Odorant-binding protein. Characterization of ligand binding

Odorant binding protein has the biochemical properties of a scavenger for 4-hydroxy-2-nonenal in mammalian nasal mucosa

Odorant binding proteins (OBP) are soluble lipocalins produced in large amounts in the nasal mucosa of several mammalian species. Although OBPs can bind a large variety of odorous compounds, direct and exclusive involvement of these proteins in olfactory perception has not been clearly demonstrated. This study investigated the binding properties and chemical resistance of OBP to the chemically reactive lipid peroxidation end-product 4-hydroxy-2-nonenal (HNE), in an attempt to establish a functional relationship between this protein and the molecular mechanisms combating free radical cellular damage. Experiments were carried out on recombinant porcine and bovine OBPs and results showed that both forms were able to bind HNE with affinities comparable with those of typical OBP ligands (K(d) = 4.9 and 9.0 microm for porcine and bovine OBP, respectively). Furthermore, OBP functionality, as determined by measuring the binding of the fluorescent ligand 1-aminoanthracene, was partially lost only when incubating HNE levels and exposure time to HNE exceeded physiological values in nasal mucosa. Finally, preliminary experiments in a simplified model resembling nasal epithelium showed that extracellular OBP can preserve the viability of an epithelial cell line derived from bovine turbinates exposed to toxic amounts of the aldehyde. These results suggest that OBP, which is expressed at millimolar levels, might reduce HNE toxicity by removing from the nasal mucus a significant fraction of the aldehyde that is produced as a consequence of direct exposure to the oxygen present in inhaled air.

Odorant binding initially occurring at the central pocket in bovine odorant-binding protein

Why bovine odorant-binding protein (OBPb), among OBP family, assumes a dimeric structure has been unclear. Here we clarified, by measuring the fluorescence of intrinsic tryptophan and tyrosine residues of intact OBPb and OBPb whose C-terminal 10 amino acids were deleted, that odorant enters the central pocket formed by the dimerization when OBPb first encounters odorant, and odorant with high affinity with OBPb subsequently enters the internal cavity (suggested binding site), releasing the pre-bound odorant. The internal cavity-bound odorant can be released by the binding of other odorants at another internal cavity or at the central pocket, depending on the binding odorants. Due to this mechanism enabled by the dimerization, OBPb is more reactive than other monomeric OBPs.

Role of the disulphide bridge in folding, stability and function of porcine odorant binding protein: spectroscopic equilibrium studies on C63A/C155A double mutant

Porcine odorant binding protein (pOBP) contains a single disulphide bridge linking residues Cys63 and Cys155. In order to get information on the role played by this crosslink in determining the structural and functional properties of the protein, we substituted these two Cys residues with two Ala residues by site directed mutagenesis and investigated the changes in folding, stability and functional features, as detected by fluorescence and circular dichroism measurements. In particular, we studied both chemical and thermal unfolding/refolding processes under equilibrium conditions, the first induced by guanidinium hydrochloride and the second by raising the temperature from 15 to 90 degrees C. Chemical unfolding curves, as obtained from intrinsic fluorescence and far-UV circular dichroism data, can be fitted by a simple two-state cooperative sigmoidal function; however, their partial overlap (C(1/2)=0.57+/-0.05 from fluorescence and 0.66+/-0.03 from CD) suggests the formation of an intermediate, which lacks tertiary structural features. Thermal unfolding was found to be reversible if the protein was heated up to 65 degrees C, but irreversible above that temperature because of aggregation. The thermodynamic unfolding parameters of this double mutant protein, when compared to those of the wild type protein, clearly point out the important role played by the disulphide bridge on the stability and function of this protein family and probably of many other lipocalins.

Crystal structures of bovine odorant-binding protein in complex with odorant molecules

The structure of bovine odorant-binding protein (bOBP) revealed a striking feature of a dimer formed by domain swapping [Tegoni, M., Ramoni, R., Bignetti, E., Spinelli, S. & Cambillau, C. (1996) Nat. Struct. Biol.3, 863-867; Bianchet, M.A., Bains, G., Pelosi, P., Pevsner, J., Snyder, S.H., Monaco, H.L. & Amzel, L.M. (1996) Nat. Struct. Biol.3, 934-939] and the presence of a naturally occuring ligand [Ramoni, R., Vincent, F., Grolli, S., Conti, V., Malosse, C., Boyer, F.D., Nagnan-Le Meillour, P., Spinelli, S., Cambillau, C. & Tegoni, M. (2001) J. Biol. Chem.276, 7150-7155]. These features led us to investigate the binding of odorant molecules with bOBP in solution and in the crystal. The behavior of odorant molecules in bOBP resembles that observed with porcine OBP (pOBP), although the latter is monomeric and devoid of ligand when purified. The odorant molecules presented K(d) values with bOBP in the micromolar range. Most of the X-ray structures revealed that odorant molecules interact with a common set of residues forming the cavity wall and do not exhibit specific interactions. Depending on the ligand and on the monomer (A or B), a single residue--Phe89--presents alternate conformations and might control cross-talking between the subunits. Crystal data on both pOBP and bOBP, in contrast with binding and spectroscopic studies on rat OBP in solution, reveal an absence of significant conformational changes involving protein loops or backbone. Thus, the role of OBP in signal triggering remains unresolved.

Unfolding and refolding of porcine odorant binding protein in guanidinium hydrochloride: equilibrium studies at neutral pH

Unfolding and refolding studies on porcine odorant binding protein (pOBP) have been performed at pH 7 in the presence of guanidinium hydrochloride (GdnHCl). Unfolding, monitored by following changes of protein fluorescence and circular dichroism (CD), was found to be a reversible process, in terms of recovered structure and function. The equilibrium transition data were fitted by a simple two-state sigmoidal function of denaturant concentration and the thermodynamic folding parameters, derived from the two techniques, were very similar (average values: C(1/2) approximately 2.4 M, m approximately 2 kcal mol(-1) M(-1), DeltaG(unf,w)(0) approximately 4.7 kcal mol(-1)). The transition was independent of protein concentration, indicating that only monomeric species are involved. Only a minor protective effect by the fluorescent ligand 1-amino-anthracene (AMA) against protein unfolding was detected, whereas dihydromyrcenol (DHM) stabilised the protein to a larger extent (DeltaC(1/2) approximately 0.5 M). Refolding was complete, when the protein, denatured with GdnHCl, was diluted with buffer. On the other hand, refolding by dialysis was largely prevented by concomitant aggregation. The present results on pOBP are compared with those on bovine OBP (bOBP) [Biochim. Biophys. Acta 1599 (2002) 90], where subunit folding is accompanied by domain swapping. We finally suggest that the generally observed two-state folding of many lipocalins is probably favoured by their beta-barrel topology.

Effects of hydration, lipids, and temperature on the binding of the volatile aroma terpenes by beta-lactoglobulin powders

The binding properties of dry proteins are relatively poorly known. Many proteins are present in emulsions and suspensions and also in dry forms. This is particularly true of dairy proteins, which are often stored and sold in powdered form. In the present work, the binding of three terpenes (alpha-terpinene, gamma-terpinene, and terpinolene), which belong to the basic aroma components, and of decane by powdered beta-lactoglobulin (BLG) was studied at different hydration levels (0.05-0.40 g of H(2)O/g of protein) and temperatures (298 and 309.5 K), in the presence or absence of lipids and small concentrations of ethanol. Vapor sorption isotherms were determined for these systems by a static method of headspace gas chromatographic analysis. A cooperative effect of hydrophobic hydration was observed for the binding of aroma terpenes and decane by the solid BLG. The temperature increase from 298 to 309.5 K reduced the observed hydration threshold of BLG by 0.05-0.08 g of H(2)O/g of protein. Lipids (1.2% w/w) in hydrated BLG gave at least a 2-fold increase in its binding affinity for the hydrocarbons studied, and synergic effects of the hydration and lipid on this affinity were observed.

Reversible unfolding of bovine odorant binding protein induced by guanidinium hydrochloride at neutral pH

An analysis of the unfolding and refolding curves at equilibrium of dimeric bovine odorant binding protein (bOBP) has been performed. Unfolding induced by guanidinium chloride (GdnHCl) is completely reversible as far as structure and ligand binding capacity are concerned. The transition curves, as obtained by fluorescence and ellipticity measurements, are very similar and have the same protein concentration-independent midpoint (C1/2 approximately 2.6 M). This result implies a sequential, rather than a concerted, unfolding mechanism, with the involvement of an intermediate. However, since it has not been detected, this intermediate must be present in small amounts or have the same optical properties of either native or denatured protein. The thermodynamic best fit parameters, obtained according to a simple two-state model, are: deltaG degrees un,w = 5.0 +/- 0.6 kcal mol(-1), m = 1.9 +/- 0.2 kcal mol(-1) M(-1) and C1/2 = 2.6 +/- 0.1 M. The presence of the ligand dihydromyrcenol has a stabilising effect against unfolding by GdnHCl, with an extrapolated deltaG degrees un,w of 22.2 +/- 0.9 kcal mol(-1), a cooperative index of 3.2 +/- 0.3 and a midpoint of 4.6 +/- 0.4 M. The refolding curves, recorded after 24 h from dilution of denaturant are not yet at equilibrium: they show an apparently lower midpoint (C1/2 = 2.2 M), but tend to overlap the unfolding curve after several days. In contrast to chromatographic unfolding data, which fail to reveal the presence of folded intermediates, chromatographic refolding data as a function of time clearly show a rapid formation of folded monomers, followed by a slower step leading to folded dimers. Therefore, according to this result, we believe that the preferential unfolding/refolding mechanism is one in which dimer dissociation occurs before unfolding rather than the reverse.

Comparative 17beta-estradiol response and lipoprotein interactions of an avian apolipoprotein

Apolipoprotein D (apo D), a member of the lipocalin protein family, has been identified and cloned in several mammalian species; its physiological functions, however, remain poorly understood. As with other lipocalins, apo D can bind small hydrophobic ligands. Lipids and hormones, such as cholesterol, arachidonic acid, and progesterone can bind to apo D; but the physiological significance of these interactions is not clear. We previously reported the existence of an avian (Gallus domesticus) apo D-like protein, and indicated a possible role for it in reproduction. This report provides a further comparative characterization of this avian protein. Evidence is presented that the putative avian apo D, like some (e.g., human) but not other (e.g., rat) mammalian apo Ds, preferentially associates with high density lipoproteins (HDL) in the circulation. These results confirm the apolipoprotein nature of the avian apo D-like protein, and indicate that it has conserved the HDL-interaction property of some mammalian apo Ds. The response of circulatory levels of the avian protein to 17beta-estradiol treatment is also examined. Large estrogen-dependent increases are known to occur in the circulatory levels of some avian apolipoproteins, such as apo B and vitellogenins, that represent major yolk precursors and nutrient sources for the embryo. Although the avian apo D-like protein is also a known yolk precursor, the minor estrogen-dependent increase observed for this apolipoprotein (less than 7% that of apo B) distinguishes it from the major yolk-precursor apolipoproteins. The response of the avian apo D-like protein to 17beta-estradiol is more like that of other yolk precursor proteins that transport regulatory molecules such as vitamin A and thyroid hormones.

G proteins and olfactory signal transduction

The olfactory system sits at the interface of the environment and the nervous system and is responsible for correctly coding sensory information from thousands of odorous stimuli. Many theories existed regarding the signal transduction mechanism that mediates this difficult task. The discovery that odorant transduction utilizes a unique variation (a novel family of G protein-coupled receptors) based upon a very common theme (the G protein-coupled adenylyl cyclase cascade) to accomplish its vital task emphasized the power and versatility of this motif. We now must understand the downstream consequences of this cascade that regulates multiple second messengers and perhaps even gene transcription in response to the initial interaction of ligand with G protein-coupled receptor.

Pheromones cause disease: pheromone/odourant transduction

This paper compares two models of the sense of smell and demonstrates that the new model has advantages over the accepted model with implications for medical research. The accepted transduction model had an odourant or pheromone contacting an aqueous sensory lymph then movement through it to a receptor membrane beneath. If the odourant or pheromone were non-soluble, the odourant/pheromone supposedly would be bound to a soluble protein in the lymph to be carried across. Thus, an odourant/carrier protein complex physically moved through the receptor lymph/mucus to interact with a membrane bound receptor. After the membranous receptor interaction, the molecule would be deactivated and any odourant/pheromone-binding protein recycled. This new electrical chemosensory model being proposed here has the pheromone or other odourant generating an electrical event in the extra-cellular mucus. Before the pheromone arrives, proteins of the 'carrier class' dissolved in the receptor mucus slowly and continuously sequester ions. A sensed pheromonal chemical species sorbs to the mucus and immediately binds to the now ion-holding dissolved protein. The binding of the pheromone to the protein causes a measurable conformational change in the pheromone/odourant-binding protein, desequestering ions. Releasing the bound ions changes the potential differences across a nearby super-sensitive dendritic membrane resulting in dendrite excitation. Pheromones will be implicated in the aetiology of the infectious, psychiatric and autoimmune diseases. This is the third article in a series of twelve to systematically explore this contention (see references 1-9).

Crystal structure of a truncated form of porcine odorant-binding protein

The odorant-binding proteins (OBPs) are a family of structurally related molecules that are found in high concentrations in the nasal mucus of vertebrates and bind with moderate affinity a large family of hydrophobic odorants. On the basis of their quaternary structure, the OBPs have been classified as monomers, homodimers, and heterodimers. Porcine OBP was believed for a long time to be a monomer under physiological conditions but there are recent data that support the existence of a monomer-dimer equilibrium. We have determined the crystal structure of a monoclinic form of porcine OBP and found that the truncated molecules, which lack the first 8 amino acids, pack in the cell as dimers that appear to have physiological relevance. The presence in the maps of electron density for an endogenous ligand has also let us identify the side chain of the amino acids that are at the ligand-binding site. In addition, an alternative way of access to the central cavity that binds the ligands is suggested by the particular packing of the molecules in this unit cell. Proteins 2001;42:201-209.

Mammalian odorant binding proteins

Odorant binding proteins (OBPs) pertain to one of the most abundant classes of proteins found in the olfactory apparatus. OBPs are a sub-class of lipocalins, defined by their property of reversibly binding volatile chemicals, that we call 'odorants'. Numerous sequences of OBPs are now available, derived from protein sequencing from nasal mucus material, or from DNA sequences. The structural knowledge of OBPs has been improved too in recent years, with the availability of two X-ray structures. The physiological role of OBPs remains, however, essentially hypothetical, and most probably, not linked to a function of odor transport. The present knowledge on OBP biochemistry, sequence and structure will be examined here in relation to the different functional hypotheses proposed for OBPs.

Why has porcine VEG protein unusually high stability and suppressed binding ability?

Von Ebner gland protein (VEGP) and odorant-binding protein (OBP) were purified from porcine lingual epithelium and nasal mucosa, respectively. Both VEGP and OBP preparations were homogeneous as indicated by SDS-PAGE, isoelectric focusing, gel-filtration and electrospray mass spectrometry. However, high-sensitivity differential scanning calorimetry (HS-DSC) yielded multiphasic denaturation thermograms for both proteins indicating their conformational heterogeneity. The unfolding transition of VEGP is observed at extremely high temperatures (about 110 degrees C), which is unexpected for a protein with significant structural homology to OBP and other lipocalins. Isothermal titration calorimetry (ITC) did not detect the binding of either aspartame or denatonium saccharide to VEGP nor did it detect binding of 2-isobutyl-3-methoxypyrazine (IBMP) to OBP. Extraction of OBP with mixed organic solvents eliminated the conformational heterogeneity and the protein showed a reversible two-state transition in HS-DSC thereafter. ITC also showed that the extracted OBP was able to bind IBMP. These results imply that tightly bound endogenous ligands increase the thermal stability of OBP and block the binding of other ligands. In contrast to OBP, the extraction of VEGP with organic solvents failed to promote binding or to establish thermal homogeneity, most likely because of the irreversible denaturation of VEGP. Thus, the elucidation of the functional behaviour of VEGP is closely related to the exhaustive purging of its endogenous ligands which otherwise very efficiently mask ligand binding sites of this protein.

Ligand-binding properties and structural characterization of a novel rat odorant-binding protein variant

After characterization of a novel odorant-binding protein (OBP) variant isolated from the rat nasal mucus, the corresponding cDNA was cloned by RT-PCR. Recombinant OBP-1F, the sequence of which is close to that of previously reported rat OBP-1, has been secreted by the yeast Pichia pastoris at a concentration of 80 mg.L-1 in a form identical to the natural protein as shown by MS, N-terminal sequencing and CD. We observed that, in contrast with porcine OBP-1, purified recombinant OBP-1F is a homodimer exhibiting two disulfide bonds (C44-C48 and C63-C155), a pairing close to that of hamster aphrodisin. OBP-1F interacts with fluorescent probe 1-aminoanthracene (1-AMA) with a dissociation constant of 0.6 +/- 0. 3 microM. Fluorescence experiments revealed that 1-AMA was displaced efficiently by molecules including usual solvents such as EtOH and dimethylsulfoxide. Owing to the large OBP-1F amounts expressed, we set up a novel biomimetic assay (volatile-odorant binding assay) to study the uptake of airborne odorants without radiolabelling and attempted to understand the odorant capture by OBP in the nasal mucus under natural conditions. The assay permitted observations on the binding of airborne odorants of different chemical structures and odors (2-isobutyl-3-methoxypyrazine, linalool, isoamyl acetate, 1-octanal, 1-octanol, dimethyl disulfide and methyl thiobutyrate). Uptake of airborne odorants in nearly physiological conditions strengthens the role of OBP as volatile hydrophobic odorant carriers in the mucus of the olfactory epithelium through the aqueous barrier towards the chemo-sensory cells.

Conformational stability and binding properties of porcine odorant binding protein

Apparently homogeneous odorant binding protein purified from pig nasal mucosa (pOBP) exhibited subunit molecular masses of 17 223, 17 447, and 17 689 (major component) Da as estimated by ESI/MS. According to gel filtration, this protein, its truncated forms, and/or its variants are homodimeric under physiologic conditions (pH 6-7, 0.1 M NaCl). The dimer if monomer equilibrium shifts toward a prevalent monomeric form at pH <4.5. Velocity sedimentation reveals a monomeric state of OBP at both pH 7.2 and 3.5, indicating a pressure-induced dissociation of the homodimer. High-sensitivity differential scanning calorimetry (HS-DSC) shows that the unfolding transition of pOBP is reversible at neutral pH. It is characterized by the transition temperature of 69.23 degrees C and an enthalpy of 391.1 kJ/mol per monomer. The transition heat capacity curve of pOBP is well-approximated by the two-state model on the level of subunit, indicating that the two monomers behave independently. Isothermal titration calorimetry (ITC) shows that at physiological pH pOBP binds 2-isobutyl-3-methoxypyrazine (IBMP) and 3,7-dimethyloctan-1-ol (DMO) with association constants of 3.19 x 10(6) and 4.94 x 10(6) M(-)(1) and enthalpies of -97.2 and -87.8 kJ/mol, respectively. The binding stoichiometry of both ligands is nearly one molecule of ligand per homodimer of pOBP. The interaction of pOBP with both ligands is enthalpically driven with an unfavorable change of entropy. The binding affinity of pOBP with IBMP does not change significantly at acidic pH, while the binding stoichiometry is nearly halved. According to HS-DSC data, the interaction with IBMP and DMO leads to a substantial stabilization of the pOBP folded structure, which is manifested by the increase in the unfolding temperature and enthalpy. The calorimetric data allow us to conclude that the mechanism of binding of the studied odorants to pOBP is not dominated by a hydrophobic effect related to any change in the hydration state of protein and ligand groups but, most likely, is driven by polar and van der Waals interactions.

Expression of major urinary protein genes in the nasal glands associated with general olfaction

Gene expression of major urinary protein (MUP) mRNAs was examined in the mouse nasal tissue. By polymerase chain reaction, we identified two cDNA segments encoding MUP 4 and MUP 5 genes in the nose. The expression level of both MUP 4 and 5 mRNAs in the nasal tissue was very high and exceeded that of the liver. Liver MUPs are excreted into the urine and are known to play an important role in pheromonal communication. We showed that nose and liver MUPs were composed of different subtypes of MUPs and that nose MUP mRNAs was detected in prepubescent periods when liver MUP mRNAs had not yet been transcripted. In situ hybridization revealed that nose MUP mRNAs are localized in the lateral wall and nasal septum and their expression pattern is identical to that of rat odorant-binding protein (OBP)-I. We also identified cDNA of mouse OBP-II gene from the nasal tissue and showed that the expression pattern of MUP gene was identical to that of OBP-II gene in the nose. These histological data indicate that nose MUPs are favorable for catching odorant molecules rather than pheromones, and may share their function with OBPs.

Optimization of the production of a honeybee odorant-binding protein by Pichia pastoris

A honeybee putative general odorant-binding protein ASP2 has been expressed in the methylotrophic yeast Pichia pastoris. It was secreted into the buffered minimal medium using either the alpha-factor preprosequence with and without the Glu-Ala-Glu-Ala spacer peptide of Saccharomyces cerevisiae or its native signal peptide. Whereas ASP2 secreted using the alpha-factor preprosequence with the spacer peptide showed N-terminal heterogeneity, the recombinant protein using the two other secretion peptides was correctly processed. Mass spectrometry showed that the protein secreted using the natural peptide sequence had a mass of 13,695.1 Da, in perfect agreement with the measured molecular mass of the native protein. These data showed a native-like processing and the three disulfide bridges formation confirmed by sulfhydryl titration analysis. After dialysis, the recombinant protein was purified by one-step anion-exchange chromatography in a highly pure form. The final expression yield after 7-day fermentation was approximately 150 mg/liter. To our knowledge, this is the first report of the use of a natural insect leader sequence for secretion with correct processing in P. pastoris. The overproduction of recombinant ASP2 should allow ligand binding and mutational analysis to understand the relationships between structure and biological function of the protein.

Odorant-binding proteins: structural aspects

Structural data on odorant-binding proteins (OBPs), both in vertebrates and in insects, are reviewed and discussed. OBPs are soluble proteins interacting with odor molecules and pheromones in the perireceptor areas, the nasal mucus in vertebrates and the sensillar lymph in insects. The physiological function of these proteins is still uncertain, but information on their structure is abundant and accurate. Based on complete amino acid sequences, several subclasses have been identified, suggesting a role in odor discrimination. The OBPs of vertebrates belong to the family of lipocalins that includes proteins involved in the delivery of pheromonal messages. Those of insects do not bear significant similarity to any other class of proteins. The three-dimensional structure of the bovine OBP is a beta-barrel, while for insect OBPs a model has been proposed, mainly containing alpha-helix motifs. In some cases the amino acid residues involved in ligand binding have been identified with the use of photoaffinity label analogues.

LUSH odorant-binding protein mediates chemosensory responses to alcohols in Drosophila melanogaster

The molecular mechanisms mediating chemosensory discrimination in insects are unknown. Using the enhancer trapping approach, we identified a new Drosophila mutant, lush, with odorant-specific defects in olfactory behavior. lush mutant flies are abnormally attracted to high concentrations of ethanol, propanol, and butanol but have normal chemosensory responses to other odorants. We show that wild-type flies have an active olfactory avoidance mechanism to prevent attraction to concentrated alcohol, and this response is defective in lush mutants. This suggests that the defective olfactory behavior associated with the lush mutation may result from a specific defect in chemoavoidance. lush mutants have a 3-kb deletion that produces a null allele of a new member of the invertebrate odorant-binding protein family, LUSH. LUSH is normally expressed exclusively in a subset of trichoid chemosensory sensilla located on the ventral-lateral surface of the third antennal segment. LUSH is secreted from nonneuronal support cells into the sensillum lymph that bathes the olfactory neurons within these sensilla. Reintroduction of a cloned wild-type copy of lush into the mutant background completely restores wild-type olfactory behavior, demonstrating that this odorant-binding protein is required in a subset of sensilla for normal chemosensory behavior to a subset of odorants. These findings provide direct evidence that odorant-binding proteins are required for normal chemosensory behavior in Drosophila and may partially determine the chemical specificity of olfactory neurons in vivo.

The structure of the monomeric porcine odorant binding protein sheds light on the domain swapping mechanism

The X-ray structure of the porcine odorant binding protein (OBPp) was determined at 2.25 A resolution. This lipocalin is a monomer and is devoid of naturally occurring bound ligand, contrary to what was observed in the case of bovine OBP [Tegoni, M., et al. (1996) Nat. Struct. Biol. 3, 863-867; Bianchet, M. A., et al. (1996) Nat. Struct. Biol. 3, 934-939]. In this latter protein, a dimer without any disulfide bridges, domain swapping was found to occur between the beta- and alpha-domains. A single Gly (121) insertion was found in OBPp when it was compared to OBPb, which may prevent domain swapping from taking place. The presence of a disulfide bridge between the OBPp beta- and alpha-domains (cysteines 63 and 155) may lock the resulting fold in a nonswapped monomeric conformation. Comparisons with other OBPs indicate that the two cysteines involved in the OBPp disulfide bridge are conserved in the sequence, suggesting that OBPp may be considered a prototypic OBP fold, and not OBPb.

2-sec-butyl-4,5-dihydrothiazole is a ligand for mouse urinary protein and rat alpha 2u-globulin: physiological and toxicological relevance

Mouse urinary protein (MUP) and alpha 2u-globulin are structurally homologous proteins that belong to a superfamily of ligand-binding proteins and represent the major urinary proteins excreted by adult male mice and rats, respectively. Although a variety of xenobiotics bind to alpha 2u-globulin and produce a male rat-specific hyaline droplet nephropathy, no endogenous ligand for this protein has been identified. Despite extensive sequence homology. MUP does not bind to hyaline droplet-inducing agents. While performing experiments with purified MUP, we observed that it presented with a strong, distinctive odor reminiscent of mouse urine. To determine whether this odor was the result of contamination or degradation or was attributed to an endogenous ligand bound to the protein, the protein was subjected to thermal desorption and any released volatile compounds were detected with a gas chromatograph equipped with an external sniff port and mass spectrometer. With this approach, two odorous compounds were detected at the sniff port by a human observer, but only one was present in sufficient mass to allow identification. This compound, which presented with the characteristic odor, was subsequently identified as 2-sec butyl-4,5-dihydrothiazole (DHT) by GC/MS/matrix isolation IR and NMR analyses. The identification of DHT was confirmed by comparing the chromatographic and spectral properties to those of the synthesized authentic compound. In direct contrast, purified urinary alpha 2u-globulin did not present with an obvious odor, and no volatile ligands were detected on this protein. Although DHT is a major endogenous ligand for MUP, it was also found to competitively inhibit the binding of [14C]d-limonene-1,2-epoxide to alpha 2u-globulin with relatively high affinity (Ki = 2.3 microM). When dosed orally to F344 rats, DHT (1 mmol/kg for 3 days) caused the characteristic exacerbation of hyaline droplets in male rat kidneys and increased renal levels of immunoreactive alpha 2u-globulin about threefold over control levels. These results indicate that despite structural homology, MUP and alpha 2u-globulin are distinguished by the presence of a volatile endogenous ligand only on the former, a distinction that may reflect differences in the physiological functions of the two proteins. Furthermore, although DHT can bind to both MUP and alpha 2u-globulin, renal toxicity was only observed in rats, thereby emphasizing the unique toxicological properties of alpha 2u-globulin in the development of hyaline droplet nephropathy.

The three-dimensional structure of bovine odorant binding protein and its mechanism of odor recognition

Odorant binding protein (OBP) is the major odorant binding component of mammalian nasal mucosa. The two structures of bovine OBP reported in this paper (one crystallized as purified and one soaked in the presence of a selenium-containing odorant) show that: (i) the OBP dimer is composed of two compact domains related by an approximate two-fold axis of symmetry; (ii) between residues 122 and 123 the polypeptide chains cross from one domain to the other such that each domain is formed by residues from both monomers; (iii) purified OBP already contains two bound odorant molecules (one per monomer)-odorant binding occurs by replacement of these molecules with the added odorant; and (iv) the structure of the odorant binding site can explain OBP's extraordinarily broad odorant specificity.

Molecular genetics of mammalian olfaction

Olfaction plays a crucial role in the survival of most animal species; it is remarkable in its ability to recognize and discriminate numerous airborne molecules, yet is one of the least understood senses. The advent of molecular genetic approaches has greatly contributed to disclosing some of the mysteries in olfaction. The identification of olfactory-specific proteins, the discovery of the large receptor gene family, and the first insight into the mechanisms governing chemosensory gene expression hold great promise for an eventually detailed understanding of a sensory system that was previously considered as hardly accessible for research at the molecular level.

Perireceptor events in olfaction

Before reaching olfactory receptor neurons, odorant molecules have to cross an aqueous interface: the nasal mucus in vertebrates and the sensillar lymph in insects. Biochemical interactions taking place between odorants and the elements of these phases are called perireceptor events. Main protein constituents of these media, in both insects and vertebrates, are OBPs (odorant-binding proteins). Another class of proteins active in the olfactory perireceptor area includes odorant-degrading enzymes. The structure and the properties of these major proteins, with particular reference to OBPs, are reviewed and their role in olfactory transduction is discussed.

Proteins that smell: pheromone recognition and signal transduction

Pheromone perception in Lepidoptera requires initial recognition and transport of the pheromone molecule by ligand-specific pheromone binding proteins (PBPs) in the moth antennae, followed by recognition of the ligand or PBP-ligand complex by a transmembrane G-protein-coupled odorant receptor protein. This signal is transduced by activation of a specific phospholipase C, intracellular release of inositol 1,4,5-trisphosphate (IP3) and IP3-gated opening of an ion channel. Individual pheromone-specific PBPs provide the initial ligand recognition event and encode ligand specificity. We have used photoaffinity labeling, cDNA library screening and cloning, protein expression, a novel binding assay and site-directed mutagenesis to define the ligand specificity of PBPs.

Differential expression of odorant-binding protein genes in rat nasal glands: implications for odorant-binding proteinII as a possible pheromone transporter

We examined the distribution and ontogeny of two odorant-binding proteins in the rat at various stages of development from newborn to adult using northern blot and in situ hybridization methods. Our results demonstrated spatial segregation between odorant-binding protein and odorant-binding proteinII in nasal glandular tissues. Odorant-binding protein messenger RNA was expressed in the glandular system opening into the nasal vestibule, whereas odorant-binding proteinII messenger RNA was seen in the posterior glands of the nasal septum and in the vomeronasal glands. In addition, odorant-binding protein and odorant-binding proteinII messenger RNA levels increased during early postnatal stages with time courses that paralleled the anatomical development of the main olfactory system and the vomeronasal system, respectively. Our results suggest that odorant-binding proteinII functions as a pheromone transporter in the vomeronasal system.

Purification and characterization of multiple forms of odorant/pheromone binding proteins in the antennae of Mamestra brassicae (Noctuidae)

Proteins extracted form the antennae of Mamestra brassicae (L.) (Lepidoptera: Noctuidae) adults were biochemically characterized as pheromone-binding proteins (PBP) and general odorant-binding proteins (GOBP). PBP and GOBP were purified by two successive and different HPLC (high performance liquid chromatography) systems and native polyacrylamide gel electrophoresis (native-PAGE). Their N-terminal sequence was determined by Edman microsequencing. The combined results showed evidence for three different PBPs in males, and two different PBPs in females. In addition, one GOBP was characterized in both males than in females antennae. In the males, two isoforms of PBP have the same N-terminal sequence, but different apparent mobilities and hydrophobicities: they could be separated by electrophoresis and reverse phase-HPLC (RP-HPLC). The other PBP sequence (SQEIM) showed particularly high homology (88%) with the PBP of Heliothis virescens, another noctuid moth. The existence of several forms of PBP in the same animal strongly supports the hypothesis of the specificity of binding between the proteins and their odorant ligands, the pheromonal compounds. The observed microdiversity at the soluble proteins level could provide a good model for studying their involvement in the initial stages of odor discrimination.

The genetic basis for specific anosmia to isovaleric acid in the mouse

The detection and discrimination of odorants in mammals is thought to be mediated by a family of 100-1000 seven transmembrane domain receptor proteins, although none of these putative olfactory receptors have been shown to bind individual odorants with high affinity. We have used a genetic approach to identify the genomic regions responsible for the differential ability of two inbred mouse strains to detect a single odorant, isovaleric acid. Results obtained with a behavioral assay were consistent with a limited number of genes conferring the ability to detect isovaleric acid. One genetic location mapped to a 0.3 cM region between D4MIT37 and D4MIT156 on mouse chromosome 4. A second locus mapped to the distal end of mouse chromosome 6. The most likely cause of the behavior difference between the two strains of mice is the loss of the receptor protein or proteins responsible for recognizing isovaleric acid. High resolution genetic mapping provides a novel approach to the identification of genes critical for the detection of particular odorants.

EP-GP and the lipocalin VEGh, two different human salivary 20-kDa proteins

Two salivary 20-kDa proteins [the human lipocalin Von Ebner's gland protein (VEGh) and extraparotid glycoprotein (EP-GP)] show several remarkable similarities and differences. The latter is identical to secretory actin-binding protein (SABP), gross cystic disease fluid protein-15 (GCDFP-15), prolactin-induced protein (PIP), and 17-kDA CD4-binding glycoprotein (gp17). Much is known about the distribution, localization, biochemical characteristics, and molecular biology of these two proteins, yet there are only few clues about their functions.

Olfactory transduction mechanisms in sheep

The enzyme adenylyl cyclase from sheep olfactory epithelium is dually regulated by GTP and is highly sensitive to the nucleotide analogues GTP gamma S and GppNHp, as well as to fluoride ions and forskolin. Many, but not all, odorants tested are able to stimulate adenylyl cyclase in a dose-dependent manner and with different potencies. Such an effect is detectable only in the presence of GTP. The odorants belonging to the putrid class are the least effective in stimulating adenylyl cyclase activity, and only furfuryl mercaptan significantly increases cAMP biosynthesis. Mixtures of two odorants, chosen among those able to activate adenylyl cyclase, induce additive or supra-additive effects, suggesting the presence of many different receptor types. The presence of an alternative olfactory signal transduction process, i.e. the inositol phospholipid second messenger system, has been evaluated. Triethylamine, a putrid odorant completely ineffective on cAMP levels, is able to significantly increase inositol phosphate accumulation, indicating the coexistence of both cAMP- and InsP3-mediated signalling pathways in sheep olfactory epithelium.

Tear lipocalins bind a broad array of lipid ligands

To identify the native ligands of tear lipocalins, tear proteins were separated by size exclusion chromatography and the lipid content in the major protein fractions identified. Lipids extracted from native tears and purified tear lipocalins comigrated with fatty acids, fatty alcohols, phospholipids, glycolipids, and cholesterol on thin layer chromatograms. Abundant stearic and palmitic acids as well as cholesterol, and lesser amounts of lauric acid were specifically identified in extracts of purified lipocalins by gas chromatography-mass spectroscopy. A preliminary study of the ligand-protein interaction was carried out using nitroxide spin-labeled lipids.

Differential expression of vomeromodulin and odorant-binding protein, putative pheromone and odorant transporters, in the developing rat nasal chemosensory mucosae

Expression of the putative pheromone and odorant transporter, vomeromodulin, was characterized in developing rat nasal mucosae using in situ hybridization and immunocytochemistry. Initial expression of vomeromodulin mRNA and protein was detected at embryonic day (E)16 in the maxillary sinus component of the lateral nasal glands. The abundance of mRNA and protein in the lateral nasal glands increased with age and reached a peak at postnatal day (P)27. Also at P27, vomeromodulin mRNA and protein expression was initiated in vomeronasal glands and posterior glands of the nasal septum. Comparison of the developmental expression of odorant-binding protein, another carrier protein synthesized in the lateral nasal glands, with that of vomeromodulin demonstrated major differences. In contrast to vomeromodulin, odorant-binding protein was not detected until postnatal day 2 in the ventral component of the lateral nasal glands and anterior glands of the nasal septum. These results suggest that the expression of vomeromodulin and odorant-binding protein is developmentally and differentially regulated and confirms the suggestion that vomeromodulin may function in olfactory and vomeronasal perireceptor processes as a transporter for pheromones and odorants. In addition, the embryonic expression of vomeromodulin suggests its involvement in olfactory perireceptor processes in utero.

Denatonium bitter tasting among transgenic mice expressing rat von Ebner's gland protein

Von Ebner's gland protein (VEGP) is a secretory protein, which is abundantly expressed in the small von Ebner's salivary glands of the tongue. VEGP as component of the perireceptor environment around taste papillae might function as transporter of hydrophobic molecules, for example bitter substances. Here we report a new approach to investigate the physiological role of VEGP by expression of the cloned rat VEGP gene in transgenic mice. Taste papillae of mice, in contrast to rats, do not contain VEGP. The founder mouse 4345 and three offspring carry the transgene as shown by PCR analysis and saliva of the transgenic mice contains high amounts of VEGP. In two-bottle preference tests, transgenic and nontransgenic siblings show significantly different capabilities to taste the bitter compound denatonium benzoate at 10 microM. The reduced sensitivity of transgenic mice to denatonium benzoate points to a clearance function of VEGP the specificity of which for taste compounds and other molecules remains to be seen.

Expression of the putative pheromone and odorant transporter vomeromodulin mRNA and protein in nasal chemosensory mucosae

In nasal chemosensory systems, glandular proteins associated with the vomeronasal and olfactory epithelia perform specific perireceptor functions associated with sensory transduction. Vomeromodulin, a recently identified glycoprotein synthesized by the lateral nasal glands, is proposed to be a pheromone transporter (Khew-Goodall et al., FASEB J 5:2976-2982, 1991). In our study, we have investigated its expression in vomeronasal, olfactory, and respiratory nasal mucosae of rats and humans using in situ hybridization and immunocytochemical techniques. In the rat, vomeromodulin mRNA and protein were localized abundantly in the glandular acini of the maxillary sinus component of the lateral nasal glands. In addition, the vomeronasal and posterior glands of the nasal septum also expressed vomeromodulin mRNA and protein. Vomeromodulin immunoreactivity was localized extracellularly in the mucus of the sensory and non-sensory epithelia of the vomeronasal organ, and in the mucociliary complex of the olfactory, respiratory, and associated nasal epithelia. In human nasal mucosae, vomeromodulin immunoreactivity was localized in the mucociliary complex of the vomeronasal and respiratory epithelia. Comparison of the localization of vomeromodulin with that of odorant-binding protein, which is also synthesized in the lateral nasal glands of rats, revealed that odorant-binding protein was expressed in a completely separate glandular region, namely the ventral component. In the septal glands, vomeromodulin was expressed in the posterior glands whereas odorant-binding protein was localized in the anterior glands. Odorant-binding protein immunoreactivity was not observed in the vomeronasal glands. In contrast, both proteins were localized in the mucus of vomeronasal, olfactory, and respiratory epithelia. Our results suggest that vomeromodulin, like odorant-binding protein, functions as a chemosensory stimulus transporter associated with perireceptor processes in vomeronasal and olfactory transduction.

Structural organization of the genes for rat von Ebner's gland proteins 1 and 2 reveals their close relationship to lipocalins

The rat von Ebner's gland protein 1 (VEGP 1) is a secretory protein, which is abundantly expressed in the small acinar von Ebner's salivary glands of the tongue. Based on the primary structure of this protein we have previously suggested that it is a member of the lipocalin superfamily of lipophilic-ligand carrier proteins. Although the physiological role of VEGP 1 is not clear, it might be involved in sensory or protective functions in the taste epithelium. Here, we report the purification of VEGP 1 and of a closely related secretory polypeptide, VEGP 2, the isolation of a cDNA clone encoding VEGP 2, and the isolation and structural characterization of the genes for both proteins. Protein purification by gel-filtration and anion-exchange chromatography using Mono Q revealed the presence of two different immunoreactive VEGP species. N-terminal sequence determination of peptide fragments isolated after protease Asp-N digestion allowed the identification of a new VEGP, named VEGP 2, in addition to the previously characterized VEGP 1. The complete VEGP 2 sequence was deduced from a cDNA clone isolated from a von Ebner's gland cDNA library. The VEGP 2 cDNA encodes a protein of 177 amino acids and is 94% identical to VEGP 1. DNA sequence analysis of the rat VEGP 1 and 2 genes isolated from rat genomic libraries revealed that both span about 4.5 kb and contain seven exons. The VEGP 1 and 2 genes are non-allelic distinct genes in the rat genome and probably arose by gene duplication. The high degree of nucleotide sequence identity in introns A-C (94-100%) points to a recent gene conversion event that included the 5' part of the genes. The genomic organization of the rat VEGP genes closely resembles that found in other lipocalins such as beta-lactoglobulin, mouse urinary proteins (MUPs) and prostaglandin D synthase, and therefore provides clear evidence that VEGPs belong to this superfamily of proteins.

Odorant-binding proteins

Odorant-binding proteins (OBPs) are low-molecular-weight soluble proteins highly concentrated in the nasal mucus of vertebrates and in the sensillar lymph of insects. Their affinity toward odors and pheromones suggests a role in olfactory perception, but their physiological function has not been clearly defined. Several members of this class of proteins have been isolated and characterized both in insects and vertebrates; in most species two or three types of OBPs are expressed in the nasal area. Vertebrates OBPs show significant sequence similarity with a superfamily of soluble carrier proteins called lipocalins. They include some proteins of particular interest that are thought to be involved in the mechanism of releasing and modulating chemical messages with pheromonal activity. The data on vertebrate OBPs are here reviewed together with the most relevant information on related proteins. Theories and models of the physiological functions of odorant-binding proteins are presented and discussed.

Ultrastructural evidence for a binding substance to the sweet-tasting protein thaumatin inside taste bud pores of rhesus monkey foliate papillae

Thaumatin is a protein that tastes intensely sweet only to Old World monkeys and to higher primates, including man. Here we used pre-embedding ultrastructural methods to study the distribution of thaumatin in apical regions of Rhesus monkey foliate papillae, using thaumatin conjugated to 5 nm gold particles. With freeze-substitution we saw that gold-labeled thaumatin bound to an electron-opaque, sponge-like secretory substance inside the taste bud pores. Labeled thaumatin was found at the surface of the secretory substance even deep inside the pore, where other, unlabeled cellular structures surrounded the substance. With freeze-fracture deep-etching the secretory substance that bound the thaumatin-gold particles appeared coarsely granular. There was no labeling of any other taste bud pore structure, including microvilli and small membrane-lined vesicles. Pre-incubation with an excess of unlabeled thaumatin inhibited binding with gold-labeled thaumatin. The results suggest that the secretory substance had the greatest affinity of all taste pore structures to the sweet-tasting compound under our experimental conditions. Therefore, gustatory reception probably involves various taste compound binding structures, microvilli, and also secretory substances like the one described here which bound thaumatin. We speculate that the secretory substance may bind taste stimuli and serve as an intermediate between stimuli and receptors. It could be involved in stimulus removal or delivery or both.

Isolation of two odorant-binding proteins from mouse nasal tissue

1. Two soluble proteins, with good affinity to tritiated 2-isobutyl-3-methoxypyrazine, have been purified from mouse nasal mucosa. 2. The first protein is a heterodimer with subunits of apparent M(r) 18 and 19 kDa and isoelectric point of 4.9; the second is a monomer of M(r) 21 kDa and isoelectric point of 4.8. 3. The characteristics of these binding proteins are compared with those of the other known OBPs and urinary proteins and their putative role is discussed.

Alpha 2u-globulin is the only member of the lipocalin protein superfamily that binds to hyaline droplet inducing agents

The rate-limiting step in chemically induced, male rat-specific hyaline droplet nephropathy is the reversible binding of a xenobiotic to alpha 2u-globulin. In this study, equilibrium saturation binding experiments were conducted to evaluate the in vitro binding of d-limonene-1,2-oxide (dLO) and 2,4,4-trimethyl-2-pentanol (TMP-OH) to alpha 2u-globulin and members of the alpha 2u-globulin protein superfamily. Both dLO and TMP-OH bound to alpha 2u-globulin, with Scatchard analysis yielding dissociation constants of 5.6 and 6.4 x 10(-7) M, respectively. The Bmax for binding (nmol bound/mg protein) was 50.7 and 61.1 for dLO and TMP-OH, respectively, yielding a molar ratio of approximately 1 for both ligands. The ability of dLO and TMP-OH to bind to human-derived alpha 1-acid glycoprotein, rat-derived retinol-binding protein, human protein-1, and bovine beta-lactoglobulin was also studied. These superfamily proteins are generally abundant in plasma, are freely filtered across the glomerulus, and can bind a wide range of ligands. However, neither dLO nor TMP-OH bound to any of the superfamily proteins. In contrast, under identical experimental conditions, alpha 1-acid glycoprotein did bind progesterone (Kd = 10(-6) M), whereas both beta-lactoglobulin and retinol-binding protein bound retinol (Kd = 10(-8) M for both proteins). These results indicate that, under conditions where alpha 2u-globulin superfamily proteins bind to established ligands, the proteins do not interact with hyaline droplet inducing agents. Thus, the interaction between male rat-specific nephrotoxicants and alpha 2u-globulin is unique to this protein. More importantly, these results provide direct evidence that the presence of the alpha 2u-globulin superfamily proteins does not predispose humans to develop hyaline droplet nephropathy and renal cancer from this class of chemicals.

Three-dimensional structure and active site of three hydrophobic molecule-binding proteins with significant amino acid sequence similarity

We review our work on bovine and human retinol-binding protein (RBP), bovine beta lactoglobulin (BLG), and bovine odorant-binding protein (OBP). These three proteins share a sequence similarity high enough to justify the proposal that their three-dimensional structure ought to be quite similar, and they also share the function of similar or even identical hydrophobic ligand binding, although with a very different degree of specificity. Thus they constitute an ideal system to exhaustively explore the question of three-dimensional structure prediction from sequence similarity and the related question of binding site prediction for similar ligands. We have used x-ray diffraction techniques on single crystals of human and bovine RBP, bovine milk BLG, and bovine nasal mucosa OBP to investigate this problem. The results of these crystallographic studies indicate that to the level of resolution so far attained, the three-dimensional structure of these three proteins is reasonably predicted from the sequence similarity. The fold is the same and structural differences are rather subtle. Finally, we present experimental evidence that the binding sites of RBP, BLG, and OBP are in different regions of the molecules. Thus, it appears that although sequence alignment has correctly predicted the protein fold, it has incorrectly predicted the hydrophobic ligand-binding sites.

Molecular cloning of putative odorant-binding and odorant-metabolizing proteins

Olfactory reception occurs via the interaction of odorants with the chemosensory cilia of the olfactory receptor cells located in the nasal epithelium. The cDNA clones from mRNA specific to olfactory mucosa were studied. One of these clones, OBPII, encodes a secretory protein with significant homology to odorant-binding protein (OBP), a protein with broad odorant-binding ability, and is expressed in the lateral nasal gland, which is the site of expression of OBP. The OBPII sequence also shows significant homology to the VEG protein, which is thought to be involved in taste transduction. OBPII is a new member of the lipophilic molecule carrier protein family. The second cDNA clone encodes a novel homologue of glutathione peroxidase, an enzyme involved in cellular biotransformation pathways. Its expression appears to be localized to the Bowman's glands, the site of several previously identified olfactory-specific biotransformation enzymes.