Sexual conjugation in yeast: A paradigm to study G-protein-coupled receptor domain structure

Expression, Purification and Refolding of a Human NaV1.7 Voltage Sensing Domain with Native-like Toxin Binding Properties

The voltage-gated sodium channel Na_V1.7 is an important target for drug development due to its role in pain perception. Recombinant expression of full-length channels and their use for biophysical characterization of interactions with potential drug candidates is challenging due to the protein size and complexity. To overcome this issue, we developed a protocol for the recombinant expression in E. coli and refolding into lipids of the isolated voltage sensing domain (VSD) of repeat II of Na_V1.7, obtaining yields of about 2 mg of refolded VSD from 1 L bacterial cell culture. This VSD is known to be involved in the binding of a number of gating-modifier toxins, including the tarantula toxins ProTx-II and GpTx-I. Binding studies using microscale thermophoresis showed that recombinant refolded VSD binds both of these toxins with dissociation constants in the high nM range, and their relative binding affinities reflect the relative IC₅₀ values of these toxins for full-channel inhibition. Additionally, we expressed mutant VSDs incorporating single amino acid substitutions that had previously been shown to affect the activity of ProTx-II on full channel. We found decreases in GpTx-I binding affinity for these mutants, consistent with a similar binding mechanism for GpTx-I as compared to that of ProTx-II. Therefore, this recombinant VSD captures many of the native interactions between Na_V1.7 and tarantula gating-modifier toxins and represents a valuable tool for elucidating details of toxin binding and specificity that could help in the design of non-addictive pain medication acting through Na_V1.7 inhibition.

An expression and purification system for the biosynthesis of adenosine receptor peptides for biophysical and structural characterization

Biophysical and structural characterization of G protein-coupled receptors (GPCRs) has been limited due to difficulties in expression, purification, and vitro stability of the full-length receptors. "Divide and conquer" approaches aimed at the NMR characterization of peptides corresponding to specific regions of the receptor have yielded insights into the structure and dynamics of GPCR activation and signaling. Though significant progress has been made in the generation of peptides that are composed of GPCR transmembrane domains, current methods utilize fusion protein strategies that require chemical cleavage and peptide separation via chromatographic means. We have developed an expression and purification system based on fusion to ketosteroid isomerase, thrombin cleavage, and tandem affinity chromatography that enables the solubilization, cleavage, and characterization in a single detergent system relevant for biophysical and structural characterization. We have applied this expression and purification system to the production and characterization of peptides of the adenosine receptor family of GPCRs in Escherichia coli. Herein, we demonstrate using a model peptide that includes extracellular loop 3, transmembrane domain 7, and a portion of the carboxy-terminus of the adenosine A(2)a receptor that the peptide is sufficiently pure for biophysical characterization, where it adopts α-helical structure. Furthermore, we demonstrate the utility of this system by optimizing the construct for thrombin processing and apply the system to peptides with more complex structures.

Mistic and TarCF as fusion protein partners for functional expression of the cannabinoid receptor 2 in Escherichia coli

G protein coupled receptors (GPCRs) are key players in signal recognition and cellular communication making them important therapeutic targets. Large-scale production of these membrane proteins in their native form is crucial for understanding their mechanism of action and target-based drug design. Here we report the overexpression system for a GPCR, the cannabinoid receptor subtype 2 (CB2), in Escherichia coli C43(DE3) facilitated by two fusion partners: Mistic, an integral membrane protein expression enhancer at the N-terminal, and TarCF, a C-terminal fragment of the bacterial chemosensory transducer Tar at the C-terminal of the CB2 open reading frame region. Multiple histidine tags were added on both ends of the fusion protein to facilitate purification. Using individual and combined fusion partners, we found that CB2 fusion protein expression was maximized only when both partners were used. Variable growth and induction conditions were conducted to determine and optimize protein expression. More importantly, this fusion protein Mistic-CB2-TarCF can localize into the E. coli membrane and exhibit functional binding activities with known CB2 ligands including CP55,940, WIN55,212-2 and SR144,528. These results indicate that this novel expression and purification system provides us with a promising strategy for the preparation of biologically active GPCRs, as well as general application for the preparation of membrane-bound proteins using the two new fusion partners described.

Large multiple transmembrane domain fragments of a G protein-coupled receptor: biosynthesis, purification, and biophysical studies

To conduct biophysical analyses on large domains of GPCRs, multimilligram quantities of highly homogeneous proteins are necessary. This communication discusses the biosynthesis of four transmembrane and five transmembrane-containing fragments of Ste2p, a GPCR recognizing the Saccharomyces cerevisiae tridecapeptide pheromone α-factor. The target fragments contained the predicted four N-terminal Ste2p[G(31) -A(198) ] (4TMN), four C-terminal Ste2p[T(155) -L(340) ] (4TMC), or five C-terminal Ste2p[I(120) -L(340) ] (5TMC) transmembrane segments of Ste2p. 4TMN was expressed as a fusion protein using a modified pMMHa vector in L-arabinose-induced Escherichia coli BL21-AI, and cleaved with cyanogen bromide. 4TMC and 5TMC were obtained by direct expression using a pET21a vector in IPTG-induced E. coli BL21(DE3) cells. 4TMC and 5TMC were biosynthesized on a preparative scale, isolated in multimilligram amounts, characterized by MS and investigated by biophysical methods. CD spectroscopy indicated the expected highly α-helical content for 4TMC and 5TMC in membrane mimetic environments. Tryptophan fluorescence showed that 5TMC integrated into the nonpolar region of 1-stearoyl-2-hydroxy-sn-glycero-3-phospho-(1'-rac-glycerol) micelles. HSQC-TROSY investigations revealed that [(15) N]-labeled 5TMC in 50% trifluoroethanol-d(2) /H(2) O/0.05%-trifluoroacetic acid was stable enough to conduct long multidimensional NMR measurements. The entire Ste2p GPCR was not readily reconstituted from the first two and last five or first three and last four transmembrane domains.

Synthesis of a-factor peptide from Saccharomyces cerevisiae and photoactive analogues via Fmoc solid phase methodology

a-Factor from Saccharomyces cerevisiae is a farnesylated dodecapeptide involved in mating. The molecule binds to a G-protein coupled receptor and hence serves as a simple system for studying the interactions between prenylated molecules and their cognate receptors. Here, we describe the preparation of a-factor and two photoactive analogues via Fmoc solid-phase peptide synthesis using hydrazinobenzoyl AM NovaGel™ resin; the structure of the synthetic a-factor was confirmed by MS-MS analysis and NMR; the structures of the analogues were confirmed by MS-MS analysis. Using a yeast growth arrest assay, the analogues were found to have activity comparable to a-factor itself.

Scanning mutagenesis of regions in the Galpha protein Gpa1 that are predicted to interact with yeast mating pheromone receptors

The mechanism by which receptors activate heterotrimeric G proteins was examined by scanning mutagenesis of the Saccharomyces cerevisiae pheromone-responsive Galpha protein (Gpa1). The juxtaposition of high-resolution structures for rhodopsin and its cognate G protein transducin predicted that at least six regions of Galpha are in close proximity to the receptor. Mutagenesis was targeted to residues in these domains in Gpa1, which included four loop regions (beta2-beta3, alpha2-beta4, alpha3-beta5, and alpha4-beta6) as well as the N and C termini. The mutants displayed a range of phenotypes from nonsignaling to constitutive activation of the pheromone pathway. The constitutive activity of some mutants could be explained by decreased production of Gpa1, which permits unregulated signaling by Gbetagamma. However, the constitutive activity caused by the F344C and E335C mutations in the alpha2-beta4 loop and F378C in the alpha3-beta5 loop was not due to decreased protein levels, and was apparently due to defects in sequestering Gbetagamma. The strongest loss of the function mutant, which was not detectably induced by a pheromone, was caused by a K314C substitution in the beta2-beta3 loop. Several other mutations caused weak signaling phenotypes. Altogether, these results suggest that residues in different interface regions of Galpha contribute to activation of signaling.

Affinity purification and characterization of a G-protein coupled receptor, Saccharomyces cerevisiae Ste2p

We present an example of expression and purification of a biologically active G-protein coupled receptor (GPCR) from yeast. An expression vector was constructed to encode the Saccharomyces cerevisiae GPCR alpha-factor receptor (Ste2p, the STE2 gene product) containing a 9-amino acid sequence of rhodopsin that served as an epitope/affinity tag. In the construct, two glycosylation sites and two cysteine residues were removed to aid future structural and functional studies. The receptor was expressed in yeast cells and was detected as a single band in a western blot indicating the absence of glycosylation. Ligand binding and signaling assays of the epitope-tagged, mutated receptor showed it maintained the full wild-type biological activity. For extraction of Ste2p, yeast membranes were solubilized with 0.5% n-dodecyl maltoside (DM). Approximately 120 microg of purified alpha-factor receptor was obtained per liter of culture by single-step affinity chromatography using a monoclonal antibody to the rhodopsin epitope. The binding affinity (K(d)) of the purified alpha-factor receptor in DM micelles was 28 nM as compared to K(d)=12.7 nM for Ste2p in cell membranes, and approximately 40% of the purified receptor was correctly folded as judged by ligand saturation binding. About 50% of the receptor sequence was retrieved from MALDI-TOF and nanospray mass spectrometry after CNBr digestion of the purified receptor. The methods described will enable structural studies of the alpha-factor receptor and may provide an efficient technique to purify other GPCRs that have been functionally expressed in yeast.

Synthesis, biosynthesis, and characterization of transmembrane domains of a G protein-coupled receptor

Peptide fragments have been widely used in biophysical studies on specific regions of integral membrane proteins. Because of their inherent insoluble nature and tendency to aggregate the preparation of such model peptides is challenging. We have developed synthetic and biosynthetic approaches to prepare peptides containing single and multiple domains of a G protein-coupled receptor. Both the synthetic and biosynthetic products can be isolated by reversed-phase high-performance liquid chromatography to near homogeneity. The biosynthetic product, a fusion protein, is processed by CNBr cleavage to yield the target peptide in various isotopic forms. The final peptides are studied by circular dichroism spectroscopy to determine their secondary structure under a variety of conditions.

A transmembrane helix-bundle from G-protein coupled receptor CB2: Biosynthesis, purification, and NMR characterization

The cannabinoid receptor subtype 2 (CB2) is a member of the G-protein coupled receptor (GPCR) superfamily. As the relationship between structure and function for this receptor remains poorly understood, the present study was undertaken to characterize the structure of a segment including the first and second transmembrane helix (TM1 and TM2) domains of CB2. To accomplish this, a transmembrane double-helix bundle from this region was expressed, purified, and characterized by NMR. Milligrams of this hydrophobic fragment of the receptor were biosynthesized using a fusion protein overexpression strategy and purified by affinity chromatography combined with reverse phase HPLC. Chemical and enzymatic cleavage methods were implemented to remove the fusion tag. The resultant recombinant protein samples were analyzed and confirmed by HPLC, mass spectrometry, and circular dichroism (CD). The CD analyses of HPLC-purified protein in solution and in DPC micelle preparations suggested predominant alpha-helical structures under both conditions. The 13C/15N double-labeled protein CB2(27-101) was further verified and analyzed by NMR spectroscopy. Sequential assignment was accomplished for more than 80% of residues. The 15N HSQC NMR results show a clear chemical shift dispersion of the amide nitrogen-proton correlation indicative of a pure double-labeled polypeptide molecule. The results suggest that this method is capable of generating transmembrane helical bundles from GPCRs in quantity and purity sufficient for NMR and other biophysical studies. Therefore, the biosynthesis of GPCR transmembrane helix bundles represents a satisfactory alternative strategy to obtain and assemble NMR structures from recombinant "building blocks."

Biosynthesis and purification of a hydrophobic peptide from transmembrane domains of G-protein-coupled CB2 receptor

A major challenge for the structural study of the seven-transmembrane G-protein-coupled receptors is to obtain a sufficient amount of purified protein at the milligram level, which is required for either nuclear magnetic resonance (NMR) spectroscopy or X-ray crystallography. In order to develop a high-yield and cost-effective method, and also to obtain preliminary structural information for the computer modeling of the three-dimensional receptor structural model, a highly hydrophobic peptide from human cannabinoid subtype 2 receptor CB2(65-101), was chosen to develop high-yield membrane protein expression and purification methods. The peptide included the second transmembrane helix with the associated loop regions of the CB2 receptor. It was over-expressed in Escherichia coli, with a modified TrpDelta LE1413 (TrpLE) leading fusion sequence and a nine-histidine tag, and was then separated and purified from the tag in a preparative scale. An experimental protocol for the chemical cleavage of membrane protein fragment was developed using cyanogen bromide to remove the TrpLE tag from the hydrophobic fusion protein. In addition, protein uniformly labeled with isotopic 15N was obtained by expression in 15N-enriched minimum media. The developed and optimized preparation scheme of expression, cleavage, and purification provided a sufficient amount of peptide for NMR structure analysis and other biophysical studies that will be reported elsewhere. The process of fusion protein cleavage following purification was monitored by high-performance liquid chromatography (HPLC) and mass spectrometry (MS), and the final sample was validated by MS and circular dichroism experiments.

Synthetic peptides as probes for conformational preferences of domains of membrane receptors

Peptide models have been widely used to investigate conformational aspects of domains of proteins since the early 1950s. A pioneer in this field was Dr. Murray Goodman, who applied a battery of methodologies to study the onset of structure in homooligopeptides. This article reviews some of Dr. Goodman's contributions, and reports recent studies using linear and constrained peptides corresponding to the first extracellular loop and linear peptides corresponding to the sixth transmembrane domain of a G-protein coupled receptor from the yeast Saccharomyces cerevisiae. Peptides containing 30-40 residues were synthesized using solid-phase methods and purified to near homogeneity by reversed phase high performance liquid chromatography. CD and NMR analyses indicated that the first extracellular loop peptides were mostly flexible in water, and assumed some helical structure near the N-terminus in trifluoroethanol and in the presence of micelles. Comparison of oligolysines with native loop residues revealed that three lysines at each terminus of a peptide corresponding to the sixth transmembrane domain of the alpha-factor receptor resulted in better aqueous solubility and greater helicity than the native loop residues.