Functional analysis of heterologous GPCR signalling pathways in yeast

GPCR Biosensors to Study Conformational Dynamics and Signaling in Drug Discovery

G protein-coupled receptors (GPCRs) are a superfamily of transmembrane signal transducers that facilitate the flow of chemical signals across membranes. GPCRs are a desirable class of drug targets, and the activation and deactivation dynamics of these receptors are widely studied. Multidisciplinary approaches for studying GPCRs, such as downstream biochemical signaling assays, cryo-electron microscopy structural determinations, and molecular dynamics simulations, have provided insights concerning conformational dynamics and signaling mechanisms. However, new approaches including biosensors that use luminescence- and fluorescence-based readouts have been developed to investigate GPCR-related protein interactions and dynamics directly in cellular environments. Luminescence- and fluorescence-based readout approaches have also included the development of GPCR biosensor platforms that utilize enabling technologies to facilitate multiplexing and miniaturization. General principles underlying the biosensor platforms and technologies include scalability, orthogonality, and kinetic resolution. Further application and development of GPCR biosensors could facilitate hit identification in drug discovery campaigns. The goals of this review are to summarize developments in the field of GPCR-related biosensors and to discuss the current available technologies.

A GPCR-based yeast biosensor for biomedical, biotechnological, and point-of-use cannabinoid determination

Eukaryotic cells use G-protein coupled receptors to sense diverse signals, ranging from chemical compounds to light. Here, we exploit the remarkable sensing capacity of G-protein coupled receptors to construct yeast-based biosensors for real-life applications. To establish proof-of-concept, we focus on cannabinoids because of their neuromodulatory and immunomodulatory activities. We construct a CB₂ receptor-based biosensor, optimize it to achieve high sensitivity and dynamic range, and prove its effectiveness in three applications of increasing difficulty. First, we screen a compound library to discover agonists and antagonists. Second, we analyze 54 plants to discover a new phytocannabinoid, dugesialactone. Finally, we develop a robust portable device, analyze body-fluid samples, and confidently detect designer drugs like JWH-018. These examples demonstrate the potential of yeast-based biosensors to enable diverse applications that can be implemented by non-specialists. Taking advantage of the extensive sensing repertoire of G-protein coupled receptors, this technology can be extended to detect numerous compounds.

GPCRs are used for diverse sensing in eukaryotes. Here the authors use GPCRs to construct yeast-based biosensors, focussing on cannabinoids, and use these to screen agonists and antagonists, as well as generate a portable detection device.

Molecular origins of the multi-donor strategy in inducing bathochromic shifts and enlarging Stokes shifts of fluorescent proteins

Long-wavelength fluorescent proteins (LWFPs) and LWFP-based sensors are indispensable tools for bioimaging and biosensing applications. However, it remains challenging to develop LWFPs with outstanding brightness and/or sensitivities, largely due to the lack of simple and effective molecular design strategies. Herein, we rationalized the molecular origins of a multi-donor strategy that affords significant bathochromic shifts and large Stokes shifts with minimal structural changes in the resulting protein fluorophores. We analyzed three key factors that affect the spectral properties of these fluorophores, including the (1) substituent position, (2) electron-donating strength, and (3) number of electron-donating groups. We further demonstrated that this simple design strategy is generalizable to various fluorophore families. We expect that this work can provide rational guidelines for developing fluorescent proteins (and small-molecule fluorophores) with long emission wavelengths and large Stokes shifts.

Exploring absent protein function in yeast: assaying post translational modification and human genetic variation

Yeast is a valuable eukaryotic model organism that has evolved many processes conserved up to humans, yet many protein functions, including certain DNA and protein modifications, are absent. It is this absence of protein function that is fundamental to approaches using yeast as an in vivo test system to investigate human proteins. Functionality of the heterologous expressed proteins is connected to a quantitative, selectable phenotype, enabling the systematic analyses of mechanisms and specificity of DNA modification, post-translational protein modifications as well as the impact of annotated cancer mutations and coding variation on protein activity and interaction. Through continuous improvements of yeast screening systems, this is increasingly carried out on a global scale using deep mutational scanning approaches. Here we discuss the applicability of yeast systems to investigate absent human protein function with a specific focus on the impact of protein variation on protein-protein interaction modulation.

Engineering G protein-coupled receptor signalling in yeast for biotechnological and medical purposes

G protein-coupled receptors (GPCRs) comprise the largest class of membrane proteins in the human genome, with a common denominator of seven-transmembrane domains largely conserved among eukaryotes. Yeast is naturally armoured with three different GPCRs for pheromone and sugar sensing, with the pheromone pathway being extensively hijacked for characterising heterologous GPCR signalling in a model eukaryote. This review focusses on functional GPCR studies performed in yeast and on the elucidated hotspots for engineering, and discusses both endogenous and heterologous GPCR signalling. Key emphasis will be devoted to studies describing important engineering parameters to consider for successful coupling of GPCRs to the yeast mating pathway. We also review the various means of applying yeast for studying GPCRs, including the use of yeast armed with heterologous GPCRs as a platform for (i) deorphanisation of orphan receptors, (ii) metabolic engineering of yeast for production of bioactive products and (iii) medical applications related to pathogen detection and drug discovery. Finally, this review summarises the current challenges related to expression of functional membrane-bound GPCRs in yeast and discusses the opportunities to continue capitalising on yeast as a model chassis for functional GPCR signalling studies.

G protein-coupled receptors expressed and studied in yeast. The adenosine receptor as a prime example

G protein-coupled receptors (GPCRs) are the largest class of membrane proteins with around 800 members in the human genome/proteome. Extracellular signals such as hormones and neurotransmitters regulate various biological processes via GPCRs, with GPCRs being the bodily target of 30-40% of current drugs on the market. Complete identification and understanding of GPCR functionality will provide opportunities for novel drug discovery. Yeast expresses three different endogenous GPCRs regulating pheromone and sugar sensing, with the pheromone pathway offering perspectives for the characterization of heterologous GPCR signaling. Moreover, yeast offers a ''null" background for studies on mammalian GPCRs, including GPCR activation and signaling, ligand identification, and characterization of disease-related mutations. This review focuses on modifications of the yeast pheromone signaling pathway for functional GPCR studies, and on opportunities and usage of the yeast system as a platform for human GPCR studies. Finally, this review discusses in some further detail studies of adenosine receptors heterologously expressed in yeast, and what Geoff Burnstock thought of this approach.

Engineering a Model Cell for Rational Tuning of GPCR Signaling

G protein-coupled receptor (GPCR) signaling is the primary method eukaryotes use to respond to specific cues in their environment. However, the relationship between stimulus and response for each GPCR is difficult to predict due to diversity in natural signal transduction architecture and expression. Using genome engineering in yeast, we constructed an insulated, modular GPCR signal transduction system to study how the response to stimuli can be predictably tuned using synthetic tools. We delineated the contributions of a minimal set of key components via computational and experimental refactoring, identifying simple design principles for rationally tuning the dose response. Using five different GPCRs, we demonstrate how this enables cells and consortia to be engineered to respond to desired concentrations of peptides, metabolites, and hormones relevant to human health. This work enables rational tuning of cell sensing while providing a framework to guide reprogramming of GPCR-based signaling in other systems.

GPCRs from fusarium graminearum detection, modeling and virtual screening - the search for new routes to control head blight disease

BACKGOUND

Fusarium graminearum (FG) is one of the major cereal infecting pathogens causing high economic losses worldwide and resulting in adverse effects on human and animal health. Therefore, the development of new fungicides against FG is an important issue to reduce cereal infection and economic impact. In the strategy for developing new fungicides, a critical step is the identification of new targets against which innovative chemicals weapons can be designed. As several G-protein coupled receptors (GPCRs) are implicated in signaling pathways critical for the fungi development and survival, such proteins could be valuable efficient targets to reduce Fusarium growth and therefore to prevent food contamination.

RESULTS

In this study, GPCRs were predicted in the FG proteome using a manually curated pipeline dedicated to the identification of GPCRs. Based on several successive filters, the most appropriate GPCR candidate target for developing new fungicides was selected. Searching for new compounds blocking this particular target requires the knowledge of its 3D-structure. As no experimental X-Ray structure of the selected protein was available, a 3D model was built by homology modeling. The model quality and stability was checked by 100 ns of molecular dynamics simulations. Two stable conformations representative of the conformational families of the protein were extracted from the 100 ns simulation and were used for an ensemble docking campaign. The model quality and stability was checked by 100 ns of molecular dynamics simulations previously to the virtual screening step. The virtual screening step comprised the exploration of a chemical library with 11,000 compounds that were docked to the GPCR model. Among these compounds, we selected the ten top-ranked nontoxic molecules proposed to be experimentally tested to validate the in silico simulation.

CONCLUSIONS

This study provides an integrated process merging genomics, structural bioinformatics and drug design for proposing innovative solutions to a world wide threat to grain producers and consumers.

Receptor Activity-modifying Protein-directed G Protein Signaling Specificity for the Calcitonin Gene-related Peptide Family of Receptors

The calcitonin gene-related peptide (CGRP) family of G protein-coupled receptors (GPCRs) is formed through the association of the calcitonin receptor-like receptor (CLR) and one of three receptor activity-modifying proteins (RAMPs). Binding of one of the three peptide ligands, CGRP, adrenomedullin (AM), and intermedin/adrenomedullin 2 (AM2), is well known to result in a Gα_s-mediated increase in cAMP. Here we used modified yeast strains that couple receptor activation to cell growth, via chimeric yeast/Gα subunits, and HEK-293 cells to characterize the effect of different RAMP and ligand combinations on this pathway. We not only demonstrate functional couplings to both Gα_s and Gα_q but also identify a Gα_i component to CLR signaling in both yeast and HEK-293 cells, which is absent in HEK-293S cells. We show that the CGRP family of receptors displays both ligand- and RAMP-dependent signaling bias among the Gα_s, Gα_i, and Gα_q/11 pathways. The results are discussed in the context of RAMP interactions probed through molecular modeling and molecular dynamics simulations of the RAMP-GPCR-G protein complexes. This study further highlights the importance of RAMPs to CLR pharmacology and to bias in general, as well as identifying the importance of choosing an appropriate model system for the study of GPCR pharmacology.

Evidence of a role for S. cerevisiae α-arrestin Art1 (Ldb19) in mating projection and zygote formations

The discovery of arrestin-mediated biased signalling mechanisms for mammalian G-protein coupled receptors (GPCRs) has revolutionized the field over the last decade. Now, with the recent demonstration of a role for α-arrestins in internalization of the yeast pheromone GPCR, Ste2p, the possibility of arrestin-mediated alternate GPCR functionalities in yeast also follows. Here, the effects of knockout and complementation of yeast α-arrestin expression during mating are reported. Although minor effects on classical pheromone-related signalling are noted for a few arrestins, much stronger effects were observed downstream of cell cycle arrest, in particular linking Ldb19 (Art1) to mediation of zygote formation. Subsequent phenotypic observations linked this activity to more pronounced projection formation in an Art1 complemented noncuple α-arrestin knockout line, compared to the knockout-line alone, or either of the Art3 or Art6 complemented lines. Together with the observation of ligand-stimulated localization of Art-GFP to the mating projection, a possible role for this arrestin-like protein in projection formation is supported. While leaving the full mechanism of alternate Art1 functionality to be elucidated, together these findings implicate Art1 in selective regulation of mating events downstream of receptor internalization and cell cycle arrest, leading to schmoo, and ultimately zygote formation.

Quantification of mutation-derived bias for alternate mating functionalities of the Saccharomyces cerevisiae Ste2p pheromone receptor

Although well documented for mammalian G-protein-coupled receptors, alternate functionalities and associated alternate signalling remain to be unequivocally established for the Saccharomyces cerevisiae pheromone Ste2p receptor. Here, evidence supporting alternate functionalities for Ste2p is re-evaluated, extended and quantified. In particular, strong mating and constitutive signalling mutations, focusing on residues S254, P258 and S259 in TM6 of Ste2p, are stacked and investigated in terms of their effects on classical G-protein-mediated signal transduction associated with cell cycle arrest, and alternatively, their impact on downstream mating projection and zygote formation events. In relative dose response experiments, accounting for systemic and observational bias, mutational-derived functional differences were observed, validating the S254L-derived bias for downstream mating responses and highlighting complex relationships between TM6-mutation derived constitutive signalling and ligand-induced functionalities. Mechanistically, localization studies suggest that alterations to receptor trafficking may contribute to mutational bias, in addition to expected receptor conformational stabilization effects. Overall, these results extend previous observations and quantify the contributions of Ste2p variants to mediating cell cycle arrest versus downstream mating functionalities.

Novobiocin and peptide analogs of α-factor are positive allosteric modulators of the yeast G protein-coupled receptor Ste2p

G protein-coupled receptors (GPCRs) are the target of many drugs prescribed for human medicine and are therefore the subject of intense study. It has been recognized that compounds called allosteric modulators can regulate GPCR activity by binding to the receptor at sites distinct from, or overlapping with, that occupied by the orthosteric ligand. The purpose of this study was to investigate the nature of the interaction between putative allosteric modulators and Ste2p, a model GPCR expressed in the yeast Saccharomyces cerevisiae that binds the tridecapeptide mating pheromone α-factor. Biological assays demonstrated that an eleven amino acid α-factor analog and the antibiotic novobiocin were positive allosteric modulators of Ste2p. Both compounds enhanced the biological activity of α-factor, but did not compete with α-factor binding to Ste2p. To determine if novobiocin and the 11-mer shared a common allosteric binding site, a biologically-active analog of the 11-mer peptide ([Bio-DOPA]11-mer) was chemically cross-linked to Ste2p in the presence and absence of novobiocin. Immunoblots probing for the Ste2p-[Bio-DOPA]11-mer complex revealed that novobiocin markedly decreased cross-linking of the [Bio-DOPA]11-mer to the receptor, but cross-linking of the α-factor analog [Bio-DOPA]13-mer, which interacts with the orthosteric binding site of the receptor, was minimally altered. This finding suggests that both novobiocin and [Bio-DOPA]11-mer compete for an allosteric binding site on the receptor. These results indicate that Ste2p may provide an excellent model system for studying allostery in a GPCR.

Functional coupling of a nematode chemoreceptor to the yeast pheromone response pathway

Sequencing of the Caenorhabditis elegans genome revealed sequences encoding more than 1,000 G-protein coupled receptors, hundreds of which may respond to volatile organic ligands. To understand how the worm's simple olfactory system can sense its chemical environment there is a need to characterise a representative selection of these receptors but only very few receptors have been linked to a specific volatile ligand. We therefore set out to design a yeast expression system for assigning ligands to nematode chemoreceptors. We showed that while a model receptor ODR-10 binds to C. elegans Gα subunits ODR-3 and GPA-3 it cannot bind to yeast Gα. However, chimaeras between the nematode and yeast Gα subunits bound to both ODR-10 and the yeast Gβγ subunits. FIG2 was shown to be a superior MAP-dependent promoter for reporter expression. We replaced the endogenous Gα subunit (GPA1) of the Saccharomyces cerevisiae (ste2Δ sst2Δ far1Δ) triple mutant ("Cyb") with a Gpa1/ODR-3 chimaera and introduced ODR-10 as a model nematode GPCR. This strain showed concentration-dependent activation of the yeast MAP kinase pathway in the presence of diacetyl, the first time that the native form of a nematode chemoreceptor has been functionally expressed in yeast. This is an important step towards en masse de-orphaning of C. elegans chemoreceptors.

Use of PKA-mediated phenotypes for genetic and small-molecule screens in Schizosaccharomyces pombe

PKA (protein kinase A) in the fission yeast Schizosaccharomyces pombe controls transcription of genes involved in metabolism, cell growth and sexual development. In the present review, we discuss phenotypes associated with either high or low PKA activity in the context of how they can be used to carry out genetic or small-molecule screens that affect components of the PKA pathway. Although our recent research has focused on the study of heterologously expressed cyclic nucleotide PDEs (phosphodiesterases), these same methods can be used to target other S. pombe proteins or their functionally equivalent orthologues that act in the PKA pathway.

Investigating G protein signalling bias at the glucagon-like peptide-1 receptor in yeast

BACKGROUND AND PURPOSE

The glucagon-like peptide 1 (GLP-1) receptor performs an important role in glycaemic control, stimulating the release of insulin. It is an attractive target for treating type 2 diabetes. Recently, several reports of adverse side effects following prolonged use of GLP-1 receptor therapies have emerged: most likely due to an incomplete understanding of signalling complexities.

EXPERIMENTAL APPROACH

We describe the expression of the GLP-1 receptor in a panel of modified yeast strains that couple receptor activation to cell growth via single Gα/yeast chimeras. This assay enables the study of individual ligand-receptor G protein coupling preferences and the quantification of the effect of GLP-1 receptor ligands on G protein selectivity.

KEY RESULTS

The GLP-1 receptor functionally coupled to the chimeras representing the human Gαs, Gαi and Gαq subunits. Calculation of the dissociation constant for a receptor antagonist, exendin-3 revealed no significant difference between the two systems. We obtained previously unobserved differences in G protein signalling bias for clinically relevant therapeutic agents, liraglutide and exenatide; the latter displaying significant bias for the Gαi pathway. We extended the use of the system to investigate small-molecule allosteric compounds and the closely related glucagon receptor.

CONCLUSIONS AND IMPLICATIONS

These results provide a better understanding of the molecular events involved in GLP-1 receptor pleiotropic signalling and establish the yeast platform as a robust tool to screen for more selective, efficacious compounds acting at this important class of receptors in the future.

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.

Bright fluorescence monitoring system utilizing Zoanthus sp. green fluorescent protein (ZsGreen) for human G-protein-coupled receptor signaling in microbial yeast cells

G-protein-coupled receptors (GPCRs) are currently the most important pharmaceutical targets for drug discovery because they regulate a wide variety of physiological processes. Consequently, simple and convenient detection systems for ligands that regulate the function of GPCR have attracted attention as powerful tools for new drug development. We previously developed a yeast-based fluorescence reporter ligand detection system using flow cytometry. However, using this conventional detection system, fluorescence from a cell expressing GFP and responding to a ligand is weak, making detection of these cells by fluorescence microscopy difficult. We here report improvements to the conventional yeast fluorescence reporter assay system resulting in the development of a new highly-sensitive fluorescence reporter assay system with extremely bright fluorescence and high signal-to-noise (S/N) ratio. This new system allowed the easy detection of GPCR signaling in yeast using fluorescence microscopy. Somatostatin receptor and neurotensin receptor (implicated in Alzheimer's disease and Parkinson's disease, respectively) were chosen as human GPCR(s). The facile detection of binding to these receptors by cognate peptide ligands was demonstrated. In addition, we established a highly sensitive ligand detection system using yeast cell surface display technology that is applicable to peptide screening, and demonstrate that the display of various peptide analogs of neurotensin can activate signaling through the neurotensin receptor in yeast cells. Our system could be useful for identifying lead peptides with agonistic activity towards targeted human GPCR(s).

The role of the RACK1 ortholog Cpc2p in modulating pheromone-induced cell cycle arrest in fission yeast

The detection and amplification of extracellular signals requires the involvement of multiple protein components. In mammalian cells the receptor of activated C kinase (RACK1) is an important scaffolding protein for signal transduction networks. Further, it also performs a critical function in regulating the cell cycle by modulating the G₁/S transition. Many eukaryotic cells express RACK1 orthologs, with one example being Cpc2p in the fission yeast Schizosaccharomyces pombe. In contrast to RACK1, Cpc2p has been described to positively regulate, at the ribosomal level, cells entry into M phase. In addition, Cpc2p controls the stress response pathways through an interaction with Msa2p, and sexual development by modulating Ran1p/Pat1p. Here we describe investigations into the role, which Cpc2p performs in controlling the G protein-mediated mating response pathway. Despite structural similarity to Gβ-like subunits, Cpc2p appears not to function at the G protein level. However, upon pheromone stimulation, cells overexpressing Cpc2p display substantial cell morphology defects, disorientation of septum formation and a significantly protracted G₁ arrest. Cpc2p has the potential to function at multiple positions within the pheromone response pathway. We provide a mechanistic interpretation of this novel data by linking Cpc2p function, during the mating response, with its previous described interactions with Ran1p/Pat1p. We suggest that overexpressing Cpc2p prolongs the stimulated state of pheromone-induced cells by increasing ste11 gene expression. These data indicate that Cpc2p regulates the pheromone-induced cell cycle arrest in fission yeast by delaying cells entry into S phase.

Quantitative analysis of human ras localization and function in the fission yeast Schizosaccharomyces pombe

Ras signalling is central to fundamental and diverse cellular processes. In higher eukaryotes ras signalling is highly complex, involving multiple isoforms, regulatory proteins and effectors. As a consequence, the study of ras activity in mammalian systems presents a number of technical challenges. The model organism Schizosaccharomyces pombe has previously proved a key system for the study of human signalling components and provides an ideal model for the study of ras, as it contains just one ras protein (Ras1p), which is non-essential and controls a number of downstream processes. Here we present data demonstrating the quantitative analysis of three distinct Ras1-related signalling outputs, utilizing the three most abundant human ras isoforms, H-Ras, N-Ras and K-Ras4B, in Sz. pombe. Further, we have characterized the localization of these three human ras isoforms in Sz. pombe, utilizing quantitative image analysis techniques. These data indicate that all three human ras isoforms are functional in fission yeast, displaying differing localization patterns which correlate strongly with function in the regulation of pheromone response and cell shape. These data demonstrate that such yeast strains could provide powerful tools for the investigation of ras biology, and potentially in the development of cancer therapies.

Membrane-displayed somatostatin activates somatostatin receptor subtype-2 heterologously produced in Saccharomyces cerevisiae

The G-protein-coupled receptor (GPCR) superfamily, which includes somatostatin receptors (SSTRs), is one of the most important drug targets in the pharmaceutical industry. The yeast Saccharomyces cerevisiae is an attractive host for the ligand screening of human GPCRs. Here, we demonstrate the utility of the technology that was developed for displaying peptide ligands on yeast plasma membrane, termed "PepDisplay", which triggers signal transduction upon GPCR activation. A yeast strain that heterologously produced human somatostatin receptor subtype-2 (SSTR2) and chimeric Gα protein was constructed along with membrane-displayed somatostatin; somatostatin was displayed on the yeast plasma membrane by linking it to the anchoring domain of the glycosylphosphatidylinositol anchored plasma membrane protein Yps1p. We demonstrate that the somatostatin displayed on the plasma membrane successfully activated human SSTR2 in S. cerevisiae. The methodology presented here provides a new platform for identifying novel peptide ligands for both liganded and orphan mammalian GPCRs.

Functional expression and characterization of the C. elegans G-protein-coupled FLP-2 Receptor (T19F4.1) in mammalian cells and yeast

Profound neuropeptide diversity characterizes the nematode nervous system, but it has proven challenging to match neuropeptide G protein-coupled receptors (GPCR) with their cognate ligands in heterologous systems. We have expressed the Caenorhabditis elegans GPCR encoded in the locus T19F4.1, previously matched with FMRFamide-like peptides encoded on the flp-2 precursor gene, in mammalian cells and in the yeast Saccharomyces cerevisiae. Pharmacological characterization revealed that the receptor is potently activated by flp-2 peptides in CHO cells (∼10 nM EC50) and in yeast (∼100 nM EC50), signaling through a Gqα pathway in each system. The yeast GPCR expression system provides a robust assay for screening for agonists of the flp-2 receptor and is the target of an ongoing high-throughput screening exercise.

Untangling the Roles of Anti-Apoptosis in Regulating Programmed Cell Death using Humanized Yeast Cells

Genetically programmed cell death (PCD) mechanisms, including apoptosis, are important for the survival of metazoans since it allows, among things, the removal of damaged cells that interfere with normal function. Cell death due to PCD is observed in normal processes such as aging and in a number of pathophysiologies including hypoxia (common causes of heart attacks and strokes) and subsequent tissue reperfusion. Conversely, the loss of normal apoptotic responses is associated with the development of tumors. So far, limited success in preventing unwanted PCD has been reported with current therapeutic approaches despite the fact that inhibitors of key apoptotic inducers such as caspases have been developed. Alternative approaches have focused on mimicking anti-apoptotic processes observed in cells displaying increased resistance to apoptotic stimuli. Hormesis and pre-conditioning are commonly observed cellular strategies where sub-lethal levels of pro-apoptotic stimuli lead to increased resistance to higher or lethal levels of stress. Increased expression of anti-apoptotic sequences is a common mechanism mediating these protective effects. The relevance of the latter observation is exemplified by the observation that transgenic mice overexpressing anti-apoptotic genes show significant reductions in tissue damage following ischemia. Thus strategies aimed at increasing the levels of anti-apoptotic proteins, using gene therapy or cell penetrating recombinant proteins are being evaluated as novel therapeutics to decrease cell death following acute periods of cell death inducing stress. In spite of its functional and therapeutic importance, more is known regarding the processes involved in apoptosis than anti-apoptosis. The genetically tractable yeast Saccharomyces cerevisiae has emerged as an exceptional model to study multiple aspects of PCD including the mitochondrial mediated apoptosis observed in metazoans. To increase our knowledge of the process of anti-apoptosis, we screened a human heart cDNA expression library in yeast cells undergoing PCD due to the conditional expression of a mammalian pro-apoptotic Bax cDNA. Analysis of the multiple Bax suppressors identified revealed several previously known as well as a large number of clones representing potential novel anti-apoptotic sequences. The focus of this review is to report on recent achievements in the use of humanized yeast in genetic screens to identify novel stress-induced PCD suppressors, supporting the use of yeast as a unicellular model organism to elucidate anti-apoptotic and cell survival mechanisms.

Membrane-displayed peptide ligand activates the pheromone response pathway in Saccharomyces cerevisiae

The budding yeast, Saccharomyces cerevisiae, is an attractive host for studying G protein-coupled receptors (GPCRs). We developed a system in which a peptide ligand specific for GPCR is displayed on yeast plasma membrane. The model system described here is based on yeast plasma membrane display of an analogue of α-factor, which is a peptide ligand for Ste2p, the GPCR that activates the yeast pheromone response pathway. α-Factor analogues, containing linkers of varying lengths and produced in yeast cells, became attached to the cell plasma membrane by linking to the glycosylphosphatidylinositol (GPI)-anchored plasma membrane protein Yps1p. We were able to demonstrate that an optimized α-factor analogue activated the pheromone response pathway in S. cerevisiae, as assessed by a fluorescent reporter assay. Furthermore, it was shown that linker length strongly influenced signalling pathway activation. To our knowledge, this is the first report documenting functional signalling by a plasma membrane-displayed ligand in S. cerevisiae.

Interaction between pheromone and its receptor of the fission yeast Schizosaccharomyces pombe examined by a force spectroscopy study

Interaction between P-factor, a peptide pheromone composed of 23 amino acid residues, and its pheromone receptor, Mam2, on the cell surface of the fission yeast Schizosaccharomyces pombe was examined by an atomic force microscope (AFM). An AFM tip was modified with P-factor derivatives to perform force curve measurements. The specific interaction force between P-factor and Mam2 was calculated to be around 120 pN at a probe speed of 1.74 μm/s. When the AFM tip was modified with truncated P-factor derivative lacking C-terminal Leu, the specific interaction between the tip and the cell surface was not observed. These results were also confirmed with an assay system using a green fluorescent protein (GFP) reporter gene to monitor the activation level of signal transduction following the interaction of Mam2 with P-factor.

The N-terminal replacement of an olfactory receptor for the development of a yeast-based biomimetic odor sensor

For the development of a biomimetic odor-sensing system, we investigated the effects of replacing the N-terminus of an olfactory receptor (OR) on its functional expression in the budding yeast, Saccharomyces cerevisiae. Using the mouse olfactory receptor OR226 (mOR226), three types of chimeric ORs were constructed by replacing N-terminal regions of mOR226 with the corresponding regions of the rat I7 receptor, which is known to be functionally expressed in yeast. The replacement of the N-terminal region of mOR226 dramatically affected the expression and localization of the receptor and improved the sensing ability of the yeast cells for the odorant. Furthermore, the replacement of the endogenous yeast G-protein α subunit (Gpa1) by the OR-specific G(olf) drastically elevated the odorant-sensing ability of the yeast cells and caused the cells to display a dose-dependent responsiveness to the odorant. Because of the suitability of yeast cells for screening large-scale libraries, the strategy presented here would be useful for the establishment of advanced biomimetic odor-sensing systems.

Evidence that prokineticin receptor 2 exists as a dimer in vivo

Prokineticins are proteins that regulate diverse biological processes including gastrointestinal motility, angiogenesis, circadian rhythm, and innate immune response. Prokineticins bind two closed related G-protein coupled receptors (GPCRs), PKR1 and PKR2. In general, these receptors act as molecular switches to relay activation to heterotrimeric G-proteins and a growing body of evidence points to the fact that GPCRs exist as homo- or heterodimers. We show here by Western-blot analysis that PKR2 has a dimeric structure in neutrophils. By heterologous expression of PKR2 in Saccharomyces cerevisiae, we examined the mechanisms of intermolecular interaction of PKR2 dimerization. The potential involvement of three types of mechanisms was investigated: coiled-coil, disulfide bridges, and hydrophobic interactions between transmembrane domains. Characterization of differently deleted or site-directed PKR2 mutants suggests that dimerization proceeds through interactions between transmembrane domains. We demonstrate that co-expressing binding-deficient and signaling-deficient forms of PKR2 can re-establish receptor functionality, possibly through a domain-swapping mechanism.

Solution- and solid-state NMR studies of GPCRs and their ligands

G protein-coupled receptors (GPCRs) represent one of the major targets of new drugs on the market given their roles as key membrane receptors in many cellular signalling pathways. Structure-based drug design has potential to be the most reliable method for novel drug discovery. Unfortunately, GPCR-ligand crystallisation for X-ray diffraction studies is very difficult to achieve. However, solution- and solid-state NMR approaches have been developed and have provided new insights, particularly focussing on the study of protein-ligand interactions which are vital for drug discovery. This review provides an introduction for new investigators of GPCRs/ligand interactions using NMR spectroscopy. The guidelines for choosing a system for efficient isotope labelling of GPCRs and their ligands for NMR studies will be presented, along with an overview of the different sample environments suitable for generation of high resolution structural information from NMR spectra.

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.

The Gα subunit signals through the Ste50 protein during the mating pheromone response in the yeast Kluyveromyces lactis

Yeast mating signal transduction pathways require a heterotrimeric G protein composed of Gα, Gβ, and Gγ subunits connected to a mitogen-activated protein kinase (MAPK) module. While in Saccharomyces cerevisiae elimination of Gα induces constitutive activation of the mating pathway, in Kluyveromyces lactis it produces partial sterility, which indicates that K. lactis Gα (KlGα) is required to positively activate mating. We use physical interaction experiments to determine that KlGα interacts with the adaptor protein KlSte50p. The Ras association (RA) domain of KlSte50p favored interaction with the GDP-bound KlGα subunit, and when the KlGα protein is constitutively activated, the interaction drops significantly. Additionally, KlSte50p strongly associates with the MAPK kinase kinase (MAPKKK) KlSte11p through its sterile alpha motif (SAM) domain. Genetic experiments placed KlSte50p downstream of the G protein α subunit, indicating that KlGα may stimulate the mating pathway via KlSte50p. Fusion of KlSte50p to the KlGβ subunit partially eliminated the requirement of KlGα for mating, indicating that one contribution of KlGα to the mating pathway is to facilitate plasma membrane anchoring of KlSte50p.

Hydrophobicity profiles in G protein-coupled receptor transmembrane helical domains

The lack of a crystallographically derived structure for all but three G (TP [guanosine triphosphate]-binding) protein-coupled receptor (GPCRs) proteins necessitates the use of computationally derived methods to determine their structures. Computational methodologies allow a mechanistic glimpse into GPCR-ligand interactions at a molecular level to better understand the initial steps leading to a protein's biologic functions, ie, protecting the ligands that activate, deactivate, or inhibit the protein, stabilizing protein structure in the membrane's lipid bilayer, and ensuring that the hydrophilic environment within the GPCR-binding pocket is maintained. Described here is a formalism that quantifies the amphiphilic nature of a helix, by determining the effective hydrophobicity (or hydrophilicity) at specific positions around it. This formalism will enable computational protein modelers to position helices so that the functional aspects of GPCRs are adequately represented in the model. Hydro-Eff®, an online tool, allows users to calculate effective helical hydrophobicities.

Generation of designer receptors exclusively activated by designer drugs (DREADDs) using directed molecular evolution

G protein-coupled receptors (GPCRs) and their signal transductions are important for both physiological and pathological processes in neuron systems. Neuronal GPCRs activated by synthetic ligands have been created by designed mutagenesis for studying their functions and signal pathways. However, these engineered GPCRs have problems, such as their high constitutive activity. To overcome this drawback, a new generation of receptors termed designer receptors exclusively activated by designer drugs (DREADDs), have been designed. DREADDs are exclusively activated by synthetic ligands, but are insensitive to their endogenous ligand and have no constitutive activity, which provides the ability to selectively modulate signal transduction of certain GPCRs in vitro and in vivo. This protocol provides detailed instructions for creating DREADDs using directed molecular evolution. The procedures to generate DREADDS include GPCR functional expression in yeast, mutant GPCR library generation, and high-throughput yeast screening. These methods are general and suitable for any GPCRs that can be functionally expressed in yeast.

The beta subunit of the heterotrimeric G protein triggers the Kluyveromyces lactis pheromone response pathway in the absence of the gamma subunit

The Kluyveromyces lactis heterotrimeric G protein is a canonical Galphabetagamma complex; however, in contrast to Saccharomyces cerevisiae, where the Ggamma subunit is essential for mating, disruption of the KlGgamma gene yielded cells with almost intact mating capacity. Expression of a nonfarnesylated Ggamma, which behaves as a dominant-negative in S. cerevisiae, did not affect mating in wild-type and DeltaGgamma cells of K. lactis. In contrast to the moderate sterility shown by the single DeltaKlGalpha, the double DeltaKlGalpha DeltaKlGgamma mutant displayed full sterility. A partial sterile phenotype of the DeltaKlGgamma mutant was obtained in conditions where the KlGbeta subunit interacted defectively with the Galpha subunit. The addition of a CCAAX motif to the C-end of KlGbeta, partially suppressed the lack of both KlGalpha and KlGgamma subunits. In cells lacking KlGgamma, the KlGbeta subunit cofractionated with KlGalpha in the plasma membrane, but in the DeltaKlGalpha DeltaKlGgamma strain was located in the cytosol. When the KlGbeta-KlGalpha interaction was affected in the DeltaKlGgamma mutant, most KlGbeta fractionated to the cytosol. In contrast to the generic model of G-protein function, the Gbeta subunit of K. lactis has the capacity to attach to the membrane and to activate mating effectors in absence of the Ggamma subunit.