Two amino acids within the alpha4 helix of Galphai1 mediate coupling with 5-hydroxytryptamine1B receptors

Latrophilin-2 mediates fluid shear stress mechanotransduction at endothelial junctions

Endothelial cell responses to fluid shear stress from blood flow are crucial for vascular development, function, and disease. A complex of PECAM-1, VE-cadherin, VEGF receptors (VEGFRs), and Plexin D1 located at cell-cell junctions mediates many of these events. However, available evidence suggests that another mechanosensor upstream of PECAM-1 initiates signaling. Hypothesizing that GPCR and Gα proteins may serve this role, we performed siRNA screening of Gα subunits and found that Gαi2 and Gαq/11 are required for activation of the junctional complex. We then developed a new activation assay, which showed that these G proteins are activated by flow. We next mapped the Gα residues required for activation and developed an affinity purification method that used this information to identify latrophilin-2 (Lphn2/ADGRL2) as the upstream GPCR. Latrophilin-2 is required for all PECAM-1 downstream events tested. In both mice and zebrafish, latrophilin-2 is required for flow-dependent angiogenesis and artery remodeling. Furthermore, endothelial-specific knockout demonstrates that latrophilin plays a role in flow-dependent artery remodeling. Human genetic data reveal a correlation between the latrophilin-2-encoding Adgrl2 gene and cardiovascular disease. Together, these results define a pathway that connects latrophilin-dependent G protein activation to subsequent endothelial signaling, vascular physiology, and disease.

Latrophilin-2 mediates fluid shear stress mechanotransduction at endothelial junctions

Endothelial cell responses to fluid shear stress from blood flow are crucial for vascular development, function and disease. A complex of PECAM-1, VE-cadherin, VEGF receptors (VEGFRs) and PlexinD1 located at cell-cell junctions mediates many of these events. But available evidence suggests that another mechanosensor upstream of PECAM-1 initiates signaling. Hypothesizing that GPCR and Gα proteins may serve this role, we performed siRNA screening of Gα subunits and found that Gαi2 and Gαq/11 are required for activation of the junctional complex. We then developed a new activation assay, which showed that these G proteins are activated by flow. We next mapped the Gα residues required for activation and developed an affinity purification method that used this information to identify latrophilin-2 (Lphn-2/ADGRL2) as the upstream GPCR. Latrophilin-2 is required for all PECAM-1 downstream events tested. In both mice and zebrafish, latrophilin-2 is required for flow-dependent angiogenesis and artery remodeling. Furthermore, endothelial specific knockout demonstrates that latrophilin plays a role in flow-dependent artery remodeling. Human genetic data reveal a correlation between the latrophilin-2-encoding Adgrl2 gene and cardiovascular disease. Together, these results define a pathway that connects latrophilin-dependent G protein activation to subsequent endothelial signaling, vascular physiology and disease.

Coevolution underlies GPCR-G protein selectivity and functionality

G protein-coupled receptors (GPCRs) regulate diverse physiological events, which makes them as the major targets for many approved drugs. G proteins are downstream molecules that receive signals from GPCRs and trigger cell responses. The GPCR-G protein selectivity mechanism on how they properly and timely interact is still unclear. Here, we analyzed model GPCRs (i.e. HTR, DAR) and Gα proteins with a coevolutionary tool, statistical coupling analysis. The results suggested that 5-hydroxytryptamine receptors and dopamine receptors have common conserved and coevolved residues. The Gα protein also have conserved and coevolved residues. These coevolved residues were implicated in the molecular functions of the analyzed proteins. We also found specific coevolving pairs related to the selectivity between GPCR and G protein were identified. We propose that these results would contribute to better understandings of not only the functional residues of GPCRs and Gα proteins but also GPCR-G protein selectivity mechanisms.

Optical approaches for single-cell and subcellular analysis of GPCR-G protein signaling

G protein-coupled receptors (GPCRs), G proteins, and their signaling associates are major signal transducers that control the majority of cellular signaling and regulate key biological functions including immune, neurological, cardiovascular, and metabolic processes. These pathways are targeted by over one-third of drugs on the market; however, the current understanding of their function is limited and primarily derived from cell-destructive approaches providing an ensemble of static, multi-cell information about the status and composition of molecules. Spatiotemporal behavior of molecules involved is crucial to understanding in vivo cell behaviors both in health and disease, and the advent of genetically encoded fluorescence proteins and small fluorophore-based biosensors has facilitated the mapping of dynamic signaling in cells with subcellular acuity. Since we and others have developed optogenetic methods to regulate GPCR-G protein signaling in single cells and subcellular regions using dedicated wavelengths, the desire to develop and adopt optogenetically amenable assays to measure signaling has motivated us to take a broader look at the available optical tools and approaches compatible with measuring single-cell and subcellular GPCR-G protein signaling. Here we review such key optical approaches enabling the examination of GPCR, G protein, secondary messenger, and downstream molecules such as kinase and lipid signaling in living cells. The methods reviewed employ both fluorescence and bioluminescence detection. We not only further elaborate the underlying principles of these sensors but also discuss the experimental criteria and limitations to be considered during their use in single-cell and subcellular signal mapping.

GPCR-Gα protein precoupling: Interaction between Ste2p, a yeast GPCR, and Gpa1p, its Gα protein, is formed before ligand binding via the Ste2p C-terminal domain and the Gpa1p N-terminal domain

G protein coupled receptors bind ligands that initiate intracellular signaling cascades via heterotrimeric G proteins. In this study, involvement of the N-terminal residues of yeast G-alpha (Gpa1p) with the C-terminal residues of a full-length or C-terminally truncated Ste2p were investigated using bioluminescence resonance energy transfer (BRET), a non-radiative energy transfer phenomenon where protein-protein interactions can be quantified between a donor bioluminescent molecule and a suitable acceptor fluorophore. Constitutive and position-dependent BRET signal was observed in the absence of agonist (α-factor). Upon the activation of the receptors with α-factor, no significant change in BRET signal was observed. The location of Ste2p-Gpa1p heterodimer was investigated using confocal fluorescence microscopy and bimolecular fluorescence complementation (BiFC) assay, a technique where two non-fluorescent fragments of a fluorescent protein reassemble in vivo to restore fluorescence property thereby directly reporting a protein-protein interaction. BiFC experiments resulted in a dimerization signal intracellularly during biosynthesis on the endoplasmic reticulum (ER) and on the plasma membrane (PM). The constitutive BRET and BiFC signals observed on ER between Ste2p and Gpa1p in their quiescent and activated states are indicative of pre-coupling between these two proteins. This study is the first to show that the extreme N-terminus of yeast G protein alpha subunit is in close proximity to its receptor. The data suggests a pre-coupled heterodimer prior to receptor activation. The images presented in this study are the first direct in vivo evidence showing the localization of receptor - G protein heterodimers during biosynthesis and before reaching the plasma membrane.

Structural mechanism of G protein activation by G protein-coupled receptor

G protein-coupled receptors (GPCRs) are a family of membrane receptors that regulate physiology and pathology of various organs. Consequently, about 40% of drugs in the market targets GPCRs. Heterotrimeric G proteins are composed of α, β, and γ subunits, and act as the key downstream signaling molecules of GPCRs. The structural mechanism of G protein activation by GPCRs has been of a great interest, and a number of biochemical and biophysical studies have been performed since the late 80's. These studies investigated the interface between GPCR and G proteins and the structural mechanism of GPCR-induced G protein activation. Recently, arrestins are also reported to be important molecular switches in GPCR-mediated signal transduction, and the physiological output of arrestin-mediated signal transduction is different from that of G protein-mediated signal transduction. Understanding the structural mechanism of the activation of G proteins and arrestins would provide fundamental information for the downstream signaling-selective GPCR-targeting drug development. This review will discuss the structural mechanism of GPCR-induced G protein activation by comparing previous biochemical and biophysical studies.

How much do we know about the coupling of G-proteins to serotonin receptors?

Serotonin receptors are G-protein-coupled receptors (GPCRs) involved in a variety of psychiatric disorders. G-proteins, heterotrimeric complexes that couple to multiple receptors, are activated when their receptor is bound by the appropriate ligand. Activation triggers a cascade of further signalling events that ultimately result in cell function changes. Each of the several known G-protein types can activate multiple pathways. Interestingly, since several G-proteins can couple to the same serotonin receptor type, receptor activation can result in induction of different pathways. To reach a better understanding of the role, interactions and expression of G-proteins a literature search was performed in order to list all the known heterotrimeric combinations and serotonin receptor complexes. Public databases were analysed to collect transcript and protein expression data relating to G-proteins in neural tissues. Only a very small number of heterotrimeric combinations and G-protein-receptor complexes out of the possible thousands suggested by expression data analysis have been examined experimentally. In addition this has mostly been obtained using insect, hamster, rat and, to a lesser extent, human cell lines. Besides highlighting which interactions have not been explored, our findings suggest additional possible interactions that should be examined based on our expression data analysis.

A transient interaction between the phosphate binding loop and switch I contributes to the allosteric network between receptor and nucleotide in Gαi1

Receptor-mediated activation of the Gα subunit of heterotrimeric G proteins requires allosteric communication between the receptor binding site and the guanine nucleotide binding site, which are separated by >30 Å. Structural changes in the allosteric network connecting these sites are predicted to be transient in the wild-type Gα subunit, making studies of these connections challenging. In the current work, site-directed mutants that alter the energy barriers between the activation states are used as tools to better understand the transient features of allosteric signaling in the Gα subunit. The observed differences in relative receptor affinity for intact Gαi1 subunits versus C-terminal Gαi1 peptides harboring the K345L mutation are consistent with this mutation modulating the allosteric network in the protein subunit. Measurement of nucleotide exchange rates, affinity for metarhodopsin II, and thermostability suggest that the K345L Gαi1 variant has reduced stability in both the GDP-bound and nucleotide-free states as compared with wild type but similar stability in the GTPγS-bound state. High resolution x-ray crystal structures reveal conformational changes accompanying the destabilization of the GDP-bound state. Of these, the conformation for Switch I was stabilized by an ionic interaction with the phosphate binding loop. Further site-directed mutagenesis suggests that this interaction between Switch I and the phosphate binding loop is important for receptor-mediated nucleotide exchange in the wild-type Gαi1 subunit.

A yeast screening method to decipher the interaction between the adenosine A2B receptor and the C-terminus of different G protein α-subunits

The expression of human G protein-coupled receptors (GPCRs) in Saccharomyces cerevisiae containing chimeric yeast/mammalian Gα subunits provides a useful tool for the study of GPCR activation. In this study, we used a one-GPCR-one-G protein yeast screening method in combination with molecular modeling and mutagenesis studies to decipher the interaction between GPCRs and the C-terminus of different α-subunits of G proteins. We chose the human adenosine A2B receptor (hA2BR) as a paradigm, a typical class A GPCR that shows promiscuous behavior in G protein coupling in this yeast system. The wild-type hA2BR and five mutant receptors were expressed in 8 yeast strains with different humanized G proteins, covering the four major classes: Gαi, Gαs, Gαq, and Gα12. Our experiments showed that a tyrosine residue (Y) at the C-terminus of the Gα subunit plays an important role in controlling the activation of GPCRs. Receptor residues R103(3.50) and I107(3.54) are vital too in G protein-coupling and the activation of the hA2BR, whereas L213(IL3) is more important in G protein inactivation. Substitution of S235(6.36) to alanine provided the most divergent G protein-coupling profile. Finally, L236(6.37) substitution decreased receptor activation in all G protein pathways, although to a different extent. In conclusion, our findings shed light on the selectivity of receptor/G protein coupling, which may help in further understanding GPCR signaling.

Structural Aspects of GPCR-G Protein Coupling

G protein-coupled receptors (GPCRs) are membrane receptors; approximately 40% of drugs on the market target GPCRs. A precise understanding of the activation mechanism of GPCRs would facilitate the development of more effective and less toxic drugs. Heterotrimeric G proteins are important molecular switches in GPCR-mediated signal transduction. An agonist-activated receptor interacts with specific sites on G proteins and promotes the release of GDP from the Gα subunit. Because of the important biological role of the GPCR-G protein coupling, conformational changes in the G protein upon receptor coupling have been of great interest. One of the most important questions was the interface between the GPCR and G proteins and the structural mechanism of GPCR-induced G protein activation. A number of biochemical and biophysical studies have been performed since the late 80s to address these questions; there was a significant breakthrough in 2011 when the crystal structure of a GPCR-G protein complex was solved. This review discusses the structural aspects of GPCR-G protein coupling by comparing the results of previous biochemical and biophysical studies to the GPCR-G protein crystal structure.

Structural features of the G-protein/GPCR interactions

BACKGROUND

The details of the functional interaction between G proteins and the G protein coupled receptors (GPCRs) have long been subjected to extensive investigations with structural and functional assays and a large number of computational studies.

SCOPE OF REVIEW

The nature and sites of interaction in the G-protein/GPCR complexes, and the specificities of these interactions selecting coupling partners among the large number of families of GPCRs and G protein forms, are still poorly defined.

MAJOR CONCLUSIONS

Many of the contact sites between the two proteins in specific complexes have been identified, but the three dimensional molecular architecture of a receptor-Gα interface is only known for one pair. Consequently, many fundamental questions regarding this macromolecular assembly and its mechanism remain unanswered.

GENERAL SIGNIFICANCE

In the context of current structural data we review the structural details of the interfaces and recognition sites in complexes of sub-family A GPCRs with cognate G-proteins, with special emphasis on the consequences of activation on GPCR structure, the prevalence of preassembled GPCR/G-protein complexes, the key structural determinants for selective coupling and the possible involvement of GPCR oligomerization in this process.

Conformational flexibility and structural dynamics in GPCR-mediated G protein activation: a perspective

Structure and dynamics of G proteins and their cognate receptors, both alone and in complex, are becoming increasingly accessible to experimental techniques. Understanding the conformational changes and timelines that govern these changes can lead to new insights into the processes of ligand binding and associated G protein activation. Experimental systems may involve the use of, or otherwise stabilize, non-native environments. This can complicate our understanding of structural and dynamic features of processes such as the ionic lock, tryptophan toggle, and G protein flexibility. While elements in the receptor's transmembrane helices and the C-terminal α5 helix of Gα undergo well-defined structural changes, regions subject to conformational flexibility may be important in fine-tuning the interactions between activated receptors and G proteins. The pairing of computational and experimental approaches will continue to provide powerful tools to probe the conformation and dynamics of receptor-mediated G protein activation.

Linking receptor activation to changes in Sw I and II of Gα proteins

G-protein coupled receptors catalyze nucleotide exchange on G proteins, which results in subunit dissociation and effector activation. In the recent β2AR-Gs structure, portions of Switch I and II of Gα are not fully elucidated. We paired fluorescence studies of receptor-Gαi interactions with the β2AR-Gs and other Gi structures to investigate changes in Switch I and II during receptor activation and GTP binding. The β2/β3 loop containing Leu194 of Gαi is located between Switches I and II, in close proximity to IC2 of the receptor and the C-terminus of Gα, thus providing an allosteric connection between these Switches and receptor activation. We compared the environment of residues in myristoylated Gαi proteins in the heterotrimer to that upon receptor activation and subsequent GTP binding. Upon receptor activation, residues in both Switch regions are less solvent-exposed, as compared to the heterotrimer. Upon GTPγS binding, the environment of several residues in Switch I resemble the receptor-bound state, while Switch II residues display effects on their environment which are consistent with their role in GTP binding and Gβγ dissociation. The ability to merge available crystal structures with solution studies is a powerful tool to gain insight into conformational changes associated with receptor-mediated Gi protein activation.

Fluorescence/bioluminescence resonance energy transfer techniques to study G-protein-coupled receptor activation and signaling

Fluorescence and bioluminescence resonance energy transfer (FRET and BRET) techniques allow the sensitive monitoring of distances between two labels at the nanometer scale. Depending on the placement of the labels, this permits the analysis of conformational changes within a single protein (for example of a receptor) or the monitoring of protein-protein interactions (for example, between receptors and G-protein subunits). Over the past decade, numerous such techniques have been developed to monitor the activation and signaling of G-protein-coupled receptors (GPCRs) in both the purified, reconstituted state and in intact cells. These techniques span the entire spectrum from ligand binding to the receptors down to intracellular second messengers. They allow the determination and the visualization of signaling processes with high temporal and spatial resolution. With these techniques, it has been demonstrated that GPCR signals may show spatial and temporal patterning. In particular, evidence has been provided for spatial compartmentalization of GPCRs and their signals in intact cells and for distinct physiological consequences of such spatial patterning. We review here the FRET and BRET technologies that have been developed for G-protein-coupled receptors and their signaling proteins (G-proteins, effectors) and the concepts that result from such experiments.

Signal protein-derived peptides as functional probes and regulators of intracellular signaling

The functionally important regions of signal proteins participating in their specific interaction and responsible for transduction of hormonal signal into cell are rather short in length, having, as a rule, 8 to 20 amino acid residues. Synthetic peptides corresponding to these regions are able to mimic the activated form of full-size signal protein and to trigger signaling cascades in the absence of hormonal stimulus. They modulate protein-protein interaction and influence the activity of signal proteins followed by changes in their regulatory and catalytic sites. The present review is devoted to the achievements and perspectives of the study of signal protein-derived peptides and to their application as selective and effective regulators of hormonal signaling systems in vitro and in vivo. Attention is focused on the structure, biological activity, and molecular mechanisms of action of peptides, derivatives of the receptors, G protein α subunits, and the enzymes generating second messengers.

Calnuc plays a role in dynamic distribution of Galphai but not Gbeta subunits and modulates ACTH secretion in AtT-20 neuroendocrine secretory cells

In AtT-20 cells ACTH secretion is regulated by both Ca²⁺ and G proteins. We previously demonstrated that calnuc, an EF-hand Ca²⁺ binding protein which regulates Alzheimer's β-amyloid precursor protein (APP) biogenesis, binds both Ca²⁺ as well as Gα subunits. Here we investigate calnuc's role in G protein-mediated regulation of ACTH secretion in AtT-20 neuroendocrine secretory cells stably overexpressing calnuc-GFP. Similar to endogenous calnuc, calnuc-GFP is mainly found in the Golgi, on the plasma membrane (PM), and associated with regulated secretion granules (RSG). By deconvolution immunofluorescence, calnuc-GFP partially colocalizes with Gαi1/2 and Gαi3 at the PM and on RSG. Cytosolic calnuc(ΔSS)-CFP with the signal sequence deleted also partially colocalizes with RSG and partially cosediments with Gαi1/2 in fractions enriched in RSG. Overexpression of calnuc-GFP specifically increases the distribution of Gαi1/2 on the PM whereas the distribution of Gβ subunits and synaptobrevin 2 (Vamp 2) is unchanged. Overexpression of calnuc-GFP or cytosolic calnuc(ΔSS)-CFP enhances ACTH secretion two-fold triggered by mastoparan or GTPγS but does not significantly affect glycosaminoglycan (GAG) chain secretion along the constitutive pathway or basal secretion of ACTH. Calnuc's facilitating effects on ACTH secretion are decreased after introducing anti-Gαi1/2, Gαi3, Gβ or calnuc IgG into permeabilized cells but not when Gα12 or preimmune IgG is introduced. The results suggest that calnuc binds to Gα subunits on the Golgi and on RSG and that overexpression of calnuc causes redistribution of Gαi subunits to the PM and RSG, indicating that calnuc plays a role in dynamic distribution of only Gα but not Gβ subunits. Thus calnuc may connect G protein signaling and calcium signaling during regulated secretion.

A sweet cycle for Arabidopsis G-proteins: Recent discoveries and controversies in plant G-protein signal transduction

Heterotrimeric G-proteins are a class of signal transduction proteins highly conserved throughout evolution that serve as dynamic molecular switches regulating the intracellular communication initiated by extracellular signals including sensory information. This property is achieved by a guanine nucleotide cycle wherein the inactive, signaling-incompetent Galpha subunit is normally bound to GDP; activation to signaling-competent Galpha occurs through the exchange of GDP for GTP (typically catalyzed via seven-transmembrane domain G-protein coupled receptors [GPCRs]), which dissociates the Gbetagamma dimer from Galpha-GTP and initiates signal transduction. The hydrolysis of GTP, greatly accelerated by "Regulator of G-protein Signaling" (RGS) proteins, returns Galpha to its inactive GDP-bound form and terminates signaling. Through extensive characterization of mammalian Galpha isoforms, the rate-limiting step in this cycle is currently considered to be the GDP/GTP exchange rate, which can be orders of magnitude slower than the GTP hydrolysis rate. However, we have recently demonstrated that, in Arabidopsis, the guanine nucleotide cycle appears to be limited by the rate of GTP hydrolysis rather than nucleotide exchange. This finding has important implications for the mechanism of sugar sensing in Arabidopsis. We also discuss these data on Arabidopsis G-protein nucleotide cycling in relation to recent reports of putative plant GPCRs and heterotrimeric G-protein effectors in Arabidopsis.

How do receptors activate G proteins?

Heterotrimeric G proteins couple the activation of heptahelical receptors at the cell surface to the intracellular signaling cascades that mediate the physiological responses to extracellular stimuli. G proteins are molecular switches that are activated by receptor-catalyzed GTP for GDP exchange on the G protein alpha subunit, which is the rate-limiting step in the activation of all downstream signaling. Despite the important biological role of the receptor-G protein interaction, relatively little is known about the structure of the complex and how it leads to nucleotide exchange. This chapter will describe what is known about receptor and G protein structure and outline a strategy for assembling the current data into improved models for the receptor-G protein complex that will hopefully answer the question as to how receptors flip the G protein switch.

Heterotrimeric G protein activation by G-protein-coupled receptors

Heterotrimeric G proteins have a crucial role as molecular switches in signal transduction pathways mediated by G-protein-coupled receptors. Extracellular stimuli activate these receptors, which then catalyse GTP-GDP exchange on the G protein alpha-subunit. The complex series of interactions and conformational changes that connect agonist binding to G protein activation raise various interesting questions about the structure, biomechanics, kinetics and specificity of signal transduction across the plasma membrane.

The signaling pathway of rhodopsin

The signal-transduction mechanism of rhodopsin was studied by molecular dynamics (MD) simulations of the high-resolution, inactive structure in an explicit membrane environment. The simulations were employed to calculate equal-time correlations of the fluctuating interaction energy of residue pairs. The resulting interaction-correlation matrix was used to determine a network that couples retinal to the cytoplasmic interface, where transducin binds. Two highly conserved motifs, D(E)RY and NPxxY, were found to have strong interaction correlation with retinal. MD simulations with restraints on each transmembrane helix indicated that the major signal-transduction pathway involves the interdigitating side chains of helices VI and VII. The functional roles of specific residues were elucidated by the calculated effect of retinal isomerization from 11-cis to all-trans on the residue-residue interaction pattern. It is suggested that Glu134 may act as a "signal amplifier" and that Asp83 may introduce a threshold to prevent background noise from activating rhodopsin.

Structural basis for nucleotide exchange on G alpha i subunits and receptor coupling specificity

Heterotrimeric G proteins are molecular switches that relay information intracellularly in response to various extracellular signals. How ligand-activated G protein-coupled receptors act at a distance to exert exchange activity on the Galpha nucleotide binding pocket is poorly understood. Here we describe the synergistic action of two peptides: one from the third intracellular loop of the D2 dopamine receptor (D2N), and a second, Galpha.GDP-binding peptide (KB-752) that mimics the proposed role of Gbetagamma in receptor-promoted nucleotide exchange. The structure of both peptides in complex with Galpha(i1) suggests that conformational changes in the beta3/alpha2 loop and beta6 strand act in concert for efficient nucleotide exchange. Two key residues in the alpha4 helix were found to define a receptor/Galpha(i) coupling specificity determinant.

Asymmetric conformational changes in a GPCR dimer controlled by G-proteins

G-protein-coupled receptors (GPCRs) are key players in cell communication. Although long considered as monomeric, it now appears that these heptahelical proteins can form homo- or heterodimers. Here, we analyzed the conformational changes in each subunit of a receptor dimer resulting from agonist binding to either one or both subunits by measuring the fluorescent properties of a leukotriene B(4) receptor dimer with a single 5-hydroxytryptophan-labeled protomer. We show that a receptor dimer with only a single agonist-occupied subunit can trigger G-protein activation. We also show that the two subunits of the receptor dimer in the G-protein-coupled state differ in their conformation, even when both are liganded by the agonist. No such asymmetric conformational changes are observed in the absence of G-protein, indicating that the interaction of the G-protein with the receptor dimer brings specific constraints that prevent a symmetric functioning of this dimer. These data open new options for the differential signaling properties of GPCR dimers.

Closely related G-protein-coupled receptors use multiple and distinct domains on G-protein alpha-subunits for selective coupling

The molecular basis of selectivity in G-protein receptor coupling has been explored by comparing the abilities of G-protein heterotrimers containing chimeric Galpha subunits, comprised of various regions of Gi1alpha, Gtalpha, and Gqalpha, to stabilize the high affinity agonist binding state of serotonin, adenosine, and muscarinic receptors. The data indicate that multiple and distinct determinants of selectivity exist for individual receptors. While the A1 adenosine receptor does not distinguish between Gi1alpha and Gtalpha sequences, the 5-HT1A and 5-HT1B serotonin and M2 muscarinic receptors can couple with Gi1 but not Gt. It is possible to distinguish domains that eliminate coupling and are defined as "critical," from those that impair coupling and are defined as "important." Domains within the N terminus, alpha4-helix, and alpha4-helix-alpha4/beta6-loop of Gi1alpha are involved in 5-HT and M2 receptor interactions. Chimeric Gi1alpha/Gqalpha subunits verify the critical role of the Galpha C terminus in receptor coupling, however, the individual receptors differ in the C-terminal amino acids required for coupling. Furthermore, the EC50 for interactions with Gi1 differ among the individual receptors. These results suggest that coupling selectivity ultimately involves subtle and cooperative interactions among various domains on both the G-protein and the associated receptor as well as the G-protein concentration.

Dissecting receptor-G protein specificity using G alpha chimeras

In conclusion, by taking advantage of the overall sequence homology and structural similarity of G alpha subunits, functional chimeric G alpha subunits can be generated and used as tools for the identification of sequence-specific factors that mediate receptor: G protein specificity. The [35S]GTP gamma S binding assay and the affinity shift activity assay are two sensitive biochemical approaches that can be used to assess receptor: G protein coupling in vitro. These in vitro assays limit confounding influences from cellular proteins and allow for the strict control of receptor: G protein ratios.

Functional interaction between bovine rhodopsin and G protein transducin

To elucidate the mechanisms of specific coupling of bovine rhodopsin with the G protein transducin (G(t)), we have constructed the bovine rhodopsin mutants whose second or third cytoplasmic loop (loop 2 or 3) was replaced with the corresponding loop of the G(o)-coupled scallop rhodopsin and investigated the difference in the activation abilities for G(t), G(o), and G(i) among these mutants and wild type. We have also prepared the Galpha(i) mutants whose C-terminal 11 or 5 amino acids were replaced with those of Galpha(t), Galpha(o), and Galpha(q) to evaluate the role of the C-terminal tail of the alpha-subunit on the specific coupling of bovine rhodopsin with G(t). Replacement of loop 2 of bovine rhodopsin with that of the scallop rhodopsin caused about a 40% loss of G(t) and G(o) activation, whereas that of loop 3 enhanced the G(o) activation four times with a 60% decrease in the G(t) activation. These results indicated that loop 3 of bovine rhodopsin is one of the regions responsible for the specific coupling with G(t). Loop 3 of bovine rhodopsin discriminates the difference of the 6-amino acid sequence (region A) at a position adjacent to the C-terminal 5 amino acids of the G protein, resulting in the different activation efficiency between G(t) and G(o). In addition, the binding of region A to loop 3 of bovine rhodopsin is essential for activation of G(t) but not G(i), even though the sequence of the region A is almost identical between Galpha(t) and Galpha(i). These results suggest that the binding of loop 3 of bovine rhodopsin to region A in Galpha(t) is one of the mechanisms of specific G(t) activation by bovine rhodopsin.

Receptor signaling and structure: insights from serotonin-1 receptors

Studies of the serotonin (5-HT) receptors have illustrated several important concepts in G-protein-mediated signaling. These concepts include G-protein specificity and cellular specificity of signaling; mechanisms of transactivation; receptor states and constitutive receptor activity; and the structural basis of coupling. The 5-HT1 receptors couple via specific G(i)/G(o) proteins to inhibitory pathways [inhibition of adenylyl cyclase (AC) activity and regulation of ion channels], but also to stimulate phospholipase C, ACII, and the mitogen-activated protein kinase (MAPK) growth-signaling pathway. 5-HT1 receptors initiate novel endocytotic and Ca(2+)-dependent pathways to activate MAPK acutely, but can downregulate MAPK on chronic activation. These pathways are often mediated via distinct G(i)/G(o)-protein subtypes. Desensitization by multiple protein kinases via receptor phosphorylation is pathway selective. Structural determination of 5-HT1 receptor and G-protein domains that mediate G-protein-specific coupling and desensitization could lead to the development of highly selective ligands that directly regulate receptor-G-protein coupling.

Enhanced contraction to 5-hydroxytryptamine is not due to "unmasking" of 5-hydroxytryptamine(1b) receptors in the mesenteric artery of the deoxycorticosterone acetate-salt rat

5-Hydroxytryptamine(1B) (5-HT(1B)) receptors have been implicated in mediating arterial contraction to 5-HT. Additionally, the 5-HT(1B) receptor has been reported to be "unmasked" by depolarizing stimuli. We hypothesized that 5-HT(1B) receptors in arteries from hypertensive animals, arteries reported to have a depolarized resting membrane potential in smooth muscle cells, are unmasked and participate in the supersensitivity observed to 5-HT in hypertension. We used the isolated tissue bath apparatus and examined the response of superior mesenteric arteries from normotensive sham and hypertensive deoxycorticosterone acetate (DOCA)-salt rats. The 5-HT(1B) agonists CP93129 and sumatriptan (10(-9) to 10(-5) mol/L) caused a maximal contraction (50+/-12% of phenylephrine [10(-5) mol/L] contraction) in arteries from DOCA-salt rats; no contraction was observed in arteries from normotensive rats. The 5-HT(1B) receptor antagonist GR55562 (100 nmol/L) inhibited both the 5-HT- (4-fold rightward shift) and CP93129-induced (11-fold rightward shift) contractions in mesenteric arteries from hypertensive DOCA-salt rats. In other experiments, arteries from normotensive rats were incubated with 15 mmol/L KCl, as a depolarizing stimulus, and then exposed to 5-HT and CP93129. In the presence of KCl, a small leftward shift to 5-HT was observed. However, the presence of a depolarizing stimulus was unable to produce changes in the 5-HT maximal response to resemble that of arteries from DOCA-salt rats, nor was contraction to CP93129 observed. These data support the conclusions that 5-HT(1B) receptors mediate contraction in mesenteric arteries from hypertensive rats and that this enhanced response to 5-HT is not due to membrane depolarization alone.

An intramolecular contact in Galpha transducin that participates in maintaining its intrinsic GDP release rate

Receptor mediated stimulation of the G protein-alpha subunit leads to exchange of GDP for GTP, activating the protein. Spontaneous GDP release from Galpha can also lead to the active state, if GTP in solution binds the nucleotide binding pocket. The purpose of this study is to evaluate the molecular determinants for maintaining the spontaneous GDP release rates between two Galpha subunits. Galpha(t) has a low rate of nucleotide release, compared to Galpha(i1). Galpha(t/i1) chimeras were used to explore the molecular basis for this behavior. The C-terminal alpha4-helix, the N-terminal 56 residues and the Switch I/II regions of Galpha(t) were shown to affect the low spontaneous GDP release rate in Galpha(t). A specific molecular contact between Asp26 and Asn191 was found in Galpha(t) that is not present in Galpha(i1). In two chimeras disrupting this interaction produced an increased spontaneous GDP release; restoring the contact present in Galpha(t) into these chimeras decreased the GDP release rate by half as compared to the original chimeras. Similarly, introduction of this contact in wild-type Galpha(i1) decreased the GDP release rate of Galpha(i1) by half. Differences in GDP release rates may reflect physiological roles these proteins play in living systems.

A novel site on the Galpha -protein that recognizes heptahelical receptors

Specific domains of the G-protein alpha subunit have been shown to control coupling to heptahelical receptors. The extreme N and C termini and a region between alpha4 and alpha5 helices of the G-protein alpha subunit are known to determine selective interaction with the receptors. The metabotropic glutamate receptor 2 activated both mouse Galpha(15) and its human homologue Galpha(16), whereas metabotropic glutamate receptor 8 activated Galpha(15) only. The extreme C-terminal 20 amino acid residues are identical between the Galpha(15) and Galpha(16) and are therefore unlikely to be involved in coupling selectivity. Our data reveal two regions on Galpha(16) that inhibit its coupling to metabotropic glutamate receptor 8. On a three-dimensional model, both regions are found in a close proximity to the extreme C terminus of Galpha(16). One module comprises alpha4 helix, alpha4-beta6 loop (L9 Loop), beta6 sheet, and alpha5 helix. The other, not described previously, is located within the loop that links the N-terminal alpha helix to the beta1 strand of the Ras-like domain of the alpha subunit. Coupling of Galpha(16) protein to the metabotropic glutamate receptor 8 is partially modulated by each module alone, whereas both modules are needed to eliminate the coupling fully.

Structural requirements for the stabilization of metarhodopsin II by the C terminus of the alpha subunit of transducin

The retinal receptor rhodopsin undergoes a conformational change upon light excitation to form metarhodopsin II (Meta II), which allows interaction and activation of its cognate G protein, transducin (G(t)). A C-terminal 11-amino acid peptide from transducin, G(talpha)-(340-350), has been shown to both bind and stabilize the Meta II conformation, mimicking heterotrimeric G(t). Using a combinatorial library we identified analogs of G(talpha)-(340-350) that bound light-activated rhodopsin with high affinity (Martin, E. L., Rens-Domiano, S., Schatz, P. J., and Hamm, H. E. (1996) J. Biol. Chem. 271, 361-366). We have made peptides with key substitutions either on the background of the native G(talpha)-(340-350) sequence or on the high affinity sequences and used the stabilization of Meta II as a tool to determine which amino acids are critical in G protein-rhodopsin interaction. Removal of the positive charge at the N termini by acylation or delocalization of the charge by K to R substitution enhances the affinity of the G(talpha)-(340-350) peptides for Meta II, whereas a decrease was observed following C-terminal amidation. Cys-347, a residue conserved in pertussis toxin-sensitive G proteins, was shown to interact with a hydrophobic site in Meta II. These studies provide further insight into the mechanism of interaction between the G(talpha) C terminus and light-activated rhodopsin.

The amino terminus of Galpha(z) is required for receptor recognition, whereas its alpha4/beta6 loop is essential for inhibition of adenylyl cyclase

G(z) couples to most of the known G(i)-linked receptors and its alpha subunit (Galpha(z)) inhibits adenylyl cyclases as efficiently as Galpha(i) subtypes. A series of chimeric Galpha subunits with different portions of Galpha(z) and Galpha(t1) (a regulator of cGMP phosphodiesterase) were constructed to study the essential structural elements of Galpha(z) that determine receptor coupling and effector interaction. The receptor-mediated functions of the chimeras were assessed in two aspects: 1) stimulation of type 2 adenylyl cyclase through the release of betagamma subunits from the chimeras, and 2) inhibition of isoproterenol-stimulated adenylyl cyclase by the chimeric Galpha subunits. The results suggested that the presence of both termini of Galpha(z) were critical for coupling to delta-opioid receptor, with the N-terminal region being more important. Moreover, a stretch of amino acids (295-319) corresponding to the alpha4/beta6 loop was identified as one of the adenylyl cyclase inhibitory domains of Galpha(z).

FTIR spectroscopy of complexes formed between metarhodopsin II and C-terminal peptides from the G-protein alpha- and gamma-subunits

Metarhodopsin II (MII) provides the active conformation of rhodopsin for interaction with the G-protein, Gt. Fourier transform infrared spectra from samples prepared by centrifugation reflect the pH dependent equilibrium between MII and inactive metarhodopsin I. C-terminal synthetic peptides (Gtalpha(340-350) and Gtgamma(60-71)farnesyl) stabilize MII. We find that both peptides cause similar spectral changes not seen with control peptides (Gtalpha (K341R, L349A) and non-farnesylated Gtgamma). The spectra reflect all the protonation dependent bands normally observed when MII is formed at acidic pH. Beside the protonation dependent bands, additional features, similar with both peptides, appear in the amide I and II regions.

Rhodopsin recognition by mutant G(s)alpha containing C-terminal residues of transducin

The C-terminal regions of the heterotrimeric G protein alpha-subunits play key roles in selective activation of G proteins by their cognate receptors. In this study, mutant G(s)alpha proteins with substitutions by C-terminal residues of transducin (G(t)alpha) were analyzed for their interaction with light-activated rhodopsin (R*) to delineate the critical determinants of the G(t)alpha/R* coupling. In contrast to G(s)alpha, a chimeric G(s)alpha/G(t)alpha protein containing only 11 C-terminal residues from transducin was capable of binding to and being potently activated by R*. Our results suggest that Cys(347) and Gly(348) are absolutely essential, whereas Asp(346) is more modestly involved in the G(t) activation by R*. In addition, the analysis of the intrinsic nucleotide exchange in mutant G(s)alpha indicated an interaction between the C terminus and the switch II region in G(t)alpha.GDP. Mutant G(s)alpha containing the G(t)alpha C terminus and substitutions of Asn(239) and Asp(240) (switch II) by the corresponding G(t)alpha residues, Glu(212) and Gly(213), displayed significant reductions in spontaneous guanosine 5'-O-(3-thiotriphosphate)-binding rates to the levels approaching those in G(t)alpha. Communication between the C terminus and switch II of G(t)alpha does not appear essential for the activational coupling between G(t) and R*, but may represent one of the mechanisms by which Galpha subunits control intrinsic nucleotide exchange.