A novel interaction between adrenergic receptors and the alpha-subunit of eukaryotic initiation factor 2B

Regulation and function of elF2B in neurological and metabolic disorders

Eukaryotic initiation factor 2B, eIF2B is a guanine nucleotide exchange, factor with a central role in coordinating the initiation of translation. During stress and disease, the activity of eIF2B is inhibited via the phosphorylation of its substrate eIF2 (p-eIF2α). A number of different kinases respond to various stresses leading to the phosphorylation of the alpha subunit of eIF2, and collectively this regulation is known as the integrated stress response, ISR. This targeting of eIF2B allows the cell to regulate protein synthesis and reprogramme gene expression to restore homeostasis. Advances within structural biology have furthered our understanding of how eIF2B interacts with eIF2 in both the productive GEF active form and the non-productive eIF2α phosphorylated form. Here, current knowledge of the role of eIF2B in the ISR is discussed within the context of normal and disease states focusing particularly on diseases such as vanishing white matter disease (VWMD) and permanent neonatal diabetes mellitus (PNDM), which are directly linked to mutations in eIF2B. The role of eIF2B in synaptic plasticity and memory formation is also discussed. In addition, the cellular localisation of eIF2B is reviewed and considered along with the role of additional in vivo eIF2B binding factors and protein modifications that may play a role in modulating eIF2B activity during health and disease.

Receptor-Ribosome Coupling: A Link Between Extrinsic Signals and mRNA Translation in Neuronal Compartments

Axons receive extracellular signals that help to guide growth and synapse formation during development and to maintain neuronal function and survival during maturity. These signals relay information via cell surface receptors that can initiate local intracellular signaling at the site of binding, including local messenger RNA (mRNA) translation. Direct coupling of translational machinery to receptors provides an attractive way to activate this local mRNA translation and change the local proteome with high spatiotemporal resolution. Here, we first discuss the increasing evidence that different external stimuli trigger translation of specific subsets of mRNAs in axons via receptors and thus play a prominent role in various processes in both developing and mature neurons. We then discuss the receptor-mediated molecular mechanisms that regulate local mRNA translational with a focus on direct receptor-ribosome coupling. We advance the idea that receptor-ribosome coupling provides several advantages over other translational regulation mechanisms and is a common mechanism in cell communication.

Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Hyperinsulinaemic hypoglycaemia: A rare association of vanishing white matter disease

We report two unrelated patients with infantile onset leukoencephalopathy with vanishing white matter (VWM) and hyperinsulinaemic hypoglycaemia. To our knowledge, this association has not been described previously. Both patients had compound heterozygous pathogenic variants in EIF2B4 detected on exome sequencing and absence of other variants which might explain the hyperinsulinism. Hypoglycaemia became apparent at 6 and 8 months, respectively, although in one patient, transient neonatal hypoglycaemia was also documented. One patient responded to diazoxide and the other was managed with continuous nasogastric feeding. We hypothesise that the pathophysiology of hyperinsulinism in VWM may involve dysregulation of transcription of genes related to insulin secretion.

Receptor-specific interactome as a hub for rapid cue-induced selective translation in axons

Extrinsic cues trigger the local translation of specific mRNAs in growing axons via cell surface receptors. The coupling of ribosomes to receptors has been proposed as a mechanism linking signals to local translation but it is not known how broadly this mechanism operates, nor whether it can selectively regulate mRNA translation. We report that receptor-ribosome coupling is employed by multiple guidance cue receptors and this interaction is mRNA-dependent. We find that different receptors associate with distinct sets of mRNAs and RNA-binding proteins. Cue stimulation of growing Xenopus retinal ganglion cell axons induces rapid dissociation of ribosomes from receptors and the selective translation of receptor-specific mRNAs. Further, we show that receptor-ribosome dissociation and cue-induced selective translation are inhibited by co-exposure to translation-repressive cues, suggesting a novel mode of signal integration. Our findings reveal receptor-specific interactomes and suggest a generalizable model for cue-selective control of the local proteome.

G Protein-Coupled Receptors As Regulators of Localized Translation: The Forgotten Pathway?

G protein-coupled receptors (GPCRs) exert their physiological function by transducing a complex signaling network that coordinates gene expression and dictates the phenotype of highly differentiated cells. Much is known about the gene networks they transcriptionally regulate upon ligand exposure in a process that takes hours before a new protein is synthesized. However, far less is known about GPCR impact on the translational machinery and subsequent mRNA translation, although this gene regulation level alters the cell phenotype in a strikingly different timescale. In fact, mRNA translation is an early response kinetically connected to signaling events, hence it leads to the synthesis of a new protein within minutes following receptor activation. By these means, mRNA translation is responsive to subtle variations of the extracellular environment. In addition, when restricted to cell subcellular compartments, local mRNA translation contributes to cell micro-specialization, as observed in synaptic plasticity or in cell migration. The mechanisms that control where in the cell an mRNA is translated are starting to be deciphered. But how an extracellular signal triggers such local translation still deserves extensive investigations. With the advent of high-throughput data acquisition, it now becomes possible to review the current knowledge on the translatome that some GPCRs regulate, and how this information can be used to explore GPCR-controlled local translation of mRNAs.

X-exome sequencing of 405 unresolved families identifies seven novel intellectual disability genes

X-linked intellectual disability (XLID) is a clinically and genetically heterogeneous disorder. During the past two decades in excess of 100 X-chromosome ID genes have been identified. Yet, a large number of families mapping to the X-chromosome remained unresolved suggesting that more XLID genes or loci are yet to be identified. Here, we have investigated 405 unresolved families with XLID. We employed massively parallel sequencing of all X-chromosome exons in the index males. The majority of these males were previously tested negative for copy number variations and for mutations in a subset of known XLID genes by Sanger sequencing. In total, 745 X-chromosomal genes were screened. After stringent filtering, a total of 1297 non-recurrent exonic variants remained for prioritization. Co-segregation analysis of potential clinically relevant changes revealed that 80 families (20%) carried pathogenic variants in established XLID genes. In 19 families, we detected likely causative protein truncating and missense variants in 7 novel and validated XLID genes (CLCN4, CNKSR2, FRMPD4, KLHL15, LAS1L, RLIM and USP27X) and potentially deleterious variants in 2 novel candidate XLID genes (CDK16 and TAF1). We show that the CLCN4 and CNKSR2 variants impair protein functions as indicated by electrophysiological studies and altered differentiation of cultured primary neurons from Clcn4(-/-) mice or after mRNA knock-down. The newly identified and candidate XLID proteins belong to pathways and networks with established roles in cognitive function and intellectual disability in particular. We suggest that systematic sequencing of all X-chromosomal genes in a cohort of patients with genetic evidence for X-chromosome locus involvement may resolve up to 58% of Fragile X-negative cases.

Association of eukaryotic translation initiation factor eIF2B with fully solubilized CXCR4

Chemokine receptors are key regulators of leukocyte trafficking but also have an important role in development, tumor growth, and metastasis. Among the chemokine receptors, CXCR4 is the only one that leads to perinatal death when genetically ablated in mice, indicating a more-widespread function in development. To identify pathways that are activated downstream of CXCR4, a solubilization protocol was elaborated, which allows for the isolation of the endogenous receptor from human cells in its near-native conformation. Solubilized CXCR4 is recognized by the conformation-sensitive monoclonal antibody 12G5 and retains the ability to bind CXCL12 in solution, which was abolished in the presence of receptor antagonists. Mass spectrometry of CXCR4 immunoprecipitates revealed a specific interaction with the pentameric eukaryotic translation initiation factor 2B. The observation that the addition of CXCL12 leads to the dissociation of eukaryotic translation initiation factor 2B from CXCR4 suggests that stimulation of the receptor may trigger the local protein synthesis required for efficient cell movement.

Biochemical effects of mutations in the gene encoding the alpha subunit of eukaryotic initiation factor (eIF) 2B associated with Vanishing White Matter disease

BACKGROUND

Leukoencephalopathy with Vanishing White Matter (VWM) is an autosomal recessive disorder caused by germline mutations in the genes EIF2B1-5, which encode the 5 subunits of the eukaryotic translation initiation factor eIF2B. To date, analysis of the biochemical effects of mutations in the EIF2B2-5 genes has been carried out, but no study has been performed on mutations in the EIF2B1 gene. This gene encodes eIF2Bα, the smallest subunit in eIF2B which has an important role in both the structure and regulation of the eIF2B complex.

METHODS

eIF2B subunits were overexpressed in HEK293 cells and isolated from the resulting cell lysates by affinity chromatography. Formation of the eIF2B complex and binding of its substrate, eIF2, was assessed by western blot. Assays of the guanine nucleotide exchange (GEF) activity were also carried out.

RESULTS

Of the 5 eIF2Bα mutations studied, we found 3 that showed loss or reduction of binding of eIF2Bα to the rest of the complex, one with increased GEF activity, and one where no effects on activity or complex formation were observed.

CONCLUSIONS

This is the first study on eIF2Bα VWM mutations. We show that some mutations cause expected decreases in GEF activity or complex formation, similar to a majority of observed VWM mutations. However, we also observe some unexpected changes which hint at other effects of these mutations on as yet undescribed functions of eIF2B.

Commercially available antibodies against human and murine histamine H₄-receptor lack specificity

Antibodies are important tools to detect expression and localization of proteins within the living cell. However, for a series of commercially available antibodies which are supposed to recognize G-protein-coupled receptors (GPCR), lack of specificity has been described. In recent publications, antisera against the histamine H₄-receptor (H₄R), which is a member of the GPCR family, have been used to demonstrate receptor expression. However, a comprehensive characterization of these antisera has not been performed yet. Therefore, the purpose of our study was to evaluate the specificity of three commercially available H₄R antibodies. Sf9 insect cells and HEK293 cells expressing recombinant murine and human H₄R, spleen cells obtained from H₄⁻/⁻ and from wild-type mice, and human CD20⁺ and CD20⁻ peripheral blood cells were analyzed by flow cytometry and Western blot using three commercially available H₄R antibodies. Our results show that all tested H₄R antibodies bind to virtually all cells, independently of the expression of H₄R, thus in an unspecific fashion. Also in Western blot, the H₄R antibodies do not bind to the specified protein. Our data underscore the importance of stringent evaluation of antibodies using valid controls, such as cells of H₄R⁻/⁻ mice, to show true receptor expression and antigen specificity. Improved validation of commercially available antibodies prior to release to the market would avoid time-consuming and expensive validation assays by the user.

Modulation of monoamine receptors by adaptor proteins and lipid rafts: role in some effects of centrally acting drugs and therapeutic agents

The monoamines and their cognate receptors are widespread in the central nervous system and are vital for normal brain function. Dysfunction in these systems underlies several psychiatric and neurological disease states, and consequently monoamines are targets of a host of pharmacotherapies. This review provides an overview on how monoamine receptors are regulated by adaptor proteins and lipid rafts with emphasis on interactions in nerve cells. Monoamine receptors have prominent intracellular loops that provide binding sites for adaptor proteins. Receptor function is further modulated by cholesterol and submembranous microdomains termed lipid rafts. These interactions determine several facets of G protein-coupled receptor (GPCR) function including trafficking, localization, and signaling. Possible roles of adaptor proteins and lipid rafts in disease states and in mediating actions of drugs and therapeutic agents are also discussed.

Systems analysis of endothelial cell plasma membrane proteome of rat lung microvasculature

BACKGROUND

Endothelial cells line all blood vessels to form the blood-tissue interface which is critical for maintaining organ homeostasis and facilitates molecular exchange. We recently used tissue subcellular fractionation combined with several multi-dimensional mass spectrometry-based techniques to enhance identification of lipid-embedded proteins for large-scale proteomic mapping of luminal endothelial cell plasma membranes isolated directly from rat lungs in vivo. The biological processes and functions of the proteins expressed at this important blood-tissue interface remain unexplored at a large scale.

RESULTS

We performed an unbiased systems analysis of the endothelial cell surface proteome containing over 1800 proteins to unravel the major functions and pathways apparent at this interface. As expected, many key functions of plasma membranes in general (i.e., cell surface signaling pathways, cytoskeletal organization, adhesion, membrane trafficking, metabolism, mechanotransduction, membrane fusion, and vesicle-mediated transport) and endothelial cells in particular (i.e., blood vessel development and maturation, angiogenesis, regulation of endothelial cell proliferation, protease activity, and endocytosis) were significantly overrepresented in this proteome. We found that endothelial cells express multiple proteins that mediate processes previously reported to be restricted to neuronal cells, such as neuronal survival and plasticity, axon growth and regeneration, synaptic vesicle trafficking and neurotransmitter metabolic process. Surprisingly, molecular machinery for protein synthesis was also detected as overrepresented, suggesting that endothelial cells, like neurons, can synthesize proteins locally at the cell surface.

CONCLUSION

Our unbiased systems analysis has led to the potential discovery of unexpected functions in normal endothelium. The discovery of the existence of protein synthesis at the plasma membrane in endothelial cells provides new insight into the blood-tissue interface and endothelial cell surface biology.

Simvastatin represses protein synthesis in the muscle-derived C₂C₁₂ cell line with a concomitant reduction in eukaryotic initiation factor 2B expression

Statins are a widely prescribed class of cholesterol lowering drugs whose use is frequently associated with muscle-related ailments. A number of mechanisms have been implicated in statin-induced myotoxicity including alterations in both protein synthesis and protein degradation. The objective of the present study was to explore the mechanism(s) contributing to the statin-induced reduction in protein synthesis in the muscle-derived C₂C₁₂ cell line. Cells were treated with 10 μM simvastatin or vehicle alone for 24 h in 1% serum. Cells exposed to simvastatin exhibited reduced rates of protein synthesis, as evidenced by [(35)S]methionine and [(35)S]cysteine incorporation into protein. The reduction in protein synthesis occurred with a concomitant decrease in expression and activity of eukaryotic initiation factor 2B (eIF2B), a regulated and rate-controlling guanine nucleotide exchange factor known to affect global rates of protein synthesis. The reductions in protein synthesis and eIF2B expression were prevented by coincubation with mevalonate. Simvastatin treatment also resulted in a proteasome-sensitive reduction in the protein expression of all the subunits of the eIF2B heteropentameric complex. Finally, increased phosphorylation of the catalytic ε-subunit at Ser(535) was observed, an event consistent with an observed reduction in eIF2B activity. These results suggest that repression of eIF2B expression and activity may contribute, at least in part, to the statin-induced reduction in protein synthesis.

Translational control by RGS2

The regulator of G protein signaling (RGS) proteins are a family of guanosine triphosphatase (GTPase)-accelerating proteins. We have discovered a novel function for RGS2 in the control of protein synthesis. RGS2 was found to bind to eIF2Bepsilon (eukaryotic initiation factor 2B epsilon subunit) and inhibit the translation of messenger RNA (mRNA) into new protein. This effect was not observed for other RGS proteins tested. This novel function of RGS2 is distinct from its ability to regulate G protein-mediated signals and maps to a stretch of 37 amino acid residues within its conserved RGS domain. Moreover, RGS2 was capable of interfering with the eIF2-eIF2B GTPase cycle, which is a requisite step for the initiation of mRNA translation. Collectively, this study has identified a novel role for RGS2 in the control of protein synthesis that is independent of its established RGS domain function.

Alveolar epithelial beta2-adrenergic receptors

beta(2)-adrenergic receptors are present throughout the lung, including the alveolar airspace, where they play an important role for regulation of the active Na(+) transport needed for clearance of excess fluid out of alveolar airspace. beta(2)-adrenergic receptor signaling is required for up-regulation of alveolar epithelial active ion transport in the setting of excess alveolar edema. The positive, protective effects of beta(2)-adrenergic receptor signaling on alveolar active Na(+) transport in normal and injured lungs provide substantial support for the use of beta-adrenergic agonists to accelerate alveolar fluid clearance in patients with cardiogenic and noncardiogenic pulmonary edema. In this review, we summarize the role of beta(2)-adrenergic receptors in the alveolar epithelium with emphasis on their role in the regulation of alveolar active Na(+) transport in normal and injured lungs.

Alix (AIP1) is a vasopressin receptor (V2R)-interacting protein that increases lysosomal degradation of the V2R

The vasopressin type 2 receptor (V2R) is a G protein-coupled receptor that plays a central role in renal water reabsorption. Termination of ligand (vasopressin) stimulation is an important physiological regulatory event, but few proteins that interact with the V2R during downregulation after vasopressin (VP) binding have been identified. Using yeast two-hybrid screening of a human kidney cDNA library, we show that a 100-kDa protein called ALG-2-interacting protein X (Alix) interacts with the last 29 amino acids of the V2R COOH terminus. This was confirmed by pull-down assays using a GST-V2R-COOH-tail fusion protein. Alix was immunolocalized in principal cells of the kidney, which also express the V2R. The function of the Alix-V2R interaction was studied by transfecting Alix into LLC-PK(1) epithelial cells expressing V2R-green fluorescent protein (GFP). Under basal conditions, V2R-GFP localized mainly at the plasma membrane. On VP treatment, V2R-GFP was internalized into perinuclear vesicles in the nontransfected cells. In contrast, V2R-GFP fluorescence was virtually undetectable 2 h after exposure to VP in cells that coexpressed Alix. Western blotting using an anti-GFP antibody showed marked degradation of the V2R after 2 h in the presence of VP and Alix, a time point at which little or no degradation was detected in the absence of Alix. In contrast, little or no degradation of the parathyroid hormone receptor was detectable in the presence or absence of Alix and/or the PTH ligand. The VP-induced disappearance of V2R-GFP was abolished by chloroquine, a lysosomal degradation inhibitor, but not by MG132, a proteosome inhibitor. These data suggest that Alix increases the rate of lysosomal degradation of V2R and may play an important regulatory role in the VP response by modulating V2R downregulation.

Regulation of alpha2AR trafficking and signaling by interacting proteins

The continuing discovery of new G protein-coupled receptor (GPCR) interacting proteins and clarification of the functional consequences of these interactions has revealed multiple roles for these events. Some of these interactions serve to scaffold GPCRs to particular cellular micro-compartments or to tether them to defined signaling molecules, while other GPCR-protein interactions control GPCR trafficking and the kinetics of GPCR-mediated signaling transduction. This review provides a general overview of the variety of GPCR-protein interactions reported to date, and then focuses on one prototypical GPCR, the alpha(2)AR, and the in vitro and in vivo significance of its reciprocal interactions with arrestin and spinophilin. It seems appropriate to recognize the life and career of Arthur Hancock with a summary of studies that both affirm and surprise our preconceived notions of how nature is designed, as his career-long efforts similarly affirmed the complexity of human biology and attempted to surprise pathological changes in that biology with novel, discovery-based therapeutic interventions. Dr. Hancock's love of life, of family, and of commitment to making the world a better place are a model of the life well lived, and truly missed by those who were privileged to know, and thus love, him.

Changes in skeletal muscle gene expression following clenbuterol administration

Background

Beta-adrenergic receptor agonists (BA) induce skeletal muscle hypertrophy, yet specific mechanisms that lead to this effect are not well understood. The objective of this research was to identify novel genes and physiological pathways that potentially facilitate BA induced skeletal muscle growth. The Affymetrix platform was utilized to identify gene expression changes in mouse skeletal muscle 24 hours and 10 days after administration of the BA clenbuterol.

Results

Administration of clenbuterol stimulated anabolic activity, as indicated by decreased blood urea nitrogen (BUN; P < 0.01) and increased body weight gain (P < 0.05) 24 hours or 10 days, respectively, after initiation of clenbuterol treatment. A total of 22,605 probesets were evaluated with 52 probesets defined as differentially expressed based on a false discovery rate of 10%. Differential mRNA abundance of four of these genes was validated in an independent experiment by quantitative PCR. Functional characterization of differentially expressed genes revealed several categories that participate in biological processes important to skeletal muscle growth, including regulators of transcription and translation, mediators of cell-signalling pathways, and genes involved in polyamine metabolism.

Conclusion

Global evaluation of gene expression after administration of clenbuterol identified changes in gene expression and overrepresented functional categories of genes that may regulate BA-induced muscle hypertrophy. Changes in mRNA abundance of multiple genes associated with myogenic differentiation may indicate an important effect of BA on proliferation, differentiation, and/or recruitment of satellite cells into muscle fibers to promote muscle hypertrophy. Increased mRNA abundance of genes involved in the initiation of translation suggests that increased levels of protein synthesis often associated with BA administration may result from a general up-regulation of translational initiators. Additionally, numerous other genes and physiological pathways were identified that will be important targets for further investigations of the hypertrophic effect of BA on skeletal muscle.

Interaction of the amyloid precursor like protein 1 with the alpha2A-adrenergic receptor increases agonist-mediated inhibition of adenylate cyclase

Alpha2-adrenergic receptor agonists exert potent analgesic and sedative/hypnotic effects. In addition, they have been shown to be neuroprotective, but the mechanisms of these actions are still poorly defined. To isolate proteins that may control alpha2-adrenergic receptor function or trafficking, we performed a two-hybrid screen using the carboxy-terminal fourth intracellular tail of the alpha2A-adrenergic receptor as bait. This screen identified the amyloid precursor like protein 1 (APLP1), a homologue of the beta-amyloid precursor protein involved in Alzheimer's disease, as alpha2A-adrenergic receptor-binding protein. GST affinity chromatography revealed that APLP1 specifically interacts with all three human alpha2-adrenergic receptor subtypes and deletion mutant analysis confined the APLP1 domain involved in binding to alpha2-adrenergic receptors to the 13 amino acid residues Ser599-Ala611. Coimmunoprecipitations of transiently transfected cells with epitope-tagged APLP1 and alpha2-adrenergic receptors confirmed the interaction. Agonist treatment tended to increase the amount of alpha2A-adrenergic receptor associated with APLP1 while coimmunoprecipitations were not affected by the state of receptor phosphorylation or cotransfection of arrestin-3. Confocal laser microscopy showed that APLP1 causes a considerable shift of the alpha2A-adrenergic receptor localization from plasma membrane to intracellular compartments. Furthermore, cotransfection of alpha2A-adrenergic receptor and APLP1 into HEK293 cells significantly increased norepinephrine mediated inhibition of adenylate cyclase activity. These results suggest a possible role of APLP1 in regulation of alpha2A-adrenergic receptor trafficking. Moreover, we speculate that this interaction may present one mechanism by which alpha2-adrenergic receptor agonists exert their neuroprotective effects.

Attenuation of Inflammatory Vascular Remodeling by Angiotensin II Type 1 Receptor–Associated Protein

To explore the role of angiotensin II Type 1 receptor–associated protein (ATRAP) in vascular remodeling, we developed transgenic mice for mouse ATRAP cDNA and examined remodeling after inflammatory vascular injury induced by polyethylene cuff placement. In ATRAP transgenic (ATRAP-Tg) mice, ATRAP mRNA was increased 3- to 4-fold in the heart, aorta, and femoral artery. ATRAP-Tg mice showed no significant change in body weight, systolic blood pressure, heart rate, and heart/body weight ratio. However, cell proliferation and neointimal formation in the injured artery were attenuated in ATRAP-Tg mice. The increase in NADPH oxidase activity and the expression of p22 phox , a reduced nicotinamide-adenine dinucleotide/reduced nicotinamide-adenine dinucleotide phosphate oxidase subunit, after cuff placement was also attenuated in ATRAP-Tg mice. Moreover, activation of extracellular signal–regulated kinase, signal transducer and activator of transcription 1, and signal transducer and activator of transcription 3 after cuff placement was significantly reduced in ATRAP-Tg mice. Pressor response and cardiac hypertrophy induced by angiotensin II infusion and pressure overload were also attenuated in ATRAP-Tg mice. These results suggest that ATRAP plays an important role in vascular remodeling as a negative regulator.

Novel interaction between the human 5-HT7 receptor isoforms and PLAC-24/eIF3k

Three 5-HT(7) receptor isoforms are expressed in rat and man, which differ in the amino acid sequence of their C-terminus. Thus far, no changes have been observed in the pharmacological profile of all three isoforms. To further elucidate the signal transduction pathway specific for these receptor variants, we screened for possible interacting proteins of the C-terminus of the h5-HT(7(a)) variant in a human foetal brain cDNA library. Using a yeast two-hybrid assay, we isolated PLAC-24/eIF3k as a possible interacting candidate. The association of PLAC-24 with all three receptor variants was observed and further reconfirmed in vivo by co-immunoprecipitation of PLAC-24 with the full-length receptor isoforms in transfected COS-7 cells. Studies with different deletion mutants of the receptor showed that the interaction between PLAC-24 and the receptor is not restricted to the C-terminus of the receptor. PLAC-24/eIF3k consists of 3 domains: an N-terminal HAM domain, a central WH domain and a C-terminal tail. We generated different domain constructs of PLAC-24, which indicated that the HAM and WH domain both interact with the 5-HT(7(a)) receptor. Overexpression of PLAC-24 in HEK293 cells, stably expressing the h5-HT(7(a)) receptor, caused a threefold augmentation in the expression levels of the receptor. Co-localisation studies in COS-7 cells showed that PLAC-24 relocates from the nucleus and perinuclear sites towards the plasma membrane upon co-expression with the receptor. On the other hand, the expression of domain variants of PLAC-24 seems to block the translocation of the receptor towards the membrane. These observations suggest that PLAC-24 may play a role in the transport and the stabilisation of newly synthesised 5-HT(7) receptor towards the plasma membrane.

Mammalian Tolloid Alters Subcellular Localization, Internalization, and Signaling of α1a-Adrenergic Receptors

In the present study, we identified the CUB5 domain of mammalian Tolloid (mTLD) as a novel protein binding to alpha(1A)-adrenergic receptor (AR) using the yeast two-hybrid system. Whereas CUB5 did not couple to either alpha(1B)-AR or alpha(1D)-AR. It was determined that amino acids 322 to 359 of alpha(1A)-AR were the major binding region for CUB5. The direct interaction between alpha(1A)-AR cytoplasmic tail and CUB5 was discovered by glutathione S-transferase pull-down assay. We confirmed the interaction of mTLD with alpha(1A)-AR in human embryonic kidney (HEK) 293 cells by immunoprecipitation, immunofluorescence, and fluorescence resonance energy transfer. Although mTLD did not affect the density and affinity of receptors in crudely prepared membranes from HEK293 cells stably expressing alpha(1A)-AR, it significantly altered the subcellular localization of the receptors. Moreover, mTLD reduced the level of cell surface alpha(1A)-ARs, delayed the initial rate of agonist-induced receptor internalization, and facilitated agonist-induced calcium transient. We have demonstrated that mTLD interacts with alpha(1A)-AR directly, alters the subcellular localization of receptor, and influences agonist-induced alpha(1A)-AR internalization and calcium signaling.

Fright is a provoking factor in vanishing white matter disease

Leukoencephalopathy with vanishing white matter is an inherited disorder with a chronic progressive disease course and additional episodes of rapid neurological deterioration. These episodes typically are provoked by febrile infections or minor head trauma. We report on two patients who experienced an episode of rapid neurological deterioration after a fright.

G protein-coupled receptors show unusual patterns of intrinsic unfolding

Intrinsically unstructured proteins (IUPs) or IUP-like regions often play key roles in controlling processes ranging from transcription to the cell cycle. In silico such proteins can be identified by their sequence properties; they have low hydrophobicity and high net charge. In this study, we applied the FoldIndex (http://bioportal.weizmann.ac.il/fldbin/findex) program to analyze human G protein-coupled receptors and compared them with membrane proteins of known structure and with IUPs. We show that human G protein-coupled receptor (GPCR) extramembranous domains include long (>50 residues) disordered segments, unlike membrane proteins of known structure. The predicted disorder occurred primarily in the N-terminal, C-terminal and third intracellular domain regions: 55, 69 and 56% of the human GPCRs were disordered in these regions, respectively. This increased flexibility may therefore be critical for GPCR function. Surprisingly, however, the kinds of residues used in GPCR unstructured regions were different than in hitherto-identified IUPs. The GPCR third intracellular loop domains contain very high percentages of Arg, Lys and His residues, especially Arg, but the percentage of Glu, Asp and Pro is no higher than in folded proteins. We propose that this has structural and functional consequences.

Beta-adrenergic receptors and their interacting proteins

The three subtypes of beta-adrenergic receptor (beta AR) all interact with G proteins as a central aspect of their signaling. The various beta AR subtypes also associate differentially with a variety of other cytoplasmic and transmembrane proteins. These beta AR-interacting proteins play distinct roles in the regulation of receptor signaling and trafficking. The specificity of beta AR associations with various binding partners can help to explain key physiological differences between beta AR subtypes. Moreover, the differential tissue expression patterns of many of the beta AR-interacting proteins may contribute to tissue-specific regulation of beta AR function.

Type 1 angiotensin II receptor-associated protein ARAP1 binds and recycles the receptor to the plasma membrane

The carboxyl terminus of the type 1 angiotensin II receptor (AT(1)) plays an important role in receptor phosphorylation, desensitization, and internalization. The yeast two-hybrid system was employed to isolate proteins associated with the carboxyl terminal region of the AT(1A) receptor. In the present study, we report the isolation of a novel protein, ARAP1, which promotes recycling of AT(1A) to the plasma membrane in HEK-293 cells. ARAP1 cDNA encodes a 493-amino-acid protein and its mRNA is ubiquitously expressed in rat tissues. A complex of ARAP1 and AT(1A) was observed by immunoprecipitation and Western blotting in HEK-293 cells. In the presence of ARAP1, recycled AT(1A) showed a significant Ca(2+) release response to a second stimulation by Ang II 30 min after the first treatment. Immunocytochemical analysis revealed co-localization of recycled AT(1A) and ARAP1 in the plasma membrane 45 min after the initial exposure to Ang II. Taken together, these results indicate a role for ARAP1 in the recycling of the AT(1) receptor to the plasma membrane with presumable concomitant recovery of receptor signal functions.

Direct and biochemical interaction between dopamine D3 receptor and elongation factor-1Bbetagamma

Novel signaling components of dopamine D3 receptor (D3R) were searched using yeast two-hybrid system, and the gamma subunit of elongation Factor-1B (eEF1Bgamma) was found to interact with D3R. This interaction was observed specifically between eEF1Bgamma and D3R but not with D2R or D4R. Immunocytochemical studies showed that D3R and eEF1Bgamma form clusters on the plasma membrane and their co-localization was evident in these clusters. The beta subunit of eEF1B (eEF1Bbeta), which forms a tight complex with eEF1Bgamma, was phosphorylated on serine residues in response to the stimulation of D3R. Phosphorylation of eEF1Bbeta was insensitive to pertussis toxin or wortmannin, however, stimulation of cellular protein kinase C (PKC) directly phosphorylated eEF1Bbeta and depletion of PKC abolished D3R-mediated phosphorylation of eEF1Bbeta. These results suggest the involvement of PKC, but not Gi/o proteins or phosphatidylinositol 3-kinase, in D3R-mediated phosphorylation of eEF1Bbeta. Stimulation of D3R did not activate PKC, but the activation of PKC resulted in the phosphorylation of D3R. These results show that PKC has a permissive role for the D3R-mediated phosphorylation of eEF1Bbeta, and suggest that PKC could modulate the mutual interaction between two protein by phosphorylating both D3R and eEF1Bbeta. Therefore, the cellular PKC level would be important for the D3R-mediated modulation of eEF1B, and for their cellular regulations such as protein synthesis or cellular proliferation.

Trafficking prerogatives of olfactory receptors

Olfactory receptors lead lives of exclusivity and privilege, the monarchs of fiefdoms organized solely to carry out their instructions. Each olfactory sensory neuron expresses one allele of one of approximately 1000 olfactory receptor genes. It is thought that olfactory receptor diversity is critical for the ability of animals to detect many thousands of odorants, but supporting functional evidence has been difficult to obtain because olfactory receptors expressed in heterologous cells are typically retained in the endoplasmic reticulum. The membrane trafficking entitlements enjoyed by olfactory receptors appear to be available only in mature olfactory sensory neurons. Evidence is accumulating that cell-type-specific accessory proteins regulate first the exit of olfactory receptors from the endoplasmic reticulum, and then the trafficking of olfactory receptors from post-Golgi compartments to the plasma membrane of the olfactory cilia where the receptors gain access to odorants. Critical olfactory receptor accessory proteins are known only in the nematode Caenorhabditis elegans, where the absence of a novel protein called ODR-4 or a clathrin adaptor, UNC-101, interferes with proper trafficking. Similar functional specificity also occurs in a parallel chemosensory system, the mammalian vomeronasal organ. Trafficking of the V2R type of vomeronasal receptors is mediated by a vomeronasal-specific family of major histocompatibility complex proteins. Removal of olfactory receptors from the plasma membrane may be regulated in a more conventional fashion because odor stimulation has been linked to receptor phosphorylation, to the function of G-protein coupled receptor kinase 3, and to an increase in vesicles retrieved from the plasma membrane.

Novel beta2-adrenergic receptor signaling pathways

The beta(2)-adrenergic receptor (beta(2)AR) is perhaps the most thoroughly investigated of all G-protein-coupled receptors. Although the classical pathway of beta(2)AR signaling involves agonist-promoted binding of the receptor to the heterotrimeric guanosine triphosphate-binding protein G(s), activation of adenylyl cyclase, and production of cyclic adenosine monophosphate (cAMP), current evidence suggests that beta(2)AR signaling is regulated by interaction with multiple proteins. These interactions fall into 3 major groups: guanosine triphosphate-binding proteins such as G(s) and G(i); protein kinases such as the cAMP-dependent protein kinase, protein kinase C, G-protein-coupled receptor kinases, and tyrosine kinases; and adaptor proteins such as arrestins, A-kinase anchoring proteins, and the Na(+)/H(+)-exchanger regulatory factor. This review discusses these various interactions with particular emphasis on their role in regulating beta(2)AR signaling and trafficking.

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.

Modulation of postendocytic sorting of G protein-coupled receptors

Recycling of the mu opioid receptor to the plasma membrane after endocytosis promotes rapid resensitization of signal transduction, whereas targeting of the delta opioid receptor (DOR) to lysosomes causes proteolytic down-regulation. We identified a protein that binds preferentially to the cytoplasmic tail of the DOR as a candidate heterotrimeric GTP-binding protein (G protein)-coupled receptor-associated sorting protein (GASP). Disruption of the DOR-GASP interaction through receptor mutation or overexpression of a dominant negative fragment of GASP inhibited receptor trafficking to lysosomes and promoted recycling. The GASP family of proteins may modulate lysosomal sorting and functional down-regulation of a variety of G protein-coupled receptors.

Chronic lithium chloride administration to rats decreases brain protein level of epsilon (epsilon) subunit of eukaryotic initiation factor-2B

The eukaryotic initiation factor-2B (eIF-2B) can regulate translation and protein synthesis. We used Western blot analysis to quantify the protein level of the catalytic epsilon (epsilon) subunit of eIF-2B in brains of rats fed lithium chloride (LiCl) for 6 weeks so as to produce a brain lithium concentration that is therapeutically effective in bipolar disorder. The ratio of eIF-2B (epsilon) to actin protein was significantly reduced (P<0.01) in LiCl-fed rats, 0.86+/-0.06 (SE) compared to 1.2+/-0.07 in control rats. These results suggest that a therapeutic level of lithium may downregulate the synthesis of proteins whose translation depends on eIF-2B.

G protein-coupled receptor interacting proteins: emerging roles in localization and signal transduction

The mechanism by which G protein-coupled receptors (GPCRs) translate extracellular signals into cellular changes initially was envisioned as a simple linear model: activation of the receptor by agonist binding leads to dissociation of the heterotrimeric GTP-binding G protein into its alpha and betagamma subunits, both of which can activate or inhibit various downstream effector molecules. The plethora of recently described multidomain scaffolding proteins and accessory/chaperone molecules that interact with GPCR, including GPCR themselves as homo- or heterodimers, provides for diverse molecular mechanisms for ligand recognition, signalling specificity, and receptor trafficking. This review will summarize the recently described GPCR-interacting proteins and their individual functional roles, as understood. Implicit in the search for the functional relevance of these interactions is the expectation that enhancement or disruption of target cell-specific events could serve as highly selective therapeutic opportunities.

Identification of novel G protein-coupled receptor-interacting proteins

Biochemical and genetic methods utilizing G protein-coupled receptor fragments have been used successfully to identify G protein-coupled receptor-interacting proteins. As noted earlier, these methods may be unable to detect interactions that require certain conformations of the native receptor protein, but have nevertheless proven quite useful in expanding our understanding of receptor regulation to include interactions with proteins other than G proteins, G protein-coupled receptor kinases, and arrestins. Undoubtedly, it is likely that all G protein-coupled receptors have their own unique constellations of associated cytoplasmic proteins, and the techniques described here should prove useful in identifying these.

Agonist-regulated Interaction between alpha2-adrenergic receptors and spinophilin

Previously, we demonstrated that the third intracellular (3i) loop of the heptahelical alpha2A-adrenergic receptor (alpha2A AR) is critical for retention at the basolateral surface of polarized Madin-Darby canine kidney II (MDCKII) cells following their direct targeting to this surface. Findings that the 3i loops of the D2 dopamine receptors interact with spinophilin (Smith, F. D., Oxford, G. S., and Milgram, S. L. (1999) J. Biol. Chem. 274, 19894-19900) and that spinophilin is enriched beneath the basolateral surface of polarized MDCK cells prompted us to assess whether alpha(2)AR subtypes might also interact with spinophilin. [35S]Met-labeled 3i loops of the alpha2A AR (Val(217)-Ala(377)), alpha2BAR (Lys(210)-Trp(354)), and alpha2CAR (Arg(248)-Val(363)) subtypes interacted with glutathione S-transferase-spinophilin fusion proteins. These interactions could be refined to spinophilin amino acid residues 169-255, in a region between spinophilin's F-actin binding and phosphatase 1 regulatory domains. Furthermore, these interactions occur in intact cells in an agonist-regulated fashion, because alpha2A AR and spinophilin coimmunoprecipitation from cells is enhanced by prior treatment with agonist. These findings suggest that spinophilin may contribute not only to alpha2 AR localization but also to agonist modulation of alpha2AR signaling.

Expression of a plant gene with sequence similarity to animal TGF-beta receptor interacting protein is regulated by brassinosteroids and required for normal plant development

Brassinosteroids (BRs) regulate the expression of numerous genes associated with plant development, and require the activity of a Ser/Thr receptor kinase to realize their effects. In animals, the transforming growth factor-beta (TGF-beta) family of peptides acts via Ser/Thr receptor kinases to have a major impact on several pathways involved in animal development and adult homeostasis. TGF-beta receptor-interacting protein (TRIP-1) was previously shown by others to be an intracellular substrate of the TGF-beta type II receptor kinase which plays an important role in TGF-beta signaling. TRIP-1 is a WD-repeat protein that also has a dual role as an essential subunit of the eukaryotic translation initiation factor eIF3 in animals, yeast and plants, thereby revealing a putative link between a developmental signaling pathway and the control of protein translation. In yeast, expression of a TRIP-1 homolog has also been closely associated with cell proliferation and progression through the cell cycle. We report here the novel observation that transcript levels of TRIP-1 homologs in plants are regulated by BR treatment under a variety of conditions, and that transgenic plants expressing antisense TRIP-1 RNA exhibit a broad range of developmental defects, including some that resemble the phenotype of BR-deficient and -insensitive mutants. This correlative evidence suggests that a WD-domain protein with reported dual functions in vertebrates and fungi might mediate some of the molecular mechanisms underlying the regulation of plant growth and development by BRs.

ATRAP, novel AT1 receptor associated protein, enhances internalization of AT1 receptor and inhibits vascular smooth muscle cell growth

We have identified a novel, membrane-located protein that interacts specifically with the carboxyl-terminal cytoplasmic domain of the AT1a receptor, which we named ATRAP (for AT1 receptor-associated protein). To further investigate the role of ATRAP in AT1 receptor function, we examined the effect of overexpression of ATRAP on angiotensin II (Ang II)-induced AT1 receptor desensitization and/or internalization, and cell proliferation in adult vascular smooth muscle cells (VSMCs). Transfection of ATRAP potentiated AT1 receptor internalization upon Ang II stimulation in these VSMCs. Moreover, we observed that AT1 receptor-induced DNA synthesis was markedly inhibited in ATRAP transfected VSMCs associated with the inhibition of the phosphorylation of signal transducers and activators of transcription (STAT) 3 and Akt. Our results suggest that ATRAP functions as a negative regulator in AT1 receptor-mediated cell proliferation in VSMCs.

The metabotropic GABAB receptor directly interacts with the activating transcription factor 4

G protein-coupled receptors regulate gene expression by cellular signaling cascades that target transcription factors and their recognition by specific DNA sequences. In the central nervous system, heteromeric metabotropic gamma-aminobutyric acid type B (GABA(B)) receptors through adenylyl cyclase regulate cAMP levels, which may control transcription factor binding to the cAMP response element. Using yeast-two hybrid screens of rat brain libraries, we now demonstrate that GABA(B) receptors are engaged in a direct and specific interaction with the activating transcription factor 4 (ATF-4), a member of the cAMP response element-binding protein /ATF family. As confirmed by pull-down assays, ATF-4 associates via its conserved basic leucine zipper domain with the C termini of both GABA(B) receptor (GABA(B)R) 1 and GABA(B)R2 at a site which serves to assemble these receptor subunits in heterodimeric complexes. Confocal fluorescence microscopy shows that GABA(B)R and ATF-4 are strongly coclustered in the soma and at the dendritic membrane surface of both cultured hippocampal neurons as well as retinal amacrine cells in vivo. In oocyte coexpression assays short term signaling of GABA(B)Rs via G proteins was only marginally affected by the presence of the transcription factor, but ATF-4 was moderately stimulated in response to receptor activation in in vivo reporter assays. Thus, inhibitory metabotropic GABA(B)Rs may regulate activity-dependent gene expression via a direct interaction with ATF-4.

Advantages of heterologous expression of human D2long dopamine receptors in human neuroblastoma SH-SY5Y over human embryonic kidney 293 cells

The human D2long dopamine receptor when expressed heterologously in a human neuronal cell line, SH-SY5Y, produced more robust functional signals than when expressed in a human embryonic kidney cell line, HEK293. Quinpirole (agonist)-induced GTPgamma(35)S binding and high affinity sites were 3 - 4 fold greater in SH-SY5Y than in HEK293 cells. N-type Ca(2+) channel currents present in SH-SY5Y cells, but not HEK293 cells, were inhibited potently by quinpirole with a half-maximal inhibitory concentration of 0.15+/-0.03 nM. Inhibition of adenylyl cyclases by agonists, on the other hand, was of similar potency and efficacy in the two cell lines. GTPgamma(35)S-Bound Galpha subunits from quinpirole-activated and solubilized membranes were monitored upon immobilization with various Galpha-specific antibodies. Galpha(i) and Galpha(o) subunits were highly labelled with GTPgamma(35)S in SH-SY5Y cells, but only Galpha(i) subunits were labelled in HEK293 cells. The additional G(o) coupling in SH-SY5Y cells could arise, at least in part, from the presence of G(o) coupled-effectors, such as the N-type Ca(2+) channel, and may contribute to robust agonist-induced GTPgamma(35)S binding, which is a reliable means for measuring ligand intrinsic efficacy. It appears that expression of neuronal G protein-coupled receptors in neuronal environments could reveal additional functional characteristics that are absent in non-neuronal cell lines. This appears to be due to, at least in part, to the presence of neuron-specific effectors. These findings underscore the importance of the cellular environment in which drug actions are examined, particularly in the face of intensive efforts to develop drugs for G protein-coupled receptors of various origins.

G-protein-independent signaling by G-protein-coupled receptors

Two classes of receptors transduce neurotransmitter signals: ionotropic receptors and heptahelical metabotropic receptors. Whereas the ionotropic receptors are structurally associated with a membrane channel, a mediating mechanism is necessary to functionally link metabotropic receptors with their respective effectors. According to the accepted paradigm, the first step in the metabotropic transduction process requires the activation of heterotrimeric G-proteins. An increasing number of observations, however, point to a novel mechanism through which neurotransmitters can initiate biochemical signals and modulate neuronal excitability. According to this mechanism metabotropic receptors induce responses by activating transduction systems that do not involve G-proteins.

p38JAB1 binds to the intracellular precursor of the lutropin/choriogonadotropin receptor and promotes its degradation

Using the C-terminal tail of the rat lutropin/choriogonadotropin receptor (rLHR) as "bait" in a yeast two-hybrid screen resulted in the identification of p38(JAB1) (a protein initially identified as a co-activator of c-Jun) as a putative rLHR binding partner. More recently p38(JAB1) has been shown to promote the degradation of a cyclin-dependent kinase inhibitor and to be a component of the COP9 signalosome. Microscopic localization of an epitope-tagged p38(JAB1) expressed in 293 cells revealed a punctuated perinuclear and cytosolic localization, while cell fractionation studies showed that most of the p38(JAB1) was in a high speed supernatant. Co-transfection of 293 cells revealed that p38(JAB1) binds to the immature 68-kDa precursor of the rLHR that resides in the endoplasmic reticulum and promotes its degradation. It does not appear to interact with the cell surface rLHR, however, and it does not affect its expression. When transfected into HeLa cells, p38(JAB1) potentiates the transcriptional activity of c-Jun, but co-transfection with rLHR prevents this effect. We conclude that p38(JAB1) interacts with the rLHR precursor and promotes its degradation. These results reveal a novel protein binding partner of the rLHR and are consistent with current views of the functions of p38(JAB1).

Identification of Gbetagamma binding sites in the third intracellular loop of the M(3)-muscarinic receptor and their role in receptor regulation

Gbetagamma binds directly to the third intracellular (i3) loop subdomain of the M(3)-muscarinic receptor (MR). In this report, we identified the Gbetagamma binding motif and G-protein-coupled receptor kinase (GRK2) phosphorylation sites in the M(3)-MR i3 loop via a strategy of deletional and site-directed mutagenesis. The Gbetagamma binding domain was localized to Cys(289)-His(330) within the M(3)-MR-Arg(252)-Gln(490) i3 loop, and the binding properties (affinity, influence of ionic strength) of the M(3)-MR-Cys(289)-His(330) i3 loop subdomain were similar to those observed for the entire i3 loop. Site-directed mutagenesis of the M(3)-MR-Cys(289)-His(330) i3 loop subdomain indicated that Phe(312), Phe(314), and a negatively charged region (Glu(324)-Asp(329)) were required for interaction with Gbetagamma. Generation of the full-length M(3)-MR-Arg(252)-Gln(490) i3 peptides containing the F312A mutation were also deficient in Gbetagamma binding and exhibited a reduced capacity for phosphorylation by GRK2. A similar, parallel strategy resulted in identification of major residues ((331)SSS(333) and (348)SASS(351)) phosphorylated by GRK2, which were just downstream of the Gbetagamma binding motif. Full-length M(3)-MR constructs lacking the 42-amino acid Gbetagamma binding domain (Cys(289)-His(330)) or containing the F312A mutation exhibited ligand recognition properties similar to wild type receptor and also effectively mediated agonist-induced increases in intracellular calcium following receptor expression in Chinese hamster ovary and/or COS 7 cells. However, the M(3)-MRDeltaCys(289)-His(330) and M(3)-MR(F312A) constructs were deficient in agonist-induced sequestration, indicating a key role for the Gbetagamma-M(3)-MR i3 loop interaction in receptor regulation and signal processing.

Modulation of dopamine D(2) receptor signaling by actin-binding protein (ABP-280)

Proteins that bind to G protein-coupled receptors have recently been identified as regulators of receptor anchoring and signaling. In this study, actin-binding protein 280 (ABP-280), a widely expressed cytoskeleton-associated protein that plays an important role in regulating cell morphology and motility, was found to associate with the third cytoplasmic loop of dopamine D(2) receptors. The specificity of this interaction was originally identified in a yeast two-hybrid screen and confirmed by protein binding. The functional significance of the D(2) receptor-ABP-280 association was evaluated in human melanoma cells lacking ABP-280. D(2) receptor agonists were less potent in inhibiting forskolin-stimulated cAMP production in these cells. Maximal inhibitory responses of D(2) receptor activation were also reduced. Further yeast two-hybrid experiments showed that ABP-280 association is critically dependent on the carboxyl domain of the D(2) receptor third cytoplasmic loop, where there is a potential serine phosphorylation site (S358). Serine 358 was replaced with aspartic acid to mimic the effects of receptor phosphorylation. This mutant (D(2)S358D) displayed compromised binding to ABP-280 and coupling to adenylate cyclase. PKC activation also generated D(2) receptor signaling attenuation, but only in ABP-containing cells, suggesting a PKC regulatory role in D(2)-ABP association. A mechanism for these results may be derived from a role of ABP-280 in the clustering of D(2) receptors, as determined by immunocytochemical analysis in ABP-deficient and replete cells. Our results suggest a new molecular mechanism of modulating D(2) receptor signaling by cytoskeletal protein interaction.

Odorant receptors: a plethora of G-protein-coupled receptors

Odorant receptors (ORs) comprise the largest family of G-protein-coupled receptors (GPCRs). They are located in the nasal epithelium, at the ciliated surface of olfactory sensory neurones, where the initial steps of the olfactory transduction cascade occur. ORs are encoded by a large and diverse multi-gene family, which has been characterized in cyclostomes, teleosts, amphibia, birds and mammals, as well as in Drosophila and Caenorhabditis elegans. Here, the range of diversity in OR and chemoreceptor structure is examined, noting that their functions are fundamentally similar to those of many neurotransmitter or neurohormone receptors. It is argued that ORs have emerged directly from other GPCRs independently in many species. According to this view, there is no structural prerequisite for OR identity and any GPCR has the potential to be or become an OR at a given point in evolution.

A kinase-regulated PDZ-domain interaction controls endocytic sorting of the beta2-adrenergic receptor

A fundamental question in cell biology is how membrane proteins are sorted in the endocytic pathway. The sorting of internalized beta2-adrenergic receptors between recycling endosomes and lysosomes is responsible for opposite effects on signal transduction and is regulated by physiological stimuli. Here we describe a mechanism that controls this sorting operation, which is mediated by a family of conserved protein-interaction modules called PDZ domains. The phosphoprotein EBP50 (for ezrinradixin-moesin(ERM)-binding phosphoprotein-50) binds to the cytoplasmic tail of the beta2-adrenergic receptor through a PDZ domain and to the cortical actin cytoskeleton through an ERM-binding domain. Disrupting the interaction of EBP50 with either domain or depolymerization of the actin cytoskeleton itself causes missorting of endocytosed beta2-adrenergic receptors but does not affect the recycling of transferrin receptors. A serine residue at position 411 in the tail of the beta2-adrenergic receptor is a substrate for phosphorylation by GRK-5 (for G-protein-coupled-receptor kinase-5) and is required for interaction with EBP50 and for proper recycling of the receptor. Our results identify a new role for PDZ-domain-mediated protein interactions and for the actin cytoskeleton in endocytic sorting, and suggest a mechanism by which GRK-mediated phosphorylation could regulate membrane trafficking of G-protein-coupled receptors after endocytosis.

Interaction of the alpha(1B)-adrenergic receptor with gC1q-R, a multifunctional protein

gC1q-R, a multifunctional protein, was found to bind with the carboxyl-terminal cytoplasmic domain of the alpha(1B)-adrenergic receptor (173 amino acids, amino acids 344-516) in a yeast two-hybrid screen of a cDNA library prepared from the rat liver. In a series of studies with deletion mutants in this region, the ten arginine-rich amino acids (amino acids 369-378) were identified as the site of interaction. The interaction was confirmed by specific co-immunoprecipitation of gC1q-R with full-length alpha(1B)-adrenergic receptors expressed on transfected COS-7 cells, as well as by fluorescence confocal laser scanning microscopy, which showed co-localization of these proteins in intact cells. Interestingly, the alpha(1B)-adrenergic receptors were exclusively localized to the region of the plasma membrane in COS-7 cells that expressed the alpha(1B)-adrenergic receptor alone, whereas gC1q-R was localized in the cytoplasm in COS-7 cells that expressed gC1q-R alone; however, in cells that co-expressed alpha(1B)-adrenergic receptors and gC1q-R, most of the alpha(1B)-adrenergic receptors were co-localized with gC1q-R in the intracellular region, and a remarkable down-regulation of receptor expression was observed. These observations suggest a new role for the previously identified complement regulatory molecule, gC1q-R, in regulating the cellular localization and expression of the alpha(1B)-adrenergic receptors.

Agonist-dependent interaction of the rat somatostatin receptor subtype 2 with cortactin-binding protein 1

We report here an interaction between the C terminus of the rat somatostatin receptor subtype 2 (SSTR2) and a protein that has recently been identified as cortactin-binding protein 1 (CortBP1). Interaction is mediated by the PDZ (PSD-95/discs large/ZO-1) domain of CortBP1. As shown by in situ hybridization, SSTR2 and cortactin-binding protein are coexpressed in the rat brain. The association between SSTR2 and the PDZ-domain of CortBP1 was verified by overlay assays and by coprecipitation after transfection in human embryonic kidney (HEK) cells. Analysis by confocal microscopy indicates that CortBP1 is distributed diffusely throughout the cytosol in transfected cells and that it becomes concentrated at the plasma membrane when SSTR2 is present. This process is largely increased when the receptor is stimulated by somatostatin; as CortBP1 interacts with the C terminus of SSTR2, our data suggest that the binding of agonist to the receptor increase the accessibility of the receptor C terminus to the PDZ domain of CortBP1. Our data for the first time establish a link between a G-protein coupled receptor and constituents of the cytoskeleton.

Cloning and characterization of ATRAP, a novel protein that interacts with the angiotensin II type 1 receptor

The carboxyl-terminal cytoplasmic domain of the angiotensin II type 1 (AT1) receptor has recently been shown to interact with several classes of cytoplasmic proteins that regulate different aspects of AT1 receptor physiology. Employing yeast two-hybrid screening of a mouse kidney cDNA library with the carboxyl-terminal cytoplasmic domain of the murine AT1a receptor as a bait, we have isolated a novel protein with a predicted molecular mass of 18 kDa, which we have named ATRAP (for AT1 receptor-associated protein). ATRAP interacts specifically with the carboxyl-terminal domain of the AT1a receptor but not with those of angiotensin II type 2 (AT2), m3 muscarinic acetylcholine, bradykinin B2, endothelin B, and beta2-adrenergic receptors. The mRNA of ATRAP was abundantly expressed in kidney, heart, and testis but was poorly expressed in lung, liver, spleen, and brain. The ATRAP-AT1a receptor association was confirmed by affinity chromatography, by specific co-immunoprecipitation of the two proteins, and by fluorescence microscopy, showing co-localization of these proteins in intact cells. Overexpression of ATRAP in COS-7 cells caused a marked inhibition of AT1a receptor-mediated activation of phospholipase C without affecting m3 receptor-mediated activation. In conclusion, we have isolated a novel protein that interacts specifically with the carboxyl-terminal cytoplasmic domain of the AT1a receptor and affects AT1a receptor signaling.