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1.  Allosteric Modulation of G Protein Coupled Receptors by Cytoplasmic, Transmembrane and Extracellular Ligands 
Pharmaceuticals (Basel, Switzerland)  2010;3(10):3324-3342.
G protein coupled receptors (GPCRs) bind diverse classes of ligands, and depending on the receptor, these may bind in their transmembrane or the extracellular domains, demonstrating the principal ability of GPCRs to bind ligand in either domains. Most recently, it was also observed that small molecule ligands can bind in the cytoplasmic domain, and modulate binding and response to extracellular or transmembrane ligands. Thus, all three domains in GPCRs are potential sites for allosteric ligands, and whether a ligand is allosteric or orthosteric depends on the receptor. Here, we will review the evidence supporting the presence of putative binding pockets in all three domains of GPCRs and discuss possible pathways of communication between these pockets.
doi:10.3390/ph3103324
PMCID: PMC3760430  PMID: 24009470
Rhodopsin; Metabotropic Glutamate Receptors; Allosteric Network; Communication; Membrane Proteins
2.  Allosteric Modulation of G Protein Coupled Receptors by Cytoplasmic, Transmembrane and Extracellular Ligands  
Pharmaceuticals  2010;3(10):3324-3342.
G protein coupled receptors (GPCRs) bind diverse classes of ligands, and depending on the receptor, these may bind in their transmembrane or the extracellular domains, demonstrating the principal ability of GPCRs to bind ligand in either domains. Most recently, it was also observed that small molecule ligands can bind in the cytoplasmic domain, and modulate binding and response to extracellular or transmembrane ligands. Thus, all three domains in GPCRs are potential sites for allosteric ligands, and whether a ligand is allosteric or orthosteric depends on the receptor. Here, we will review the evidence supporting the presence of putative binding pockets in all three domains of GPCRs and discuss possible pathways of communication between these pockets.
doi:10.3390/ph3103324
PMCID: PMC3760430  PMID: 24009470
rhodopsin; metabotropic glutamate receptors; allosteric network; communication; membrane proteins
3.  The Cytoplasmic Rhodopsin-Protein Interface: Potential for Drug Discovery 
Current Drug Targets  2012;13(1):3-14.
The mammalian dim-light photoreceptor rhodopsin is a prototypic G protein coupled receptor (GPCR), interacting with the G protein, transducin, rhodopsin kinase, and arrestin. All of these proteins interact with rhodopsin at its cytoplasmic surface. Structural and modeling studies have provided in-depth descriptions of the respective interfaces. Overlap and thus competition for binding surfaces is a major regulatory mechanism for signal processing. Recently, it was found that the same surface is also targeted by small molecules. These ligands can directly interfere with the binding and activation of the proteins of the signal transduction cascade, but they can also allosterically modulate the retinal ligand binding pocket. Because the pocket that is targeted contains residues that are highly conserved across Class A GPCRs, these findings imply that it may be possible to target multiple GPCRs with the same ligand(s). This is desirable for example in complex diseases such as cancer where multiple GPCRs participate in the disease networks.
PMCID: PMC3275648  PMID: 21777183
G protein coupled receptors; allostery; conformational changes; docking; protein-protein interactions
4.  The expression patterns of gremlin 1 and noggin in normal adult and tumor tissues 
Gremlin 1 and noggin are inhibitors of bone morphogenetic protein (BMP) signaling. They are vital during early development but their role in adult tissues has remained largely unresolved. The BMP signaling pathway has also been implicated in tumorigenesis, however with emphasis on the role of the ligands and receptors. We performed a concurrent survey of gremlin 1 and noggin protein expression in multiple normal and cancer samples, using immunohistochemistry on tissue microarrays containing 96 samples from 34 different normal organs/tissue sites and 208 samples of 34 different tumor types. In majority of both normal and tumor samples, gremlin 1 and noggin expression was negative or weak. However, normal stomach and skin demonstrated distinct gremlin 1 and noggin expression indicating a role in adult tissues. Likewise, strong expression of both antagonists was detected in Leydig cells of testis. In the tumor panel, the expression patterns were more variable but elevated BMP antagonist expression was detected for the first time in few cases, such as glioblastoma, hepatocellular carcinoma and diffuse B-cell lymphoma for gremlin 1 and renal granular cell tumor and thyroid papillary carcinoma for noggin. Even though gremlin 1 and noggin were not widely expressed in adult tissues, in a subset of organs their expression pattern indicated a potential role in normal tissue homeostasis as well as in malignancies.
PMCID: PMC3693206  PMID: 23826422
Gremlin 1; noggin; cancer; normal tissue; immunohistochemistry
5.  FoldGPCR: structure prediction protocol for the transmembrane domain of G protein-coupled receptors from class A 
Proteins  2010;78(10):2189-2201.
Building reliable structural models of G protein-coupled receptors (GPCRs) is a difficult task due to the paucity of suitable templates, low sequence identity, and the wide variety of ligand specificities within the superfamily. Template-based modeling is known to be the most successful method for protein structure prediction. However, refinement of homology models within 1–3 Å Cα RMSD of the native structure remains a major challenge. Here we address this problem by developing a novel protocol (foldGPCR) for modeling the transmembrane (TM) region of GPCRs in complex with a ligand, aimed to accurately model the structural divergence between the template and target in the TM helices. The protocol is based on predicted conserved inter-residue contacts between the template and target, and exploits an all-atom implicit membrane force field. The placement of the ligand in the binding pocket is guided by biochemical data. The foldGPCR protocol is implemented by a stepwise hierarchical approach, in which the TM helical bundle and the ligand are assembled by simulated annealing trials in the first step, and the receptor-ligand complex is refined with replica exchange sampling in the second step. The protocol is applied to model the human β2-adrenergic receptor (β2AR) bound to carazolol, using contacts derived from the template structure of bovine rhodopsin. Comparison to the X-ray crystal structure of the β2AR shows that our protocol is particularly successful in accurately capturing helix backbone irregularities and helix-helix packing interactions that distinguish rhodopsin from β2AR.
doi:10.1002/prot.22731
PMCID: PMC2933064  PMID: 20544957
class A GPCR; structure prediction; simulated annealing; ligand binding; implicit solvent; membrane protein
6.  Domain coupling in GPCRs: the engine for induced conformational changes 
Recent solved structures of G protein-coupled receptors (GPCRs) provide insights into variation of the structure and molecular mechanisms of GPCR activation. In this review we provide evidence for the emerging paradigm of domain coupling facilitated by intrinsic disorder of the ligand-free state in GPCRs. The structure-function and dynamic studies suggest that ligand-bound GPCRs exhibit multiple active conformations in initiating cellular signals. Long-range intra-molecular and inter-molecular interactions at distant sites on the same receptor are crucial factors that modulate signaling function of GPCRs. Positive or negative coupling between the extracellular, the transmembrane and the intracellular domains facilitates cooperativity of activating “switches” as requirements for the functional plasticity of GPCRs. Awareness that allosteric ligands robustly affect domain coupling provides a novel mechanistic basis for rational drug development, small molecule antagonism and GPCR regulation by classical, as well as non-classical modes.
doi:10.1016/j.tips.2011.09.007
PMCID: PMC3273637  PMID: 22037017
7.  Endogenous lipid activated G protein-coupled receptors: Emerging structural features from crystallography and molecular dynamics simulations 
Class A G-protein coupled receptors (GPCRs) are thought to have a common topology that includes seven transmembrane alpha helices (TMHs) that are arranged to form a closed bundle. This bundle forms the ligand binding pocket into which ligands are commonly thought to enter via the extracellular milieu. This ligand approach direction makes sense for GPCRs that have small positively charged ligands, such as the beta-2-adrenergic or the dopamine D2 receptor. However, there is a growing sub-group of Class A GPCRs that bind lipid-derived endogenous ligands, such as the cannabinoid CB1 and CB2 receptors (with endogenous ligands, N-arachidonoylethanolamine (anandamide) and sn-2-arachidonylglycerol (2-AG)) and the S1P1-5 receptors (with endogenous ligand, sphingosine-1-phosphate). Even the widely studied Class A GPCR, rhodopsin, binds a highly lipophillic chromophore (11-cis-retinal). For these receptors, ligand approach from the extracellular milieu has seemed unlikely given that the ligands of these receptors readily partition into lipid or are actually synthesized in the lipid bilayer. The recent X-ray-crystal structure of the sub-type 1 sphingosine-1-phosphate receptor (S1P1) provides important information on the key structural variations that may be the hallmarks for a Class A GPCR that binds lipid-derived ligands. These include an extracellular domain that is closed off to the extracellular milieu and the existence of an opening between transmembrane helices that may serve as a portal for ligand entry via the lipid bilayer. This review examines structural aspects that the cannabinoid receptors may share with the S1P1 receptor based upon sequence homology. This review also examines experimental and simulation results that suggest ligand entry via a lipid portal is quite likely for this emerging sub-group.
doi:10.1016/j.chemphyslip.2013.01.009
PMCID: PMC4034697  PMID: 23485612
Cannabinoid; Sphingosine-1-phosphate; GPCR; Crystal structure; Lipid portal
8.  The 2.6 Å Crystal Structure of a Human A2A Adenosine Receptor Bound to an Antagonist 
Science (New York, N.Y.)  2008;322(5905):1211-1217.
The adenosine class of G protein-coupled receptors mediates the important role of extracellular adenosine in many physiological processes and is antagonized by caffeine. We have determined the crystal structure of the human A2A adenosine receptor in complex with a high affinity subtype-selective antagonist, ZM241385, to 2.6 Å resolution. Four disulfide bridges in the extracellular domain combined with a subtle repacking of the transmembrane helices relative to the adrenergic and rhodopsin receptor structures defines a pocket distinct from that of other structurally determined GPCRs. The arrangement allows for the binding of the antagonist in an extended conformation perpendicular to the membrane plane. The binding site highlights an integral role for the extracellular loops, together with the helical core in ligand recognition by this class of GPCRs, and suggests a role for ZM241385 in restricting the movement of a tryptophan residue important in the activation mechanism of the class A receptors.
doi:10.1126/science.1164772
PMCID: PMC2586971  PMID: 18832607
9.  ss-TEA: Entropy based identification of receptor specific ligand binding residues from a multiple sequence alignment of class A GPCRs 
BMC Bioinformatics  2011;12:332.
Background
G-protein coupled receptors (GPCRs) are involved in many different physiological processes and their function can be modulated by small molecules which bind in the transmembrane (TM) domain. Because of their structural and sequence conservation, the TM domains are often used in bioinformatics approaches to first create a multiple sequence alignment (MSA) and subsequently identify ligand binding positions. So far methods have been developed to predict the common ligand binding residue positions for class A GPCRs.
Results
Here we present 1) ss-TEA, a method to identify specific ligand binding residue positions for any receptor, predicated on high quality sequence information. 2) The largest MSA of class A non olfactory GPCRs in the public domain consisting of 13324 sequences covering most of the species homologues of the human set of GPCRs. A set of ligand binding residue positions extracted from literature of 10 different receptors shows that our method has the best ligand binding residue prediction for 9 of these 10 receptors compared to another state-of-the-art method.
Conclusions
The combination of the large multi species alignment and the newly introduced residue selection method ss-TEA can be used to rapidly identify subfamily specific ligand binding residues. This approach can aid the design of site directed mutagenesis experiments, explain receptor function and improve modelling. The method is also available online via GPCRDB at http://www.gpcr.org/7tm/.
doi:10.1186/1471-2105-12-332
PMCID: PMC3162937  PMID: 21831265
10.  TM-MOTIF: an alignment viewer to annotate predicted transmembrane helices and conserved motifs in aligned set of sequences 
Bioinformation  2011;7(5):214-221.
Multiple sequence alignments become biologically meaningful only if conserved and functionally important residues and secondary structural elements preserved can be identified at equivalent positions. This is particularly important for transmembrane proteins like G-protein coupled receptors (GPCRs) with seven transmembrane helices. TM-MOTIF is a software package and an effective alignment viewer to identify and display conserved motifs and amino acid substitutions (AAS) at each position of the aligned set of homologous sequences of GPCRs. The key feature of the package is to display the predicted membrane topology for seven transmembrane helices in seven colours (VIBGYOR colouring scheme) and to map the identified motifs on its respective helices /loop regions. It is an interactive package which provides options to the user to submit query or pre-aligned set of GPCR sequences to align with a reference sequence, like rhodopsin, whose structure has been solved experimentally. It also provides the possibility to identify the nearest homologue from the available inbuilt GPCR or Olfactory Receptor cluster dataset whose association is already known for its receptor type.
Availability
The database is available for free at mini@ncbs.res.in
PMCID: PMC3218415  PMID: 22125389
Transmembrane helices; Membrane topology; Amino acid conservation and substitutions; GPCR cluster association
11.  Ligand-specific regulation of the extracellular surface of a G protein coupled receptor 
Nature  2010;463(7277):108-112.
G protein coupled receptors (GPCRs) are seven transmembrane proteins that mediate the majority of cellular responses to hormones and neurotransmitters. They are the largest group of therapeutic targets for a broad spectrum of diseases. Recent crystal structures of GPCRs1,2,3,4,5 reveal structural conservation extending from the orthosteric ligand binding site in the transmembrane core to the cytoplasmic G protein coupling domains. In contrast, the extracellular surface (ECS) of GPCRs is remarkably diverse, and therefore represents an ideal target for the discovery of subtype-selective drugs. However, little is known about the functional role of the ECS in receptor activation, or about conformational coupling of this surface to the native ligand binding pocket. Here we use NMR spectroscopy to investigate ligand-specific conformational changes around a central structural feature in the ECS of the β2 adrenergic receptor: a salt bridge linking extracellular loops (ECLs) 2 and 3. Small molecule drugs that bind within the transmembrane core and exhibit different efficacies towards G protein activation (agonist, neutral antagonist, and inverse agonist) also stabilize distinct conformations of the ECS. We thereby demonstrate conformational coupling between the ECS and the orthosteric binding site, showing that drugs targeting this diverse surface could function as allosteric modulators with high subtype selectivity. Moreover, these studies provide new insight into the dynamic behavior of GPCRs not addressable by static, inactive-state crystal structures.
doi:10.1038/nature08650
PMCID: PMC2805469  PMID: 20054398
12.  Rice G-protein coupled receptor (GPCR) 
Plant Signaling & Behavior  2011;6(8):1079-1086.
The majority of transmembrane signal transduction in response to diverse external stimuli is mediated by G-protein coupled receptors (GPCRs) and are the principal signal transducers. GPCRs are characterized by seven membrane-spanning domains with an extracellular N-terminus and a cytoplasmic C-terminus which functions along with GTP-binding protein in a highly coordinated fashion. Role of heterotrimeric G-proteins in abiotic stresses has been reported, but the response of GPCR is not yet well characterized. In the present study we report the isolation of one putative GPCR (966 bp) from Indica rice (Oryza sativa cv. Indica group Swarna) and described its transcriptional regulation under abiotic stresses. Amino acid sequence analyses shows the presence of typical heptahelical transmembrane spanning domains with extracellular N-terminus involved in ligand binding and cytoplasm facing C-terminus that binds with heterotrimeric G-protein. Sequence analysis also confirmed the presence of all signature motifs required for functional GPCR. Domain and site prediction shows the presence of myristoylation sites for membrane association and protein kinase C sites for its desensitization. The transcript levels of rice GPCR was induced following NaCl and ABA treatments. However, in drought condition the expression profile of GPCR upregulated during early exposure which subsequently decreased. On the other hand it seems no significant effect due to cold and heat stress. These findings provide a direct evidence for transcriptional regulation of rice GPCR under abiotic stress conditions. These findings also suggest that GPCR can be exploited for promoting stress tolerance in plants.
doi:10.4161/psb.6.8.15771
PMCID: PMC3260697  PMID: 21778827
abiotic stress; G-protein coupled receptor; myristoylation; protein kinase C; real-time PCR; rice; signal transduction
13.  Signaling through G protein coupled receptors 
Plant Signaling & Behavior  2009;4(10):942-947.
Heterotrimeric G proteins (Gα, Gβ/Gγ subunits) constitute one of the most important components of cell signaling cascade. G Protein Coupled Receptors (GPCRs) perceive many extracellular signals and transduce them to heterotrimeric G proteins, which further transduce these signals intracellular to appropriate downstream effectors and thereby play an important role in various signaling pathways. GPCRs exist as a superfamily of integral membrane protein receptors that contain seven transmembrane α-helical regions, which bind to a wide range of ligands. Upon activation by a ligand, the GPCR undergoes a conformational change and then activate the G proteins by promoting the exchange of GDP/GTP associated with the Gα subunit. This leads to the dissociation of Gβ/Gγ dimer from Gα. Both these moieties then become free to act upon their downstream effectors and thereby initiate unique intracellular signaling responses. After the signal propagation, the GTP of Gα-GTP is hydrolyzed to GDP and Gα becomes inactive (Gα-GDP), which leads to its re-association with the Gβ/Gγ dimer to form the inactive heterotrimeric complex. The GPCR can also transduce the signal through G protein independent pathway. GPCRs also regulate cell cycle progression. Till to date thousands of GPCRs are known from animal kingdom with little homology among them, but only single GPCR has been identified in plant system. The Arabidopsis GPCR was reported to be cell cycle regulated and also involved in ABA and in stress signaling. Here I have described a general mechanism of signal transduction through GPCR/G proteins, structure of GPCRs, family of GPCRs and plant GPCR and its role.
PMCID: PMC2801357  PMID: 19826234
heterotrimeric G proteins; GPCRs; seven-transmembrane receptors; signal transduction; stress signaling
14.  Screening for GPCR Ligands Using Surface Plasmon Resonance 
ACS Medicinal Chemistry Letters  2011;2(7):549-554.
G-protein coupled receptors (GPCRs) are a class of drug targets of primary importance. However, receptor assays are based on measurement of either ligand displacement or downstream functional responses, rather than direct observation of ligand binding. Issues of allosteric modulation, probe dependence, and functional selectivity create challenges in selecting suitable assays formats. Therefore, a method that directly measures GPCR–ligand interactions, independent of binding site, probe, and signaling pathway would be a useful primary and orthogonal screening method. We have developed a GPCR biosensor assay protocol that offers the opportunity for high-throughput label-free screening that directly measures GPCR–ligand interactions. The biosensor-based direct screening method identifies the interaction of both orthosteric and allosteric ligands with solubilized, native GPCRs, in a label-free and cell-free environment, thus overcoming the limitations of indirect and displacement assay methods. We exemplify the method by the discovery of novel ligands for the chemokine receptor, CCR5, that are ligand efficient fragments.
doi:10.1021/ml2000017
PMCID: PMC3137231  PMID: 21765967
Surface plasmon resonance; G-protein coupled receptors; CCR5; allosteric; fragments
15.  Screening for GPCR Ligands Using Surface Plasmon Resonance 
ACS Medicinal Chemistry Letters  2011;2(7):549-554.
G-protein coupled receptors (GPCRs) are a class of drug targets of primary importance. However, receptor assays are based on measurement of either ligand displacement or downstream functional responses, rather than direct observation of ligand binding. Issues of allosteric modulation, probe dependence, and functional selectivity create challenges in selecting suitable assays formats. Therefore, a method that directly measures GPCR–ligand interactions, independent of binding site, probe, and signaling pathway would be a useful primary and orthogonal screening method. We have developed a GPCR biosensor assay protocol that offers the opportunity for high-throughput label-free screening that directly measures GPCR–ligand interactions. The biosensor-based direct screening method identifies the interaction of both orthosteric and allosteric ligands with solubilized, native GPCRs, in a label-free and cell-free environment, thus overcoming the limitations of indirect and displacement assay methods. We exemplify the method by the discovery of novel ligands for the chemokine receptor, CCR5, that are ligand efficient fragments.
doi:10.1021/ml2000017
PMCID: PMC3137231  PMID: 21765967
Surface plasmon resonance; G-protein coupled receptors; CCR5; allosteric; fragments
16.  Crystal Structure of Oligomeric β1-Adrenergic G Protein- Coupled Receptors in Ligand-Free Basal State 
G protein-coupled receptors (GPCRs) mediate transmembrane signaling. Before ligand binding, GPCRs exist in a basal state. Crystal structures of several GPCRs bound with antagonists or agonists have been solved. However, the crystal structure of the ligand-free basal state of a GPCR, the starting point of GPCR activation and function, has not been determined. Here we report the X-ray crystal structure of the first ligand-free basal state of a GPCR in a lipid membrane-like environment. Oligomeric turkey β1-adrenergic receptors display two alternating dimer interfaces. One interface involves the transmembrane domain (TM) 1, TM2, the C-terminal H8, and the extracellular loop 1. The other interface engages residues from TM4, TM5, the intracellular loop 2 and the extracellular loop 2. Structural comparisons show that this ligand-free state is in an inactive conformation. This provides the structural information regarding GPCR dimerization and oligomerization.
doi:10.1038/nsmb.2504
PMCID: PMC3618578  PMID: 23435379
17.  Allosteric and Biased G Protein-Coupled Receptor Signaling Regulation: Potentials for New Therapeutics 
G protein-coupled receptors (GPCRs) are seven-transmembrane proteins that participate in many aspects of the endocrine function and are important targets for drug development. They transduce signals mainly, but not exclusively, via hetero-trimeric G proteins, leading to a diversity of intracellular signaling cascades. Ligands binding at the hormone orthosteric sites of receptors have been classified as agonists, antagonists, and/or inverse agonists based on their ability to mainly modulate G protein signaling. Accumulating evidence also indicates that such ligands, alone or in combination with other ones such as those acting outside the orthosteric hormone binding sites (e.g., allosteric modulators), have the ability to selectively engage subsets of signaling responses as compared to the natural endogenous ligands. Such modes of functioning have been variously referred to as “functional selectivity” or “ligand-biased signaling.” In this review, we provide an overview of the current knowledge regarding GPCR-biased signaling and their functional regulation with a focus on the evolving concept that receptor domains can also be targeted to allosterically bias signaling, and discuss the usefulness of such modes of regulation for the design of more efficient therapeutics.
doi:10.3389/fendo.2014.00068
PMCID: PMC4021147  PMID: 24847311
G protein-coupled receptors; allosterism; biased signaling; functional selectivity; receptor domains
18.  Computing Highly Correlated Positions Using Mutual Information and Graph Theory for G Protein-Coupled Receptors 
PLoS ONE  2009;4(3):e4681.
G protein-coupled receptors (GPCRs) are a superfamily of seven transmembrane-spanning proteins involved in a wide array of physiological functions and are the most common targets of pharmaceuticals. This study aims to identify a cohort or clique of positions that share high mutual information. Using a multiple sequence alignment of the transmembrane (TM) domains, we calculated the mutual information between all inter-TM pairs of aligned positions and ranked the pairs by mutual information. A mutual information graph was constructed with vertices that corresponded to TM positions and edges between vertices were drawn if the mutual information exceeded a threshold of statistical significance. Positions with high degree (i.e. had significant mutual information with a large number of other positions) were found to line a well defined inter-TM ligand binding cavity for class A as well as class C GPCRs. Although the natural ligands of class C receptors bind to their extracellular N-terminal domains, the possibility of modulating their activity through ligands that bind to their helical bundle has been reported. Such positions were not found for class B GPCRs, in agreement with the observation that there are not known ligands that bind within their TM helical bundle. All identified key positions formed a clique within the MI graph of interest. For a subset of class A receptors we also considered the alignment of a portion of the second extracellular loop, and found that the two positions adjacent to the conserved Cys that bridges the loop with the TM3 qualified as key positions. Our algorithm may be useful for localizing topologically conserved regions in other protein families.
doi:10.1371/journal.pone.0004681
PMCID: PMC2650788  PMID: 19262747
19.  Structure of the human smoothened receptor 7TM bound to an antitumor agent 
Nature  2013;497(7449):338-343.
The smoothened (SMO) receptor, a key signal transducer in the Hedgehog (Hh) signaling pathway is both responsible for the maintenance of normal embryonic development and implicated in carcinogenesis. The SMO receptor is classified as a class Frizzled (class F) G protein-coupled receptor (GPCR), although the canonical Hh signaling pathway involves the transcription factor Gli and the sequence similarity with class A GPCRs is less than 10%. Here we report the crystal structure at 2.5 Å resolution of the transmembrane domain of the human SMO receptor bound to the small molecule antagonist LY2940680. Although the SMO receptor shares the seven transmembrane helical (7TM) fold, most conserved motifs for class A GPCRs are absent, and the structure reveals an unusually complex arrangement of long extracellular loops stabilized by four disulfide bonds. The ligand binds at the extracellular end of the 7TM bundle and forms extensive contacts with the loops.
doi:10.1038/nature12167
PMCID: PMC3657389  PMID: 23636324
20.  Molecular Architecture of G Protein-Coupled Receptors 
Drug development research  1996;37(1):1-38.
This review of the current literature on mutations in G protein-coupled receptors (GPCRs) of the rhodopsin-related family intends to draw inferences from amino acid sequences for single receptors and multiple sequence alignments with regard to the molecular architecture of this class of receptors. For this purpose a comprehensive list of mutations within the transmembrane helical regions (TMs; over 390 mutations from 38 different receptor subtypes) and their effects on function was compiled, and an alignment of known GPCR sequences (over 150 separate sequences) was made. Regions most prominently involved in ligand binding are located in TMs 3, 5, 6, and 7. Position 3.32 in TM3 is occupied by a D in all biogenic amine receptors (sequence conservation) but may be occupied by uncharged residues in other receptors while its role in ligand binding is analogous (function conservation). TMs 5, 6, and 7 display considerable sequence conservation throughout the majority of GPCRs investigated, but not necessarily at those positions involved in ligand binding. However, considerable function conservation is observed for positions 5.42 (frequently hydrophilic), 5.46 (small amino acids required for agonist binding to “small ligand” receptors), 6.52 and 7.39 (high variability), and 7.43 (frequently aromatic). A general conclusion of this review is that there is overwhelming conservation of structure-function correlates among GPCRs. Thus, it is now possible to cross-correlate the results of mutagenesis studies between GPCRs of different subfamilies, and to use those results to predict the function of specific residues in new GPCR sequences.
doi:10.1002/(SICI)1098-2299(199601)37:1<1::AID-DDR1>3.0.CO;2-S
PMCID: PMC3171971  PMID: 21921973
G protein-coupled receptors; GPCRs; transmembrane domains; mutagenesis; sequence alignment; identifiers; molecular architecture; molecular structure
21.  Gremlin-1 Induces BMP-Independent Tumor Cell Proliferation, Migration, and Invasion 
PLoS ONE  2012;7(4):e35100.
Gremlin-1, a bone morphogenetic protein (BMP) antagonist, is overexpressed in various cancerous tissues but its role in carcinogenesis has not been established. Here, we report that gremlin-1 binds various cancer cell lines and this interaction is inhibited by our newly developed gremlin-1 antibody, GRE1. Gremlin-1 binding to cancer cells was unaffected by the presence of BMP-2, BMP-4, and BMP-7. In addition, the binding was independent of vascular endothelial growth factor receptor-2 (VEGFR2) expression on the cell surface. Addition of gremlin-1 to A549 cells induced a fibroblast-like morphology and decreased E-cadherin expression. In a scratch wound healing assay, A549 cells incubated with gremlin-1 or transfected with gremlin-1 showed increased migration, which was inhibited in the presence of the GRE1 antibody. Gremlin-1 transfected A549 cells also exhibited increased invasiveness as well as an increased growth rate. These effects were also inhibited by the addition of the GRE1 antibody. In conclusion, this study demonstrates that gremlin-1 directly interacts with cancer cells in a BMP- and VEGFR2-independent manner and can induce cell migration, invasion, and proliferation.
doi:10.1371/journal.pone.0035100
PMCID: PMC3325980  PMID: 22514712
22.  Structural insights into G-protein-coupled receptor activation☆ 
G-protein-coupled receptors (GPCRs) are the largest family of eukaryotic plasma membrane receptors, and are responsible for the majority of cellular responses to external signals. GPCRs share a common architecture comprising seven transmembrane (TM) helices. Binding of an activating ligand enables the receptor to catalyze the exchange of GTP for GDP in a heterotrimeric G protein. GPCRs are in a conformational equilibrium between inactive and activating states. Crystallographic and spectroscopic studies of the visual pigment rhodopsin and two b-adrenergic receptors have defined some of the conformational changes associated with activation.
doi:10.1016/j.sbi.2008.09.010
PMCID: PMC4019673  PMID: 18957321
23.  Receptor Oligomerization in Family B1 of G-Protein-Coupled Receptors: Focus on BRET Investigations and the Link between GPCR Oligomerization and Binding Cooperativity 
The superfamily of the seven transmembrane G-protein-coupled receptors (7TM/GPCRs) is the largest family of membrane-associated receptors. GPCRs are involved in the pathophysiology of numerous human diseases, and they constitute an estimated 30–40% of all drug targets. During the last two decades, GPCR oligomerization has been extensively studied using methods like bioluminescence resonance energy transfer (BRET) and today, receptor–receptor interactions within the GPCR superfamily is a well-established phenomenon. Evidence of the impact of GPCR oligomerization on, e.g., ligand binding, receptor expression, and signal transduction indicates the physiological and pharmacological importance of these receptor interactions. In contrast to the larger and more thoroughly studied GPCR subfamilies A and C, the B1 subfamily is small and comprises only 15 members, including, e.g., the secretin receptor, the glucagon receptor, and the receptors for parathyroid hormone (PTHR1 and PTHR2). The dysregulation of several family B1 receptors is involved in diseases, such as diabetes, chronic inflammation, and osteoporosis which underlines the pathophysiological importance of this GPCR subfamily. In spite of this, investigation of family B1 receptor oligomerization and especially its pharmacological importance is still at an early stage. Even though GPCR oligomerization is a well-established phenomenon, there is a need for more investigations providing a direct link between these interactions and receptor functionality in family B1 GPCRs. One example of the functional effects of GPCR oligomerization is the facilitation of allosterism including cooperativity in ligand binding to GPCRs. Here, we review the currently available data on family B1 GPCR homo- and heteromerization, mainly based on BRET investigations. Furthermore, we cover the functional influence of oligomerization on ligand binding as well as the link between oligomerization and binding cooperativity.
doi:10.3389/fendo.2012.00062
PMCID: PMC3355942  PMID: 22649424
GPCRs; family B1; oligomerization; BRET; binding cooperativity
24.  Genetic variations in human G protein-coupled receptors: Implications for drug therapy 
AAPS PharmSci  2001;3(3):54-80.
Numerous genes encode G protein-coupled receptors (GPCRs)-a main molecular target for drug therapy. Estimates indicate that the human genome contains approximately 600 GPCR genes. This article addresses therapeutic implications of sequence variations in GPCR genes. A number of inactivating and activating receptor mutations have been shown to cause a variety of (mostly rare) genetic disorders. However, pharmacogenetic and pharmacogenomic studies on GPCRs are scarce, and therapeutic relevance of variant receptor alleles often remains unclear. Confounding factors in assessing the therapeutic relevance of variant GPCR alleles include 1) interaction of a single drug with multiple closely related receptors, 2) poorly defined binding pockets that can accommodate drug ligands in different orientations or at alternative receptor domains, 3) possibility of multiple receptor conformations with distinct functions, and 4) multiple signaling pathways engaged by a single receptor. For example, antischizophrenic drugs bind to numerous receptors, several of which might be relevant to therapeutic outcome. Without knowing accurately what role a given receptor subtype plays in clinical outcome and how a sequence variation affects drug-induced signal transduction, we cannot predict the therapeutic relevance of a receptor variant. Genome-wide association studies with single nucleotide polymorphisms could identify critical target receptors for disease susceptibility and drug efficacy or toxicity.
doi:10.1208/ps030322
PMCID: PMC2751017  PMID: 11741273
G Protein-Coupled; Receptors; Drug Therapy; Pharmacogenomics; Pharmacogenetics
25.  Luciferase activity under direct ligand-dependent control of a muscarinic acetylcholine receptor 
BMC Biotechnology  2009;9:46.
Background
Controlling enzyme activity by ligand binding to a regulatory domain of choice may have many applications e.g. as biosensors and as tools in regulating cellular functions. However, until now only a small number of ligand-binding domains have been successfully linked to enzyme activity. G protein-coupled receptors (GPCR) are capable of recognizing an extraordinary structural variety of extracellular signals including inorganic and organic molecules. Ligand binding to GPCR results in conformational changes involving the transmembrane helices. Here, we assessed whether ligand-induced conformational changes within the GPCR helix bundle can be utilized to control the activity of an integrated enzyme.
Results
As a proof of principle, we inserted the luciferase amino acid sequence into the third intracellular loop of the M3 muscarinic acetylcholine receptor. This fusion protein retained both receptor and enzyme function. Receptor blockers slightly but significantly reduced enzyme activity. By successive deletion mutagenesis the enzyme activity was optimally coupled to ligand-induced conformational helix movements.
Conclusion
Our results demonstrate that in engineered GPCR-enzyme chimeras, intracellular enzyme activity can be directly controlled by a GPCR serving as the extracellular ligand-binding domain.
doi:10.1186/1472-6750-9-46
PMCID: PMC2689208  PMID: 19450256

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