Expression and purification of the Saccharomyces cerevisiae alpha-factor receptor (Ste2p), a 7-transmembrane-segment G protein-coupled receptor

Identification of novel ErbB3-interacting factors using the split-ubiquitin membrane yeast two-hybrid system

Analysis of membrane protein interactions is difficult because of the hydrophobic nature of these proteins, which often renders conventional biochemical and genetic assays fruitless. This is a substantial problem because proteins that are integral or associated with membranes represent approximately one-third of all proteins in a typical eukaryotic cell. We have shown previously that the modified split-ubiquitin system can be used as a genetic assay for the in vivo detection of interactions between the two characterized yeast transmembrane proteins, Ost1p and Wbp1p. This so-called split-ubiquitin membrane yeast two-hybrid (YTH) system uses the split-ubiquitin approach in which reconstitution of two ubiquitin halves is mediated by a protein-protein interaction. Here we converted the split-ubiquitin membrane YTH system into a generally applicable in vivo screening approach to identify interacting partners of a particular mammalian transmembrane protein. We have demonstrated the effectiveness of this approach by using the mammalian ErbB3 receptor as bait and have identified three previously unknown ErbB3-interacting proteins. In addition, we have confirmed one of the newly found interactions between ErbB3 and the membrane-associated RGS4 protein by coimmunoprecipitating the two proteins from human cells. We expect the split-ubiquitin membrane YTH technology to be valuable for the identification of potential interacting partners of integral membrane proteins from many model organisms.

Tyr266 in the sixth transmembrane domain of the yeast alpha-factor receptor plays key roles in receptor activation and ligand specificity

To identify interactions between Ste2p, a G protein-coupled receptor of the yeast Saccharomyces cerevisiae, and its tridecapeptide ligand, alpha-factor (WHWLQLKPGQPMY), a variety of alpha-factor analogues were used in conjunction with site-directed mutagenesis of a targeted portion of Ste2p transmembrane domain six. Alanine substitution of residues in the 262-270 region of Ste2p did not affect pheromone binding or signal transduction, except for the Y266A mutant, which did not transduce signal yet exhibited only a small decrease in alpha-factor binding affinity. Substitutions with Ser, Leu, or Lys at Y266 also generated signaling-defective receptors. In contrast, Phe or Trp substitution at Y266 retained receptor function, suggesting that aromaticity at this position was critical. When coexpressed with WT receptor, the Y266A receptor exhibited a strong dominant-negative phenotype, indicating that this mutant bound G protein. A partial tryptic digest revealed that, in the presence of agonist, a different digestion profile for Y266A receptor was generated in comparison to that for WT receptor. The difference in trypsin-sensitive sites and their negative dominance indicated that the Y266A receptor was not able to switch into an "activated" conformation upon ligand binding. In comparison to WT Ste2p, the mutantY266A receptor showed increased binding affinity for N-terminal, alanine-substituted alpha-factor analogues (residues 1-4) and the antagonist [desW(1),desH(2)]alpha-factor. A substantial decrease in affinity was observed for alpha-factor analogues with Ala substitutions from residues 5-13. The results suggest that Y266 is part of the binding pocket that recognizes the N-terminal portion of alpha-factor and is involved in the transformation of Ste2p into an activated state upon agonist binding.

Residues in the first extracellular loop of a G protein-coupled receptor play a role in signal transduction

The Saccharomyces cerevisiae pheromone, alpha-factor (WHWLQLKPGQPMY), and Ste2p, its G protein-coupled receptor, were used as a model system to study ligand-receptor interaction. Cys-scanning mutagenesis on each residue of EL1, the first extracellular loop of Ste2p, was used to generate a library of 36 mutants with a single Cys residue substitution. Mutation of most residues of EL1 had only negligible effects on ligand affinity and biological activity of the mutant receptors. However, five mutants were identified that were either partially (L102C and T114C) or severely (N105C, S108C, and Y111C) compromised in signaling but retained binding affinities similar to those of wild-type receptor. Three-dimensional modeling, secondary structure predictions, and subsequent circular dichroism studies on a synthetic peptide with amino acid sequence corresponding to EL1 suggested the presence of a helix corresponding to EL1 residues 106 to 114 followed by two short beta-strands (residues 126 to 135). The distinctive periodicity of the five residues with a signal-deficient phenotype combined with biophysical studies suggested a functional involvement in receptor activation of a face on a 3(10) helix in this region of EL1. These studies indicate that EL1 plays an important role in the conformational switch that activates the Ste2p receptor to initiate the mating pheromone signal transduction pathway.

Study of the binding environment of alpha-factor in its G protein-coupled receptor using fluorescence spectroscopy

Mating in Saccharomyces cerevisiae is induced by the interaction of alpha-factor (W1H2W3L4Q5L6K7P8G9Q10P11M12Y13) with its cognate G protein-coupled receptor (Ste2p). Fifteen fluorescently labeled analogs of alpha-factor in which the 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) group was placed at the alphaN-terminus and in side-chains at positions 1, 3, 4, 6, 7, 12 and 13 were synthesized and assayed for biological activity and receptor affinity. Eleven of the analogs retained 6-60% of the biological activity of the alpha-factor, as judged using a growth arrest assay. The binding affinities depended on the position of NBD attachment in the peptide and the distance of the tag from the backbone. Derivatization of the positions 3 and 7 side-chains with the NBD group resulted in analogs with affinities of 17-35% compared with that of alpha-factor. None of the other NBD-containing agonists had sufficient receptor affinity or strong enough emission for fluorescence analysis. The position 3 and 7 analogs were investigated using fluorescence spectroscopy and collisional quenching by KI in the presence of Ste2p in yeast membranes. The results showed that the lambda max of NBD in the position 7 side-chain shifted markedly to the blue (510 nm) when separated by 4 or 6 bonds from the peptide backbone and that this probe was shielded from quenching by KI. In contrast, separation by 3, 5, 10 or more bonds resulted in lambda max ( approximately 540 nm) and collisional quenching constants consistent with increasing degrees of exposure. The NBD group in the position 3 side-chain was also found to be blue shifted (lambda max=520 nm) and shielded from solvent. These results indicate that the position 7 side-chain is likely interacting with a pocket formed by extracellular domains of Ste2p, whereas the side-chain of Trp3 is in a hydrophobic pocket possibly within the transmembrane region of the receptor.

Identification of a contact region between the tridecapeptide alpha-factor mating pheromone of Saccharomyces cerevisiae and its G protein-coupled receptor by photoaffinity labeling

Saccharomyces cerevisiae haploid cells communicate with their opposite mating type through peptide pheromones (alpha-factor and a-factor) that activate G protein-coupled receptors (GPCRs). S. cerevisiaewas used as a model system for the study of peptide-responsive GPCRs. Here, we detail the synthesis and characterization of a number of alpha-factor (Trp-His-Trp-Leu-Gln-Leu-Lys-Pro-Gly-Gln-Pro-Met-Tyr) pheromone analogues containing the photo-cross-linkable group 4-benzoyl-L-phenylalanine (Bpa). Following characterization, one analogue, [Bpa(1), Tyr(3), Arg(7), Phe(13)]alpha-factor, was radioiodinated and used as a probe for Ste2p, the GPCR for alpha-factor. Binding of the di-iodinated probe was saturable (K(d) = 200 nM) and competable by alpha-factor. Cross-linking into Ste2p was specific for this receptor and reversed by the wild-type pheromone. Chemical and enzymatic cleavage of the receptor/radioprobe complex indicated that cross-linking occurred on a portion of Ste2p spanning residues 251-294 which encompasses transmembrane domain 6, the extracellular loop between transmembrane domains 6 and 7, and transmembrane domain 7. This fragment was verified using T7-epitope-tagged Ste2p and a biotinylated, photoactivatable alpha-factor. After cross-linking with the biotinylated photoprobe and trypsin cleavage, the cross-linked receptor fragment was revealed by both an anti T7-epitope antibody and a biotin probe. This is the first determination of a specific contact region between a Class IV GPCR and its ligand. The results demonstrate that Bpa alpha-factor probes are useful in determining contacts between alpha-factor and Ste2p and initiate mapping of the ligand binding site of this GPCR.

Expression of GATA-2 in human pituitary adenomas

Transcription factor GATA-2 contains two copies of a highly conserved zinc finger domain and plays unique roles at an early stage of hematopoietic differentiation. In the mouse pituitary gland, Pit-1-GATA-2 protein-protein interaction has been shown to lead to gene-specific actions to obtain cell-specific roles. In this study, we investigated the expression of GATA-2 and Pit-1 in human pituitary adenomas using reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemical techniques. By immunohistochemical analysis, GATA-2 was detected in all of the gonadotropin-subunit (Gn-su)-positive adenomas (n = 8) and in four of five thyroid-stimulating hormone (TSH)-secreting adenomas, but its incidence was low in the other types of adenomas. Pit-1 protein was detected in 4 of 5 TSH-secreting adenomas and in 10 of 10 growth hormone (GH)-secreting adenomas. By RT-PCR analysis, GATA-2 was detected in all Gn-su-positive adenomas and TSH-secreting adenomas, and Pit-1 was detected in all TSH-secreting adenomas and GH-secreting adenomas. These results suggested that GATA-2 contributes to the functional expression and the differentiation of Gn-su-positive adenomas and the TSH-secreting adenomas and that the interaction between GATA-2 and Pit-1 can lead to gene-specific action and differentiation of TSH-secreting adenomas. It is further speculated that GATA-2 and transcriptional interaction with Pit-1 play roles in the functional differentiation of specific pituitary adenomas.

Critical residues for the specificity of cofactors and substrates in human estrogenic 17beta-hydroxysteroid dehydrogenase 1: variants designed from the three-dimensional structure of the enzyme

Human estrogenic 17beta-hydroxysteroid dehydrogenase is an NADP(H)-preferring enzyme. It possesses 11- and 4-fold higher specificity toward NADP(H) over NAD(H) for oxidation and reduction, respectively, as demonstrated by kinetic studies. To elucidate the roles of the amino acids involved in cofactor specificity, we generated variants by site-directed mutagenesis. The results showed that introducing a positively charged residue, lysine, at the Ser12 position increased the enzyme's preference for NADP(H) more than 20-fold. Substitution of the negatively charged residue, aspartic acid, into the Leu36 position switched the enzyme's cofactor preference from NADPH to NAD with a 220-fold change in the ratio of the specificity toward the two cofactors in the case of oxidation. This variant dramatically abolished the enzyme's reductase function and stimulated its dehydrogenase activity, as shown by enzyme activity in intact cells. The substrate-binding pocket was also studied with four variants: Ser142Gly, Ser142Cys, His221Ala, and Glu282Ala. The Ser142Gly variant abolished most of the enzyme's oxidation and reduction activities. The residual reductase activity in vitro is less than 2% that of the wild-type enzyme. However, the Ser142Cys variant was fully inactive, both as a partially purified protein and in intact cells. This suggests that the bulky sulfhydryl group of cysteine entirely disrupted the catalytic triad and that the Ser142 side chain is important for maintaining the integrity of this triad. His221 variation weakened the apparent affinity for estrone, as demonstrated by a 30-fold increase in Michaelis-Menten constant, supporting its important role in substrate binding. This residue may play an important role in substrate inhibition via the formation of a dead-end complex. The formerly suggested importance of Glu282 could not be confirmed.

One-step purification of recombinant yeast 6-phosphofructo-2-kinase after the identification of contaminants by MALDI-TOF MS

His-tagged yeast 6-phosphofructo-2-kinase was overexpressed in the yeast strain DFY658 under the control of the Gal1 promoter. Here we describe a simple and fast purification protocol for the recombinant enzyme under native conditions using a HiTrap affinity column loaded with CuSO(4). The use of MALDI-TOF MS after in-gel-digestion enabled us to identify a critical contamination of the end product as yeast alcohol dehydrogenase1 (Adh1p). After identification this contaminant could be efficiently removed by carrying out the washing steps at 25 degrees C instead of at 4 degrees C. To reduce the cellular proteolytic activities a low phosphate concentration in the growth medium was applied. This simple modification of the yeast cell growth conditions increased significantly the yield of the recombinant protein.

Probing the binding domain of the Saccharomyces cerevisiae alpha-mating factor receptor with rluorescent ligands

Three analogues of the alpha-mating factor pheromone of Saccharomyces cerevisiae containing the 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) group were synthesized that had high binding affinity to the receptor and retained biological activity. The fluorescence emission maximum of the NBD group in [K7(NBD),Nle(12)]-alpha-factor was blue shifted by 35 nm compared to buffer when the pheromone bound to its receptor. Fluorescence quenching experiments revealed that the NBD group in [K7(NBD),Nle(12)]-alpha-factor bound to the receptor was shielded from collision with iodide anion when in aqueous buffer. In contrast, the emission maximum of NBD in [K7(ahNBD),Nle(12)]-alpha-factor or [Orn7(NBD),Nle(12)]-alpha-factor was not significantly shifted and iodide anion efficiently quenched the fluorescence of these derivatives when they were bound to receptor. The fluorescence investigation suggests that when the alpha-factor is bound to its receptor, K7 resides in an environment that has both hydrophobic and hydrophilic groups within a few angstroms of each other.

Ultrasensitive magnetic biosensor for homogeneous immunoassay

A technique is described for specific, sensitive, quantitative, and rapid detection of biological targets by using superparamagnetic nanoparticles and a "microscope" based on a high-transition temperature dc superconducting quantum interference device (SQUID). In this technique, a mylar film to which the targets have been bound is placed on the microscope. The film, at room temperature and atmospheric pressure, is typically 40 micrometer from the SQUID, which is at 77 K in a vacuum. A suspension of magnetic nanoparticles carrying antibodies directed against the target is added to the mixture in the well, and 1-s pulses of magnetic field are applied parallel to the SQUID. In the presence of this aligning field the nanoparticles develop a net magnetization, which relaxes when the field is turned off. Unbound nanoparticles relax rapidly by Brownian rotation and contribute no measurable signal. Nanoparticles that are bound to the target on the film are immobilized and undergo Néel relaxation, producing a slowly decaying magnetic flux, which is detected by the SQUID. The ability to distinguish between bound and unbound labels allows one to run homogeneous assays, which do not require separation and removal of unbound magnetic particles. The technique has been demonstrated with a model system of liposomes carrying the FLAG epitope. The SQUID microscope requires no more than (5 +/- 2) x 10(4) magnetic nanoparticles to register a reproducible signal.

Homo-oligomeric complexes of the yeast alpha-factor pheromone receptor are functional units of endocytosis

alpha-Factor receptors from Saccharomyces cerevisiae are G-protein-coupled receptors containing seven transmembrane segments. Receptors solubilized with the detergent n-dodecyl beta-D-maltoside were found to sediment as a single 8S species in glycerol density gradients. When the membranes from cells coexpressing two differentially tagged receptors were solubilized with detergent and subjected to immunoprecipitation, we found that the antibodies specific for either epitope tag resulted in precipitation of both tagged species. Coprecipitation was not a consequence of incomplete detergent extraction because the abundant plasma membrane protein Pma1 did not coprecipitate with the receptors. Moreover, the receptor complexes were present prior to detergent extraction because coimmunoprecipitation was not observed when cells expressing the single tagged species were mixed prior to membrane preparation. Treatment of cultures with alpha-factor had little effect on the extent of oligomerization as judged by the sedimentation behavior of the receptor complexes and by the efficiency of coimmunoprecipitation. The ability of receptor complexes to undergo ligand-mediated endocytosis was evaluated by using membrane fractionation and fluorescence microscopy. Mutant receptors that fail to bind alpha-factor (Ste2-S184R) or lack the endocytosis signal (Ste2-T326) became competent for ligand-mediated endocytosis when they were expressed in cells containing wild-type receptors. Coimmunoprecipitation experiments indicated that the C-terminal cytoplasmic domain and intermolecular disulfide bonds were unnecessary for oligomer formation. We conclude that alpha-factor receptors form homo-oligomers and that these complexes are subject to ligand-mediated endocytosis. Furthermore, we show for the first time that unoccupied receptors participate in these endocytosis-competent complexes.

Position 13 analogs of the tridecapeptide mating pheromone from Saccharomyces cerevisiae: design of an iodinatable ligand for receptor binding

Analogs of the alpha-factor tridecapeptide mating pheromone (WHWLQLKPGQPMY) from Saccharomyces cerevisiae in which Tyr13 was replaced with Phe, p-F-Phe, m-F-Phe, p-NO2-Phe, p-NH2-Phe or Ser were synthesized and purified to >99% homogeneity. These analogs were bioassayed using a growth arrest assay and a gene induction assay and evaluated for their ability to compete with binding of tritiated alpha-factor to its receptor Ste2p. The results showed that the phenolic OH of Tyr13 is not required for either biological activity or receptor recognition. Analogs containing fluorine, amino, nitro or a hydrogen in place of OH had 80-120% of the biological activity of the parent pheromone in the gene induction assay and had receptor affinities from nearly equal to 6-fold lower than that of alpha-factor. In contrast, substitution of Ser or Ala at position 13 resulted in a >100-fold decrease in receptor affinity suggesting that the aromatic ring is involved in binding to the receptor. The lack of a strict requirement for Tyr13 allowed the design of several multiple replacement analogs in which Phe or p-F-Phe were substituted at position 13 and Tyr was placed in other positions of the peptide. These analogs could then be iodinated and used in the development of a highly sensitive receptor-binding assay. One potential receptor ligand [Tyr(125I)1,Nle12, Phe13] alpha-factor exhibited saturable binding with a KD of 81 nM and was competed by alpha-factor for binding in a whole-cell assay. Thus a new family of radioactive ligands for the alpha-factor receptor has been revealed. These ligands should be extremely useful in defining active site residues during mutagenesis and cross-linking studies.

Membrane-bound human 3beta-hydroxysteroid dehydrogenase: overexpression with His-tag using a baculovirus system and single-step purification

The membrane-bound human 3beta-hydroxysteroid dehydrogenase type 1 (3beta-HSD1) was overexpressed with His(6)-tag, using a baculovirus expression system, and then purified by nickel-chelated affinity chromatography. Overexpression of 3beta-HSD1 was confirmed by enzyme assay and Western blot analysis. The protein was purified to more than 95% homogeneity by a single-step Ni(2+)-chelated affinity chromatography after solubilization of the membrane-bound protein with the detergent C(12)E(8). High yield was repeatedly obtained, with 3-4 mg of homogeneous and active 3beta-HSD1 from 1 x 10(9) of infected Sf9 cells. The kinetic study showed a K(m) of 1.7 microM and a V(max) of 50 nmol/min/mg of purified protein using dehydroepiandrosterone as the substrate. The above preparation will facilitate the structure-function study of this important enzyme.

Mutations affecting ligand specificity of the G-protein-coupled receptor for the Saccharomyces cerevisiae tridecapeptide pheromone

Random mutations were generated in the G-protein-coupled receptor (Ste2p) for the tridecapeptide pheromone (alpha-factor) of Saccharomyces cerevisiae. These mutants were screened for variants that responded to antagonists. Because multiple mutations were detected in each mutant receptor recovered from the screen, site-directed mutagenesis was used to create single-site mutant receptors. Three receptors containing mutations F55V, S219P, and S259P were analyzed for their biological responses to various alpha-factor analogs and for their ligand binding profiles. Cells expressing each of the mutant receptors responded to alpha-factor as well as or better than wild-type cells in a growth arrest assay. In contrast, the binding of alpha-factor to the F55V and S219P mutant receptors was at least 10-fold reduced in comparison to wild-type receptor indicating a complex non-linear correlation between binding affinity and biological activity. Cells expressing mutant receptors responded to some normally inactive analogs in biological assays, despite the fact that these analogs had a low affinity for Ste2p. The analysis of these mutant receptors confirms previous findings that the first and sixth transmembrane regions of Ste2p are important for ligand interaction, ligand specificity, and/or receptor activation to initiate the signal transduction pathway. Changes in binding affinity of pheromone analogs to wild-type and mutant receptors indicate that residue 55 of Ste2p is involved with both ligand binding and signal transduction.

Production of G-protein-coupled receptors in yeast

Yeasts combine the advantages of fast and easy handling with the potential to perform eukaryotic post-translational modifications and are for this reason interesting hosts for heterologous production of G-protein-coupled receptors. The possibility to connect foreign receptors to a yeast internal MAP kinase pathway was used to establish yeast-based systems for high-throughput screening of compound libraries. In addition, yeasts have the potential for high level production of G-protein-coupled receptors. In this field, non-Saccharomyces yeasts seems to be interesting alternatives to S. cerevisiae, as well as to systems based on higher eukaryotic cells.

Synthesis, biological activity, and conformational analysis of peptidomimetic analogues of the Saccharomyces cerevisiae alpha-factor tridecapeptide

Biochemical and biophysical investigations on the Saccharomyces cerevisiae alpha-factor indicate that this tridecapeptide mating pheromone (WHWLQLKPGQPMY) might adopt a type II beta-turn in the center of the peptide when it binds to its G protein-coupled receptor. To test this hypothesis we synthesized analogues of alpha-factor incorporating a (R or S)-gamma-lactam conformational constraint [3-(R or S)-amino-2-oxo-1-pyrrolidineacetamido] in place of the Pro-Gly at residues 8 and 9 of the peptide and tested their biological activities and receptor binding. Analogues were purified to >99% homogeneity as evidenced by high-performance liquid chromatography and capillary electrophoresis and characterized by amino acid analysis, mass spectrometry, and nuclear magnetic resonance (NMR) spectroscopy. The restricted alpha-factor analogue WHWLQLK[(R)-gamma-lactam]QP[Nle]Y was more active than its lactam-containing diastereomeric homologue WHWLQLK[(S)-gamma-lactam]QP[Nle]Y and about equally active with the [Nle12]-alpha-factor in growth arrest and FUS1-lacZ gene induction assays. Both lactam analogues competed with tritiated [Nle12]-alpha-factor for binding to the alpha-factor receptor (Ste2p) with the (R)-gamma-lactam-containing peptide having 7-fold higher affinity than the (S)-gamma-lactam-containing homologue. Two-dimensional NMR spectroscopy and modeling analysis gave evidence that the (R)-gamma-analogue is a flexible peptide that assumes a transient gamma-turn structure around the lactam moiety. The results represent the first example of an alpha-factor analogue containing a peptidomimetic constraint that is as active as the native pheromone. The correlation between activity and structure provides further evidence that the biologically active conformation of the molecule contains a turn in the middle of the pheromone. This study provides new insights into the structural basis of alpha-factor activity and adds to the repertoire of conformationally biasing constraints that can be used to maintain and even enhance biological activity in peptide hormones.

The human D1A dopamine receptor: heterologous expression in Saccharomyces cerevisiae and purification of the functional receptor

Functional human D1A dopamine receptor has been expressed in Saccharomyces cerevisiae. The primary sequence of the receptor was modified to include two affinity tags at the C-terminus of the protein, a FLAG tag (DYKDDDDK), and a His6 tag (HHHHHH). These modifications allowed for purification to near homogeneity using immobilized metal affinity chromatography and immunoaffinity chromatography. Radioligand binding demonstrated that the purified and reconstituted receptor binds the antagonist [3H]SCH23390 with an affinity (KD = 8.0 +/- 3.2 nM) comparable to that of the native receptor.

Cytoplasmic tail phosphorylation of the alpha-factor receptor is required for its ubiquitination and internalization

G protein-coupled (GPC) receptors are phosphorylated in response to ligand binding, a modification that promotes receptor desensitization or downregulation. The alpha-factor pheromone receptor (Ste2p) of Saccharomyces cerevisiae is a GPC receptor that is hyperphosphorylated and ubiquitinated upon binding alpha-factor. Ubiquitination triggers Ste2p internalization into the endocytic pathway. Here we demonstrate that phosphorylation of Ste2p promotes downregulation by positively regulating ubiquitination and internalization. Serines and a lysine are essential elements of the Ste2p SINNDAKSS internalization signal that can mediate both constitutive and ligand-stimulated endocytosis. The SINNDAKSS serines are required for receptor phosphorylation which, in turn, facilitates ubiquitination of the neighboring lysine. Constitutive phosphorylation is required to promote constitutive internalization, and is also a prerequisite for ligand-induced phosphorylation at or near the SINNDAKSS sequence. Mutants defective in yeast casein kinase I homologues are unable to internalize alpha-factor, and do not phosphorylate or ubiquitinate the receptor, indicating that these kinases play a direct or indirect role in phosphorylating the receptor. Finally, we provide evidence that the primary function of phosphorylation controlled by the SINNDAKSS sequence is to trigger receptor internalization, demonstrating that phosphorylation-dependent endocytosis is an important mechanism for the downregulation of GPC receptor activity.

Gs alpha meets its target--shedding light on a key signal transduction event

The recently determined crystal structure of Gs alpha bound to a catalytically active form of adenylyl cyclase reveals the location of the enzyme's active site and provides the first view of heterotrimeric G protein alpha subunit activating a downstream effector. Comparison with the structure of a catalytically inactive form of adenylyl cyclase suggests a plausible allosteric mechanism whereby the synergistic activators Gs alpha and forskolin stimulate the activity of adenylyl cyclase.