Functional expression of rat M5 muscarinic acetylcholine receptor in yeast

Which yeast species shall I choose? Saccharomyces cerevisiae versus Pichia pastoris (review)

Having decided on yeast as a production host, the choice of species is often the first question any researcher new to the field will ask. With over 500 known species of yeast to date, this could pose a significant challenge. However, in reality, only very few species of yeast have been employed as host organisms for the production of recombinant proteins. The two most widely used, Saccharomyces cerevisiae and Pichia pastoris, are compared and contrasted here.

Heterologous expression of GPCRs in fission yeast

In this chapter, we describe methods to heterologously express G protein-coupled receptors (GPCRs) in the fission yeast Schizosaccharomyces (Sz.) pombe. GPCRs regulate a diverse range of biological processes in all eukaryotic cells, including plants, insects, humans, and yeast. The high degree of conservation between GPCRs from different organisms has facilitated the development of a large number of model systems to enable study of this pharmaceutically important family of cell-surface receptors. Of the many model systems available for investigating GPCRs, yeast have proven to be one of the more attractive. Yeasts' amenability to both genetic and biochemical manipulation, a reduced number of endogenous GPCRs and their relative low culturing costs has facilitated their use in many high-throughput drug screens. Given the high number of detailed methods relating to the expression of GPCRs within budding yeast, we have focused our attention on the use of fission yeast as a model system. We describe the methods used and provide examples from our own experiences of expressing a number of human GPCRs in Sz. pombe cells.

Application of the split-ubiquitin membrane yeast two-hybrid system to investigate membrane protein interactions

The characterization of protein-protein interactions provides the foundation for further studies concerning protein complex function and regulation. Since the advent of the yeast two-hybrid assay, many additional genetic systems based upon the principle of protein fragment complementation have been designed. One such system, the split-ubiquitin membrane yeast two-hybrid system (MbYTH), is able to analyze the interaction status between two integral membrane proteins. This ability of the MbYTH system augments genetic analysis of protein interactions by covering for the inherent limitation of the yeast two-hybrid system when studying membrane protein interactions. Herein, we provide a description of the MbYTH method and detailed protocols in order to monitor protein interactions and discover novel interacting partners using the MbYTH system.

Green fluorescent protein as a reporter of human mu-opioid receptor overexpression and localization in the methylotrophic yeast Pichia pastoris

The human mu-opioid receptor (HuMOR) was fused in its N-terminus end to the green fluorescent protein (GFP) or/and to the c-myc and six histidines tags in its C-terminus end, and expressed in the methylotrophic yeast Pichia pastoris. Neither the C- nor the N-terminal tagging of the receptor does modify its pharmacological properties as compared to the untagged receptor. Expression levels of fusion receptors determined by GFP fluorescence measurements strongly correlates with the number of sites expressed per cell detected through saturation studies (Bmax value), thus showing that GFP is an efficient and reliable reporter of the HuMOR functional expression. The N- and C-terminus tags have allowed to show that the entire molecule is overexpressed. They have permitted in-situ localization experiments using fluorescence and electron microscopy techniques and have shown a dense intracellular labelling. Above all, the quantification of expression levels made possible through fluorescence intensity analysis, have revealed that huge amounts of receptor are produced that could not be detected through classical binding experiments: for a Bmax value of 1 pmol mg(-1) of receptor determined through binding studies, 16 pmol were found in membrane preparations using fluorescence and 100 pmol in whole cells. These results should be very useful for large-scale production and structural biology of HuMOR, and other G-protein coupled receptors (GPCRs).

Neurotensin induces mating in Saccharomyces cerevisiae cells that express human neurotensin receptor type 1 in place of the endogenous pheromone receptor

Heterologous expression of the human neurotensin receptor type I (hNT1-R) has been achieved in the yeast Saccharomyces cerevisiae. Immunoanalysis of membranes prepared from cells expressing a c-myc-tagged version of hNT1-R revealed multiple c-myc cross-reacting polypeptides of high molecular mass, suggesting that hNT1-R was glycosylated in yeast. High-affinity binding sites for 125I-labeled-[monoiodo-Tyr3]neurotensin ([125I-Tyr3]NT) were detected on hNT1-R-expressing cells with Kd and Bmax values of 3.2 nM and of 500 receptors per cell, respectively. Competition binding studies of neurotensin with SR142948 and SR48692, two nonpeptidic antagonists of hNT1-R, indicated that the yeast-produced recombinant receptor displayed the same pharmacological properties as hNT1-R expressed in mammalian cells. Interestingly, neurotensin activated the pheromone pathway in hNT1-R-expressing cells in a dose-dependent fashion, as revealed by a beta-galactosidase activity assay with a pheromone-responsive Fus1:lacZ construct. Mutational inactivation of the SST2 and STE2 genes increased the level of beta-galactosidase activity in response to neurotensin by twofold. Recombinant hNT1-R-producing cells, which lacked the endogenous G-protein-coupled receptor for the alpha pheromone, mated with wild-type MATalpha haploid cells in response to neurotensin, leading to bona fide diploid zygote formation. This is the first report of a mammalian receptor that can replace the endogenous pheromone receptor when produced in yeast, by signaling a fully effective, agonist-induced, mating process.

Functional expression of M(1), M(3) and M(5) muscarinic acetylcholine receptors in yeast

The goal of this study was to functionally express the three G(q)-coupled muscarinic receptor subtypes, M(1), M(3) and M(5), in yeast (Saccharomyces cerevisiae). Transformation of yeast with expression constructs coding for the full-length receptors resulted in very low numbers of detectable muscarinic binding sites (B(max) < 5 fmol/mg). Strikingly, deletion of the central portion of the third intracellular loops of the M(1), M(3) and M(5) muscarinic receptors resulted in dramatic increases in B(max) values (53-214 fmol/mg). To monitor productive receptor/G-protein coupling, we used specifically engineered yeast strains that required agonist-stimulated receptor/G-protein coupling for cell growth. These studies showed that the shortened versions of the M(1), M(3) and M(5) receptors were unable to productively interact with the endogenous yeast G protein alpha-subunit, Gpa1p, or a Gpa1 mutant subunit that contained C-terminal mammalian Galpha(s) sequence. In contrast, all three receptors gained the ability to efficiently couple to a Gpa1/Galpha(q) hybrid subunit containing C-terminal mammalian Galpha(q) sequence, indicating that the M(1), M(3) and M(5) muscarinic receptors retained proper G-protein coupling selectivity in yeast. This is the first study to report the expression of muscarinic receptors in a coupling-competent form in yeast. The strategy described here, which involves structural modification of both receptors and co-expressed G proteins, should facilitate the functional expression of other classes of G protein-coupled receptors in yeast.

Nonselective coupling of the human mu-opioid receptor to multiple inhibitory G-protein isoforms

The human mu-opioid receptor was expressed in Saccharomyces cerevisiae. Binding of [3H]diprenorphine to yeast spheroplasts was specific and saturable (Kd = 1 nm, Bmax = 0.2-1 pmol x mg-1 of membrane proteins). Inhibition of [3H]diprenorphine binding by antagonists and agonists with varying opioid selectivities (mu, delta and kappa) occurred with the same order of potency as in mammalian tissues. Affinities of antagonists were the same with yeast spheroplasts as in reference tissues whereas those of agonists, except etorphine and buprenorphine, were 10-fold to 100-fold lower. Addition of heterotrimeric Gi,o-proteins purified from bovine brain shifted the mu-opioid receptor into a high-affinity state for agonists. Using individually purified Galpha-subunits re-associated with betagamma-dimers, we showed that alphao1, alphao2, alphai1, alphai2 and alphai3 reconstituted high-affinity agonist binding with equal efficiency. This suggests that the structural determinants of the mu-opioid receptor responsible for G-protein coupling are not able to confer a high degree of specificity towards any member of the Gi,o family. The selective effects of opioid observed in specialized tissues upon opioid stimulation may be a result of regulation of G-protein activity by cell-specific factors which should conveniently be analysed using the reconstitution assay described here.

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.

Pharmacological characterisation of the D2 dopamine receptor expressed in the yeast Schizosaccharomyces pombe

The rat D2(long) dopamine receptor has been expressed in the fission yeast Schizosaccharomyces pombe at levels of about 1 pmol/mg of protein. The recombinant receptor, analysed in ligand binding experiments, exhibits properties typical of a D2 dopamine receptor and the affinities of antagonists agree with values obtained for the receptor expressed in mammalian systems although the affinities of some antagonists are lower. Substituted benzamide antagonists show lower affinities in the absence of sodium ions whereas clozapine and classical antagonists mostly show higher affinities. Agonist binding is insensitive to the effects of GTP indicating lack of a stable interaction with G-proteins.

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.

Heterologous expression of G-protein-coupled receptors: human opioid receptors under scrutiny

G-protein-coupled receptors whose topology shows seven transmembrane domains form the largest known family of receptors involved in higher organism signal transduction. Despite increasing knowledge on the functioning mechanisms of these receptors, almost no structural data are available but only a few models. Structural studies using a wide range of physical and biochemical techniques may require fairly large (up to several milligrams) amounts of purified protein. Since such quantities are not naturally available, overexpression is prerequisite. Heterologous expression systems are then assayed for maximal production of a protein facsimile. Heterologous systems may also provide interesting alternatives for receptor functional studies in a different cellular context. Opioid receptors will be used as an example to discuss aspects related to the choice and suitability of several different expression systems for the intended analysis of G-protein-coupled receptor properties. General implications will be outlined.

Comparative biochemical and pharmacological characterization of the mouse 5HT5A 5-hydroxytryptamine receptor and the human beta2-adrenergic receptor produced in the methylotrophic yeast Pichia pastoris

Over the last few years, Pichia pastoris has been developed into a powerful expression system for a multitude of foreign genes. Here, we demonstrate that the P. pastoris expression system has similar power to the baculovirus expression system in high-level production of two G-protein-coupled receptors, the mouse 5HT5A 5-hydroxtryptamine receptor and the human beta2-adrenergic receptor. Different expression plasmids were constructed in which the cDNAs of the two receptors were cloned under the transcriptional control of the highly inducible promoter of the P. pastoris alcohol oxidase 1 (AOX1) gene. In three expression plasmids, the receptors were fused to the Saccharomyces cerevisiae alpha-factor prepropeptide and also to the c-myc tag or the FLAG tag to permit immunological detection of the receptors. After transformation into P. pastoris strains KM71 and SMD 1163, recombinant clones were selected and tested for the production of the 5HT5A receptor and the beta2-adrenergic receptor by radioligand binding using [N-methyl-3H]lysergic acid diethylamide and [5,7-3H](-)CGP-12177 respectively. The production level of the 5HT5A receptor was improved by a factor of three by fusion with the alpha-factor prepropeptide. Also, the higher gene dosage resulting from multiple insertions of the expression cassette led to an improvement in production by a factor of two for both receptors. The addition of the adrenergic antagonist alprenolol to the culture medium had a positive effect on the number of specific binding sites detectable in clones producing the beta2-adrenergic receptor. For the 5HT5A receptor the addition of yohimbine resulted in a similar but smaller effect. Binding assays revealed that approx. 25 pmol of beta2-adrenergic receptor and approx. 40 pmol of 5HT5A receptor per mg of membrane protein in crude membrane preparations were produced. The pharmacological profiles for the heterologously produced receptors, estimated by ligand-displacement analysis using certain adrenergic and serotoninergic agonists and antagonists, were comparable with those reported for the receptors expressed in mammalian systems. Immunoblot analysis of the 5HT5A receptor revealed an apparent molecular mass about 20 kDa higher than expected from the amino acid sequence. Here, the Kex2 endopeptidase failed to process the alpha-factor leader correctly. Blocking glycosylation in vivo by tunicamycin or in vitro deglycosylation of membranes by endoglycosidase H resulted in correct processing. In contrast, the beta2-adrenergic receptor fusion to the alpha-factor leader was correctly processed by the internal Kex2 endopeptidase. The Kex2-processed beta2-adrenergic receptor was not glycosylated. In conclusion, the high-level production of the two receptors in P. pastoris will allow their purification in quantities sufficient for subsequent biophysical and structural studies.

G-protein-coupled receptors in Saccharomyces cerevisiae: high-throughput screening assays for drug discovery

G-protein-coupled receptors are an important class of therapeutic drug targets by virtue of their roles in the regulation of diverse cellular functions. Recent advances in the expression of heterologous G-protein-coupled receptors in the yeast Saccharomyces cerevisiae have led to the development of sensitive and selective assays of their ligand-induced activation. Implementation of this new technology in the high-throughput screening of compound libraries has enabled the discovery of novel ligands for the G-protein-coupled somatostatin receptor. This article describes the broad applicability of the technology and its use in drug discovery.

Gene expression systems in the development of high-throughput screens

Recent advances in the development of combinatorial automated chemical synthesis, robotic sample handling, and data collection and analysis have significantly increased the number of compounds available for screening against potential therapeutic targets. The implementation of highly sensitive in vitro biochemical and cell-based high-throughput screening assays is essential to facilitate the rapid identification of selective and potent lead molecules from compound libraries. The ability to easily produce functional proteins in sufficient quantities for in vitro biochemical assays and to devise useful cell-based systems is dependent on the successful application of a variety of gene expression systems.

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

A plasmid vector was developed that permitted high-level expression of a functional form of the Saccharomyces cerevisiae alpha-factor receptor (the STE2 gene product) tagged at its C-terminal end with an epitope (FLAG) and a His6 tract. When expressed in yeast from this plasmid, Ste2p was produced at a level at least 3-fold higher than that reported previously for any other 7-transmembrane-segment receptor expressed in the same cells. For purification, isolated cell membranes containing the overexpressed receptor were solubilized with detergent under specific conditions and subjected to immobilized metal affinity chromatography. Yields as high as 1 mg of nearly homogeneous (95%) receptor were routinely obtained even from relatively small scale preparations (60 g of frozen cell paste). The purified receptor was reconstituted into artificial phospholipid vesicles. Radioligand binding studies demonstrated that the purified receptor, in the reconstituted vesicles, bound its tridecapeptide ligand (alpha-factor) with a KD (155 nM) consistent with the affinity expected for this receptor in the absence of its associated G protein. Efficient restoration of ligand binding activity upon reconstitution required the addition of solubilized membranes prepared from a yeast strain lacking the receptor. Sufficient amounts of active material can be obtained by this procedure to allow physical studies of this receptor and other 7-transmembrane-segment receptors expressed in this system.

Functional expression of bovine opsin in the methylotrophic yeast Pichia pastoris

The methylotrophic yeast Pichia pastoris was examined for functional expression of bovine opsin. An expression plasmid was constructed where the bovine opsin gene was placed downstream from the P. pastoris alcohol oxidase 1 gene promoter and fused at its amino-terminus to the acid phosphatase secretion signal. Quantitative-competitive PCR analysis of a stable yeast transformant showed that one copy of the opsin gene was integrated into the yeast genome. The expression level in this transformant corresponded to approximately 0.3 mg of opsin per liter of cell culture (A600 = 1.0). Sucrose density sedimentation analysis indicated that the opsin was associated exclusively with the membrane fraction. Similar to retinal opsin, P. pastoris-expressed opsin migrated as a single band of approximately 37 kDa on SDS-PAGE and showed high mannose N-glycosylation. A portion of the expressed opsin (approximately 4-15%) reacted with 11-cis-retinal to form the rhodopsin chromophore (lambda max 500 nm), and after purification showed ground and excited state spectral characteristics indistinguishable from those of the native pigment. Further, the metarhodopsin-II-mediated G-protein-activating potential of yeast expressed rhodopsin was similar to that of native rhodopsin. These results show that P. pastoris cells have the capacity to functionally express bovine opsin.

Expression and pharmacological characterization of the human mu-opioid receptor in the methylotrophic yeast Pichia pastoris

The human mu-opioid receptor cDNA from which the 32 amino-terminal codons were substituted by the Saccharomyces cerevisiae alpha-mating factor signal sequence has been expressed in the methylotrophic yeast Pichia pastoris using the host promoter of the alcohol oxidase-1 gene. Cell membranes exhibited specific and saturable binding of the opioid antagonist [3H]diprenorphine (Kd = 0.2 nM and Bmax = 400 fmol/mg protein or 800 sites/cell). Competition studies with non-selective, and mu-, delta- and kappa-selective opioid agonists and antagonists revealed a typical mu-opioid receptor binding profile, suggesting proper folding of the protein in yeast membranes.

Expression of the human muscarinic receptor gene m2 in Dictyostelium discoideum

We have expressed a functional human muscarinic M2 receptor, under the control of the homologous discoidin I gamma promoter, in the cellular slime mold Dictyostelium discoideum. The use of a contact site A leader peptide ensured insertion of the newly synthesized receptor protein into the plasma membrane. Due to the characteristics of the discoidin I gamma promoter, the M2 receptor is expressed during late growth and early development. The heterologously expressed M2 receptors show binding characteristics similar to authentic receptors. Membranes as well as whole cells can be used in ligand binding assays.

Functional coupling of a mammalian somatostatin receptor to the yeast pheromone response pathway

A detailed analysis of structural and functional aspects of G-protein-coupled receptors, as well as discovery of novel pharmacophores that exert their effects on members of this class of receptors, will be facilitated by development of a yeast-based bioassay. To that end, yeast strains that functionally express the rat somatostatin receptor subtype 2 (SSTR2) were constructed. High-affinity binding sites for somatostatin ([125I-Tyr-11]S-14) comparable to those in native tissues were detected in yeast membrane extracts at levels equivalent to the alpha-mating pheromone receptor (Ste2p). Somatostatin-dependent growth of strains modified by deletion of genes encoding components of the pheromone response pathway was detected through induction of a pheromone-responsive HIS3 reporter gene, enabling cells to grow on medium lacking histidine. Dose-dependent growth responses to S-14 and related SSTR2 subtype-selective agonists that were proportional to the affinity of the ligands for SSTR2 were observed. The growth response required SSTR2, G alpha proteins, and an intact signal transduction pathway. The sensitivity of the bioassay was affected by intracellular levels of the G alpha protein. A mutation in the SST2 gene, which confers supersensitivity to pheromone, was found to significantly enhance the growth response to S-14. In sst2 delta cells, SSTR2 functionally interacted with both a chimeric yeast/mammalian G alpha protein and the yeast G alpha protein, Gpa1p; to promote growth. These yeast strains should serve as a useful in vivo reconstitution system for examination of molecular interactions of the G-protein-coupled receptors and G proteins.

Overexpression of integral membrane proteins for structural studies

Determination of the structure of integral membrane proteins is a challenging task that is essential to understand how fundamental biological processes (such as photosynthesis, respiration and solute translocation) function at the atomic level. Crystallisation of membrane proteins in 3D has led to the determination of four atomic resolution structures [photosynthetic reaction centres (Allenet al. 1987; Changet al. 1991; Deisenhofer & Michel, 1989; Ermleret al. 1994); porins (Cowanet al. 1992; Schirmeret al. 1995; Weisset al. 1991); prostaglandin H2synthase (Picotet al. 1994); light harvesting complex (McDermottet al. 1995)], and crystals of membrane proteins formed in the plane of the lipid bilayer (2D crystals) have produced two more structures [bacteriorhodopsin (Hendersonet al. 1990); light harvesting complex (Kühlbrandtet al. 1994)].

High-level expression of the human neurokinin-1 receptor in mammalian cell lines using the Semliki Forest virus expression system

Human neurokinin-1 receptor cDNA was introduced into the pSFV1 Semliki Forest virus (SFV) vector and the in vitro transcribed RNA was electroporated into BHK cells with pSFV-Helper RNA. This procedure resulted in the packaging of a high-titer SFV-NK-1 virus stock containing approximately 5 x 10(9) infective units/ml. Infection of baby hamster kidney, COS-7, Chinese hamster ovary and human osteosarcoma cells yielded high levels of human neurokinin-1 receptor expression as assessed by [3H]substance P binding. The maximal receptor expression level obtained was 4 x 10(6) receptors/cell and studies of the post-infection time indicated that a high level of receptor expression was observed 10-24 h post-infection. The human neurokinin-1 receptor expressed in infected baby hamster kidney, COS-7 and Chinese hamster ovary cells was able to stimulate Ca2+ mobilization indicating functional coupling to guanine-nucleotide-binding proteins. The application of the Semliki Forest virus expression system will permit the rapid and efficient production of large quantities of receptor protein for both pharmacological and structural studies.