Interaction sites of the COOH-terminal region of the gamma subunit of cGMP phosphodiesterase with the GTP-bound alpha subunit of transducin

The molecular architecture of photoreceptor phosphodiesterase 6 (PDE6) with activated G protein elucidates the mechanism of visual excitation

Photoreceptor phosphodiesterase 6 (PDE6) is the central effector of the visual excitation pathway in both rod and cone photoreceptors, and PDE6 mutations that alter PDE6 structure or regulation can result in several human retinal diseases. The rod PDE6 holoenzyme consists of two catalytic subunits (Pαβ) whose activity is suppressed in the dark by binding of two inhibitory γ-subunits (Pγ). Upon photoactivation of rhodopsin, the heterotrimeric G protein (transducin) is activated, resulting in binding of the activated transducin α-subunit (Gt_α) to PDE6, displacement of Pγ from the PDE6 active site, and enzyme activation. Although the biochemistry of this pathway is understood, a lack of detailed structural information about the PDE6 activation mechanism hampers efforts to develop therapeutic interventions for managing PDE6-associated retinal diseases. To address this gap, here we used a cross-linking MS-based approach to create a model of the entire interaction surface of Pγ with the regulatory and catalytic domains of Pαβ in its nonactivated state. Following reconstitution of PDE6 and activated Gt_α with liposomes and identification of cross-links between Gt_α and PDE6 subunits, we determined that the PDE6-Gt_α protein complex consists of two Gt_α-binding sites per holoenzyme. Each Gt_α interacts with the catalytic domains of both catalytic subunits and induces major changes in the interaction sites of the Pγ subunit with the catalytic subunits. These results provide the first structural model for the activated state of the transducin-PDE6 complex during visual excitation, enhancing our understanding of the molecular etiology of inherited retinal diseases.

The N termini of the inhibitory γ-subunits of phosphodiesterase-6 (PDE6) from rod and cone photoreceptors differentially regulate transducin-mediated PDE6 activation

Phosphodiesterase-6 (PDE6) plays a central role in both rod and cone phototransduction pathways. In the dark, PDE6 activity is suppressed by its inhibitory γ-subunit (Pγ). Rhodopsin-catalyzed activation of the G protein transducin relieves this inhibition and enhances PDE6 catalysis. We hypothesized that amino acid sequence differences between rod- and cone-specific Pγs underlie transducin's ability to more effectively activate cone-specific PDE6 than rod PDE6. To test this, we analyzed rod and cone Pγ sequences from all major vertebrate and cyclostome lineages and found that rod Pγ loci are far more conserved than cone Pγ sequences and that most of the sequence differences are located in the N-terminal region. Next we reconstituted rod PDE6 catalytic dimer (Pαβ) with various rod or cone Pγ variants and analyzed PDE6 activation upon addition of the activated transducin α-subunit (Gt_α*-GTPγS). This analysis revealed a rod-specific Pγ motif (amino acids 9-18) that reduces the ability of Gt_α*-GTPγS to activate the reconstituted PDE6. In cone Pγ, Asn-13 and Gln-14 significantly enhanced Gt_α*-GTPγS activation of cone Pγ truncation variants. Moreover, we observed that the first four amino acids of either rod or cone Pγ contribute to Gt_α*-GTPγS-mediated activation of PDE6. We conclude that physiological differences between rod and cone photoreceptor light responsiveness can be partially ascribed to ancient, highly conserved amino acid differences in the N-terminal regions of Pγ isoforms, demonstrating for the first time a functional role for this region of Pγ in the differential activation of rod and cone PDE6 by transducin.

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.

Vigabatrin-induced retinal toxicity is partially mediated by signaling in rod and cone photoreceptors

Vigabatrin (VGB) is a commonly prescribed antiepileptic drug designed to inhibit GABA-transaminase, effectively halting seizures. Unfortunately, VGB treatment is also associated with the highest frequencies of peripheral visual field constriction of any of the antiepileptic drugs and the mechanisms that lead to these visual field defects are uncertain. Recent studies have demonstrated light exposure exacerbates vigabatrin-induced retinal toxicity. We further assessed this relationship by examining the effects of vigabatrin treatment on the retinal structures of mice with genetically altered photoreception. In keeping with previous studies, we detected increased toxicity in mice exposed to continuous light. To study whether cone or rod photoreceptor function was involved in the pathway to toxicity, we tested mice with mutations in the cone-specific Gnat2 or rod-specific Pde6g genes, and found the mutations significantly reduced VGB toxicity. Our results confirm light is a significant enhancer of vigabatrin toxicity and that a portion of this is mediated, directly or indirectly, by phototransduction signaling in rod and cone photoreceptors.

Functional mapping of interacting regions of the photoreceptor phosphodiesterase (PDE6) γ-subunit with PDE6 catalytic dimer, transducin, and regulator of G-protein signaling9-1 (RGS9-1)

The cGMP phosphodiesterase (PDE6) involved in visual transduction in photoreceptor cells contains two inhibitory γ-subunits (Pγ) which bind to the catalytic core (Pαβ) to inhibit catalysis and stimulate cGMP binding to the GAF domains of Pαβ. During visual excitation, interaction of activated transducin with Pγ relieves inhibition. Pγ also participates in a complex with RGS9-1 and other proteins to accelerate the GTPase activity of activated transducin. We studied the structural determinants for these important functions of Pγ. First, we identified two important sites in the middle region of Pγ (amino acids 27-38 and 52-54) that significantly stabilize the overall binding affinity of Pγ with Pαβ. The ability of Pγ to stimulate noncatalytic cGMP binding to the GAF domains of PDE6 has been localized to amino acids 27-30 of Pγ. Transducin activation of PDE6 catalysis critically depends on the presence of Ile54 in the glycine-rich region of Pγ in order to relieve inhibition of catalysis. The central glycine-rich region of Pγ is also required for transducin to increase cGMP exchange at the GAF domains. Finally, Thr-65 and/or Val-66 of Pγ are critical residues for Pγ to stimulate GTPase activity of transducin in a complex with RGS9-1. We propose that the glycine-rich region of Pγ is a primary docking site for PDE6-interacting proteins involved in the activation/inactivation pathways of visual transduction. This functional mapping of Pγ with its binding partners demonstrates the remarkable versatility of this multifunctional protein and its central role in regulating the activation and lifetime of visual transduction.

Complementary interactions of the rod PDE6 inhibitory subunit with the catalytic subunits and transducin

Activation of the cyclic GMP phosphodiesterase (PDE6) by transducin is the central event of visual signal transduction. How the PDE6 inhibitory gamma-subunit (Pgamma) interacts with the catalytic subunits (Palphabeta) and the transducin alpha-subunit (alpha(t)) in this process is not entirely clear. Here we have investigated this issue, taking advantage of site-specific label transfer from throughout the full-length Pgamma molecule to both alpha(t) and Palphabeta. The interaction profiling and pull-down experiments revealed that the Pgamma C- terminal domain accounted for the major interaction with alpha(t) bound with guanosine 5'-3-O-(thio)triphosphate (alpha(t)GTPgammaS) in comparison with the central region, whereas an opposite pattern was observed for the Pgamma-Palphabeta interaction. This complementary feature was further exhibited when both alpha(t)GTPgammaS and Palphabeta were present and competing for Pgamma interaction, with the Pgamma C-terminal domain favoring alpha(t), whereas the central region demonstrated a preference for Palphabeta. Furthermore, alpha(t)GTPgammaS co-immunoprecipitated with PDE6 and vice versa in a Pgamma-dependent manner. Either Palphabeta or alpha(t)GTPgammaS could be pulled down by the Btn-Pgamma molecules on streptavidin beads that were saturated by the other partner, indicating simultaneous binding of these two partners to Pgamma. These data together indicate that complementary Pgamma interactions with its two targets facilitate the alpha(t).PDE6 "transducisome" formation. Thus, our study provides new insights into the molecular mechanisms of PDE6 activation.

The retinal cGMP phosphodiesterase gamma-subunit - a chameleon

Intrinsically disordered proteins (IDPs) represent an emerging class of proteins (or domains) that are characterized by a lack of ordered secondary and tertiary structure. This group of proteins has recently attracted tremendous interest primarily because of a unique feature: they can bind to different targets due to their structural plasticity, and thus fulfill diverse functions. The inhibitory gamma-subunit (PDEgamma) of retinal PDE6 is an intriguing IDP, of which unique protein properties are being uncovered. PDEgamma critically regulates the turn on as well as the turn off of visual signaling through alternate interactions with the PDE6 catalytic core, transducin, and the regulator of G protein signaling RGS9-1. The intrinsic disorder of PDEgamma does not compromise, but rather, optimizes its functionality. PDEgamma "curls up" when free in solution but "stretches out" when binding with the PDE6 catalytic core. Conformational changes of PDEgamma also likely occur in its C-terminal PDE6-binding region upon interacting with transducin during PDE6 activation. Growing evidence shows that PDEgamma is also a player in non-phototransduction pathways, suggesting additional protein targets. Thus, PDEgamma is highly likely to be adaptive in its structure and function, hence a "chameleon".

Juxtaposition of the steroid binding domain-like I and II regions constitutes a ligand binding site in the sigma-1 receptor

sigma-1 receptors represent unique binding sites that are capable of interacting with a wide range of compounds to mediate different cellular events. The composition of the ligand binding site of this receptor is unclear, since no NMR or crystal structures are available. Recent studies in our laboratory using radiolabeled photoreactive ligands suggested that the steroid binding domain-like I (SBDLI) (amino acids 91-109) and the steroid binding domain-like II (SBDLII) (amino acids 176-194) regions are involved in forming the ligand binding site(s) ( Chen, Y., Hajipour, A. R., Sievert, M. K., Arbabian, M., and Ruoho, A. E. (2007) Biochemistry 46, 3532-3542 ; Pal, A., Hajipour, A. R., Fontanilla, D., Ramachandran, S., Chu, U. B., Mavlyutov, T., and Ruoho, A. E. (2007) Mol. Pharmacol. 72, 921-933 ). In this report, we have further addressed this issue by utilizing our previously developed sulfhydryl-reactive, cleavable, radioiodinated photocross-linking reagent: methanesulfonothioic acid, S-((4-(4-amino-3-[125I]iodobenzoyl) phenyl)methyl) ester (Guo, L. W., Hajipour, A. R., Gavala, M. L., Arbabian, M., Martemyanov, K. A., Arshavsky, V. Y., and Ruoho, A. E. (2005) Bioconjugate Chem. 16, 685-693). This photoprobe was shown to derivatize the single cysteine residues as mixed disulfides at position 94 in the SBDLI region of the wild type guinea pig sigma-1 receptor (Cys94) and at position 190 in the SBDLII region of a mutant guinea pig sigma-1 receptor (C94A,V190C), both in a sigma-ligand (haloperidol or (+)-pentazocine)-sensitive manner. Significantly, photocross-linking followed by Endo Lys-C cleavage under reducing conditions and intramolecular radiolabel transfer from the SBDLI to the SBDLII region in the wild type receptor and, conversely, from the SBDLII to the SBDLI region in the mutant receptor were observed. These data support a model in which the SBDLI and SBDLII regions are juxtaposed to form, at least in part, a ligand binding site of the sigma-1 receptor.

The N terminus of GTP gamma S-activated transducin alpha-subunit interacts with the C terminus of the cGMP phosphodiesterase gamma-subunit

Dynamic regulation of G-protein signaling in the phototransduction cascade ensures the high temporal resolution of vision. In a key step, the activated alpha-subunit of transducin (Galphat-GTP) activates the cGMP phosphodiesterase (PDE) by binding the inhibitory gamma-subunit (PDEgamma). Significant progress in understanding the interaction between Galphat and PDEgamma was achieved by solving the crystal structure of the PDEgamma C-terminal peptide bound to Galphat in the transition state for GTP hydrolysis (Slep, K. C., Kercher, M. A., He, W., Cowan, C. W., Wensel, T. G., and Sigler, P. B. (2001) Nature 409, 1071-1077). However, some of the structural elements of each molecule were absent in the crystal structure. We have probed the binding surface between the PDEgamma C terminus and activated Galphat bound to guanosine 5'-O-(3-thio)-triphosphate (GTPgammaS) using a series of full-length PDEgamma photoprobes generated by intein-mediated expressed protein ligation. For each of seven PDEgamma photoprobe species, expressed protein ligation allowed one benzoyl-L-phenylalaine substitution at selected hydrophobic C-terminal positions, and the addition of a biotin affinity tag at the extreme C terminus. We have detected photocross-linking from several PDEgamma C-terminal positions to the Galphat-GTPgammaS N terminus, particularly from PDEgamma residue 73. The overall percentage of cross-linking to the Galphat-GTPgammaSN terminus was analyzed using a far Western method for examining Galphat-GTPgammaS proteolytic digestion patterns. Furthermore, mass spectrometric analysis of cross-links to Galphat from a benzoyl-phenylalanine replacement at PDEgamma position 86 localized the region of photoinsertion to Galphat N-terminal residues Galphat-(22-26). This novel Galphat/PDEgamma interaction suggests that the transducin N terminus plays an active role in signal transduction.

Asymmetric interaction between rod cyclic GMP phosphodiesterase gamma subunits and alphabeta subunits

Rod phosphodiesterase (PDE6) is the central effector enzyme in vertebrate visual transduction. Holo-PDE6 consists of two similar catalytic subunits (Palphabeta) and two identical inhibitory subunits (Pgamma). Palphabeta is the only heterodimer in the PDE superfamily, yet its significance for the function of PDE6 is poorly understood. An unequal interaction of Pgamma with Pbeta as compared with Palpha in the PDE6 complex has not been reported. We investigated the interaction interface between full-length Pgamma and Palphabeta, by differentiating Pgamma interaction with each individual Palphabeta subunit through radiolabel transfer from various positions throughout the entire Pgamma molecule. The efficiency of radiolabel transfer indicates that the close vicinity of serine 40 on Pgamma makes a major contribution to the interaction with Palphabeta. In addition, a striking asymmetry of interaction between the Pgamma polycationic region and the Palphabeta subunits was observed when the stoichiometry of Pgamma versus the Palphabeta dimer was below 2. Preferential photolabeling on Pbeta from Pgamma position 40 and on Palpha from position 30 increased while lowering the Pgamma/Palphabeta ratio, but diminished when the Pgamma/Palphabeta ratio was over 2. Our finding leads to the conclusion that two classes of Pgamma binding sites exist on Palphabeta, each composed of GAF domains in both Palpha and Pbeta, differing from the conventional models suggesting that each Pgamma binds only one of the Palphabeta catalytic subunits. This new model leads to insight into how the unique Palphabeta heterodimer contributes to the sophisticated regulation in visual transduction through interaction with Pgamma.

Cone dystrophy phenotype associated with a frameshift mutation (M280fsX291) in the alpha-subunit of cone specific transducin (GNAT2)

AIM

To describe the phenotype of a three generation consanguineous Pakistani family containing six individuals with autosomal recessive cone dystrophy caused by mutation in GNAT2.

METHODS

Five of the six affected individuals underwent an ophthalmological examination, electrodiagnostic testing, fundus photography, autofluorescence imaging, and detailed psychophysical testing.

RESULTS

All five examined patients had a history of nystagmus from infancy, photophobia, defective colour vision, and poor visual acuity. The nystagmus in three of the individuals had lessened with time. Fundus examination revealed an abnormal foveal appearance, without frank atrophy or pigmentation. Electroretinography (ERG) revealed absent ISCEV cone flicker ERGs with some preservation of responses to short wavelength stimulation. Rod ERGs showed no definite abnormality, but maximal (mixed rod-cone) response a-wave amplitudes were mildly subnormal. Rudimentary residual colour vision was detected in three individuals. There is clinical evidence of progressive visual acuity reduction in two older individuals.

CONCLUSION

Mutation in the alpha-subunit of cone specific transducin (GNAT2) is characterised by an infantile onset cone dystrophy. Some affected individuals may show deterioration of visual acuity with time.

Chemical cleavage of proteins on membranes

Described in this unit are five basic protocols that are widely used for specific and efficient chemical cleavage of proteins bound to membranes. Cyanogen bromide (CNBr) cleaves at methionine (Met) residues; BNPS-skatole cleaves at tryptophan (Trp) residues; formic acid cleaves at aspartic acid-proline (Asp-Pro) peptide bonds; hydroxylamine cleaves at asparagine-glycine (Asn-Gly) peptide bonds, and 2-nitro-5-thiocyanobenzoic acid (NTCB) cleaves at cysteine (Cys) residues. Because the above loci are at relatively low abundance in most proteins, digestion with these agents will yield relatively long peptides. In addition, Alternate Protocol an describes CNBr cleavage of PVDF-bound protein previously analyzed by Edman degradation. Finally, a Support Protocol discusses preferred methods of separating and analyzing peptide fragments generated by the chemical cleavage reactions described in the basic protocols.

Structural determinants for regulation of phosphodiesterase by a G protein at 2.0 A

A multitude of heptahelical receptors use heterotrimeric G proteins to transduce signals to specific effector target molecules. The G protein transducin, Gt, couples photon-activated rhodopsin with the effector cyclic GMP phosophodiesterase (PDE) in the vertebrate phototransduction cascade. The interactions of the Gt alpha-subunit (alpha(t)) with the inhibitory PDE gamma-subunit (PDEgamma) are central to effector activation, and also enhance visual recovery in cooperation with the GTPase-activating protein regulator of G-protein signalling (RGS)-9 (refs 1-3). Here we describe the crystal structure at 2.0 A of rod transducin alpha x GDP x AlF4- in complex with the effector molecule PDEgamma and the GTPase-activating protein RGS9. In addition, we present the independently solved crystal structures of the RGS9 RGS domain both alone and in complex with alpha(t/i1) x GDP x AlF4-. These structures reveal insights into effector activation, synergistic GTPase acceleration, RGS9 specificity and RGS activity. Effector binding to a nucleotide-dependent site on alpha(t) sequesters PDEgamma residues implicated in PDE inhibition, and potentiates recruitment of RGS9 for hydrolytic transition state stabilization and concomitant signal termination.

A regulator of G protein signaling interaction surface linked to effector specificity

Proteins of the regulator of G protein signaling (RGS) family accelerate GTP hydrolysis by the alpha subunits (G(alpha)) of G proteins, leading to rapid recovery of signaling cascades. Many different RGS proteins can accelerate GTP hydrolysis by an individual G(alpha), and GTP hydrolysis rates of different G(alpha)s can be enhanced by the same RGS protein. Consequently, the mechanisms for specificity in RGS regulation and the residues involved remain unclear. Using the evolutionary trace (ET) method, we have identified a cluster of residues in the RGS domain that includes the RGS-G(alpha) binding interface and extends to include additional functionally important residues on the surface. One of these is within helix alpha3, two are in alpha5, and three are in the loop connecting alpha5 and alpha6. A cluster of surface residues on G(alpha) previously identified by ET, and composed predominantly of residues from the switch III region and helix alpha3, is spatially contiguous with the ET-identified residues in the RGS domain. This cluster includes residues proposed to interact with the gamma subunit of G(talpha)'s effector, cGMP phosphodiesterase (PDEgamma). The proximity of these clusters suggests that they form part of an interface between the effector and the RGS-G(alpha) complex. Sequence variations in these residues correlate with PDEgamma effects on GTPase acceleration. Because ET identifies residues important for all members of a protein family, these residues likely form a general site for regulation of G protein-coupled signaling cascades, possibly by means of effector interactions.

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.

The helical domain of a G protein alpha subunit is a regulator of its effector

The alpha subunit (Galpha) of heterotrimeric G proteins is a major determinant of signaling selectivity. The Galpha structure essentially comprises a GTPase "Ras-like" domain (RasD) and a unique alpha-helical domain (HD). We used the vertebrate phototransduction model to test for potential functions of HD and found that the HD of the retinal transducin Galpha (Galphat) and the closely related gustducin (Galphag), but not Galphai1, Galphas, or Galphaq synergistically enhance guanosine 5'-gamma[-thio]triphosphate bound Galphat (GalphatGTPgammaS) activation of bovine rod cGMP phosphodiesterase (PDE). In addition, both HDt and HDg, but not HDi1, HDs, or HDq attenuate the trypsin-activated PDE. GalphatGDP and HDt attenuation of trypsin-activated PDE saturate with similar affinities and to an identical 38% of initial activity. These data suggest that interaction of intact Galphat with the PDE catalytic core may be caused by the HD moiety, and they indicate an independent site(s) for the HD moiety of Galphat within the PDE catalytic core in addition to the sites for the inhibitory Pgamma subunits. The HD moiety of GalphatGDP is an attenuator of the activated catalytic core, whereas in the presence of activated GalphatGTPgammaS the independently expressed HDt is a potent synergist. Rhodopsin catalysis of Galphat activation enhances the PDE activation produced by subsaturating levels of Galphat, suggesting a HD-moiety synergism from a transient conformation of Galphat. These results establish HD-selective regulations of vertebrate retinal PDE, and they provide evidence demonstrating that the HD is a modulatory domain. We suggest that the HD works in concert with the RasD, enhancing the efficiency 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.

Molecular determinants of selectivity in 5-hydroxytryptamine1B receptor-G protein interactions

The recognition between G protein and cognate receptor plays a key role in specific cellular responses to environmental stimuli. Here we explore specificity in receptor-G protein coupling by taking advantage of the ability of the 5-hydroxytryptamine1B (5-HT1B) receptor to discriminate between G protein heterotrimers containing Galphai1 or Galphat. Gi1 can interact with the 5-HT1B receptor and stabilize a high affinity agonist binding state of this receptor, but Gt cannot. A series of Galphat/Galphai1 chimeric proteins have been generated in Escherichia coli, and their functional integrity has been reported previously (Skiba, N. P., Bae, H., and Hamm, H. E. (1996) J. Biol. Chem. 271, 413-424). We have tested the functional coupling abilities of the Galphat/Galphai1 chimeras to 5-HT1B receptors using high affinity agonist binding and receptor-stimulated guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) binding. In the presence of betagamma subunits, amino acid residues 299-318 of Galphai1 increase agonist binding to the 5-HT1B receptor and receptor stimulation of GTPgammaS binding. Moreover, Galphai1 containing only Galphat amino acid sequences from this region does not show any coupling ability to 5-HT1B receptors. Our studies suggest that the alpha4 helix and alpha4-beta6 loop region of Galphas are an important region for specific recognition between receptors and Gi family members.

Peptide mapping of the [125I]Iodoazidoketanserin and [125I]2-N-[(3'-iodo-4'-azidophenyl)propionyl]tetrabenazine binding sites for the synaptic vesicle monoamine transporter

The full-length cDNA for the rat recombinant synaptic vesicle monoamine transporter (rVMAT2) containing a COOH-terminal polyhistidine epitope was engineered into baculovirus DNA for expression in Spodoptera frugiperda (Sf9) cells. Using this recombinant baculovirus and cultured Sf9 cells, rVMAT2 has been expressed to high levels and purified to >95% homogeneity using immobilized Ni2+-affinity chromatography followed by lectin (concanavalin A) chromatography. Purified transporter was photolabeled using [125I]-7-azido-8-iodoketanserin ([125I]AZIK) and [125I]2-N-[(3'-iodo-4'-azidophenyl)propionyl]tetrabenazine ([125I]TBZ-AIPP). Both [125I]AZIK and [125I]TBZ-AIPP photoaffinity labeling of purified rVMAT2 were protectable by 10 microM tetrabenazine (TBZ), 10 microM 7-aminoketanserin, and 1 mM concentrations of the transporter substrates dopamine, norepinephrine, and serotonin. Radiolabeled peptides were generated using enzymatic and chemical methods, purified using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and NH2-terminal microsequenced. Radiosequencing of [125I]AZIK-labeled rVMAT2 indicated derivatization of Lys-20 in the NH2 terminus, just prior to putative transmembrane domain 1 (TMD1). [125I]TBZ-AIPP derivatized a segment of rVMAT2 between Gly-408 and Cys-431 in TMD10 and 11. These data implicate juxtaposition of TMD1 and 10/11.

Regulation of transducin GTPase activity by human retinal RGS

The intrinsic GTPase activity of transducin controls inactivation of the effector enzyme, cGMP phosphodiesterase (PDE), during turnoff of the visual signal. The inhibitory gamma-subunit of PDE (Pgamma), an unidentified membrane factor and a retinal specific member of the RGS family of proteins have been shown to accelerate GTP hydrolysis by transducin. We have expressed a human homologue of murine retinal specific RGS (hRGSr) in Escherichia coli and investigated its role in the regulation of transducin GTPase activity. As other RGS proteins, hRGSr interacted preferentially with a transitional conformation of the transducin alpha-subunit, GtalphaGDPAlF4-, while its binding to GtalphaGTPgammaS or GtalphaGDP was weak. hRGSr and Pgamma did not compete for the interaction with GtalphaGDPAlF4-. Affinity of the Pgamma-GtalphaGDPAlF4- interaction was modestly enhanced by addition of hRGSr, as measured by a fluorescence assay of GtalphaGDPAlF4- binding to Pgamma labeled with 3-(bromoacetyl)-7-diethylaminocoumarin (PgammaBC). Binding of hRGSr to GtalphaGDPAlF4- complexed with PgammaBC resulted in a maximal approximately 40% reduction of BC fluorescence allowing estimation of the hRGSr affinity for GtalphaGDPAlF4- (Kd 35 nM). In a single turnover assay, hRGSr accelerated GTPase activity of transducin reconstituted with the urea-stripped rod outer segment (ROS) membranes by more than 10-fold to a rate of 0.23 s-1. Addition of Pgamma to the reconstituted system reduced the GTPase level accelerated by hRGSr (kcat 0.085 s-1). The GTPase activity of transducin and the PDE inactivation rates in native ROS membranes in the presence of hRGSr were elevated 3-fold or more regardless of the membrane concentrations. In ROS suspensions containing 30 microM rhodopsin these rates exceeded 0.7 s-1. Our data suggest that effects of hRGSr on transducin's GTPase activity are attenuated by Pgamma but independent of a putative membrane GTPase activating protein factor. The rate of transducin GTPase activity in the presence of hRGSr is sufficient to correlate it with in vivo turnoff kinetics of the visual cascade.

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.

The retinal specific protein RGS-r competes with the gamma subunit of cGMP phosphodiesterase for the alpha subunit of transducin and facilitates signal termination

In vertebrate photoreceptor cells, transducin mediates signaling between rhodopsin and cGMP phosphodiesterase by transiently binding its gamma subunit (PDEgamma). For the termination of signaling GTP hydrolysis by the transducin alpha subunit (TDalpha) GTPase is required. This reaction can be accelerated by GTPase-activating proteins (GAPs), e.g. PDEgamma. Recently we identified a second retinal GAP that interacts with TDalpha, RGS-r. Here we compare the GAP function of RGS-r and PDEgamma. Both proteins stimulated single turnover GTPase of TDalpha; however, RGS-r was more effective than PDEgamma. When added together, PDEgamma competitively inhibited the RGS-r-stimulated GTPase. In addition, the interaction of TDalpha in its GTP-bound form (TDalphaGTPgammaS), the transition state (TDalphaGDP*AMF) and the GDP-bound form (TDalphaGDP) with RGS-r and PDE, respectively, was measured by surface plasmon resonance. PDEgamma displayed highest affinity for TDalphaGTPgammaS, weaker affinity for TDalphaGDP*AMF, and weakest affinity for TDalphaGDP. RGS-r exhibited only a comparable high affinity for TDalphaGDP*AMF. We conclude that the observed competition between RGS-r and PDEgamma for TDalpha occurs when the hydrolysis of GTP is initiated. By competing with PDEgamma and removing it from TDalpha as well as increasing Pi release, RGS-r apparently facilitates signal termination and TDalpha recycling.