactions between the two proteins. The amino-terminal 1-45 segment containing the central polycationic region binds to G alpha t's alpha 3 helix and alpha 3/beta 5 loop, while the COOH-terminal region of P gamma, 63-87, binds in concert to the conserved switch regions. The first interaction provides specific interaction with both the GDP- and GTP-liganded G alpha t, while the second one is conserved between G alpha t and G alpha i1 and dependent on the activated conformation.

Regulatory GTPases

The past year has witnessed a tremendous increase in our understanding of the structures and interactions of the GTPases. The highlights include crystal structures of G alpha subunits, as well as the first complex between a GTPase (Rap1A) and an effector molecule (c-Raf1 Ras-binding domain). In the field of elongation factors (EFs), three very important structures have been determined: EF-G, the ternary complex of EF-Tu.GTP with aminoacyl-tRNA, and the EF-Tu.EF-Ts complex.

Functional expression of the taste specific G-protein, alpha-gustducin

The taste-specific G-protein alpha-subunit, alpha-gustducin, was expressed using a baculovirus based system. alpha-Gustducin was demonstrated to be myristoylated and was also palmitoylated in insect larval cells. Recombinant alpha-gustducin was purified to homogeneity. Neither receptors nor effectors that interact with gustducin in taste are known. However, alpha-gustducin has a close structural similarity to the visual G-protein, alpha-transducin. Therefore alpha-gustducin was reconstituted with components of the visual system to determine the degree of its functional similarity with alpha-transducin. Despite the fact that the sequences of alpha-gustducin and alpha-transducin share only 80% identity with each other, the interactions and functions of these two proteins were quantitatively identical. These included the interaction with receptor, bovine rhodopsin, with effector, bovine retinal cyclic GMP-phosphodiesterase, and with bovine brain and retinal G-protein beta gamma-heterodimers; receptor-catalysed GDP-GTP exchange and the intrinsic GTPase activity of alpha-gustducin and alpha-transducin were also identical. Gi alpha which is 70% identical with alpha-transducin interacts with different receptor and effector proteins and has very different guanine-nucleotide binding properties. Therefore, the functional equivalence of alpha-gustducin and alpha-transducin suggest that taste buds are likely to contain receptor and effector proteins that share many properties with their retinal equivalents.

A cyclic-nucleotide-suppressible conductance activated by transducin in taste cells

Taste can be divided into four primary sensations: salty, sour, sweet and bitter. Salty and sour are directly transduced by apical channels, whereas sweet and bitter utilize cyclic nucleotide second messengers. We have shown that rod transducin is present in mammalian taste receptor cells, where it is activated by a bitter receptor and in turn activates a phosphodiesterase. Here we introduce into frog taste cells peptides derived from transducin's phosphodiesterase-interaction region, which cause an inward whole-cell current in a subset of cells. We find that the peptides' effects are reversibly suppressed by IBMX and forskolin, indicative of a transducin-activated phosphodiesterase. Cyclic nucleotides suppress the whole-cell current, indicating that cyclic nucleotides may regulate taste-cell conductance. IBMX modifies taste-cell responses to two taste stimuli, implicating phosphodiesterase in taste transduction. Submicromolar cyclic nucleotides directly suppress the conductance of inside-out patches derived from the taste-cell plasma membrane, independently of protein phosphorylation. The channels are unusual in that they are suppressed, rather than activated by cyclic nucleotides. We propose that transducin, via phosphodiesterase, decreases cyclic nucleotide levels to activate the cyclic-nucleotide-suppressible conductance, leading to Ca2+ influx and taste-cell depolarization.

Coupling of bitter receptor to phosphodiesterase through transducin in taste receptor cells

The rod and cone transducins are specific G proteins originally thought to be present only in photoreceptor cells of the vertebrate retina. Transducins convert light stimulation of photoreceptor opsins into activation of cyclic GMP phosphodiesterase (reviewed in refs. 5-7). A transducin-like G protein, gustducin, has been identified and cloned from rat taste cells. We report here that rod transducin is also present in vertebrate taste cells, where it specifically activates a phosphodiesterase isolated from taste tissue. Furthermore, the bitter compound denatonium in the presence of taste-cell membranes activates transducin but not Gi. A peptide that competitively inhibits rhodopsin activation of transducin also blocks taste-cell membrane activation of transducin, arguing for the involvement of a seven-transmembrane-helix G-protein-coupled receptor. These results suggest that rod transducin transduces bitter taste by coupling taste receptor(s) to taste-cell phosphodiesterase. Phosphodieterase-mediated degradation of cyclic nucleotides may lead to taste-cell depolarization through the recently identified cyclic-nucleotide-suppressible conductance.