Chemosensory signals and their receptors in the olfactory neural system

Chemical communication is widely used among various organisms to obtain essential information from their environment required for life. Although a large variety of molecules have been shown to act as chemical cues, the molecular and neural basis underlying the behaviors elicited by these molecules has been revealed for only a limited number of molecules. Here, we review the current knowledge regarding the signaling molecules whose flow from receptor to specific behavior has been characterized. Discussing the molecules utilized by mice, insects, and the worm, we focus on how each organism has optimized its reception system to suit its living style. We also highlight how the production of these signaling molecules is regulated, an area in which considerable progress has been recently made.

Dimethyl Trisulfide as a Characteristic Odor Associated with Fungating Breast Cancer Wounds

Background: Some advanced cancer patients suffer from unpleasant odor from their fungating wounds. The fungating wounds have been reported to occur in about 5% of patients with cancer. These wounds are usually superinfected with bacteria, and therefore, the infected area tends to emit malodor. However, the odorant(s) causing the malodor has not been revealed. Here we aim to determine the chemical identity of the cancer wound-derived odor(s). Results: We examined three female patients with breast cancer (B1; stage IV, B2; stage IIIB, B3; stage IV) and two male patients with head and neck cancer (H1; stage III, H2; stage IVA). First, we evaluated the intensity and quality of body odors emitted from the fungating wounds of patients. All of them had a similar pungent sulfury odor. In addition to the sulfury odor, B1and B2 had a cheese-like odor and B3, H1 and H2 had a rotten fish odor. Next, we analyzed the malodor emitted from the wounds of each patient. Sterile gauze pads were placed on fungating wounds of the patients for 6-12h. The head-space volatiles of pads were extracted onto SPME fibers for 2h. The compounds on the SPME fibers were then analyzed by gas chromatography-mass spectrometry-olfactometry (GC-MS-O) that enabled us to examine mass spectra and odor qualities of individual GC-separated odorants simultaneously. GC-MS-O analysis and evaluation of body odors were performed by three persons. As a result, the sulfury odor, which was the same as the odor we identified in body odors of all patients, came out at the retention time (RT) of 19.46min. Cheese-like odors in B1 and B2 patients came out at RT=24.01min, 25.47min and 26.08min. The structure of dimethyl trisulfide (DMTS) was predicted by the mass spectrum of the peak around 19.5min. The mass spectrum and the RT of authentic DMTS were identical to those of the peak compound, demonstrating that the sulfury odor at 19.46min was DMTS. Comparing the odor quality and intensity of various concentrations of DMTS solutions and cancer samples, we conclude that DMTS is emitted from the cancer wounds at the level significant enough to make ones feel uncomfortable. We also identified the structure of compounds with cheese-like odors: isobutyric acid for the cheese odor (24.01min), butyric acid for the cheese and vomit odor (25.47min), and isovaleric acid for the cheese and feet odor (26.08min). Discussion: We identified DMTS as the main odorant that caused the severe malodor in some advanced cancer patients. DMTS has been found in volatiles emitted from vegetables such as cooked onion. DMTS is also reported to be produced by aerobes such as Pseudomonas aeruginosa that reside in leg ulcers. Thus, although the source of DMTS found in the fungating cancer wounds in this study remains to be elucidated, DMTS may be a product of infected bacteria in fungating wounds. To improve the QOL of patients, the development of the way to prevent or reduce the DMTS odor is awaited. Indeed, almost all patients with fungating wounds are suffering from this malodor. Our results provide new insights into the better strategies toward the treatment of malodor in cancer patients.

Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 5042.

Insect olfaction: receptors, signal transduction, and behavior

The insect olfactory system is a suitable model for exploring molecular function of odorant receptors, axonal projection of olfactory receptor neurons onto secondary neurons, and the neural circuit for odor perception. Recent progress in the study of insect olfaction revealed that the heteromeric insect olfactory receptor complex forms a cation nonselective ion channel directly gated by odor or pheromone ligands independent of known G-protein signaling pathways. Despite fundamental differences in transduction machineries between insects and vertebrates, the anatomical and functional features of insect odor-coding strategy are similar and thus justify any consideration of mammalian olfaction in the study of insects. The understanding of the molecular mechanism of insect olfaction will help in the development of insect repellents for controlling insect pest and vector populations for a wide range of pathogens.

Molecular bases of odor discrimination: Reconstitution of olfactory receptors that recognize overlapping sets of odorants

The vertebrate olfactory system discriminates a wide variety of odorants by relaying coded information from olfactory sensory neurons in the olfactory epithelium to olfactory cortical areas of the brain. Recent studies have shown that the first step in odor discrimination is mediated by approximately 1000 distinct olfactory receptors, which comprise the largest family of G-protein-coupled receptors. In the present study, we used Ca(2+) imaging and single-cell reverse transcription-PCR techniques to identify mouse olfactory neurons responding to an odorant and subsequently to clone a receptor gene from the responsive cell. The functionally cloned receptors were expressed in heterologous systems, demonstrating that structurally related olfactory receptors recognized overlapping sets of odorants with distinct affinities and specificities. Our results provide direct evidence for the existence of a receptor code in which the identities of different odorants are specified by distinct combinations of odorant receptors that possess unique molecular receptive ranges. We further demonstrate that the receptor code for an odorant changes with odorant concentration. Finally, we show that odorant receptors in human embryonic kidney 293 cells couple to stimulatory G-proteins such as Galphaolf, resulting in odorant-dependent increases in cAMP. Odor discrimination is thus determined by differences in the receptive ranges of the odorant receptors that together encode specific odorant molecules.

Functional cloning and reconstitution of vertebrate odorant receptors

The olfactory systems of vertebrates have a remarkable capacity to recognize and discriminate thousands of different odorant molecules. The initial step in the process of odorant perception is the recognition of volatile odorant molecules by a group of roughly one thousand G protein-coupled odorant receptors that are expressed on the surface of olfactory neuronal cilia. The aims of this study were to obtain functional evidence that these putative odorant receptors recognize and respond to specific odorant molecules, and to elucidate the mechanisms of odorant discrimination in vertebrate olfaction at a receptor level. In order to identify odorant receptors that specifically recognize a particular odorant of interest, we developed a functional cloning strategy in an odorant-directed manner by combining Ca2+-recording and single cell RT-PCR techniques. We then adopted an adenovirus-mediated expression system or a chimeric receptor approach to reconstitute the functionally cloned receptors for further biochemical analyses. We herein describe how we obtained experimental evidence for a combinatory mechanism of odorant recognition by examining the diversity of odorant receptors that recognize a particular odorant of interest, and by determining ligand specificity and structure-function relationships for individual odorant receptors.

RACK1, a protein kinase C anchoring protein, coordinates the binding of activated protein kinase C and select pleckstrin homology domains in vitro

The pleckstrin homology (PH) domain, identified in numerous signaling proteins including the beta-adrenergic receptor kinase (betaARK), was found to bind to various phospholipids as well as the beta subunit of heterotrimeric G proteins (Gbeta) [Touhara, K., et al. (1994) J. Biol. Chem. 269, 10217-10220]. Several PH domain-containing proteins are also substrates of protein kinase C (PKC). Because RACK1, an anchoring protein for activated PKC, is homologous to Gbeta (both contain seven repeats of the WD-40 motif), we determined (i) whether a direct interaction between various PH domains and RACK1 occurs and (ii) the effect of PKC on this interaction. We found that recombinant PH domains of several proteins exhibited differential binding to RACK1. Activated PKC and the PH domain of beta-spectrin or dynamin-1 concomitantly bound to RACK1. Although PH domains bind acidic phospholipids, the interaction between various PH domains and RACK1 was not dependent on the phospholipid activators of PKC, phosphatidylserine and 1, 2-diacylglycerol. Binding of these PH domains to RACK1 was also not affected by either inositol 1,4,5-triphosphate (IP(3)) or phosphatidylinositol 4,5-bisphosphate (PIP(2)). Our in vitro data suggest that RACK1 binds selective PH domains, and that PKC regulates this interaction. We propose that, in vivo, RACK1 may colocalize the kinase with its PH domain-containing substrates.

Functional identification and reconstitution of an odorant receptor in single olfactory neurons

The olfactory system is remarkable in its capacity to discriminate a wide range of odorants through a series of transduction events initiated in olfactory receptor neurons. Each olfactory neuron is expected to express only a single odorant receptor gene that belongs to the G protein coupled receptor family. The ligand-receptor interaction, however, has not been clearly characterized. This study demonstrates the functional identification of olfactory receptor(s) for specific odorant(s) from single olfactory neurons by a combination of Ca2+-imaging and reverse transcription-coupled PCR analysis. First, a candidate odorant receptor was cloned from a single tissue-printed olfactory neuron that displayed odorant-induced Ca2+ increase. Next, recombinant adenovirus-mediated expression of the isolated receptor gene was established in the olfactory epithelium by using green fluorescent protein as a marker. The infected neurons elicited external Ca2+ entry when exposed to the odorant that originally was used to identify the receptor gene. Experiments performed to determine ligand specificity revealed that the odorant receptor recognized specific structural motifs within odorant molecules. The odorant receptor-mediated signal transduction appears to be reconstituted by this two-step approach: the receptor screening for given odorant(s) from single neurons and the functional expression of the receptor via recombinant adenovirus. The present approach should enable us to examine not only ligand specificity of an odorant receptor but also receptor specificity and diversity for a particular odorant of interest.

Identification of myosin II as a binding protein to the PH domain of protein kinase B

Myosin II was identified as a binding protein to the pleckstrin homology (PH) domain of protein kinase B (PKB) in CHO cell extract by using the glutathione S-transferase-fusion protein as a probe. When myosin II purified from rabbit skeletal muscle was employed, myosin II was shown to bind almost exclusively to the PH domain of PKB among the PH domain fusion proteins examined. The purified myosin II bound to the PH domain of PKB with a Kd value of 1.1 x 10(-7) M. Studies with a series of truncated molecules indicated that the whole structure of the PH domain is required for the binding of myosin II, and the binding to the PH domain was inhibited by phosphatidylinositol 4,5-bisphosphate. These results suggest that myosin II is a specific binding protein to the PH domain of particular proteins including PKB.

Effects of mutations in pleckstrin homology domain on beta-adrenergic receptor kinase activity in intact cells

beta-Adrenergic receptor kinase (betaARK) plays a pivotal role in phosphorylating and desensitizing G protein coupled receptors by virtue of pleckstrin homology (PH) domain-mediated membrane translocation. betaARK is localized to the specific membrane compartment by betagamma subunits of G proteins (Gbetagamma) and phosphatidylinositol phosphates that specifically and coordinately bind to the carboxyl and amino terminus half, respectively, of the betaARK PH domain. To determine the function of the betaARK PH domain in intact cells, various point mutations were incorporated in the betaARK PH domain and the constructs were tested for their ability to agonist-dependently phosphorylate the muscarinic acetylcholine receptor or alpha-adrenergic receptor in COS-7 cells. It was found that selected mutations (i.e., W643A, L647A, and an Ala-insertion following Trp643) completely abolished betaARK's ability to phosphorylate the receptors in whole-cell labeling experiments. These residues are located in the carboxyl-terminal alpha-helix of the PH domain that is essential for binding to Gbetagamma. This site-directed mutation study provides molecular information on the mechanism and significance of the betaARK PH domain function in the intact cell system.

Binding of multiple ligands to pleckstrin homology domain regulates membrane translocation and enzyme activity of beta-adrenergic receptor kinase

Pleckstrin homology (PH) domains are discrete structural modules present in numerous proteins involved in signal transduction processes. In the case of the beta-adrenergic receptor kinase (betaARK), PH domain-mediated binding of two ligands, the betagamma subunits of heterotrimeric G proteins (Gbetagamma) and phosphatidylinositol 4,5-bisphosphate (PIP2), has been shown to be required for the kinase function. In this study, the ability of Gbetagamma and PIP2 to affect membrane localization of betaARK is used to define the ligand binding characteristics of the betaARK PH domain. The binding of these ligands to the PH domain of the intact kinase is shown to be cooperative, Gbetagamma increasing the affinity of the PH domain for PIP2. Notably, although PIP2-dependent membrane association of betaARK is observed at high concentrations of this lipid, in the absence of Gbetagamma, no receptor phosphorylation is observed. Peptides derived from the receptor intracellular loop inhibit the receptor phosphorylation without affecting the membrane translocation of the kinase complex, suggesting that betaARK activity does not necessarily correlate with the amount of betaARK associated with the membrane. These results point to a distinct role for each PH domain ligand in betaARK-mediated receptor phosphorylation. Strikingly, the ligand binding characteristics of the isolated betaARK PH domain fused to glutathione S-transferase are very different from those of the PH domain of the intact kinase. Thus, in contrast to the native protein, the isolated PH domain binds Gbetagamma and PIP2 independently and with no apparent cooperativity. That protein environment plays an important role in determining the ligand binding characteristics of a particular PH domain highlights the potential risks of inferring mechanisms from studies of isolated PH domains.

G-protein-coupled receptors and their regulation: activation of the MAP kinase signaling pathway by G-protein-coupled receptors

G-protein-coupled receptors that mediate cellular responses to a variety of humoral, endothelial-, or platelet-derived substances are able to stimulate MAP kinase activity. In transfected model systems, G-protein-coupled receptors that couple to pertussis toxin-insensitive G proteins of the Gq/11 family mediate this activation predominantly via a PKC-dependent mechanism. In contrast, activation of MAP kinase by receptors that couple to pertussis toxin-sensitive Gi proteins is PKC-independent and requires downstream activation of the low-molecular-weight G protein, Ras. This pathway can be inhibited by coexpression of peptides that sequester Gbetagamma subunits, and is mimicked by overexpression of Gbetagamma subunits. This Ras-dependent MAP kinase activation requires tyrosine phosphorylation of "docking proteins," including the shc adapter protein, and depends upon recruitment of Grb2/Sos1 complexes to the plasma membrane, thus resembling the pathway of MAP kinase activation employed by the receptor tyrosine kinases. Other molecules, including PI-3-kinases and phosphotyrosine phosphatases, probably also contribute to Gbetagamma-subunit-mediated assembly of a mitogenic signaling complex. Identification of the G-protein-coupled, receptor-regulated tyrosine kinase(s), and the means by which the mitogenic signaling complex is assembled at the plasma membrane, remain subjects of further study.

Role of mitogen-activated protein kinase pathway in prostaglandin F2alpha-induced rat puerperal uterine contraction

In this study, prostaglandin (PG) F2alpha was found to activate mitogen-activated protein (MAP) kinase and MAP kinase kinase (MEK) in cultured rat puerperal uterine myometrial cells. PGF2alpha stimulation also led to an increase in phosphorylation of raf-1, son of sevenless (SOS), and Shc. Furthermore, we examined the mechanism by which PGF2alpha induced MAP kinase phosphorylation. Both pertussis toxin (10 ng/ml), which inactivates Gi/Go proteins, and expression of a peptide derived from the carboxyl terminus of the beta-adrenergic receptor kinase 1 (betaARK1), which specifically blocks signaling mediated by the betagamma subunits of G proteins, blocked the PGF2alpha-induced activation of MAP kinase. Ritodrine (1 microM), which is known to relax uterine muscle contraction, attenuated PGF2alpha-induced tyrosine phosphorylation of MAP kinase. Moreover, to examine the role of MAP kinase pathway in uterine contraction, an inhibitor of MEK activity, PD098059, was used. Although MEK inhibitor had no effect on PGF2alpha-induced calcium mobilization, this inhibitor partially inhibited PGF2alpha-induced uterine contraction. These results provide evidence that PGF2alpha stimulates the MAP kinase signaling pathway in cultured rat puerperal uterine myometrial cells through Gbetagamma protein, suggesting that this new pathway may play an important role in the biological action of PGF2alpha on these cells.

Phosducin, potential role in modulation of olfactory signaling

Phosducin, which tightly binds betagamma-subunits of heterotrimeric G-proteins, has been conjectured to play a role in regulating second messenger signaling cascades, but to date its specific function has not been elucidated. Here we demonstrate a potential role for phosducin in regulating olfactory signal transduction. In isolated olfactory cilia certain odorants elicit a rapid and transient cAMP response, terminated by a concerted process which requires the action of two protein kinases, protein kinase A (PKA) and a receptor-specific kinase (GRK3) (Schleicher, S., Boekhoff, I. Arriza, J., Lefkowitz, R. J., and Breer, H. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 1420-1424). The mechanism of action of GRK3 involves a Gbetagamma-mediated translocation of the kinase to the plasma membrane bound receptors (Pitcher, J. A., Inglese, J., Higgins, J. B. , Arriza, J. L., Casey, P. J., Kim, C., Benovic, J. L., Kwatra, M. M. , Caron, M. G., and Lefkowitz, R. J. (1992) Science 257, 1264-1267). A protein with a molecular mass of 33 kDa that comigrates on SDS gels with recombinant phosducin and which is immunoreactive with phosducin antibodies is present in olfactory cilia. Recombinant phosducin added to permeabilized olfactory cilia preparations strongly inhibits termination of odorant-induced cAMP response and odorant-induced membrane translocation of GRK3. In addition, the cAMP analogue dibutyryl cAMP stimulates membrane targeting of the receptor kinase. This effect is presumably due to PKA-mediated phosphorylation of phosducin, which diminishes its affinity for binding to the Gbetagamma-subunit, thereby making Gbetagamma available to function as a membrane anchor for GRK3. A specific PKA inhibitor blocks the odorant-induced translocation of the receptor kinase. Consistent with this formulation, a non-phosphorylatable mutant of phosducin (phosducin Ser-73 --> Ala) is an even more effective inhibitor of desensitization and membrane targeting of GRK3 than the wild-type protein. A phosducin mutant that mimics phosphorylated phosducin (phosducin Ser-73 --> Asp) lacks this property and in fact recruits GRK3 to the membrane and potentiates desensitization. These results suggest that phosducin may act as a phosphorylation-dependent switch in second messenger signaling cascades, regulating the kinetics of desensitization processes by controlling the activity of Gbetagamma-dependent GRKs.

Partially purified RhoA-stimulated phospholipase D activity specifically binds to phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PIP2) is absolutely required for the ADP-ribosylation factor-stimulated phospholipase D (PLD) activity. In the present study, partially purified rat brain PLD was found to be activated by another PLD activator, RhoA, when PIP2, but not other acidic phospholipids, was included in vesicles comprising phosphatidylethanolamine (PE) and the PLD substrate phosphatidyicholine (PC) (PE/PC vesicles), demonstrating the absolute requirement of PIP2 for the RhoA-stimulated PLD activation, too. It is interesting that the RhoA-dependent PLD activity in the partially purified preparation was drastically decreased after the preparation was incubated with and separated from PE/PC vesicles containing PIP2. The PLD activity was extracted by higher concentrations of NaCl from the vesicles containing PIP2 that were incubated with and then separated from the partially purified PLD preparation. These results demonstrate that RhoA-dependent PLD binds to PE/PC vesicles with PIP2. The degree of binding of the RhoA-dependent PLD activity to the vesicles was totally dependent on the amount of PIP2 in the vesicles and correlated well with the extent of the enzyme activation. Further-more, it was found that a recombinant peptide of the pleckstrin homology domain of beta-adrenergic receptor kinase fused to glutathione S-transferase, which specifically binds to PIP2, inhibited the PIP2-stimulated, RhoA-dependent PLD activity in a concentration-dependent manner. From these results, it is concluded that in vitro rat brain PLD translocates to the vesicles containing PIP2, owing to its specific interaction with PIP2, to access its substrate PC, thereby catalyzing the hydrolysis of PC. PLD appears to localize exclusively on plasma membranes of cells and tissues. An aminoglycoside, neomycin, that has high affinity for PIP2 effectively extracted the RhoA-dependent PLD activity from rat brain membranes. This indicates that PIP2 serves as an anchor to localize PLD on plasma membranes in vivo.

G protein beta gamma subunits stimulate phosphorylation of Shc adapter protein

The mechanism of mitogen-activated protein (MAP) kinase activation by pertussis toxin-sensitive Gi-coupled receptors is known to involve the beta gamma subunits of heterotrimeric G proteins (G beta gamma), p21ras activation, and an as-yet-unidentified tyrosine kinase. To investigate the mechanism of G beta gamma-stimulated p21ras activation, G beta gamma-mediated tyrosine phosphorylation was examined by overexpressing G beta gamma or alpha 2-C10 adrenergic receptors (ARs) that couple to Gi in COS-7 cells. Immunoprecipitation of phosphotyrosine-containing proteins revealed a 2- to 3-fold increase in the phosphorylation of two proteins of approximately 50 kDa (designated as p52) in G beta gamma-transfected cells or in alpha 2-C10 AR-transfected cells stimulated with the agonist UK-14304. The latter response was pertussis toxin sensitive. These proteins (p52) were also specifically immunoprecipitated with anti-Shc antibodies and comigrated with two Shc proteins, 46 and 52 kDa. The G beta gamma- or alpha 2-C10 AR-stimulated p52 (Shc) phosphorylation was inhibited by coexpression of the carboxyl terminus of beta-adrenergic receptor kinase (a G beta gamma-binding pleckstrin homology domain peptide) or by the tyrosine kinase inhibitors genistein and herbimycin A, but not by a dominant negative mutant of p21ras. Worthmannin, a specific inhibitor of phosphatidylinositol 3-kinase (PI3K) inhibited phosphorylation of p52 (Shc), implying involvement of PI3K. These results suggest that G beta gamma-stimulated Shc phosphorylation represents an early step in the pathway leading to p21ras activation, similar to the mechanism utilized by growth factor tyrosine kinase receptors.

Receptor-tyrosine-kinase- and G beta gamma-mediated MAP kinase activation by a common signalling pathway

Mitogen-activated protein (MAP) kinases mediate the phosphorylation and activation of nuclear transcription factors that regulate cell growth. MAP kinase activation may result from stimulation of either tyrosine-kinase (RTK) receptors, which possess intrinsic tyrosine kinase activity, or G-protein-coupled receptors (GPCR). RTK-mediated mitogenic signalling involves a series of SH2- and SH3-dependent protein-protein interactions between tyrosine-phosphorylated receptor, Shc, Grb2 and Sos, resulting in Ras-dependent MAP kinase activation. The beta gamma subunits of heterotrimeric G proteins (G beta gamma) also mediate Ras-dependent MAP kinase activation by an as-yet unknown mechanism. Here we demonstrate that activation of MAP kinase by Gi-coupled receptors is preceded by the G beta gamma-mediated tyrosine phosphorylation of Shc, leading to an increased functional association between Shc, Grb2 and Sos. Moreover, disruption of the Shc-Grb2-Sos complex blocks G beta gamma-mediated MAP kinase activation, indicating that G beta gamma does not mediate MAP kinase activation by a direct interaction with Sos. These results indicate that G beta gamma-mediated MAP kinase activation is initiated by a tyrosine phosphorylation event and proceeds by a pathway common to both GPCRs and RTKs.

G beta gamma subunits mediate mitogen-activated protein kinase activation by the tyrosine kinase insulin-like growth factor 1 receptor

The receptors for insulin-like growth factor 1 (IGF1) and insulin are related heterotetrameric proteins which, like the epidermal growth factor (EGF) receptor, possess intrinsic ligand-stimulated tyrosine protein kinase activity. In Rat 1 fibroblasts, stimulation of mitogen-activated protein (MAP) kinase via the IGF1 receptor and the Gi-coupled receptor for lysophosphatidic acid (LPA), but not via the EGF receptor, is sensitive both to pertussis toxin treatment and to cellular expression of a specific G beta gamma subunit-binding peptide. The IGF1, LPA, and EGF receptor-mediated signals are all sensitive to inhibitors of tyrosine protein kinases, require p21ras activation, and are independent of protein kinase C. These data suggest that some tyrosine kinase growth factor receptors (e.g. IGF1 receptor) and classical G protein-coupled receptors (e.g. LPA receptor) employ a similar mechanism for mitogenic signaling that involves both tyrosine phosphorylation and G beta gamma subunits derived from pertussis toxin-sensitive G proteins.

Mutational analysis of the pleckstrin homology domain of the beta-adrenergic receptor kinase. Differential effects on G beta gamma and phosphatidylinositol 4,5-bisphosphate binding

The beta gamma subunits of heterotrimeric G proteins (G beta gamma) play a variety of roles in cellular signaling, one of which is membrane targeting of the beta-adrenergic receptor kinase (beta ARK). This is accomplished via a physical interaction of G beta gamma and a domain within the carboxyl terminus of beta ARK which overlaps with a pleckstrin homology (PH) domain. The PH domain of beta ARK not only binds G beta gamma but also interacts with phosphatidylinositol 4,5-bisphosphate (PIP2). Based on previous mapping of the G beta gamma binding region of beta ARK, and conserved residues within the PH domain, we have constructed a series of mutants in the carboxyl terminus of beta ARK in order to determine important residues involved in G beta gamma and PIP2 binding. To examine the effects of mutations on G beta gamma binding, we employed three different methodologies: direct G beta gamma binding to GST fusion proteins; the ability of GST fusion proteins to inhibit G beta gamma-mediated beta ARK translocation to rhodopsin-enriched rod outer segments; and the ability of mutant peptides expressed in cells to inhibit G beta gamma-mediated inositol phosphate accumulation. Direct PIP2 binding was also assessed on mutant GST fusion proteins. Ala residue insertion following Trp643 completely abolished the ability of beta ARK to bind G beta gamma, suggesting that a proper alpha-helical conformation is necessary for the G beta gamma.beta ARK interaction. In contrast, this insertional mutation had no effect on PIP2 binding. Both G beta gamma binding and PIP2 binding were abolished following Ala replacement of Trp643, suggesting that this conserved residue within the last subdomain of the PH domain is crucial for both interactions. Other mutations also produced differential effects on the physical interactions of the beta ARK carboxyl terminus with G beta gamma and PIP2. These results suggest that the last PH subdomain and its neighboring sequences within the carboxyl terminus of beta ARK, including Trp643, Leu647, and residues Lys663-Arg669, are critical for G beta gamma binding while Trp643 and residues Asp635-Glu639 are important for the PH domain to form the correct structure for binding to PIP2.

[PH domain: a new functional domain]

A pleckstrin homology (PH) domain is an approximately 100 amino acid region of sequence homology present in numerous proteins involved in signal transduction and growth control. The three dimensional structures of several PH domains demonstrate that they consist of a beta-barrel of seven antiparallel beta-sheets and a carboxyl-terminal amphiphilic alpha-helix. Several ligands capable of binding to PH domains have been identified including phosphatidylinositol 4,5-bisphosphate and the beta gamma subunits of heterotrimeric G proteins, which bind to the amino and carboxyl-termini of the PH domain, respectively. Furthermore, several isoforms of protein kinase C appear to bind to some PH domains. A general function of PH domains may be to anchor PH domain-containing proteins to the appropriate membrane-compartment. The membrane localization of PH domain-containing proteins may require cooperative multiple ligand binding to the PH domain. Finally, the heterogeneity of sequences among various PH domains may prove to be the basis for differences in the regulation and specificity of PH domain-ligand interaction in a fashion similar to SH2 and SH3 domains. The function of the PH domain and the mechanisms of PH domain action seem to be quite complex.

Effect of cellular expression of pleckstrin homology domains on Gi-coupled receptor signaling

Pleckstrin homology (PH) domains are 90-110 amino acid regions of protein sequence homology that are found in a variety of proteins involved in signal transduction and growth control. We have previously reported that the PH domains of several proteins, including beta ARK1, PLC gamma, IRS-1, Ras-GRF, and Ras-GAP, expressed as glutathione S-transferase fusion proteins, can reversibly bind purified bovine brain G beta gamma subunits in vitro with varying affinity. To determine whether PH domain peptides would behave as antagonists of G beta gamma subunit-mediated signal transduction in intact cells, plasmid minigene constructs encoding these PH domains were prepared, which permit transient cellular expression of the peptides. Pertussis toxin-sensitive, G beta gamma subunit-mediated inositol phosphate (IP) production was significantly inhibited in COS-7 cells transiently coexpressing the alpha 2-C10 adrenergic receptor (AR) and each of the PH domain peptides. Pertussis toxin-insensitive, Gq alpha subunit-mediated IP production via coexpressed M1 muscarinic acetylcholine receptor (M1 AChR) was attenuated only by the PLC gamma PH domain peptide, suggesting that the inhibitory effect of most of the PH domain peptides was G beta gamma subunit-specific. Stimulation of the mitogen-activated protein (MAP) kinase pathway by Gi-coupled receptors in COS-7 cells has been reported to require activation of p21ras and to be independent of protein kinase C. Since several proteins involved in activation contain PH domains, the effect of PH domain peptide expression on alpha 2-C10 AR-mediated p21ras-GTP exchange and MAP kinase activation as well as direct G beta gamma subunit-mediated activation of MAP kinase was determined. In each assay, coexpression of the PH domain peptides resulted in significant inhibition. Increasing G beta gamma subunit expression surmounted PH domain peptide-mediated inhibition of MAP kinase activation. These data suggest that the PH domain peptides behave as specific antagonists of G beta gamma-mediated signaling in intact cells and that interactions between PH domains and G beta gamma subunits or structurally related proteins may play a role in the activation of mitogenic signaling pathways by G protein-coupled receptors.

Pleckstrin homology domain-mediated membrane association and activation of the beta-adrenergic receptor kinase requires coordinate interaction with G beta gamma subunits and lipid

The pleckstrin homology (PH) domain is an approximately 100-amino-acid region of sequence homology present in numerous proteins of diverse functions, which forms a discrete structural module. Several ligands capable of binding to PH domain-containing proteins have been identified including phosphatidylinositol 4,5-bisphosphate (PIP2) and the G beta gamma subunits of heterotrimeric G proteins (G beta gamma), which bind to the amino and carboxyl termini of the PH domain, respectively. Here we report that the binding of G beta gamma and lipid to the PH domain of the beta-adrenergic receptor kinase (beta ARK) synergistically enhances agonist-dependent receptor phosphorylation and that both PH domain-binding ligands are required for membrane association of the kinase. PIP2 and to a lesser extent phosphatidylinositol 4-phosphate, phosphatidylinositol, and phosphatidic acid were the only lipids tested capable, in the presence of G beta gamma, of enhancing beta ARK activity. In contrast, the Km and Vmax for phosphorylation of a soluble beta ARK substrate (casein) was not altered in either the presence or absence of G beta gamma and/or PIP2. A fusion protein of the beta ARK containing an intact PH domain inhibits G beta gamma/PIP2-dependent beta ARK activity. In contrast, a mutant fusion protein in which a tryptophan residue, invariant in all PH domain sequences, is mutated to alanine shows no inhibitory activity. The requirement for the simultaneous presence of two PH domain binding ligands represents a previously unappreciated mechanism for effecting membrane localization of a protein and may have relevance to other PH domain-containing proteins.

G beta gamma interactions with PH domains and Ras-MAPK signaling pathways

G-protein-coupled receptor signaling and receptor tyrosine kinase (RTK) signaling are two mechanisms of transmembrane communication used by numerous extracellular agents and stimuli. The beta gamma-subunit complex of G proteins mediates many of the functions associated with G-protein-coupled receptor signaling and may even provide a means to link G proteins to RTK-initiated cascades. This connection may be mediated by the pleckstrin homology domain, a modular domain found in many signaling proteins that interact with G beta gamma.

Determination of the G beta gamma-binding domain of phosducin. A regulatable modulator of G beta gamma signaling

Although a role for the beta gamma-subunits of heterotrimeric G proteins (G beta gamma) in signal transduction by several cellular systems has been established, the structural features of cellular proteins interacting with G beta gamma have yet to be fully elucidated. The G beta gamma-binding region of beta-adrenergic receptor kinase (beta ARK), a cytosolic enzyme recruited to the membrane receptor substrate by G beta gamma, has been localized to the carboxyl terminus of the enzyme. Here, we demonstrate that the amino terminus of phosducin, a 33-kDa G beta gamma-binding retinal phosphoprotein, contains sequences homologous with the G beta gamma-binding domain of beta ARK. Accordingly, a glutathione S-transferase-fusion protein containing only the amino-terminal 105 amino acids of phosducin displayed G beta gamma binding ability. This domain of phosducin contains a protein kinase A (PKA) phosphorylation site, and upon phosphorylation, the binding of full-length phosducin to G beta gamma is reduced. In addition, transient expression of phosducin in COS-7 cells significantly inhibits G beta gamma-mediated phosphoinositide hydrolysis. This inhibitory effect is completely reversed by pretreatment of cells with dibutyryl cAMP, an activator of PKA. Thus, the binding of G beta gamma to phosducin can be regulated by PKA-phosphorylation in an intact cell model system.

Larva lights: a decade of photoaffinity labeling with juvenile hormone analogues

The introduction of photoaffinity labeling into the mode of action of insect hormones and pheromones started 12 yr ago with the photoaffinity labeling of juvenile hormone binding proteins (JHBPs) from cockroaches in the laboratory of the late John K. Koeppe. Applying this technique to Manduca sexta led ultimately to a three-laboratory collaborative project that has begun to dissect the molecular basis for JH transport, metabolism, and nuclear binding and gene activation in Lepidoptera. This review provides (1) a history of the first experiments; (2) an idea of the breadth of the technique in the arthropod classes Insecta, Crustacea, and Arachnida; and (3) evidence for the depth of the technique in unearthing key details about three different types of the molecular action of JH in M. sexta.

A nuclear juvenile hormone-binding protein from larvae of Manduca sexta: a putative receptor for the metamorphic action of juvenile hormone

A 29-kDa nuclear juvenile hormone (JH)-binding protein from the epidermis of Manduca sexta larvae was purified by using the photoaffinity analog for JH II ([3H]epoxyhomofarnesyldiazoacetate) and partially sequenced. A 1.1-kb cDNA was isolated by using degenerate oligonucleotide primers for PCR based on these sequences. The cDNA encoded a 262-amino acid protein that showed no similarity with other known proteins, except for short stretches of the interphotoreceptor retinoid-binding protein, rhodopsin, and human nuclear protein p68. Recombinant baculovirus containing this cDNA made a 29-kDa protein that was covalently modified by [3H]epoxyhomofarnesyldiazoacetate and specifically bound the natural enantiomer of JH I (Kd = 10.7 nM). This binding was inhibited by the natural JHs but not by methoprene. Immunocytochemical analysis showed localization of this 29-kDa protein to epidermal nuclei. Both mRNA and protein are present during the intermolt periods; during the larval molt, the mRNA disappears but the protein persists. Later when cells become pupally committed, both the mRNA and protein disappear with a transient reappearance near pupal ecdysis. The properties of this protein are consistent with its being the receptor necessary for the antimetamorphic effects of JH.

Functionally active targeting domain of the beta-adrenergic receptor kinase: an inhibitor of G beta gamma-mediated stimulation of type II adenylyl cyclase

The beta-adrenergic receptor kinase (beta ARK) phosphorylates its membrane-associated receptor substrates, such as the beta-adrenergic receptor, triggering events leading to receptor desensitization. beta ARK activity is markedly stimulated by the isoprenylated beta gamma subunit complex of heterotrimeric guanine nucleotide-binding proteins (G beta gamma), which translocates the kinase to the plasma membrane and thereby targets it to its receptor substrate. The amino-terminal two-thirds of beta ARK1 composes the receptor recognition and catalytic domains, while the carboxyl third contains the G beta gamma binding sequences, the targeting domain. We prepared this domain as a recombinant His6 fusion protein from Escherichia coli and found that it had both independent secondary structure and functional activity. We demonstrated the inhibitory properties of this domain against G beta gamma activation of type II adenylyl cyclase both in a reconstituted system utilizing Sf9 insect cell membranes and in a permeabilized 293 human embryonic kidney cell system. Gi alpha-mediated inhibition of adenylyl cyclase was not affected. These data suggest that this His6 fusion protein derived from the carboxyl terminus of beta ARK1 provides a specific probe for defining G beta gamma-mediated processes and for studying the structural features of a G beta gamma-binding domain.

Binding of G protein beta gamma-subunits to pleckstrin homology domains

Ligand-induced activation of many receptors leads to dissociation of the alpha- and beta gamma-subunit complexes of heterotrimeric G proteins, both of which regulate a variety of effector molecules involved in cellular signaling processes. In one case, a cytosolic enzyme, the beta-adrenergic receptor kinase (beta ARK) binds to the dissociated, prenylated, membrane-anchored beta gamma-subunits of heterotrimeric G proteins (G beta gamma) and is thereby targeted to its membrane-bound receptor substrate. Quite recently, numerous proteins involved in cellular signal transduction have been shown to contain sequences homologous with a "domain" originally identified in the protein "pleckstrin" (pleckstrin homology domain; PH domain) and subsequently found in the G beta gamma interaction region of the beta ARK sequence. Here we demonstrate that glutathione S-transferase-fusion proteins, containing sequences encompassing the PH domain of nine proteins from this group, bind G beta gamma to varying extents. Binding of G beta gamma to these fusion proteins was documented either by a direct binding assay or by ability to block G beta gamma-mediated membrane translocation of beta ARK1. G beta gamma binding to these fusion proteins was inhibited by the alpha subunit of Go (Go alpha), indicating that the binding of G beta gamma to G alpha and the PH domain-containing fusion proteins is mutually exclusive. Studies with a series of truncated PH domains derived from the Ras-guanine-nucleotide-releasing factor indicate that the G beta gamma binding domain includes only the C-terminal portion of the PH domain and sequences just distal to this. Protein-protein interactions between G beta gamma and PH domain-containing proteins may play a significant role in cellular signaling analogous to that previously demonstrated for Src homology 2 and 3 domains.

Juvenile hormone epoxide hydrolase. Photoaffinity labeling, purification, and characterization from tobacco hornworm eggs

Juvenile hormone epoxide hydrolase (JHEH), which may play a pivotal role in regulating insect juvenile hormone (JH) titer along with JH esterase, was identified in tobacco hornworm (Manduca sexta) eggs by using photoaffinity analogs of JHs. The UV light-induced covalent labeling with [3H]epoxyhomofarnesyl diazoacetate, a JHII analog, revealed a membrane-associated 50-kDa protein that was selectively and specifically labeled. This 50-kDa protein was copurified 171-fold with the JHEH activity to homogeneity through DEAE-Sephacel, Mono Q, and hydroxylapatite columns, which led us to conclude that the labeled 50-kDa protein was a JHEH. The steady-state kinetics of the purified microsomal JHEH showed that it followed Michaelis-Menten kinetics with Km values of 0.61, 0.55, and 0.28 microM for JHI, II, and III, respectively, and that JHIII showed a significantly higher Vmax than JHI or JHII. JH acid was also converted to the corresponding diol at a rate 4-fold slower than the corresponding JH. Thus, the differences in the binding of substrate and the rate of turnover by JHEH were affected by the epoxyfarnesoate ester moiety of JH and the difference between the cis-11-methyl group of JHIII versus the cis-11-ethyl group of JHI and II. Purified JHEH showed optimal enzyme activity at pH 7.5-8.5. Interestingly, the presence of recombinant M. sexta JH binding protein (JHBP) dramatically decreased the degradation of JH by JHEH in vitro. Since the cytosolic JHBP in eggs closely resembles the hemolymph JHBP, we suggest that cytosolic JHBP may play a role in protecting JHs from JHEH in vivo. Furthermore, JHEH may play a significant role in the secondary metabolism of JH acid generated by JH esterase.

Ligand binding by a recombinant insect juvenile hormone binding protein

A cDNA for the hemolymph juvenile hormone binding protein (JHBP) of larval Manduca sexta has been isolated, sequenced, and expressed in an insect cell line. A recombinant baculovirus, containing the JHBP cDNA fused to the p10 promoter of Autographa californica nuclear polyhedrosis virus, was constructed. Insect cells (Sf9) infected with this virus secreted recombinant JHBP (rJHBP) into the medium (> 50 micrograms/mL), and cotranslational removal of an 18 amino acid leader sequence was observed. rJHBP was cross-reactive with an antiserum prepared to the hemolymph JHBP and was specifically labeled by [3H]EHDA, a photoaffinity analog of JH II, demonstrating that rJHBP was an isoform of the previously reported 32-kDa JHBP [Lerro, K. A., & Prestwich, G.D. (1990) J. Biol. Chem. 265, 19800-19806]. rJHBP was purified from insect cell medium to homogeneity by ion-exchange and gel-filtration chromatography. The purified rJHBP had a higher affinity (KD = 11 nM for JH I and KD = 42 nM for JH II) than that reported for crude hemolymph JHBP (KD = 80 nM for JH I). The circular dichroism (CD) spectrum of purified rJHBP indicated 34% alpha-helix and 23% beta-sheet. The CD spectra of rJHBP in the presence and absence of JH II were the same, indicating no change in secondary structure induced by ligand binding. Thus, the rJHBP expressed in insect cells binds JHs and is suitable for structural and functional analysis.

Biosynthesis and endocrine control of the production of the German cockroach sex pheromone 3,11-dimethylnonacosan-2-one

The biosynthesis and endocrine regulation of sex pheromone production in the female German cockroach (Blattella germanica) were determined. Radio-TLC and radio-GLC were used to demonstrate the metabolism of 3,11-dimethylnonacosane, a major cuticular lipid component, to the corresponding alkan-2-ol and methyl ketone. [11,12-3H2]-3,11-Dimethylnonacosan-2-ol was efficiently metabolized to the methyl ketone, and radio-GLC showed that the methyl ketone product from both experiments was coeluted with a methyl ketone standard. A comparison of the metabolism of the labeled dimethylalkane and dimethylalkan-2-ol by age and sex showed that both males and females from day 1 through day 9 after adult emergence readily metabolized the alcohol to the corresponding methyl ketone, whereas only females of 5-9 days postemergence efficiently converted the labeled dimethylalkane to the corresponding methyl ketone. Application of the juvenile hormone analog hydroprene induced significant increases in the conversion of the labeled hydrocarbon to the methyl ketone in starved adult females as well as in females fed a protein-free diet, conditions under which endogenous juvenile hormone biosynthesis is nearly undetectable. These data show that the methyl ketone sex pheromone is formed by the hydroxylation and oxidation of the 3,11-dimethylalkane at the 2 position, show that the age- and sex-specific step in this process is the conversion of 3,11-dimethylnonacosane to 3,11-dimethylnonacosan-2-ol, and provide evidence that juvenile hormone regulates sex pheromone production in the German cockroach.

Binding site mapping of a photoaffinity-labeled juvenile hormone binding protein

The juvenile hormone binding protein (JHBP) of larval Manduca sexta was labeled by a photoaffinity analog of JH II and purified by preparative IEF and ion-exchange HPLC. The purified [3H]EHDA-labeled JHBP was selectively cleaved by CNBr and by endoproteinases Lys-C and Glu-C. The radioactive peptides were separated by tricine SDS-PAGE and sequenced after blotting to a PVDF membrane. The sequence revealed that Ala184-Asn226 contained a primary binding site of [3H]EHDA. Furthermore, peptide mapping indicated that Asp1-Glu34 also contained a second covalent attachment site of [3H]EHDA. Labeling of the N-terminal region increased when the photolysis was performed at lower temperature. Since Ala184-Asn226 is predicted to be a hydrophobic beta-sheet region, it may participate in the recognition of lipophilic backbone of JH. Five out of six cysteines are located in these two regions, consistent with a model in which the two binding regions connected by disulfide bridges provide a two-sided binding pocket for juvenile hormone.