Distinct sequence elements control the specificity of G protein activation by muscarinic acetylcholine receptor subtypes

Hijacking of internal calcium dynamics by intracellularly residing viral rhodopsins

Rhodopsins are ubiquitous light-driven membrane proteins with diverse functions, including ion transport. Widely distributed, they are also coded in the genomes of giant viruses infecting phytoplankton where their function is not settled. Here, we examine the properties of OLPVR1 (Organic Lake Phycodnavirus Rhodopsin) and two other type 1 viral channelrhodopsins (VCR1s), and demonstrate that VCR1s accumulate exclusively intracellularly, and, upon illumination, induce calcium release from intracellular IP3-dependent stores. In vivo, this light-induced calcium release is sufficient to remote control muscle contraction in VCR1-expressing tadpoles. VCR1s natively confer light-induced Ca2+ release, suggesting a distinct mechanism for reshaping the response to light of virus-infected algae. The ability of VCR1s to photorelease calcium without altering plasma membrane electrical properties marks them as potential precursors for optogenetics tools, with potential applications in basic research and medicine.

The "www" of Xenopus laevis Oocytes: The Why, When, What of Xenopus laevis Oocytes in Membrane Transporters Research

After 50 years, the heterologous expression of proteins in Xenopus laevis oocytes is still essential in many research fields. New approaches and revised protocols, but also classical methods, such as the two-electrode voltage clamp, are applied in studying membrane transporters. New and old methods for investigating the activity and the expression of Solute Carriers (SLC) are reviewed, and the kinds of experiment that are still useful to perform with this kind of cell are reported. Xenopus laevis oocytes at the full-grown stage have a highly efficient biosynthetic apparatus that correctly targets functional proteins at the defined compartment. This small protein factory can produce, fold, and localize almost any kind of wild-type or recombinant protein; some tricks are required to obtain high expression and to verify the functionality. The methodologies examined here are mainly related to research in the field of membrane transporters. This work is certainly not exhaustive; it has been carried out to be helpful to researchers who want to quickly find suggestions and detailed indications when investigating the functionality and expression of the different members of the solute carrier families.

A Collision Coupling Model Governs the Activation of Neuronal GIRK1/2 Channels by Muscarinic-2 Receptors

The G protein-activated Inwardly Rectifying K⁺-channel (GIRK) modulates heart rate and neuronal excitability. Following G-Protein Coupled Receptor (GPCR)-mediated activation of heterotrimeric G proteins (Gαβγ), opening of the channel is obtained by direct binding of Gβγ subunits. Interestingly, GIRKs are solely activated by Gβγ subunits released from Gα_i/o-coupled GPCRs, despite the fact that all receptor types, for instance Gα_q-coupled, are also able to provide Gβγ subunits. It is proposed that this specificity and fast kinetics of activation stem from pre-coupling (or pre-assembly) of proteins within this signaling cascade. However, many studies, including our own, point towards a diffusion-limited mechanism, namely collision coupling. Here, we set out to address this long-standing question by combining electrophysiology, imaging, and mathematical modeling. Muscarinic-2 receptors (M2R) and neuronal GIRK1/2 channels were coexpressed in Xenopus laevis oocytes, where we monitored protein surface expression, current amplitude, and activation kinetics. Densities of expressed M2R were assessed using a fluorescently labeled GIRK channel as a molecular ruler. We then incorporated our results, along with available kinetic data reported for the G-protein cycle and for GIRK1/2 activation, to generate a comprehensive mathematical model for the M2R-G-protein-GIRK1/2 signaling cascade. We find that, without assuming any irreversible interactions, our collision coupling kinetic model faithfully reproduces the rate of channel activation, the changes in agonist-evoked currents and the acceleration of channel activation by increased receptor densities.

Structures of the M1 and M2 muscarinic acetylcholine receptor/G-protein complexes

Muscarinic acetylcholine receptors are G protein-coupled receptors that respond to acetylcholine and play important signaling roles in the nervous system. There are five muscarinic receptor subtypes (M1R to M5R), which, despite sharing a high degree of sequence identity in the transmembrane region, couple to different heterotrimeric GTP-binding proteins (G proteins) to transmit signals. M1R, M3R, and M5R couple to the G_{q/ 11} family, whereas M2R and M4R couple to the G_{i/ o} family. Here, we present and compare the cryo-electron microscopy structures of M1R in complex with G₁₁ and M2R in complex with G_oA The M1R-G₁₁ complex exhibits distinct features, including an extended transmembrane helix 5 and carboxyl-terminal receptor tail that interacts with G protein. Detailed analysis of these structures provides a framework for understanding the molecular determinants of G-protein coupling selectivity.

CB1 cannabinoid receptor-phosphorylated fourth intracellular loop structure-function relationships

A peptide comprising the juxtamembrane C-terminal intracellular loop 4 (IL4) of the CB₁ cannabinoid receptor possesses three Serine residues (Ser402, Ser411 and Ser415). Here we report the effect of Ser phosphorylation on the CB₁ IL4 peptide conformation and cellular signaling functions using nuclear magnetic resonance spectroscopy, circular dichroism, G protein activation and cAMP production. Circular dichroism studies indicated that phosphorylation at various Ser residues induced helical structure in different environments. NMR data indicates that helical content varies in the order of IL4pSer411 > IL4pSer415 > IL4 > IL4pSer402. The efficacy of phosphorylated IL4 peptides in activating Go and Gi3 ([³⁵S]GTPγS binding) and inhibiting cAMP accumulation in N18TG2 cells were correlated with helicity changes. Treatment of cells with bradykinin, which activates PKC, augmented CB₁-mediated inhibition of cAMP accumulation, and this was reversed by a PKC inhibitor, suggesting that phosphorylation of serine might be a physiologically relevant modification in vivo. We conclude that phosphorylation-dependent alterations of helicity of CB₁ IL4 peptides can increase efficacy of G protein signaling.

Unraveling a molecular determinant for clathrin-independent internalization of the M2 muscarinic acetylcholine receptor

Endocytosis and postendocytic sorting of G-protein-coupled receptors (GPCRs) is important for the regulation of both their cell surface density and signaling profile. Unlike the mechanisms of clathrin-dependent endocytosis (CDE), the mechanisms underlying the control of GPCR signaling by clathrin-independent endocytosis (CIE) remain largely unknown. Among the muscarinic acetylcholine receptors (mAChRs), the M4 mAChR undergoes CDE and recycling, whereas the M2 mAChR is internalized through CIE and targeted to lysosomes. Here we investigated the endocytosis and postendocytic trafficking of M2 mAChR based on a comparative analysis of the third cytoplasmic domain in M2 and M4 mAChRs. For the first time, we identified that the sequence (374)KKKPPPS(380) servers as a sorting signal for the clathrin-independent internalization of M2 mAChR. Switching (374)KKKPPPS(380) to the i3 loop of the M4 mAChR shifted the receptor into lysosomes through the CIE pathway; and therefore away from CDE and recycling. We also found another previously unidentified sequence that guides CDE of the M2 mAChR, (361)VARKIVKMTKQPA(373), which is normally masked in the presence of the downstream sequence (374)KKKPPPS(380). Taken together, our data indicate that endocytosis and postendocytic sorting of GPCRs that undergo CIE could be sequence-dependent.

Microsecond Molecular Dynamics Simulations Provide Insight into the Allosteric Mechanism of the Gs Protein Uncoupling from the β2 Adrenergic Receptor

Experiments have revealed that in the β(2) adrenergic receptor (β(2)AR)-Gs protein complex the α subunit (Gαs) of the Gs protein can adopt either an "open" conformation or a "closed" conformation. In the "open" conformation the Gs protein prefers to bind to the β(2)AR, while in the "closed" conformation an uncoupling of the Gs protein from the β(2)AR occurs. However, the mechanism that leads to such different behaviors of the Gs protein remains unclear. Here, we report results from microsecond molecular dynamics simulations and community network analysis of the β(2)AR-Gs complex with Gαs in the "open" and "closed" conformations. We observed that the complex is stabilized differently in the "open" and "closed" conformations. The community network analysis reveals that in the "closed" conformation there exists strong allosteric communication between the β(2)AR and Gβγ, mediated by Gαs. We suggest that such high information flows are necessary for the Gs protein uncoupling from the β(2)AR.

Active-state models of ternary GPCR complexes: determinants of selective receptor-G-protein coupling

Based on the recently described crystal structure of the β₂ adrenergic receptor - G_s-protein complex, we report the first molecular-dynamics simulations of ternary GPCR complexes designed to identify the selectivity determinants for receptor-G-protein binding. Long-term molecular dynamics simulations of agonist-bound β2AR-Gα_s and D2R-Gα_i complexes embedded in a hydrated bilayer environment and computational alanine-scanning mutagenesis identified distinct residues of the N-terminal region of intracellular loop 3 to be crucial for coupling selectivity. Within the G-protein, specific amino acids of the α5-helix, the C-terminus of the Gα-subunit and the regions around αN-β1 and α4-β6 were found to determine receptor recognition. Knowledge of these determinants of receptor-G-protein binding selectivity is essential for designing drugs that target specific receptor/G-protein combinations.

Distinct roles of M1 and M3 muscarinic acetylcholine receptors controlling oscillatory and non-oscillatory [Ca2+]i increase

We examined ACh-induced [Ca2+]i dynamics in pancreatic acinar cells prepared from mAChR subtype-specific knockout (KO) mice. ACh did not induce any [Ca2+]i increase in the cells isolated from M1/M3 double KO mice. In the cells from M3KO mice, ACh (0.3-3 μM) caused a monotonic [Ca2+]i increase. However, we found characteristic oscillatory [Ca2+]i increases in cells from M1KO mice in lower concentrations of ACh (0.03-0.3 μM). We investigated the receptor specific pattern of [Ca2+]i increase in COS-7 cells transfected with M1 or M3 receptors. ACh induced the oscillatory [Ca2+]i increase in M3 expressing cells, but not in cells expressing M1, which exhibited monotonic [Ca2+]i increases. IP3 production detected in fluorescent indicator co-transfected cells was higher in M1 than in M3 expressing cells. From the examination of four types of M1/M3 chimera receptors we found that the carboxyl-terminal region of M3 was responsible for the generation of Ca2+ oscillations. The present results suggest that the oscillatory Ca2+ increase in response to M3 stimulation is dependent upon a moderate IP3 increase, which is suitable for causing Ca(2+)-dependent IP3-induced Ca2+ release. The C-terminal domain of M3 may contribute as a regulator of the efficiency of Gq and PLC cooperation.

Crosstalk between GABAB and mGlu1a receptors reveals new insight into GPCR signal integration

G protein-coupled receptors (GPCRs) have critical functions in intercellular communication. Although a wide range of different receptors have been identified in the same cells, the mechanism by which signals are integrated remains elusive. The ability of GPCRs to form dimers or larger hetero-oligomers is thought to generate such signal integration. We examined the molecular mechanisms responsible for the GABA(B) receptor-mediated potentiation of the mGlu receptor signalling reported in Purkinje neurons. We showed that this effect does not require a physical interaction between both receptors. Instead, it is the result of a more general mechanism in which the betagamma subunits produced by the Gi-coupled GABA(B) receptor enhance the mGlu-mediated Gq response. Most importantly, this mechanism could be generally applied to other pairs of Gi- and Gq-coupled receptors and the signal integration varied depending on the time delay between activation of each receptor. Such a mechanism helps explain specific properties of cells expressing two different Gi- and Gq-coupled receptors activated by a single transmitter, or properties of GPCRs naturally coupled to both types of the G protein.

The NALCN ion channel is activated by M3 muscarinic receptors in a pancreatic beta-cell line

A previously uncharacterized putative ion channel, NALCN (sodium leak channel, non-selective), has been recently shown to be responsible for the tetrodotoxin (TTX)-resistant sodium leak current implicated in the regulation of neuronal excitability. Here, we show that NALCN encodes a current that is activated by M3 muscarinic receptors (M3R) in a pancreatic β-cell line. This current is primarily permeant to sodium ions, independent of intracellular calcium stores and G proteins but dependent on Src activation, and resistant to TTX. The current is recapitulated by co-expression of NALCN and M3R in human embryonic kidney-293 cells and in Xenopus oocytes. We also show that NALCN and M3R belong to the same protein complex, involving the intracellular I–II loop of NALCN and the intracellular i3 loop of M3R. Taken together, our data show the molecular basis of a muscarinic-activated inward sodium current that is independent of G-protein activation, and provide new insights into the properties of NALCN channels.

A novel mechanism of hippocampal LTD involving muscarinic receptor-triggered interactions between AMPARs, GRIP and liprin-alpha

BACKGROUND

Long-term depression (LTD) in the hippocampus can be induced by activation of different types of G-protein coupled receptors, in particular metabotropic glutamate receptors (mGluRs) and muscarinic acetylcholine receptors (mAChRs). Since mGluRs and mAChRs activate the same G-proteins and isoforms of phospholipase C (PLC), it would be expected that these two forms of LTD utilise the same molecular mechanisms. However, we find a distinct mechanism of LTD involving GRIP and liprin-alpha.

RESULTS

Whilst both forms of LTD require activation of tyrosine phosphatases and involve internalisation of AMPARs, they use different molecular interactions. Specifically, mAChR-LTD, but not mGluR-LTD, is blocked by peptides that inhibit the binding of GRIP to the AMPA receptor subunit GluA2 and the binding of GRIP to liprin-alpha. Thus, different receptors that utilise the same G-proteins can regulate AMPAR trafficking and synaptic efficacy via distinct molecular mechanisms.

CONCLUSION

Our results suggest that mAChR-LTD selectively involves interactions between GRIP and liprin-alpha. These data indicate a novel mechanism of synaptic plasticity in which activation of M1 receptors results in AMPAR endocytosis, via a mechanism involving interactions between GluA2, GRIP and liprin-alpha.

Unique helical conformation of the fourth cytoplasmic loop of the CB1 cannabinoid receptor in a negatively charged environment

The proximal portion of the C-terminus of the CB(1) cannabinoid receptor is a primary determinant for G-protein activation. A 17 residue proximal C-terminal peptide (rodent CB1 401-417), the intracellular loop 4 (IL4) peptide, mimicked the receptor's G-protein activation domain. Because of the importance of the cationic amino acids to G-protein activation, the three-dimensional structure of the IL4 peptide in a negatively charged sodium dodecyl sulfate (SDS) micellar environment has been studied by two-dimensional proton nuclear magnetic resonance (2D (1)H NMR) spectroscopy and distance geometry calculations. Unambiguous proton NMR assignments were carried out with the aid of correlation spectroscopy (DQF-COSY and TOCSY) and nuclear Overhauser effect spectroscopy (NOESY and ROESY) experiments. The distance constraints were used in torsion angle dynamics algorithm for NMR applications (DYANA) to generate a family of structures which were refined using restrained energy minimization and dynamics. In water, the IL4 peptide prefers an extended conformation, whereas in SDS micelles, 3(10)-helical conformation is induced. The predominance of 3(10)-helical domain structure in SDS represents a unique difference compared with structure in alternative environments, which can significantly impact global electrostatic surface potential on the cytoplasmic surface of the CB(1) receptor and might influence the signal to the G-proteins.

Calmodulin potentiates G beta gamma activation of phospholipase C-beta3

Phospholipase C-beta (PLC-beta) isozymes (EC 3.1.4.11) hydrolyze the membrane phospholipid phosphatidylinositol-4,5-bisphosphate to generate intracellular second messenger signaling molecules inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) in response to receptor activation and other cellular stimuli. PLCbeta1 and PLCbeta3 isozymes were previously demonstrated to bind the calcium-sensitive molecule calmodulin [McCullar JS, Larsen SA, Millimaki RA, Filtz TM. Calmodulin is a phospholipase C-{beta} interacting protein. J Biol Chem 2003;278(36):33708-13]. We have now shown through fluorescence anisotropy that calmodulin/PLCbeta3 affinities increase with increasing calcium in a physiologically relevant concentration range. The bimolecular affinity constants for calmodulin interaction with PLCbeta1 or PLCbeta3 were estimated as 260 and 200 nM, respectively, from fluorescence anisotropy data. There was no effect of calmodulin on basal or G alpha q-stimulated catalytic activity for either isozyme. However, the interaction between calmodulin and PLCbeta3 leads to potentiation of activation by the G-protein beta gamma dimer in an in vitro assay. 1321N1 cells treated with calmodulin inhibitors concurrent with and post-stimulation of muscarinic receptors significantly reduced [3H]PIP hydrolysis. Together these data are suggestive of cooperative role for calmodulin in the G-protein beta gamma dimer-stimulated activity of PLCbeta3.

Movement of 'gating charge' is coupled to ligand binding in a G-protein-coupled receptor

Activation by agonist binding of G-protein-coupled receptors (GPCRs) controls most signal transduction processes. Although these receptors span the cell membrane, they are not considered to be voltage sensitive. Recently it was shown that both the activity of GPCRs and their affinity towards agonists are regulated by membrane potential. However, it remains unclear whether GPCRs intrinsically respond to changes in membrane potential. Here we show that two prototypical GPCRs, the m2 and m1 muscarinic receptors (m2R and m1R), display charge-movement-associated currents analogous to 'gating currents' of voltage-gated channels. The gating charge-voltage relationship of m2R correlates well with the voltage dependence of the affinity of the receptor for acetylcholine. The loop that couples m2R and m1R to their G protein has a crucial function in coupling voltage sensing to agonist-binding affinity. Our data strongly indicate that GPCRs serve as sensors for both transmembrane potential and external chemical signals.

Conserved motifs in somatostatin, D2-dopamine, and alpha 2B-adrenergic receptors for inhibiting the Na-H exchanger, NHE1

Receptor subtypes within families of G protein-coupled receptors that are activated by similar ligands can regulate distinct intracellular effectors. We identified conserved motifs within intracellular domains 2 and 3 of selective subtypes of several G protein-coupled receptor families that confer coupling to the Na-H exchanger, NHE1. A T(s,p)V motif within intracellular domain 2 and a QQ(r) motif within intracellular domain 3 are shared by the somatostatin receptor subtypes SSTR1, -3, and -4, which couple to the inhibition of NHE1, but not by SSTR2 and -5, which do not signal to NHE1. Only the collective substitution of cognate SSTR2 residues with these two motifs conferred the ability of mutant SSTR2 to inhibit NHE1. Both motifs are present in D(2)-dopamine receptors, which inhibit NHE1, and in alpha(2B)-adrenergic receptors, which couple to the inhibition of NHE1, but not in alpha(2A)-adrenergic receptors, which do not regulate NHE1. These findings indicate that motifs shared by different subfamilies of G protein-coupled receptors, but not necessarily by receptor subtypes within a subfamily, can confer coupling to a common effector.

The first inner loop of endothelin receptor type B is necessary for specific coupling to Galpha 13

Endothelin (EDN) receptor type B (EDNRB) activates serum response factor (SRF) via G(q/11) and G(12/13) G proteins. In this study, we investigated the involvement of intracellular loop sequences of EDNRB in coupling to these G proteins. EDNRB mutants were generated and tested for their abilities to activate SRF in NIH3T3 cells and in the mouse embryonic fibroblast cell line (F(q/11)) lacking both Galpha(q) and Galpha(11). EDNRB can activate SRF in NIH3T3 cells via G(q/11), although it can only activate SRF through G(12/13) in F(q/11) cells. Mutants with mutations in the second and third inner loops of EDNRB functioned in the same manner in both cell lines, either able or unable to activate SRF. This finding suggests that the second and third inner loops of EDNRB either participate or not in coupling to both G(q/11) and G(12/13) but are not specific for either one. However, in the first inner loop, a substitution of three Ala residues for Met(128)-Arg(129)-Asn(130) abolished the ability to activate SRF only in F(q/11) cells, suggesting that this mutation might specifically disrupt the coupling to G(12/13) rather than to G(q/11). Further characterization of this first inner loop mutant revealed that exogenous expression of Galpha(12) or Galpha(q) could restore SRF activation, whereas the expression of Galpha(13) did not. Therefore, we conclude that although the three intracellular loops of EDNRB may be involved in coupling to G proteins, residues Met(128)-Arg(129)-Asn(130) in the first intracellular loop are specifically required for activation of Galpha(13).

Role of the intracellular domains of CXCR4 in SDF-1-mediated signaling

The CXCR4 chemokine receptor is a G(i) protein-coupled receptor that triggers multiple intracellular signals in response to stromal cell-derived factor 1 (SDF-1), including calcium mobilization and p44/42 extracellular signal-regulated kinases (ERK1/2). Transduced signals lead to cell chemotaxis and are terminated through receptor internalization depending on phosphorylation of the C terminus part of CXCR4. Receptor endocytosis is also required for some receptors to stimulate ERK1/2 and to migrate through a chemokine gradient. In this study, we explored the role played by the 3 intracellular loops (ICL1-3) and the C terminus domain of CXCR4 in SDF-1-mediated signaling by using human embryonic kidney (HEK)-293 cells stably expressing wild-type or mutated forms of CXCR4. ICL3 of CXCR4 is specifically involved in G(i)-dependent signals such as calcium mobilization and ERK activation, but does not trigger CXCR4 internalization after SDF-1 binding, indicating that ERK phosphorylation is independent of CXCR4 endocytosis. Surprisingly, ICL2, with or without the aspartic acid, arginine, and tyrosine (DRY) motif, is dispensable for G(i) signaling. However, ICL2 and ICL3, as well as the C terminus part of CXCR4, are needed to transduce SDF-1-mediated chemotaxis, suggesting that this event involves multiple activation pathways and/or cooperation of several cytoplasmic domains of CXCR4.

Structure of the third intracellular loop of the human cannabinoid 1 receptor

The third cytoplasmic loop (IC3) is a determinant in the dynamic life cycle of G protein-coupled receptors, including the activation, internalization, desensitization, and resensitization processes. Here, we characterize the structural features of the IC3 of the cannabinoid 1 receptor (CB1) in micelle solution using heteronuclear, (1)H,(15)N-high-resolution NMR methods. The IC3 construct was designed to contain one-third of each of the transmembrane helices (TMs 5 and 6) to tether the protein to the hydrophobic portion of the micelle. Indeed, the NMR analysis illustrates prominent alpha-helices at the N-terminus (G1-R10) and C-terminus (Q37-T47) of the IC3 receptor domain, corresponding to the cytoplasmic termini of TM5 and TM6. The structural features of the central portion of the IC3 consist of a small alpha-helix, adjacent to the terminus of TM5. The remainder is mostly unstructured as indicated by the NMR-based observables (NOEs and chemical shifts). Despite the lack of secondary structure, the hydrophobic triplet of isoleucine residues in the center of the IC3 is found in molecular dynamics simulations to associate with the lipid environment, producing two smaller loops out of the IC3. Previous studies examining mastoparan and related peptides and their ability to activate G proteins have concluded an alpha-helix is required for efficient binding and activation. Our structural results for the IC3 of CB1 would then suggest that in the intact receptor the G protein is activated by the alpha-helices of the cytoplasmic ends of TM5 or TM6 and not the unstructured central region of the IC3.

Novel interaction between the M4 muscarinic acetylcholine receptor and elongation factor 1A2

The activation of the muscarinic acetylcholine receptor (mAChR) family, consisting of five subtypes (M1-M5), produces a variety of physiological effects throughout the central nervous system. However, the role of each individual subtype remains poorly understood. To further elucidate signal transduction pathways for specific subtypes, we used the most divergent portion of the subtypes, the intracellular third (i3) loop, as bait to identify interacting proteins. Using a brain pull-down assay, we identify elongation factor 1A2 (eEF1A2) as a specific binding partner to the i3 loop of M4, and not to M1 or M2. In addition, we demonstrate a direct interaction between these proteins. In the rat striatum, the M4 mAChR colocalizes with eEF1A2 in the soma and neuropil. In PC12 cells, endogenous eEF1A2 co-immunoprecipitates with the endogenous M4 mAChR, but not with the endogenous M1 mAChR. In our in vitro model, M4 dramatically accelerates nucleotide exchange of eEF1A2, a GTP-binding protein. This indicates the M4 mAChR is a guanine exchange factor for eEF1A2. eEF1A2 is an essential GTP-binding protein for protein synthesis. Thus, our data suggest a novel role for M4 in the regulation of protein synthesis through its interaction with eEF1A2.

Identification of G protein-coupled, inward rectifier potassium channel gene products from the rat anterior pituitary gland

Dopamine (DA) is a physiological regulator of PRL secretion, exerting tonic inhibitory control. DA activates an inward rectifier K(+) (IRK) channel in rat lactotropes, causing membrane hyperpolarization and inhibition of Ca(2+)-dependent action potentials. Both the activation of this effector K(+) channel and the inhibition of PRL release are mediated by D(2)-type receptor activation and pertussis toxin- sensitive G proteins. To study the molecular basis of this physiologically relevant channel, a homology-based PCR approach was employed to identify members of the IRK channel family expressed in the anterior pituitary gland. Nondegenerate primers corresponding to regions specific for IRK channels known to be G protein activated (GIRKs; gene subfamily Kir 3.0) were synthesized and used in the PCR with reverse transcribed female rat anterior pituitary messenger RNA as the template. PCR products of predicted sizes for Kir 3.1, 3.2, and 3.4 were consistently observed by ethidium bromide staining after 16 amplification cycles. The identities of the products were confirmed by subcloning and sequencing. Expression of each of these gene products in anterior pituitary was confirmed by Northern blot analysis. Functional analysis of the GIRK proteins was performed in the heterologous expression system, Xenopus laevis oocytes. Macroscopic K(+) currents were examined in oocytes injected with different combinations of Kir 3.0 complementary RNA (cRNA) and G protein subunit (beta(1)gamma(2)) cRNA. The current-voltage relationships demonstrated strong inward rectification for each individual and pairwise combination of GIRK channel subunits. Oocytes coinjected with any pair of GIRK subunit cRNA exhibited significantly larger inward K(+) currents than oocytes injected with only one GIRK channel subtype. Ligand-dependent activation of only one of the GIRK combinations (GIRK1 and GIRK4) was observed when channel subunits were coexpressed with the D(2) receptor in Xenopus oocytes. Dose-response data fit to a Michaelis-Menten equation gave an apparent K(d) similar to that for DA binding in anterior pituitary tissue. GIRK1 and GIRK4 proteins were coimmunoprecipitated from anterior pituitary lysates, confirming the presence of native GIRK1/GIRK4 oligomers in this tissue. These data indicate that GIRK1 and GIRK4 are excellent candidate subunits for the D(2)-activated, G protein-gated channel in pituitary lactotropes, where they play a critical role in excitation-secretion coupling.

Reactive oxygen species regulate signaling pathways induced by M1 muscarinic receptors in PC12M1 cells

Activation of the m1 muscarinic receptor subtype in rat pheochromocytoma (PC12) cells stably expressing cloned m1 muscarinic acetylcholine receptors was previously shown to induce morphological changes and growth arrest. However, the signaling pathways which lead to these effects were not identified. In an attempt to characterize the intracellular signaling that might be involved in the muscarinic-induced effects, we investigated the role of reactive oxygen species in the regulation of these processes. Stimulation of the muscarinic receptor in these cells increased the intracellular concentrations of reactive oxygen species. Muscarinic activation induced intracellular signaling pathways that involve activation of Ras, extracellular signal-regulated kinase (ERK), and p38. These pathways were partially blocked when reactive oxygen species (ROS) production was prevented by the antioxidant N-acetylcysteine. Other muscarinic-induced signals, such as activation of c-Jun NH(2)-terminal kinase (JNK) or an increase in the binding activity of the transcription factors nuclear factor-kappa B and activator protein-1, were inhibited by the antioxidant dicoumarol. N-Acetylcysteine also blocked the growth arrest and changes in cell shape induced by stimulation of the muscarinic receptor in PC12M1 cells. These findings suggest that ROS act as second messengers in muscarinic-induced cellular signaling. Moreover, generation of ROS appears to be an early and critical intermediary event, which occurs immediately after stimulation of the muscarinic receptor and affects in a variety of mechanisms the muscarinic-mediated cellular signaling.

Receptors coupling to G proteins: is there a signal behind the sequence?

Upon the binding of their ligands, G protein-coupled receptors couple to the heterotrimeric G proteins to transduce a signal. One receptor family may couple to a single G protein subtype and another family to several ones. Is there a signal in the receptor sequence that can give an indication of the G protein subtype selectivity? We used a sequence analysis method on biogenic amine and adenosine receptors and concluded that a weak signal can be detected in receptor families where specialization for coupling to a given G protein occurred during a recent divergent evolutionary process. Proteins 2000;41:448-459.

Dominance of G(s) in doubly G(s)/G(i)-coupled chimaeric A(1)/A(2A) adenosine receptors in HEK-293 cells

A(1) adenosine receptors inhibit adenylate cyclase by activating G(i)/G(o), whereas A(2A) receptors activate G(s). We examined how regions of A(1) and A(2A) receptors regulate coupling to G-proteins by constructing chimaeras in which the third intracellular loops (3ICL or L) and/or the C-termini (or T) were switched. Pertussis toxin (PTX) was used in membrane radioligand binding assays to calculate the fraction of recombinant receptors coupled to G(i)/G(o) and in whole cells to differentially influence agonist-stimulated cAMP accumulation. Switching A(1)/A(2A) 3ICL domains results in receptors that maintain binding selectivity for ligands but are doubly coupled. Receptor chimaeras with an A(1) 3ICL sequence (A(2A)/A(1)L or A(2A)/A(1)LT) respond to agonist stimulation with elevated cAMP despite being coupled predominantly to G(i)/G(o). These chimaeras have basal cAMP levels lower than those of wild-type A(2A) receptors, similar to wild-type A(1) receptors. The A(1) C-terminus modulates the coupling of receptors with A(1) 3ICL such that A(2A)/A(1)LT is better coupled to G(i)/G(o) than A(2A)/A(1)L. The C-terminus has little impact on coupling to receptors containing A(2A) 3ICL sequence. Our results show that the C-terminus sequence selectively facilitates coupling to G(i)/G(o) mediated by A(1) 3ICL and not by other intracellular domains that favour G(i) coupling. The C-terminus sequence has little or no effect on coupling to G(s). For doubly G(s)/G(i)-coupled adenosine receptors in HEK-293 cells, G(s)-mediated stimulation predominates over G(i)/G(o)-mediated inhibition of adenylate cyclase. We discuss the signalling consequences of simultaneously activating opposing G-proteins within single cells.

The role of receptor structure in determining adenosine receptor activity

Adenosine produces a wide variety of physiological effects through the activation of cell surface adenosine receptors (ARs). ARs are members of the G-protein-coupled receptor family, and currently, four subtypes, the A1AR, A2AAR, A2BAR, and A3AR, are recognized. This review focuses on the role of receptor structure in governing various facets of AR activity. Ligand-binding properties of ARs are primarily dictated by amino acids in the transmembrane domains of the receptors, although a role for extracellular domains of certain ARs has been suggested. Studies have identified certain amino acids conserved amongst AR subtypes that are critical for ligand recognition, as well as additional residues that may differentiate between agonist and antagonist ligands. Receptor regions responsible for activation of Gs have been identified for the A2AAR. The location of these intracellular sites is consistent with findings described for other G-protein-coupled receptors. Site-directed mutagenesis has been employed to analyze the structural basis for the differences in the kinetics of the desensitization response displayed by various AR subtypes. For the A2AAR and A3AR, agonist-stimulated phosphorylation of the AR, presumably via a G-protein receptor kinase, has been shown to occur. For these AR subtypes, intracellular regions or individual amino acids that may be targets for this phosphorylation have been identified. Finally, the role of A1AR gene structure in regulating the expression of this AR subtype is reviewed.

Expression and function of the gonadotropin-releasing hormone receptor are dependent on a conserved apolar amino acid in the third intracellular loop

The coupling of agonist-activated heptahelical receptors to their cognate G proteins is often dependent on the amino-terminal region of the third intracellular loop. Like many G protein-coupled receptors, the gonadotropin-releasing hormone (GnRH) receptor contains an apolar amino acid in this region at a constant distance from conserved Pro and Tyr/Asn residues in the fifth transmembrane domain (TM V). An analysis of the role of this conserved residue (Leu(237)) in GnRH receptor function revealed that the binding affinities of the L237I and L237V mutant receptors were unchanged, but their abilities to mediate GnRH-induced inositol phosphate signaling, G protein coupling, and agonist-induced internalization were significantly impaired. Receptor expression at the cell surface was reduced by replacement of Leu(237) with Val, and abolished by replacement with Ala, Arg, or Asp residues. These results are consistent with molecular modeling of the TM V and VI regions of the GnRH receptor, which predicts that Leu(237) is caged by several apolar amino acids (Ile(233), Ile(234), and Val(240) in TM V, and Leu(262), Leu(265), and Val(269) in TM VI) to form a tight hydrophobic cluster. These findings indicate that the conserved apolar residue (Leu(237)) in the third intracellular loop is an important determinant of GnRH receptor expression and activation, and possibly that of other G protein-coupled receptors.

Cloning and characterization of rat leukotriene B(4) receptor

Leukotriene B(4) (LTB(4)) is a potent chemoattractant for neutrophils and eosinophils. cDNAs for LTB(4) receptor (BLT) have been cloned from human, mouse, and guinea pig. Here we report the isolation of BLT from rat genomic library. Rat BLT consists of 351 amino acids with homologies of 80.2, 93.2, and 71.6%, to human, mouse, and guinea pig BLT, respectively. When expressed in human embryonic kidney (HEK)-293 cells, rat BLT showed a specific and high-affinity binding to LTB(4) with a Kd value of 0.68 nM (mean, n = 3). Northern blot analysis showed that BLT is exclusively expressed in polymorphonuclear leukocytes. Furthermore, the expression of BLT was high in proteosepeptone-activated peritoneal macrophages, while the resident macrophages did not show significant expression. The present results suggest important roles of LTB(4) in macrophage recruitment and activation.

Association of the D2 dopamine receptor third cytoplasmic loop with spinophilin, a protein phosphatase-1-interacting protein

Signaling through D2 class dopamine receptors is crucial to correct brain development and function, and dysfunction of this system is implicated in major neurological disorders such as Parkinson's disease and schizophrenia. To investigate potential novel mechanisms of D2 receptor regulation, the third cytoplasmic loop of the D2 dopamine receptor was used to screen a rat hippocampal yeast two-hybrid library. Spinophilin, a recently characterized F-actin and protein phosphatase-1-binding protein with a single PDZ domain was identified as a protein that specifically associates with this region of D2 receptors. A direct interaction between spinophilin and the D2 receptor was confirmed in vitro using recombinant fusion proteins. The portion of spinophilin responsible for interacting with the D2 third cytoplasmic loop was narrowed to a region that does not include the actin-binding domain, the PDZ domain, or the coiled-coil. This region is distinct from the site of interaction with protein phosphatase-1, and both D2 receptors and protein phosphatase-1 may bind spinophilin at the same time. The interaction is not mediated via the unique 29-amino acid insert in D2long; both D2long and D2short third cytoplasmic loops interact with spinophilin in vitro and in yeast two-hybrid assays. Expression of D2 receptors containing an extracellular hemagglutinin epitope in Madin-Darby canine kidney cells results in co-localization of receptor and endogenous spinophilin as determined by immunocytochemistry using antibodies directed against spinophilin and the HA tag. We hypothesize that spinophilin is important for establishing a signaling complex for dopaminergic neurotransmission through D2 receptors by linking receptors to downstream signaling molecules and the actin cytoskeleton.

Cloning of a new mouse two-P domain channel subunit and a human homologue with a unique pore structure

Mouse KCNK6 is a new subunit belonging to the TWIK channel family. This 335-amino acid polypeptide has four transmembrane segments, two pore-forming domains, and a Ca2+-binding EF-hand motif. Expression of KCNK6 transcripts is principally observed in eyes, lung, stomach and embryo. In the eyes, immunohistochemistry reveals protein expression only in some of the retina neurons. Although KCNK6 is able to dimerize as other functional two-P domain K+ channels when it is expressed in COS-7 cells, it remains in the endoplasmic reticulum and is unable to generate ionic channel activity. Deletions, mutations, and chimera constructions suggest that KCNK6 is not an intracellular channel but rather a subunit that needs to associate with a partner, which remains to be discovered, in order to reach the plasma membrane. A closely related human KCNK7-A subunit has been cloned. KCNK7 displays an intriguing GLE sequence in its filter region instead of the G(Y/F/L)G sequence, which is considered to be the K+ channel signature. This subunit is alternatively spliced and gives rise to the shorter forms KCNK7-B and -C. None of the KCNK7 structures can generate channel activity by itself. The KCNK7 gene is situated on chromosome 11, in the q13 region, where several candidate diseases have been identified.

Muscarinic acetylcholine receptors: structural basis of ligand binding and G protein coupling

Muscarinic acetylcholine receptors (m1-m5) were studied by a combined molecular genetic/pharmacologic approach to elucidate the molecular characteristics of the ligand binding site and of the receptor domains involved in G protein coupling. Site-directed mutagenesis studies of the rat m3 muscarinic receptor suggest that the acetylcholine binding domain is formed by a series of hydrophilic amino acids located in the "upper" half of transmembrane domains (TM) III, V, VI, and VII. Moreover, we showed that mutational modification of a TM VI Asn residue (Asn507 in the rat m3 receptor sequence) which is characteristic for the muscarinic receptor family has little effect on high-affinity acetylcholine binding and receptor activation, but results in dramatic reductions in binding affinities for certain subclasses of muscarinic antagonists. The N-terminal portion of the third intracellular loop (i3) of muscarinic and other G protein-coupled receptors has been shown to play a central role in determining the G protein coupling profile of a given receptor subtype. Insertion mutagenesis studies with the rat m3 muscarinic receptor suggest that this region forms an amphiphilic alpha-helix and that the hydrophobic side of this helix represents an important G protein recognition surface. Further mutational analysis of this receptor segment showed that Tyr254 located at the N-terminus of the i3 loop of the m3 muscarinic receptor plays a key role in muscarinic receptor-induced Gq activation. The studies described here, complemented by biochemical and biophysical approaches, should eventually lead to a detailed structural model of the ligand-receptor-G protein complex.

Cellular and molecular basis for electrical rhythmicity in gastrointestinal muscles

Regulation of gastrointestinal (GI) motility is intimately coordinated with the modulation of ionic conductance expressed in GI smooth muscle and nonmuscle cells. Interstitial cells of Cajal (ICC) act as pacemaker cells and possess unique ionic conductances that trigger slow wave activity in these cells. The slow wave mechanism is an exclusive feature of ICC: Smooth muscle cells may lack the basic ionic mechanisms necessary to generate or regenerate slow waves. The molecular identification of the components for these conductances provides the foundation for a complete understanding of the ionic basis for GI motility. In addition, this information will provide a basis for the identification or development of therapeutics that might act on these channels. It is much easier to study these conductances and develop blocking drugs in expression systems than in native GI muscle cells. This review focuses on the relationship between ionic currents in native GI smooth muscle cells and ICC and their molecular counterparts.

Regulation of Gi by the CB1 cannabinoid receptor C-terminal juxtamembrane region: structural requirements determined by peptide analysis

A CB1 cannabinoid receptor peptide fragment from the C-terminal juxtamembrane region autonomously inhibits adenylyl cyclase activity in a neuroblastoma membrane preparation. The cannabinoid receptor antagonist, SR141716A, failed to block the response. The peptide was able to evoke the response in membranes from Chinese hamster ovary (CHO) cells that do not express the CB1 receptor. These studies are consistent with a direct activation of Gi by the peptide. To test the importance of a BXBXXB sequence, Lys403 was acetylated, resulting in a peptide having similar affinity but reduced efficacy. N-Terminal truncation of Arg401 resulted in a 6-fold loss of affinity, which was not further reduced by sequential truncation of up to the first seven amino acids, four of which are charged. N-Terminal-truncated peptides exhibited maximal activity, suggesting that Gi activation can be conferred by the remaining amino acids. Truncation of the C-terminal Glu417 or substitution of Glu417 by a Leu or of Arg401 by a Norleucine reduced activity at 100 microM. The C-terminal juxtamembrane peptide was constrained to a loop peptide by placement of Cys residues at both terminals and disulfide coupling. This modification reduced the affinity 3-fold but yielded near-maximal efficacy. Blocking the Cys termini resulted in a loss of efficacy. Circular dichroism spectropolarimetry revealed that all C-terminal juxtamembrane peptide analogues exist in a random coil conformation in an aqueous environment. A hydrophobic environment (trifluoroethanol) failed to induce alpha-helix formation in the C-terminal juxtamembrane peptide but did so in less active peptides. The anionic detergent sodium dodecyl sulfate induced alpha-helix formation in all analogues except the loop peptide, where it induces a left-handed PII conformation. It is concluded that alpha-helix formation is not required for Gi activation.

A conserved arginine in the distal third intracellular loop of the mu-opioid receptor is required for G protein activation

In the present study, the functional significance of the intracellular C-terminal loop of the mu-opioid receptor in activating Gi proteins was determined by constructing a C-terminal deletion mutant mu(C delta 45) receptor, which lacks the carboxyl 45 amino acids. When the truncated mu(C delta 45) receptor was stably expressed in human embryonic kidney (HEK) 293 cells, the efficacy and the potency of [D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin (DAMGO), a specific mu-opioid receptor agonist, to inhibit forskolin-stimulated adenylate cyclase activity were not significantly affected. Similar to other G-coupled receptors, the third cytoplasmic loop of the mu-opioid receptor contains conserved basic residues (R276/R277/R280) at the C-terminal segment. Mutating these basic residues to neutral amino acids (L276/M277/L280) greatly impaired the ability of DAMGO to inhibit forskolin-stimulated cyclic AMP formation. Replacing R276/R277 with L276/M277 did not affect the efficacy and potency by which DAMGO inhibits the adenylate cyclase activity. In HEK 293 cells stably expressing mutant (R280L) mu-opioid receptors, the ability of DAMGO to inhibit forskolin-stimulated cyclic AMP production was greatly reduced. These results suggest that the intracellular carboxyl tail of the mu-opioid receptor does not play a significant role in activating Gi proteins and that the arginine residue (R280) at the distal third cytoplasmic loop is required for Gi activation by the mu-opioid receptor.

Molecular basis of receptor/G-protein-coupling selectivity

Molecular cloning studies have shown that G-protein-coupled receptors form one of the largest protein families found in nature, and it is estimated that approximately 1000 different such receptors exist in mammals. Characteristically, when activated by the appropriate ligand, an individual receptor can recognize and activate only a limited set of the many structurally closely related heterotrimeric G-proteins expressed within a cell. To understand how this selectivity is achieved at a molecular level has become the focus of an ever increasing number of laboratories. This review provides an overview of recent structural, molecular genetic, biochemical, and biophysical studies that have led to novel insights into the molecular mechanisms governing receptor-mediated G-protein activation and receptor/G-protein coupling selectivity.

Structural correlates for down-regulation of m1 and m2 muscarinic receptor subtypes

Three chimeric receptors stably expressed in murine fibroblast (B82) cells were used to examine how different parts of the rat muscarinic m1 and m2 receptors contribute to the down-regulation process. The MCH7 chimeric m2 receptor contained a fragment between VIth TM and C-terminal end derived from the m1 receptor. The MCH3 and MCH5 receptors have exchanged N-terminal and third intracellular loop regions of the MCH7 receptor. Fibroblast cells stably expressing individual muscarinic wild type (m1, m2) or chimeric (MCH3, MCH5, or MCH7) receptors were treated with plain medium (control) or medium containing carbachol for 24 h. Receptor density changes were measured by [3H](-)1-N-methyl-3-quinuclidinyl benzilate ([3H](-)MQNB) saturation binding studies. There was a significant loss of receptor density, different for each receptor studied, following carbachol treatment relative to control cells. We related this loss of [3H](-)MQNB binding to the number of amino acids derived from m1 or m2 receptors for each constructed chimera and to the affinity of carbachol to the receptors studied. We demonstrate that: 1) the region from the VIth TMD to the end of C-terminal controls the extent of m1 and m2 receptor down-regulation; 2) the overall receptor conformation and the interaction between intracellular portions of the receptor influence the extent of receptor down-regulation; and 3) resistance to down-regulation by carbachol correlates with the affinity of carbachol to the muscarinic receptor construct.

The second intracellular loop of the m5 muscarinic receptor is the switch which enables G-protein coupling

We have completed a systematic search of the intracellular loops of a muscarinic acetylcholine receptor for domains that govern G-protein coupling. A unique feature of the second intracellular (i2) loop was an ordered cluster of residues where diverse substitutions cause constitutive activation. A second group of residues in i2 was identified where mutations compromised receptor/G-protein coupling. The residues of each group alternate and are spaced three to four positions apart, suggesting an alpha-helical structure where these groups form opposing faces of the helix. We propose that the constitutively activating face normally constrains the receptor in the "off-state," while the other face couples G-proteins in the "on-state." Therefore, the i2 loop functions as the switch enabling G-protein activation.

Asymmetrical contributions of subunit pore regions to ion selectivity in an inward rectifier K+ channel

We have investigated aspects of ion selectivity in K+ channels by functional expression of wild-type and mutant heteromultimeric G protein-coupled inward-rectifier K+ (GIRK) channels in Xenopus oocytes. Within the K+ channel pore (P) region signature sequence, a large number of point mutations in GIRK1 and GIRK4 subunits have been made at a key tyrosine residue--the "signature" tyrosine of the GYG. Studies of mutant GIRK1/GIRK4 heteromultimers reveal that the GIRK1 and GIRK4 subunits contribute asymmetrically to K+ selectivity. The signature tyrosine of GIRK1 can be mutated to many different residues while retaining selectivity; in contrast, the analogous position in GIRK4 must be tyrosine for maximum selectivity. Other residues of the P region also contribute to selectivity, and studies with GIRK1/GIRK4 chimeras reveal that an intact, heteromultimeric P region is necessary and sufficient for optimal K+ selectivity. We propose that the GIRK1 and GIRK4 P regions play roles similar to the two P regions of an emerging family of K+ channels whose subunits each have two P regions connected in tandem. We find different consequences between similar mutations in inward-rectifier and voltage-gated K+ channels, which suggests that the pore structures and selectivity mechanisms in the two classes of channel may not be identical. We confirm that GIRK4 subunits alone can form functional channels in oocytes, but we find that these channels are measurably permeable to Na2+ and Ca2+.

Genetics of colorectal cancer

Recent years have brought great advances in the understanding of the pathogenesis of colorectal cancers. The elucidation of the underlying genetic alterations that produce these cancers has made it possible to broadly categorize this disease into two major types, hereditary and sporadic. The hereditary cancers have been divided further into recognized syndromes, mainly familial adenomatous polyposis and hereditary nonpolyposis colorectal cancer. These syndromes have unique clinical pictures and different prognoses in addition to their different genetic bases. Sporadic cancers have been found to have analogous genetic alterations which propel them along the progression from normal tissue to benign adenoma/dysplasia to malignancy. These advances in genetics can potentially lead to clinically useful advances in detection, treatment, and, ultimately, prevention of colorectal cancer.

Arrestin-independent internalization of the m1, m3, and m4 subtypes of muscarinic cholinergic receptors

To understand what processes contribute to the agonist-induced internalization of subtypes of muscarinic acetylcholine receptors, we analyzed the role of arrestins. Whereas the m2 mAChR has been shown to undergo augmented internalization when arrestins 2 and 3 are overexpressed (Pals-Rylaarsdam, R., Gurevich, V. V., Lee, K. B., Ptasienski, J. A., Benovic, J. L., and Hosey, M. M. (1997) J. Biol. Chem. 272, 23682-23689), the agonist-induced internalization of m1, m3, and m4 mAChRs was unchanged when arrestins 2 or 3 were overexpressed in transiently transfected HEK-tsA201 cells. Furthermore, when a dominant-negative arrestin was used to interrupt endogenous arrestin function, there was no change in the internalization of the m1, m3, and m4 mAChR whereas the internalization of the beta2 adrenergic receptor was completely blocked. Wild-type and GTPase-deficient dominant-negative dynamin were used to determine which endocytic machinery played a role in the endocytosis of the subtypes of mAChRs. Interestingly, when dynamin function was blocked by overexpression of the GTPase-deficient dynamin, agonist- induced internalization of the the m1, m3, and m4 mAChRs was suppressed. These results suggested that the internalization of the m1, m3, and m4 mAChRs occurs via an arrestin-independent but dynamin-dependent pathway. To ascertain whether domains that confer arrestin sensitivity and dynamin insensitivity could be functionally exchanged between subtypes of mAChRs, chimeric m2/m3 receptors were analyzed for their properties of agonist-induced internalization. The results demonstrated that the third intracellular loop of the m2 mAChR conferred arrestin sensitivity and dynamin insensitivity to the arrestin-insensitive, dynamin-sensitive m3 mAChR while the analogous domain of the m3 mAChR conferred arrestin resistance and dynamin sensitivity to the previously arrestin-sensitive, dynamin-insensitive m2 mAChR.

The third intracellular loop of the rat gonadotropin-releasing hormone receptor couples the receptor to Gs- and G(q/11)-mediated signal transduction pathways: evidence from loop fragment transfection in GGH3 cells

The GnRH receptor (GnRH-R) belongs to the rhodopsin/beta-adrenergic family of G protein-coupled receptors. The intracellular domains of these receptors, particularly the regions closest to the plasma membrane in intracellular loops 2 (2i) and 3 (3i) as well as some regions located in the membrane-proximal end of the COOH-terminus, are frequently important sites for G protein coupling and specificity determination. Although studies in mouse and human GnRH-R have identified loop 2i as a critical determinant for coupling the receptor to the G(q/11)-mediated signal transduction pathway, given the functional similarity among the members of this particular G protein-coupled receptor subfamily and the fact that the GnRH-R lacks the typical intracellular COOH-terminal domain of its superfamily (a potential site for G protein coupling), we investigated the possibility that loop 3i of this receptor also participates in GnRH-R coupling to G proteins. GGH(3)1' cells, a pituitary-derived cell line that expresses a functional rat GnRH-R coupled to both Gs and G(q/11) proteins, were transiently transfected with a plasmid DNA containing a complementary DNA (cDNA) coding for the entire loop 3i of the GnRH-R as well as with other expression plasmids containing cDNAs encoding loop 3i of other Gs-, G(i/o)-, or G(q/11)-coupled receptors. The effects of coexpression of these loops with the wild-type GnRH-R on inositol phosphate (IP) production, cAMP accumulation, and PRL release were then examined. Transfection of GGH(3)1' cells with the cDNA for loop 3i of the rat GnRH-R (efficiency, 35-45%) maximally inhibited buserelin-stimulated IP turnover by 20% as well as cAMP accumulation and PRL secretion by 30%. This attenuation in cellular responses to a GnRH agonist was statistically significant (P < 0.05) compared with the responses exhibited by GGH(3)1' cells transfected with a control plasmid and stimulated with the same GnRH agonist. Transfection of minigenes coding for loop 3i of the M1Ach-muscarinic and the alpha1B-adrenergic (G(q/11)-coupled) receptors resulted in 25-55% inhibition of maximal GnRH-evoked IP turnover. Paradoxically, loop 3i from the M1Ach-muscarinic receptor also maximally inhibited GnRH agonist-stimulated cAMP accumulation and PRL release by 40% (both effects mediated through activation of the Gs protein). Transfection of loop 3i from the D1A -dopamine receptor (coupled to the Gs protein) produced a selective attenuation (40%) in Gs-mediated cellular responses. In contrast, receptor/G protein coupling appeared unaffected by expression of loop 3i domains derived from two receptors coupled to G(i/o) proteins (M2Ach-muscarinic and alpha2A-adrenergic receptors). These data indicate that the third intracellular loop of the rat GnRH-R is involved in receptor G(q/11) protein coupling and/or selectivity, and in the GGH(3)1' cell line, this loop is also involved in signal transduction mediated through the Gs protein pathway.

Structure/function relationships of a G-protein coupling pocket formed by the third intracellular loop of the m5 muscarinic receptor

Using random saturation mutagenesis, we have previously identified the amino acids K439, A440, and A441 in the C-terminus of the third intracellular loop (Ci3) of the m5 muscarinic receptor as being critical for G-protein coupling [Burstein, E. S., Spalding, T. A., Hill-Eubanks, D., and Brann, M. R. (1995) J. Biol. Chem. 270, 3141-3146]. In the present study, we have constructed a series of point mutants at each of these residues and characterized their functional phenotypes in order to define the structure/function relationships of each of these residues for G-protein coupling. Although a wide variety of substitutions were tolerated at K439, most caused significant increases in the EC50 of carbachol and decreases in the maximum response (Rmax). Only other basic residues were well tolerated (<10-fold increase in EC50, >70% of wild type). Acidic substitutions had the largest effects, reducing Rmax to under 20% of wild type. At A440, only the conservative substitution threonine was well tolerated. Substitutions by hydrophobic, polar, and basic residues caused 10-80-fold increases in EC50 values and in many cases also significantly reduced Rmax (<70% of wild type). In contrast, at A441 mutations selectively affected EC50 but not Rmax values. Previously we identified I216, Y217, T220, and R223 as the residues in the N-terminus of the i3 loop of m5 (Ni3) that are critical for G-protein coupling [Burstein, E. S., Spalding, T. S., and Brann, M. R. (1996) J. Biol. Chem. 271, 2882-2885]. To investigate whether there were additive contributions of Ni3 and Ci3 to G-protein coupling, the functional responses of two double mutants, R223E/K439E and Y217S/A441T, were evaluated. Though these mutations were tolerated individually, both double mutant receptors produced almost indetectable responses. Little or no changes in expression levels or ligand binding properties were detected, suggesting the observed effects were caused primarily by changes receptor/G-protein coupling. We conclude that K439 participates in G-protein activation through an ionic mechanism, that A440 fulfills a structural role forming part of the G-protein coupling pocket, and that A441 contributes to receptor affinity for G-proteins. We propose that the third intracellular loop forms a G-protein coupling pocket comprised of a positively charged "lip" and a hydrophobic core.

A cluster of basic residues in the carboxyl-terminal tail of the short metabotropic glutamate receptor 1 variants impairs their coupling to phospholipase C

Among phospholipase C-coupled metabotropic glutamate receptors (mGluRs), some have a surprisingly long carboxyl-terminal intracellular domain (mGluR1a, -5a, and -5b), and others have a short one (mGluR1b, -1c, and -1d). All mGluR1 sequences are identical up to 46 residues following the 7th transmembrane domain, followed by 313, 20, 11, and 26 specific residues in mGluR1a, mGluR1b, mGluR1c, and mGluR1d, respectively. Several functional differences have been described between the long isoforms (mGluR1a, -5a, and -5b) and the short ones (mGluR1b, -1c, and -1d). Compared with the long receptors, the short ones induce slower increases in intracellular Ca2+, are activated by higher concentration of agonists, and do not exhibit constitutive, agonist-independent activity. To identify the residues responsible for these functional properties, a series of truncated, chimeric, and mutated receptors were constructed. We found that the deletion of the last 19 carboxyl-terminal residues in mGluR1c changed its properties into those of mGluR1a. Moreover, the exchange of the long carboxyl-terminal domain of mGluR5a with that of mGluR1c generated a chimeric receptor that possessed functional properties similar to those of mGluR1c. Mutagenesis of specific residues within the 19 carboxyl-terminal residues of mGluR1c revealed the importance of a cluster of 4 basic residues in defining the specific properties of this receptor. Since this cluster is part of the sequence common to all mGluR1 variants, we conclude that the long carboxyl-terminal domain of mGluR1a suppresses the inhibitory action of this sequence element.

Two basic amino acids in the second inner loop of the interleukin-8 receptor are essential for Galpha16 coupling

The involvement of basic residues of interleukin(IL)-8 receptors in coupling to the Gi and G16 proteins was investigated by using a series of IL-8 receptor mutants. Substitution of the basic amino acids in the third inner loop of the receptor does not alter the abilities of the receptor mutants to activate recombinant Galpha16 or phosphoinositide-specific phospholipase C (PLC) beta2 expressed in COS-7 cells. However, an IL-8 receptor mutant with double mutations at residues Lys158 and Arg159 of the second inner loop loses its abilities to activate Galpha16 but retains its ability to activate PLC beta2. The activation of PLC beta2 by an IL-8 receptor that is sensitive to pertussis toxin has been previously demonstrated to be mediated through Gbetagamma. Surprisingly, the IL-8 receptor mutants with substitution of Ala for either residue Lys158 or Arg159 can still activate Galpha16, which suggests that either of the two basic residues in the second inner loop of the IL-8 receptor is sufficient for Galpha16 coupling.

The rat mGlu1d receptor splice variant shares functional properties with the other short isoforms of mGlu1 receptor

Three splice variants of the rat metabotropic glutamate receptor 1 (mGlu1a, 1b and 1c receptors) have been characterized so far. All have the same sequence up to the 46th residue following the 7th transmembrane domain, followed by different carboxyl-terminal tails. Whereas mGlu1b and mGlu1c receptors possess a short intracellular carboxyl-terminal tail, the mGlu1a receptor has a very long one. Compared to cells expressing mGlu1b or mGlu1c receptors, a higher agonist potency and basal phospholipase C activity were detected in cells expressing mGlu1a receptors. Another variant with a short carboxyl-terminal tail, the HmGlu1d receptor, has been recently isolated from human brain. Here we show that the mGlu1d receptor variant also exists in the rat. Like all rat mGlu1 receptor variants, the mGlu1d receptor activates phospholipase C upon stimulation with mGlu1 receptor agonists. Although the rank order of agonist potency is the same on mGlu1a and mGlu1d receptors, agonists are less potent in stimulating phospholipase C in mGlu1d receptor-expressing cells than in cells expressing mGlu1a receptors. Moreover, like the other short variants it has no significant constitutive activity. These results indicate that the mGlu1d receptor shares similar functional properties with the other short mGlu1 receptor splice variants, and further suggests that the long carboxyl-terminal tail of the mGlu1a receptor increases phospholipase C coupling efficacy.