Purification of a bacterially expressed herpes simplex virus type 1 origin binding protein for use in posttranslational processing studies.

The origin binding protein (OBP) encoded by the UL9 open reading frame of herpes simplex virus type 1 (HSV-1) plays an essential role in productive infection by promoting the initiation of viral DNA synthesis. In this study, OBP was inducibly expressed in Escherichia coli and purified to homogeneity using a two-step chromatographic separation procedure. The properties of this recombinant OBP (rOBP) were found to be indistinguishable from those of the virus-encoded protein. Since rOBP was synthesized in bacterial cells, it lacked the posttranslational processing which normally occurs in OBP produced in HSV-1-infected mammalian cells and could therefore be exploited in experiments which addressed the effects of protein modification on OBP function. As an initial study, the impact of phosphorylation on enzymatic activity was examined using rOBP which had been treated with a panel of purified cellular kinases. rOBP was found to act as a substrate for nearly all of the kinases tested in (32)P-labeled phosphate transfer assays. However, only phosphorylation by protein kinase A (PKA, or cAMP-dependent protein kinase) was shown to significantly alter the enzymatic properties of rOBP, as it increased by five- to eightfold the ATPase activity associated with this protein. Activation of this critical viral DNA replication enzyme by a cAMP-dependent kinase such as PKA may be of some relevance in the natural course of HSV-1 infections, since reactivation of latent virus is thought to involve both signal transduction events and the induction of viral DNA synthesis. Thus, the expression and purification strategy outlined in this work provides an economical source of unmodified HSV-1 OBP that should prove useful in future in vitro studies.     

Complexes of porcine odorant binding protein with odorant molecules belonging to different chemical classes

Porcine odorant binding protein (pOBP) is a monomer of 157 amino acid residues, purified in abundance from pig nasal mucosa. In contrast to the observation on lipocalins as retinol binding protein (RBP), major urinary protein (MUP) or bovine odorant binding protein (bOBP), no naturally occurring ligand was found in the beta-barrel cavity of pOBP. Porcine OBP was therefore chosen as a simple model for structure/function studies with odorant molecules. In competition experiments with tritiated pyrazine, the affinity of pOBP towards several odorant molecules belonging to different chemical classes has been found to be of the micromolar order, with a 1:1 stoichiometry. The X-ray structures of pOBP complexed to these molecules were determined at resolution between 2.15 and 1.4 A. As expected, the electron density of the odorant molecules was observed into the hydrophobic beta-barrel of the lipocalin. Inside this cavity, very few specific interactions were established between the odorant molecule and the amino acid side-chains, which did not undergo significant conformational change. The high B-factors observed for the odorant molecules as well as the existence of alternative conformations reveal a non-specific mode of binding of the odorant molecules in the cavity.     

Porcine odorant-binding protein selectively binds to a human olfactory receptor

Odorant-binding proteins (OBPs) represent a highly abundant class of proteins secreted in the nasal mucus by the olfactory neuroepithelium. These proteins display binding affinity for a variety of odorant molecules, thereby assuming the role of carrier during olfactory perception. However, no specific interaction between OBP and olfactory receptors (ORs) has yet been shown and early events in olfaction remain so far poorly understood at a molecular level. Two human ORs, OR 17-209 and OR 17-210, were fused to a Green Fluorescent Protein and stably expressed in COS-7 cell lines. Interaction with OBP was investigated using a highly purified radioiodinated porcine OBP (pOBP) preparation, devoid of any ligand in its binding cavity. No specific binding of the pOBP tracer could be detected with OR 17-209. In contrast, OR 17-210 exhibited specific saturable binding (K(d) = 9.48 nM) corresponding to the presence of a single class of high-affinity binding sites (B(max) = 65.8 fmol/mg prot). Association and dissociation kinetics further confirmed high-affinity interaction between pOBP and OR 17-210. Autoradiographic studies of labeled pOBP to newborn mouse slices revealed the presence of multiple specific binding sites located mainly in olfactory tissue but also in several other peripheral tissues. Our data thus demonstrate a high-affinity interaction between OBP and OR, indicating that under physiological conditions, ORs may be specifically associated with an OBP partner in the absence of odorant. This provides further evidence of a novel role for OBP in the mechanism of olfactory perception.     

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.     

Differential expression of odorant-binding proteins in the mandibular glands of the honey bee according to caste and age

Odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) mediate both perception and release of chemical stimuli in insects. The genome of the honey bee contains 21 genes encoding OBPs and 6 encoding CSPs. Using a proteomic approach, we have investigated the expression of OBPs and CSPs in the mandibular glands of adult honey bees in relation to caste and age. OBP13 is mostly expressed in young individuals and in virgin queens, while OBP21 is abundant in older bees and is prevalent in mated queens. OBP14, which had been found in larvae, is produced in hive workers' glands. Quite unexpectedly, the mandibular glands of drones also contain OBPs, mainly OBP18 and OBP21. We have expressed three of the most represented OBPs and studied their binding properties. OBP13 binds with good specificity oleic acid and some structurally related compounds, OBP14 is better tuned to monoterpenoid structures, while OBP21 binds the main components of queen mandibular pheromone as well as farnesol, a compound used as a trail pheromone in the honey bee and other hymenopterans. The high expression of different OBPs in the mandibular glands suggests that such proteins could be involved in solubilization and release of semiochemicals.     

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.     

中华蜜蜂气味结合蛋白AcerOBP14的表达及配体结合特性分析

[目的]气味结合蛋白(odorant binding proteins,OBPs)在昆虫寄主定位、产卵地选择等行为中发挥着重要作用,明确中华蜜蜂Apis cerana cerana AcerOBP14与配体的结合特性有助于阐明中华蜜蜂嗅觉识别的分子机制.[方法]通过qRT-PCR测定OBP14在20日龄中华蜜蜂成年工蜂...     

Biochemical and chemical supports for a transnatal olfactory continuity through sow maternal fluids

Recognition of the mother is of major importance for the survival of mammalian neonates. This recognition is based, immediately after birth, on the detection of odours that have been learned by the fetus in utero. If the ethological basis of a transnatal olfactory continuity is well established, little is known on the nature of its olfactory cues, and nothing about the presence of potential carrier proteins in the maternal fluids such as amniotic fluid, colostrum and milk. We have identified the components of the pig putative maternal pheromone in these fluids of the sow. We also used a ligand-oriented approach to functionally characterize carrier proteins for these compounds in the maternal fluids. Six proteins were identified, using binding assay, immunodetection and peptide mapping by mass spectrometry. These proteins are known to transport hydrophobic ligands in biological fluids. Among them, alpha-1 acid glycoprotein (AGP) and odorant-binding protein (OBP) have been described in the oral sphere of piglets as being involved in the detection of pig putative maternal pheromone components. These are the first chemical and biochemical data supporting a transnatal olfactory continuity between the fetal and the postnatal environments.     

Molecular and functional characterization of an odorant binding protein of the Asian elephant,Elephas maximus: implications for the role of lipocalins in mammalian olfaction

The sex pheromone present in the pre-ovulatory urine of female Asian elephants is the simple lipid (Z)-7-dodecen-1-yl acetate (Z7-12:Ac). Using radiolabeled probes, we have identified a pheromone binding protein that is abundant in the mucus of the trunk; this protein is homologous to a class of lipocalins known as odorant binding proteins (OBPs). To test five previously proposed roles for the OBP in chemosensory perception, we determined the equilibrium dissociation constant of the OBP-pheromone complex, as well as the association and dissociation rates. Using a mathematical model in conjunction with experimental data, we suggest that the binding and release of the pheromone by the OBP are too slow for the OBP to function in transporting the pheromone through the mucus that covers the olfactory sensory epithelium. Our data indicate that the elephant OBP only modestly increases the solubility of the pheromone in the mucus. Our results are most consistent with the notion that elephant OBP functions as a scavenger of the pheromone and possibly other ligands, including odorants. In light of these findings, and published results for other mammalian OBP-ligand complexes, a general model for the role of OBPs in mammalian olfaction is proposed. Moreover, the potential implications of these findings for interaction of Z7-12:Ac with insect antennal proteins are discussed.     

Phosphorylation of the herpes simplex virus type 1 origin binding protein

The herpes simplex virus type 1 (HSV-1) origin binding protein (OBP), the product of the UL9 gene, is one of seven HSV-encoded proteins required for viral DNA replication. OBP performs multiple functions characteristic of a DNA replication initiator protein, including origin-specific DNA binding and ATPase and helicase activities, as well as the ability to interact with viral and cellular proteins involved in DNA replication. Replication initiator proteins in other systems, including those of other DNA viruses, are known to be regulated by phosphorylation; however, the role of phosphorylation in OBP function has been difficult to assess due to the low level of OBP expression in HSV-infected cells. Using a metabolic labeling and immunoprecipitation approach, we obtained evidence that OBP is phosphorylated during HSV-1 infection. Kinetic analysis of metabolically labeled cells indicated that the levels of OBP expression and phosphorylation increased at approximately 4 h postinfection. Notably, when expressed from a transfected plasmid, a recombinant baculovirus, or a recombinant adenovirus (AdOBP), OBP was phosphorylated minimally, if at all. In contrast, superinfection of AdOBP-infected cells with an OBP-null mutant virus increased the level of OBP phosphorylation approximately threefold, suggesting that HSV-encoded viral or HSV-induced cellular factors enhance the level of OBP phosphorylation. Using HSV mutants inhibited at sequential stages of the viral life cycle, we demonstrated that this increase in OBP phosphorylation is dependent on early protein synthesis and is independent of viral DNA replication. Based on gel mobility shift assays, phosphorylation does not appear to affect the ability of OBP to bind to the HSV origins.     

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.     

Construction and analysis of cDNA libraries from the antennae of male and female cotton bollworms Helicoverpa armigera (Hübner) and expression analysis of putative odorant-binding protein genes

Two high-quality cDNA libraries were constructed from female and male antennae of the cotton bollworm Helicoverpa armigera (Hübner). The titers were approximately 2.0 × 10? pfu/ml for females and 2.3 × 10? pfu/ml for males, and this complies with the test requirement. From the libraries, 1750 male ESTs and 1640 female ESTs were sequenced and further analyzed. We identified 15 olfactory genes (12 are new), and 14 of them have the characteristic six conserved cysteine residues. With the exception of OBP9, all the genes were classified as classical OBP genes. By alignment and cluster analysis, the 14 classical OBPs were divided into pheromone binding protein (PBP) genes, odorant binding protein (OBP) genes, general odorant binding protein 1 (GOBP1) genes, general odorant binding protein 2 (GOBP2) genes and antennae binding protein (ABP) genes. Among these genes, we obtained three PBP genes (PBP1-PBP3) including two new PBP genes, one new ABP gene, nine new OBP genes (OBP1-OBP9), one known GOBP1 gene and one known GOBP2 gene. Furthermore, the expression patterns of these 14 classical OBP genes were investigated in various tissues by real-time quantitative polymerase chain reaction (qPCR). The results indicated that some OBP genes are expressed differently in different sexes and tissues, but most of them are highly expressed in antennae.     

绿盲蝽气味结合蛋白AlucOBP7的表达及气味结合特性

气味结合蛋白(odorant binding proteins, OBPs) 在昆虫嗅觉识别中起着重要的作用, 尤其是在运输外界脂溶性气味分子通过嗅觉感器淋巴液到达嗅觉受体(olfactory receptors, ORs)的过程中发挥关键作用。明确OBPs在昆虫同外界进行信息交流过程中的作用有利于阐明昆虫嗅觉识别的机制, 同时可为利用干扰昆虫嗅觉识别来进行害虫防治奠定理论基础。本研究克隆了一个绿盲蝽Apolygus lucorum (Meyer-Dür)气味结合蛋白AlucOBP7基因（GenBank登录号: JQ675724）, 并进行了原核表达, 以1-NPN为荧光探针采用荧光竞争结合实验研究了AlucOBP7蛋白和10种棉花挥发物及 6种性信息素类似物的结合能力。结果表明： 在10种棉花挥发物中, AlucOBP7能够和2 己酮及水杨酸甲酯有效结合, 结合常数分别为55.13 μmol/L和28.26 μmol/L。在6种盲蝽性信息素类似物中, 4-氧代-反-2-己烯醛和AlucOBP7 具有较强的结合能力, 结合常数为23.14 μmol/L。丁酸乙酯、 丁酸丁酯及己酸己酯也能够和AlucOBP7 有效结合, 但结合能力中等, 结合常数分别为30.58, 39.26和35.81 μmol/L。初步推测, AlucOBP7 可能是绿盲蝽性信息素结合蛋白(pheromone binding proteins, PBPs), 并在感受性信息素和植物挥发物的过程中发挥双重功能。     

Drosophila OBP LUSH is required for activity of pheromone-sensitive neurons

Odorant binding proteins (OBPs) are extracellular proteins localized to the chemosensory systems of most terrestrial species. OBPs are expressed by nonneuronal cells and secreted into the fluid bathing olfactory neuron dendrites. Several members have been shown to interact directly with odorants, but the significance of this is not clear. We show that the Drosophila OBP lush is completely devoid of evoked activity to the pheromone 11-cis vaccenyl acetate (VA), revealing that this binding protein is absolutely required for activation of pheromone-sensitive chemosensory neurons. lush mutants are also defective for pheromone-evoked behavior. Importantly, we identify a genetic interaction between lush and spontaneous activity in VA-sensitive neurons in the absence of pheromone. The defects in spontaneous activity and VA sensitivity are reversed by germline transformation with a lush transgene or by introducing recombinant LUSH protein into mutant sensilla. These studies directly link pheromone-induced behavior with OBP-dependent activation of a subset of olfactory neurons.     

Construction and analysis of cDNA libraries from the antennae of male and female cotton bollworms Helicoverpa armigera (Hübner) and expression analysis of putative odorant-binding protein genes

Two high-quality cDNA libraries were constructed from female and male antennae of the cotton bollworm Helicoverpa armigera (Hübner). The titers were approximately 2.0 × 10⁶ pfu/ml for females and 2.3 × 10⁶ pfu/ml for males, and this complies with the test requirement. From the libraries, 1750 male ESTs and 1640 female ESTs were sequenced and further analyzed. We identified 15 olfactory genes (12 are new), and 14 of them have the characteristic six conserved cysteine residues. With the exception of OBP9, all the genes were classified as classical OBP genes. By alignment and cluster analysis, the 14 classical OBPs were divided into pheromone binding protein (PBP) genes, odorant binding protein (OBP) genes, general odorant binding protein 1 (GOBP1) genes, general odorant binding protein 2 (GOBP2) genes and antennae binding protein (ABP) genes. Among these genes, we obtained three PBP genes (PBP1–PBP3) including two new PBP genes, one new ABP gene, nine new OBP genes (OBP1–OBP9), one known GOBP1 gene and one known GOBP2 gene. Furthermore, the expression patterns of these 14 classical OBP genes were investigated in various tissues by real-time quantitative polymerase chain reaction (qPCR). The results indicated that some OBP genes are expressed differently in different sexes and tissues, but most of them are highly expressed in antennae.     

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.     

NMR mapping of the recombinant mouse major urinary protein I binding site occupied by the pheromone 2-sec-butyl-4,5-dihydrothiazole.

The interactions between the mouse major urinary protein isoform MUP-I and the pheromone 2-sec-butyl-4,5-dihydrothiazole have been characterized in solution. (15)N-labeled and (15)N, (13)C-doubly-labeled recombinant MUP-I were produced in a bacterial expression system and purified to homogeneity. Racemic 2-sec-butyl-4, 5-dihydrothiazole was produced synthetically. An equilibrium diffusion assay and NMR titration revealed that both enantiomers of the pheromone bind to the recombinant protein with a stoichiometry of 1 equiv of protein to 1 equiv of racemic pheromone. A micromolar dissociation constant and slow-exchange regime dissociation kinetics were determined for the pheromone-protein complex. (1)H, (15)N, and (13)C chemical shifts of MUP-I were assigned using triple resonance and (15)N-correlated 3D NMR experiments. Changes in protein (1)H(N) and (15)N(H) chemical shifts upon addition of pheromone were used to identify the ligand binding site. Several amide signals, corresponding to residues on one side of the binding site, were split into two peaks in the saturated protein-ligand complex. Similarly, two overlapping ligand spin systems were present in isotope-filtered NMR spectra of labeled protein bound to unlabeled pheromone. The two sets of peaks were attributed to the two possible chiralities of the pheromone. Intermolecular NOEs indicated that the orientation of the pheromone in the MUP-I binding cavity is opposite to that modeled in a previous X-ray structure.     

Bacterial expression and photoaffinity labeling of a pheromone binding protein.

The first high-level production of a binding-active odorant binding protein is described. The expression cassette polymerase chain reaction was used to generate a DNA fragment encoding the pheromone binding protein (PBP) of the male moth Antheraea polyphemus. Transformation of Escherichia coli cells with a vector containing this construct generated clones which, when induced with isopropyl beta-D-thiogalactopyranoside, produced the 14-kDa PBP in both the soluble fraction and in inclusion bodies. Purification of the soluble recombinant PBP by preparative isoelectric focusing and gel filtration gave > 95% homogeneous protein, which was immunoreactive with an anti-PBP antiserum and exhibited specific, pheromone-displaceable covalent modification by the photoaffinity label [3H]6E,11Z-hexadecadienyl diazoacetate. Recombinant PBP was indistinguishable from the insect-derived PBP, as determined by both native and denaturing gel electrophoresis, immunoreactivity, and photoaffinity labeling properties. Moreover, the insoluble inclusion body protein could be solubilized, refolded, and purified by the same procedures to give a recombinant PBP indistinguishable from the soluble PBP. Proton NMR spectra of the soluble and refolded protein provide further evidence that they possess the same folded structure.     

Wake up and smell the pheromones

Odorant binding proteins (OBPs) are abundant proteins of unknown function expressed at high levels in insect and vertebrate chemosensory organs. In this issue of Neuron, Xu et al. show that Drosophila OBP76a is necessary for fruit flies to respond to the aggregation pheromone 11-cis vaccenyl acetate. The results suggest a mechanism by which this OBP is intimately involved in pheromone signal transduction.