Expression and functional purification of a glycosylation deficient version of the human adenosine 2a receptor for structural studies

Detergent-free purification and reconstitution of functional human serotonin transporter (SERT) using diisobutylene maleic acid (DIBMA) copolymer

Structure and function analysis of human membrane proteins in lipid bilayer environments is acutely lacking despite the fundame1ntal cellular importance of these proteins and their dominance of drug targets. An underlying reason is that detailed study usually requires a potentially destabilising detergent purification of the proteins from their host membranes prior to subsequent reconstitution in a membrane mimic; a situation that is exacerbated for human membrane proteins due to the inherent difficulties in overexpressing suitable quantities of the proteins. We advance the promising styrene maleic acid polymer (SMA) extraction approach to introduce a detergent-free method of obtaining stable, functional human membrane transporters in bilayer nanodiscs directly from yeast cells. We purify the human serotonin transporter (hSERT) following overexpression in Pichia pastoris using diisobutylene maleic acid (DIBMA) as a superior method to traditional detergents or the more established styrene maleic acid polymer. hSERT plays a pivotal role in neurotransmitter regulation being responsible for the transport of the neurotransmitter 5-hydroxytryptamine (5-HT or serotonin). It is representative of the neurotransmitter sodium symporter (NSS) family, whose importance is underscored by the numerous diseases attributed to their malfunction. We gain insight into hSERT activity through an in vitro transport assay and find that DIBMA extraction improves the thermostability and activity of hSERT over the conventional detergent method.

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The non-aromatic amphipathic polymer DIBMA can be successfully employed to purify human membrane proteins.

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DIBMA solubilisation of hSERT from yeast membranes and resultant nanodisc thermostability is comparable to SMA.

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DIBMA and SMA encapsulated hSERT lipid-nanodiscs exhibit higher binding activity than hSERT DDMCHS micelles.

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Proteoliposomes reconstituted with hSERT-DIBMALPs possess higher transport activity than comparable DDMCHS reconstitutions.

Expression and purification of recombinant G protein-coupled receptors: A review

Given their extensive role in cell signalling, GPCRs are significant drug targets; despite this, many of these receptors have limited or no available prophylaxis. Novel drug design and discovery significantly rely on structure determination, of which GPCRs are typically elusive. Progress has been made thus far to produce sufficient quantity and quality of protein for downstream analysis. As such, this review highlights the systems available for recombinant GPCR expression, with consideration of their advantages and disadvantages, as well as examples of receptors successfully expressed in these systems. Additionally, an overview is given on the use of detergents and the styrene maleic acid (SMA) co-polymer for membrane solubilisation, as well as purification techniques.

Functional characterisation of G protein-coupled receptors

Characterisation of receptors can involve either assessment of their ability to bind ligands or measure receptor activation as a result of agonist or inverse agonist interactions. This review focuses on G protein-coupled receptors (GPCRs), examining techniques that can be applied to both receptors in membranes and after solubilisation. Radioligand binding remains a widely used technique, although there is increasing use of fluorescent ligands. These can be used in a variety of experimental designs, either directly monitoring ligand itself with techniques such as fluorescence polarisation or indirectly via resonance energy transfer (fluorescence/Forster resonance energy transfer, FRET and bioluminescence resonance energy transfer, BRET). Label free techniques such as isothermal titration calorimetry (ITC) and surface plasmon resonance (SPR) are also increasingly being used. For GPCRs, the main measure of receptor activation is to investigate the association of the G protein with the receptor. The chief assay measures the receptor-stimulated binding of GTP or a suitable analogue to the receptor. The direct association of the G protein with the receptor has been investigated via resonance energy techniques. These have also been used to measure ligand-induced conformational changes within the receptor; a variety of experimental techniques are available to incorporate suitable donors and acceptors within the receptor.

Photoaffinity Labeling of the Human A2A Adenosine Receptor and Cross-link Position Analysis by Mass Spectrometry

Photoaffinity labeling (PAL) is widely used for the identification of ligand-binding proteins and elucidation of ligand-binding sites. PAL has also been employed for the characterization of G protein-coupled receptors (GPCRs); however, a limited number of reports has successfully identified their cross-linked amino acids. This report is the first of its kind to determine the cross-link position of the human A_2A adenosine receptor by PAL with the novel diazirine-based photoaffinity probe 9.

The synthesis of recombinant membrane proteins in yeast for structural studies

Historically, recombinant membrane protein production has been a major challenge meaning that many fewer membrane protein structures have been published than those of soluble proteins. However, there has been a recent, almost exponential increase in the number of membrane protein structures being deposited in the Protein Data Bank. This suggests that empirical methods are now available that can ensure the required protein supply for these difficult targets. This review focuses on methods that are available for protein production in yeast, which is an important source of recombinant eukaryotic membrane proteins. We provide an overview of approaches to optimize the expression plasmid, host cell and culture conditions, as well as the extraction and purification of functional protein for crystallization trials in preparation for structural studies.

G-protein coupled receptor solubilization and purification for biophysical analysis and functional studies, in the total absence of detergent

G-protein coupled receptors (GPCRs) constitute the largest class of membrane proteins and are a major drug target. A serious obstacle to studying GPCR structure/function characteristics is the requirement to extract the receptors from their native environment in the plasma membrane, coupled with the inherent instability of GPCRs in the detergents required for their solubilization. In the present study, we report the first solubilization and purification of a functional GPCR [human adenosine A_2A receptor (A_2AR)], in the total absence of detergent at any stage, by exploiting spontaneous encapsulation by styrene maleic acid (SMA) co-polymer direct from the membrane into a nanoscale SMA lipid particle (SMALP). Furthermore, the A_2AR–SMALP, generated from yeast (Pichia pastoris) or mammalian cells, exhibited increased thermostability (∼5°C) compared with detergent [DDM (n-dodecyl-β-D-maltopyranoside)]-solubilized A_2AR controls. The A_2AR–SMALP was also stable when stored for prolonged periods at 4°C and was resistant to multiple freeze-thaw cycles, in marked contrast with the detergent-solubilized receptor. These properties establish the potential for using GPCR–SMALP in receptor-based drug discovery assays. Moreover, in contrast with nanodiscs stabilized by scaffold proteins, the non-proteinaceous nature of the SMA polymer allowed unobscured biophysical characterization of the embedded receptor. Consequently, CD spectroscopy was used to relate changes in secondary structure to loss of ligand binding ([³H]ZM241385) capability. SMALP-solubilization of GPCRs, retaining the annular lipid environment, will enable a wide range of therapeutic targets to be prepared in native-like state to aid drug discovery and understanding of GPCR molecular mechanisms.

It is universally acknowledged that exposing cell-surface receptors to detergent is detrimental. We have used a polymer to extract the receptor and surrounding lipid as a nanoparticle that provides a novel solution compatible with purification and receptor-based drug discovery assays.

Membrane protein expression and analysis in yeast

This protocol describes the expression and analysis of membrane proteins produced in yeast, as illustrated with Yarrowia lipolytica and Pichia pastoris. Step by step, we explain how to generate a yeast strain expressing the membrane protein of interest, how to prepare a membrane protein sample from yeast, and how to analyze the expression levels using SDS-PAGE and Western blotting. In the final section, we describe how to perform a radioligand binding assay, which quantifies the amount of the protein folded in a ligand-binding competent state.

Functional recombinant protein is present in the pre-induction phases of Pichia pastoris cultures when grown in bioreactors, but not shake-flasks

Background

Pichia pastoris is a widely-used host for recombinant protein production; expression is typically driven by methanol-inducible alcohol oxidase (AOX) promoters. Recently this system has become an important source of recombinant G protein-coupled receptors (GPCRs) for structural biology and drug discovery. The influence of diverse culture parameters (such as pH, dissolved oxygen concentration, medium composition, antifoam concentration and culture temperature) on productivity has been investigated for a wide range of recombinant proteins in P. pastoris. In contrast, the impact of the pre-induction phases on yield has not been as closely studied. In this study, we examined the pre-induction phases of P. pastoris bioreactor cultivations producing three different recombinant proteins: the GPCR, human A_2a adenosine receptor (hA_2aR), green fluorescent protein (GFP) and human calcitonin gene-related peptide receptor component protein (as a GFP fusion protein; hCGRP-RCP-GFP).

Results

Functional hA_2aR was detected in the pre-induction phases of a 1 L bioreactor cultivation of glycerol-grown P. pastoris. In a separate experiment, a glycerol-grown P. pastoris strain secreted soluble GFP prior to methanol addition. When glucose, which has been shown to repress AOX expression, was the pre-induction carbon source, hA_2aR and GFP were still produced in the pre-induction phases. Both hA_2aR and GFP were also produced in methanol-free cultivations; functional protein yields were maintained or increased after depletion of the carbon source. Analysis of the pre-induction phases of 10 L pilot scale cultivations also demonstrated that pre-induction yields were at least maintained after methanol induction, even in the presence of cytotoxic concentrations of methanol. Additional bioreactor data for hCGRP-RCP-GFP and shake-flask data for GFP, horseradish peroxidase (HRP), the human tetraspanins hCD81 and CD82, and the tight-junction protein human claudin-1, demonstrated that bioreactor but not shake-flask cultivations exhibit recombinant protein production in the pre-induction phases of P. pastoris cultures.

Conclusions

The production of recombinant hA_2aR, GFP and hCGRP-RCP-GFP can be detected in bioreactor cultivations prior to methanol induction, while this is not the case for shake-flask cultivations of GFP, HRP, hCD81, hCD82 and human claudin-1. This confirms earlier suggestions of leaky expression from AOX promoters, which we report here for both glycerol- and glucose-grown cells in bioreactor cultivations. These findings suggest that the productivity of AOX-dependent bioprocesses is not solely dependent on induction by methanol. We conclude that in order to maximize total yields, pre-induction phase cultivation conditions should be optimized, and that increased specific productivity may result in decreased biomass yields.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-014-0127-y) contains supplementary material, which is available to authorized users.

Overexpression of membrane proteins from higher eukaryotes in yeasts

Heterologous expression and characterisation of the membrane proteins of higher eukaryotes is of paramount interest in fundamental and applied research. Due to the rather simple and well-established methods for their genetic modification and cultivation, yeast cells are attractive host systems for recombinant protein production. This review provides an overview on the remarkable progress, and discusses pitfalls, in applying various yeast host strains for high-level expression of eukaryotic membrane proteins. In contrast to the cell lines of higher eukaryotes, yeasts permit efficient library screening methods. Modified yeasts are used as high-throughput screening tools for heterologous membrane protein functions or as benchmark for analysing drug-target relationships, e.g., by using yeasts as sensors. Furthermore, yeasts are powerful hosts for revealing interactions stabilising and/or activating membrane proteins. We also discuss the stress responses of yeasts upon heterologous expression of membrane proteins. Through co-expression of chaperones and/or optimising yeast cultivation and expression strategies, yield-optimised hosts have been created for membrane protein crystallography or efficient whole-cell production of fine chemicals.

Enhanced membrane protein expression by engineering increased intracellular membrane production

Background

Membrane protein research is frequently hampered by the low natural abundance of these proteins in cells and typically relies on recombinant gene expression. Different expression systems, like mammalian cells, insect cells, bacteria and yeast are being used, but very few research efforts have been directed towards specific host cell customization for enhanced expression of membrane proteins. Here we show that by increasing the intracellular membrane production by interfering with a key enzymatic step of lipid synthesis, enhanced expression of membrane proteins in yeast is achieved.

Results

We engineered the oleotrophic yeast, Yarrowia lipolytica, by deleting the phosphatidic acid phosphatase, PAH1, which led to massive proliferation of endoplasmic reticulum (ER) membranes. For all eight tested representatives of different integral membrane protein families, we obtained enhanced protein accumulation levels and in some cases enhanced proteolytic integrity in the ∆pah1 strain. We analysed the adenosine A_2AR G-protein coupled receptor case in more detail and found that concomitant induction of the unfolded protein response in the ∆pah1 strain enhanced the specific ligand binding activity of the receptor. These data indicate an improved quality control mechanism for membrane proteins accumulating in yeast cells with proliferated ER.

Conclusions

We conclude that redirecting the metabolic flux of fatty acids away from triacylglycerol- and sterylester-storage towards membrane phospholipid synthesis by PAH1 gene inactivation, provides a valuable approach to enhance eukaryotic membrane protein production. Complementary to this improvement in membrane protein quantity, UPR co-induction further enhances the quality of the membrane protein in terms of its proper folding and biological activity. Importantly, since these pathways are conserved in all eukaryotes, it will be of interest to investigate similar engineering approaches in other cell types of biotechnological interest, such as insect cells and mammalian cells.

Screening for high-yielding Pichia pastoris clones: the production of G protein-coupled receptors as a case study

Pichia pastoris is an established host for the production of a wide range of recombinant proteins including membrane proteins. The system has a particularly good track record for the production of G protein-coupled receptors (GPCRs). Generation and screening of expression clones with this system use standard molecular biology techniques. Multiple clones can be generated and screened in a matter of a few weeks making this similar to Escherichia coli in terms of speed. In addition, basic buffer components and the lack of expensive equipment make small-scale expression screening in P. pastoris very cost-effective. Here we describe the procedures used for small-scale GPCR production screening.

Platform for the rapid construction and evaluation of GPCRs for crystallography in Saccharomyces cerevisiae

BACKGROUND

Recent successes in the determination of G-protein coupled receptor (GPCR) structures have relied on the ability of receptor variants to overcome difficulties in expression and purification. Therefore, the quick screening of functionally expressed stable receptor variants is vital.

RESULTS

We developed a platform using Saccharomyces cerevisiae for the rapid construction and evaluation of functional GPCR variants for structural studies. This platform enables us to perform a screening cycle from construction to evaluation of variants within 6-7 days. We firstly confirmed the functional expression of 25 full-length class A GPCRs in this platform. Then, in order to improve the expression level and stability, we generated and evaluated the variants of the four GPCRs (hADRB2, hCHRM2, hHRH1 and hNTSR1). These stabilized receptor variants improved both functional activity and monodispersity. Finally, the expression level of the stabilized hHRH1 in Pichia pastoris was improved up to 65 pmol/mg from negligible expression of the functional full-length receptor in S. cerevisiae at first screening. The stabilized hHRH1 was able to be purified for use in crystallization trials.

CONCLUSIONS

We demonstrated that the S. cerevisiae system should serve as an easy-to-handle and rapid platform for the construction and evaluation of GPCR variants. This platform can be a powerful prescreening method to identify a suitable GPCR variant for crystallography.

Purification of the human G protein-coupled receptor adenosine A(2a)R in a stable and functional form expressed in Pichia pastoris

The isolation of membrane proteins with the aim of producing highly pure, homogeneous, stable, and functional material remains challenging, and it is often necessary to develop protein-specific purification protocols by trial and error. One key tool that is required in the development of a suitable protocol is a functional assay. This unit describes a range of different protocols for isolation of the human adenosine A2a receptor (A(2a)R). These protocols show the importance of developing a robust method for comparing the quality of protein obtained by a combination of biophysical analyses including SDS-PAGE, analytical size-exclusion chromatography, and functional analysis. One of the keys to isolating and maintaining a functional receptor, found not only in the optimal protocol described here but in other published examples, is that there should be no more than two chromatographic steps.

Radioligand binding analysis as a tool for quality control of GPCR production for structural characterization: adenosine A(2a)R as a template for study

Functional characterization of G protein-coupled receptors is essential to ascertain the suitability of a protein target for downstream studies and to help develop optimal expression and isolation procedures. Radioligand binding analysis is a well-established technique, which allows direct measurement of the amount of functional receptor in a sample. It can be readily applied to both membrane-bound and soluble receptor samples and is an ideal method for monitoring the amount of functional protein at each stage in the expression and isolation process. This unit presents protocols for the radioligand binding analysis of the human adenosine A(2a) receptor and provides examples of how these assays can be used at several stages to help optimize expression, solubilization, and isolation procedures.

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.

Setting up a bioreactor for recombinant protein production in yeast

Scale-up from shake flasks to bioreactors allows for the more reproducible, high-yielding production of recombinant proteins in yeast. The ability to control growth conditions through real-time monitoring facilitates further optimization of the process. The setup of a 3-L stirred-tank bioreactor for such an application is described.

A purified C-terminally truncated human adenosine A(2A) receptor construct is functionally stable and degradation resistant

Recent high resolution structures of modified G-protein coupled receptors (GPCRs) have provided major insight into the mechanisms of receptor-ligand binding. However understanding of the complete mechanism of GPCR function remains limited. This study characterised C-terminally truncated versions of the human adenosine A(2A) receptor (A(2A)R) with a view to producing protein suitable for structural studies. The constructs terminated at residue A316, removing the intracellular C-terminal tail, or V334, producing a C-terminal tail equivalent in length to that of rhodopsin. Higher levels of functional receptor before and after solubilisation were obtained for both C-terminally truncated constructs compared to the wild-type receptor (WT) as assessed by radioligand binding analysis using [(3)H]ZM241385. The construct which yielded the highest level of functional receptor, V334 A(2A)R, was purified in DDM to high homogeneity with a final yield of 2 mg/L. Binding analysis revealed that the purified receptor had a specific activity of 20.2+/-1.2 nmol/mg, close to the theoretical maximum. Pure V334 A(2A)R was resistant to degradation over 15 days when stored at 4 degrees C or 20 degrees C and showed remarkable functional stability when stored at 4 degrees C, retaining 84% of initial functionality after 30 days. This construct is an excellent candidate for structural studies.

Tuning microbial hosts for membrane protein production

The last four years have brought exciting progress in membrane protein research. Finally those many efforts that have been put into expression of eukaryotic membrane proteins are coming to fruition and enable to solve an ever-growing number of high resolution structures. In the past, many skilful optimization steps were required to achieve sufficient expression of functional membrane proteins. Optimization was performed individually for every membrane protein, but provided insight about commonly encountered bottlenecks and, more importantly, general guidelines how to alleviate cellular limitations during microbial membrane protein expression. Lately, system-wide analyses are emerging as powerful means to decipher cellular bottlenecks during heterologous protein production and their use in microbial membrane protein expression has grown in popularity during the past months.

This review covers the most prominent solutions and pitfalls in expression of eukaryotic membrane proteins using microbial hosts (prokaryotes, yeasts), highlights skilful applications of our basic understanding to improve membrane protein production. Omics technologies provide new concepts to engineer microbial hosts for membrane protein production.

Altering the ribosomal subunit ratio in yeast maximizes recombinant protein yield

Background

The production of high yields of recombinant proteins is an enduring bottleneck in the post-genomic sciences that has yet to be addressed in a truly rational manner. Typically eukaryotic protein production experiments have relied on varying expression construct cassettes such as promoters and tags, or culture process parameters such as pH, temperature and aeration to enhance yields. These approaches require repeated rounds of trial-and-error optimization and cannot provide a mechanistic insight into the biology of recombinant protein production. We published an early transcriptome analysis that identified genes implicated in successful membrane protein production experiments in yeast. While there has been a subsequent explosion in such analyses in a range of production organisms, no one has yet exploited the genes identified. The aim of this study was to use the results of our previous comparative transcriptome analysis to engineer improved yeast strains and thereby gain an understanding of the mechanisms involved in high-yielding protein production hosts.

Results

We show that tuning BMS1 transcript levels in a doxycycline-dependent manner resulted in optimized yields of functional membrane and soluble protein targets. Online flow microcalorimetry demonstrated that there had been a substantial metabolic change to cells cultured under high-yielding conditions, and in particular that high yielding cells were more metabolically efficient. Polysome profiling showed that the key molecular event contributing to this metabolically efficient, high-yielding phenotype is a perturbation of the ratio of 60S to 40S ribosomal subunits from approximately 1:1 to 2:1, and correspondingly of 25S:18S ratios from 2:1 to 3:1. This result is consistent with the role of the gene product of BMS1 in ribosome biogenesis.

Conclusion

This work demonstrates the power of a rational approach to recombinant protein production by using the results of transcriptome analysis to engineer improved strains, thereby revealing the underlying biological events involved.

Large-scale functional expression of WT and truncated human adenosine A2A receptor in Pichia pastoris bioreactor cultures

Background

The large-scale production of G-protein coupled receptors (GPCRs) for functional and structural studies remains a challenge. Recent successes have been made in the expression of a range of GPCRs using Pichia pastoris as an expression host. P. pastoris has a number of advantages over other expression systems including ability to post-translationally modify expressed proteins, relative low cost for production and ability to grow to very high cell densities. Several previous studies have described the expression of GPCRs in P. pastoris using shaker flasks, which allow culturing of small volumes (500 ml) with moderate cell densities (OD600 ~15). The use of bioreactors, which allow straightforward culturing of large volumes, together with optimal control of growth parameters including pH and dissolved oxygen to maximise cell densities and expression of the target receptors, are an attractive alternative. The aim of this study was to compare the levels of expression of the human Adenosine 2A receptor (A_2AR) in P. pastoris under control of a methanol-inducible promoter in both flask and bioreactor cultures.

Results

Bioreactor cultures yielded an approximately five times increase in cell density (OD600 ~75) compared to flask cultures prior to induction and a doubling in functional expression level per mg of membrane protein, representing a significant optimisation. Furthermore, analysis of a C-terminally truncated A_2AR, terminating at residue V334 yielded the highest levels (200 pmol/mg) so far reported for expression of this receptor in P. pastoris. This truncated form of the receptor was also revealed to be resistant to C-terminal degradation in contrast to the WT A_2AR, and therefore more suitable for further functional and structural studies.

Conclusion

Large-scale expression of the A_2AR in P. pastoris bioreactor cultures results in significant increases in functional expression compared to traditional flask cultures.

A novel, generic and effective method for the rapid purification of G protein-coupled receptors

G protein-coupled receptors (GPCRs) constitute the largest family of membrane receptors and are of major therapeutic importance. Structure determination of G protein-coupled receptors and other applications require milligram quantities of purified receptor proteins on a regular basis. Recombinant GPCRs fused to a heterologous biotinylation domain were produced in the yeast Pichia pastoris. We describe an efficient method for their rapid purification that relies on the capture of these receptors with streptavidin immobilized on agarose beads, and their subsequent release by enzymatic digestion with TEV protease. This method has been applied to several GPCRs belonging to the class A rhodopsin subfamily, leading to high yields of purified proteins; it represents a method of choice for biochemical and biophysical studies when large quantities of purified GPCRs are needed.

BHLH32 modulates several biochemical and morphological processes that respond to Pi starvation in Arabidopsis

P(i) (inorganic phosphate) limitation severely impairs plant growth and reduces crop yield. Hence plants have evolved several biochemical and morphological responses to P(i) starvation that both enhance uptake and conserve use. The mechanisms involved in P(i) sensing and signal transduction are not completely understood. In the present study we report that a previously uncharacterized transcription factor, BHLH32, acts as a negative regulator of a range of P(i) starvation-induced processes in Arabidopsis. In bhlh32 mutant plants in P(i)-sufficient conditions, expression of several P(i) starvation-induced genes, formation of anthocyanins, total P(i) content and root hair formation were all significantly increased compared with the wild-type. Among the genes negatively regulated by BHLH32 are those encoding PPCK (phosphoenolpyruvate carboxylase kinase), which is involved in modifying metabolism so that P(i) is spared. The present study has shown that PPCK genes are rapidly induced by P(i) starvation leading to increased phosphorylation of phosphoenolpyruvate carboxylase. Furthermore, several Arabidopsis proteins that regulate epidermal cell differentiation [TTG1 (TRANSPARENT TESTA GLABRA1), GL3 (GLABRA3) and EGL3 (ENHANCER OF GL3)] positively regulate PPCK gene expression in response to P(i) starvation. BHLH32 can physically interact with TTG1 and GL3. We propose that BHLH32 interferes with the function of TTG1-containing complexes and thereby affects several biochemical and morphological processes that respond to P(i) availability.

High-level expression in Saccharomyces cerevisiae enables isolation and spectroscopic characterization of functional human adenosine A2a receptor

The G-protein coupled receptors (GPCRs) are a class of membrane proteins that trigger cellular responses to external stimuli, and are believed to be targets for nearly half of all pharmaceutical drugs on the market. However, little is known regarding their folding and cellular interactions, as well as what factors are crucial for their activity. Further structural characterization of GPCRs has largely been complicated by problems with expression, purification, and preservation of activity in vitro. Previously, we have demonstrated high-level expression (approximately 4mg/L of culture) of functional human adenosine A(2)a receptor fused to a green fluorescent protein (A(2)aR-GFP) from Saccharomyces cerevisiae. In this work, we re-engineered A(2)aR with a purification tag, developed an adequate purification scheme, and performed biophysical characterization on purified receptors. Milligram amounts per liter of culture of A(2)aR and A(2)aR-GFP were functionally expressed in S. cerevisiae, with a C-terminal deca-histidine tag. Lysis procedures were developed for optimal membrane protein solubilization and recovery through monitoring fluorescence of A(2)aR-GFP-His(10). One-step purification of the protein was achieved through immobilized metal affinity chromatography. After initial solubilization in n-dodecyl-beta-d-maltoside (DDM), a combination of added cholesterol hemisuccinate (CHS) in 3-(3-cholamidopropyl)-dimethylammoniopropane sulfonate (CHAPS) was required to stabilize the functional state of the protein. Isolated A(2)aR under these conditions was found to be largely alpha-helical, and properly incorporated into a mixed-micelle environment. The A(2)a-His(10) receptor was purified in quantities of 6+/-2mg/L of culture, with ligand-binding yields of 1mg/L, although all protein bound to xanthine affinity resin. This represents the highest purified total and functional yields for A(2)aR yet achieved from any heterologous expression system.

Heterologous GPCR expression: a bottleneck to obtaining crystal structures

G protein-coupled receptors (GPCRs) are an important, medically relevant class of integral membrane proteins. Laboratories throughout all disciplines of science devote time and energy into developing practical methods for the discovery, isolation, and characterization of these proteins. Since the crystal structure of rhodopsin was solved 6 years ago, the race to determine high-resolution structures of more GPCRs has gained momentum. Since certain GPCRs are currently produced at sufficient levels for X-ray crystallography trials, it is speculated that heterologous expression of GPCRs may no longer be a bottleneck in obtaining crystal structures. This Review focuses on the current approaches in heterologous expression of GPCRs and explores the problems associated with obtaining crystal structures from GPCRs expressed in different systems. Although milligram amounts of certain GPCRs are attainable, the majority of GPCRs are still either produced at very low levels or not at all. Developing reliable expression techniques for GPCRs is still a major priority for the structural characterization of GPCRs.