Protein structure encodes the ligand binding specificity in pheromone binding proteins

Sexual attraction in the silkworm moth: structure of the pheromone-binding-protein-bombykol complex

BACKGROUND

Insects use volatile organic molecules to communicate messages with remarkable sensitivity and specificity. In one of the most studied systems, female silkworm moths (Bombyx mori) attract male mates with the pheromone bombykol, a volatile 16-carbon alcohol. In the male moth's antennae, a pheromone-binding protein conveys bombykol to a membrane-bound receptor on a nerve cell. The structure of the pheromone-binding protein, its binding and recognition of bombykol, and its full role in signal transduction are not known.

RESULTS

The three-dimensional structure of the B. mori pheromone-binding protein with bound bombykol has been determined by X-ray diffraction at 1.8 A resolution.

CONCLUSIONS

The pheromone binding protein of B. mori has six helices, and bombykol binds in a completely enclosed hydrophobic cavity formed by four antiparallel helices. Bombykol is bound in this cavity through numerous hydrophobic interactions, and sequence alignments suggest critical residues for specific pheromone binding.

Disulfide structure of the pheromone binding protein from the silkworm moth, Bombyx mori

Disulfide bond formation is the only known posttranslational modification of insect pheromone binding proteins (PBPs). In the PBPs from moths (Lepidoptera), six cysteine residues are highly conserved at positions 19, 50, 54, 97, 108 and 117, but to date nothing is known about their respective linkage or redox status. We used a multiple approach of enzymatic digestion, chemical cleavage, partial reduction with Tris-(2-carboxyethyl)phosphine, followed by digestion with endoproteinase Lys-C to determine the disulfide connectivity in the PBP from Bombyx mori (BmPBP). Identification of the reaction products by on-line liquid chromatography-electrospray ionization mass spectrometry (LC/ESI-MS) and protein sequencing supported the assignment of disulfide bridges at Cys-19-Cys-54, Cys-50-Cys-108 and Cys-97-Cys-117. The disulfide linkages were identical in the protein obtained by periplasmic expression in Escherichia coli and in the native BmPBP.

A new class of hexahelical insect proteins revealed as putative carriers of small hydrophobic ligands

BACKGROUND

THP12 is an abundant and extraordinarily hydrophilic hemolymph protein from the mealworm Tenebrio molitor and belongs to a group of small insect proteins with four highly conserved cysteine residues. Despite their sequence homology to odorant-binding proteins and pheromone-binding proteins, the function of these proteins is unclear.

RESULTS

The first three-dimensional structure of THP12 has been determined by multidimensional NMR spectroscopy. The protein has a nonbundle helical structure consisting of six alpha helices. The arrangement of the alpha helices has a 'baseball glove' shape. In addition to the hydrophobic core, electrostatic interactions make contributions to the overall stability of the protein. NMR binding studies demonstrated the binding of small hydrophobic ligands to the single hydrophobic groove in THP12. Comparing the structure of THP12 with the predicted secondary structure of homologs reveals a common fold for this new class of insect proteins. A search with the program DALI revealed extensive similarity between the three-dimensional structure of THP12 and the N-terminal domain (residues 1-95) of recoverin, a member of the family of calcium-binding EF-hand proteins.

CONCLUSIONS

Although the biological function of this new class of proteins is as yet undetermined, a general role as alpha-helical carrier proteins for small hydrophobic ligands, such as fatty acids or pheromones, is proposed on the basis of NMR-shift perturbation spectroscopy.

Intraspecific nucleotide variation at the pheromone binding protein locus in the turnip moth, Agrotis segetum

Inter- and intraspecific amino acid variability in the pheromone binding proteins (PBPs) of the Lepidoptera is believed to contribute to a molecular mechanism of pheromone blend discrimination. Messenger RNA coding for PBP sequence in Agrotis segetum (Noctuidae) was cloned, and nucleotide and inferred amino acid variation across a 769-bp region of a PBP locus was studied in two populations. A single gene copy was fully sequenced, revealing an intron/exon structure conserved with distant saturniids. While several nucleotide substitutions are predicted to result in amino acid replacement, tests for the presence of natural selection suggest that the observed variation is neutral. A phylogenetic analysis provides evidence that the two populations are in the process of genetic isolation.

Olfactory reception in invertebrates

Recent progress in understanding the principles and mechanisms in olfaction is the result of multidisciplinary research efforts that explored chemosensation by using a variety of model organisms. Studies on invertebrates, notably nematodes, insects, and crustaceans, to which diverse experimental approaches can be applied, have greatly helped elucidate various aspects of olfactory signaling. From the converging results of genetic, molecular, and physiological studies, a common set of chemosensory mechanisms emerges. Recognition and discrimination of odorants as well as chemo-electrical transduction and processing of olfactory signals appear to be mediated by fundamentally similar mechanisms in phylogenetically diverse animals. The common challenge of organisms to decipher the world of odors was apparently met by a phylogenetically conserved strategy. Thus, comparative studies should continue to provide important contributions toward an understanding of the sense of smell.

Identification and cloning of odorant binding proteins from the scarab beetle Phyllopertha diversa

Wehave identified, cloned, and characterized two odorant binding proteins from the pale brown chafer, Phyllopertha diversa. One of the proteins (OBP1, 116 amino acids long) showed high amino acid identity (>90%) to two previously identified PBPs from scarab beetles. The second protein (OBP2) showed limited sequence similarity to lepidopteran and dipteran OBPs, but contained only 133 amino acids. Both proteins showed the occurrence of six highly conserved cysteines; electrospray mass spectral data suggested they are all bound in three disulfide bonds. During purification, OBP2 separated into several isoforms; N-terminal amino acid sequencing and electrospray ionization mass spectrometry demonstrated that they are different conformations of the same protein. In the native gel electrophoresis binding experiments, none of the OBPs bound 1, 3-dimethyl-2,4-(1H,3H)-quinazolinedione but different isoforms showed different binding affinities for (R)-japonilure, a pheromone from related scarab beetles, and bombykol, the pheromone from the silkworm moth, Bombyx mori. OBP1 also bound (R)-japonilure.

Recombinant pheromone binding protein 1 from Mamestra brassicae (MbraPBP1). Functional and structural characterization

Pheromone binding proteins (PBPs) are small proteins (17 kDa on average) present at high concentrations ( approximately 10 mM) in the sensillum lymph of Lepidoptera antennae, where they play a key role in the perception of pheromones. By expression in Escherichia coli, we have obtained large quantities (2-3 mg.L-1) of pure, soluble, Mamestra brassicae PBP1 (MbraPBP1). These quantities are compatible with the requirements of X-ray and NMR studies. The recombinant protein has been characterized by native-polyacrylamide gel electrophoresis, Western blotting, N-terminal sequencing, mass spectrometry, gel filtration, circular dichroism, and NMR. Moreover, the recombinant MbraPBP1 has been shown to be able to bind the specific pheromone and a structural analogue, Z11-16:TFMK (cis-11-hexadecenyl trifluoromethyl ketone), in displacement experiments. Our results on MbraPBP1 confirm and extend previous findings on PBPs. MbraPBP1 and two PBPs from different species have been found to exist as dimers under nondenaturing conditions. The CD and structural prediction data confirm a markedly helical structure for insect PBPs rather than the beta-barrel fold found in vertebrates odorant binding proteins. We have tentatively identified the location of the helices and the short beta-strands with respect to the binding site. Currently we have obtained small diffracting crystals of the recombinant MbraPBP1 and determined their space group and molecular content.

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.

Pheromone binding proteins in the European and Asian corn borers: no protein change associated with pheromone differences

Pheromone binding proteins (PBPs) are thought to play a role in the recognition of sex pheromone in male moth antennae. By binding selectively to different components of pheromone blends, these PBPs could play a role in differentiating between structurally related compounds. In this study we have characterized the pheromone binding proteins of two pheromone strains of the European corn borer (Ostrinia nubilalis) and also the closely related Asian corn borer (O. furnacalis). We have been able to detect only one PBP gene, which encodes a mature protein that is identical in amino acid sequence in individuals from different pheromone strains and different species. This result suggests that the PBP is not detecting differences between the two isomeric compounds of the European corn borer pheromone or the difference in double bond position between the pheromone molecules of the European and Asian corn borers.

Functional characterization of a new class of odorant-binding proteins in the moth Mamestra brassicae

A new protocol of binding assay allowed us to functionally characterize two additional odorant-binding proteins in antennae of the moth Mamestra brassicae. These proteins have no N-terminal sequence homology with the moth pheromone-binding proteins and general odorant-binding proteins previously described. One of the two proteins designated MbraAOBP2 is between 60 and 73% similar in N-terminal to several proteins characterized in chemosensory organs of Diptera, Hymenoptera, Lepidoptera, and Phasmids, indicating that these proteins constitute a new group of odorant-binding proteins. A particularly high similarity between MbraAOBP2 and ejaculatory bulb specific protein III of Drosophila suggested that vaccenyl acetate could be a specific ligand for these proteins. As a matter of fact, MbraAOBP2 bound vaccenyl acetate in vitro, but we failed to detect any receptor cell in long and short sensilla trichodea of males. This protocol could be used as a rapid method to identify new odorant-binding proteins in chemosensory organs or tissues.

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.

Pheromone-binding proteins of scarab beetles

We have characterized pheromone binding proteins (PBPs) present in the antennae of several species of scarab beetles. In most cases there was only one class of PBP, which was expressed in both sexes. Both Anomala osakana++ and Popillia japonica possess a single PBP, highly homologous to each other. In species the same PBP seems to recognize both enantiomers of japonilure, which have opposite biological functions, i.e., the sex pheromone and the behavioral antagonist (stop signal). The purified PBP of A. osakana binds both enantiomers apparently with the same low affinity. Unexpectedly, these ligands were bound by moth PBPs, which utilize pheromones with unrelated structures. These findings suggest that the ligand specificity in this class of proteins may not be as high as it has been postulated.

Olfactory coding in a compound nose. Coexpression of odorant-binding proteins in Drosophila

Odorant-binding proteins (OBPs) are small, soluble proteins present in the aqueous medium surrounding olfactory receptor neurons. Their function in olfaction is unknown: they have been proposed to facilitate the transit of hydrophobic molecules to olfactory receptors, to deactivate the odorant stimulus, and/or to play a role in chemosensory coding. We have examined the genomic organization and expression patterns of two olfactory-specific genes (OS-E and OS-F) of Drosophila melanogaster, the products of which are members of a protein family in Drosophila sharing sequence similarity with moth OBPs. We found that the OS-E and OS-F transcription units are located < 1 kb apart. They are oriented in the same direction and display a similar intron-exon organization. Expression of both OS-E and OS-F proteins is spatially restricted to the ventrolateral region of the Drosophila antenna. Within this region, both OS-E and OS-F proteins are expressed within two different types of sensory hairs: in most, if not all, sensilla trichodea and in approximately 40% of the interspersed small sensilla basiconica. We consistently observe that OS-E and OS-F are coexpressed, indicating that an individual sensillum can contain more than one odorant-binding protein. This finding has potential implications for the roles of odorant-binding proteins in olfactory coding.

Odorant-binding proteins: expression and function

Odorant-binding proteins (OBPs) are a major constituent of the aqueous perireceptor compartment in vertebrates and in insects. Although different in primary structure, they are supposed to serve similar functions in both animal groups: (i) OBPs may act as solubilizers and carriers of the lipophilic odorants in the aqueous mucus or sensillum lymph; (ii) OBPs may act in addition as peripheral filters in odor discrimination by selectively binding certain classes of odorants; (iii) OBPs may present the stimulus molecule in a particular way to the receptor proteins to facilitate signal transduction; (iv) OBPs may clean the perireceptor space from unwanted and toxic compounds; (v) OBPs may rapidly deactivate odorants after stimulation of the receptors. Experimental evidence in favor of this multiple role of OBPs is reviewed.

Molecular cloning of two pheromone binding proteins in the cabbage armyworm Mamestra brassicae

Two cDNA clones encoding pheromone binding proteins (PBPs) were isolated from antennal cDNA of Mamestra brassicae by reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends-PCR (RACE-PCR) performed with specific primers deduced from the N-terminal sequences of two PBPs previously reported. The deduced protein sequences of the two PBPs showed a strong relationship between primary structures and functional properties of the corresponding mature proteins.

Cloning and characterization of a cDNA encoding a male-specific serum protein of the Mediterranean fruit fly, Ceratitis capitata, with sequence similarity to odourant-binding proteins

Male-specific serum proteins (MSSPs) are low molecular weight proteins which accumulate in high amounts in the haemolymph of adult males of the medfly Ceratitis capitata. By screening an expression library with anti-MSSP antibodies, we have isolated and determined the nucleotide sequence of a cDNA clone coding for one of the male-specific polypeptides (MSSP-alpha). The MSSP-alpha mRNA encodes a polypeptide of 144 amino acids with a secretory signal sequence of sixteen amino acids. Southern analysis indicated that there are multiple copies of MSSP genes in the medfly genome. Northern analysis showed that the MSSP mRNAs are synthesized only in adult males. The accumulation pattern of these mRNAs during development suggests that the expression of the MSSP genes is developmentally regulated at both transcriptional and translational levels. The predicted peptide sequence of MSSP-alpha shows significant similarity to a group of pheromone- and general odourant-binding proteins of insects.

Attracted or repelled?--a matter of two neurons, one pheromone binding protein, and a chiral center

Two species of scarab beetles, the Osaka beetle (Anomala osakana) and the Japanese beetle (Popillia japonica), utilize the opposite enantiomers of japonilure, (Z)-5-(1-decenyl)oxacyclopentan-2-one, as their sex pheromones. Each species produces only one of the enantiomers that functions as its own sex pheromone and as a very strong behavioral antagonist for the other species. Using an integrated approach we tested whether the discrimination of these two opposite signals is due to selective filtering by pheromone binding proteins or whether it originates in the specificity of ligand-receptor interactions. We found that the antennae of each of these two scarab species contain only a single pheromone binding protein, which associates with both enantiomers to a similar extent. The single neuron recording technique, on the other hand, showed that both species possess olfactory receptor neurons, colocalized in one sensillum, extremely specific to either (R)- or (S)-japonilure. Therefore, pheromone binding proteins (PBPs) alone cannot perform the task of chiral discrimination; enantiomeric specificity must be achieved by the interaction of the pheromone or the appropriate ligand-PBP complex with membrane receptors.

Elements of the olfactory signaling pathways in insect antennae

Owing to their enormous ability to recognize airborne molecules, insects have long been used as model systems for studying various aspects of olfaction. Modern biological techniques have opened new avenues for exploring the molecular mechanisms underlying the complex signaling processes in chemosensory neurons. Biochemical and molecular analyses have allowed the identification of molecular elements of the olfactory reaction pathways and have shed light on mechanisms that account for the sensitivity and specificity of the chemosensory system.

Coexpression of two odorant-binding protein homologs in Drosophila: implications for olfactory coding

Odorant-binding proteins (OBPs) are small soluble proteins present in the aqueous medium surrounding olfactory receptor neurons. Their function in olfaction is still unknown: they have been proposed to facilitate the transit of hydrophobic molecules to olfactory receptors, to deactivate the odorant stimulus, and/or to play a role in chemosensory coding. In this study we examine the genomic organization and expression patterns of two olfactory-specific genes (OS-E and OS-F) of Drosophila melanogaster, the products of which are members of a protein family in Drosophila sharing sequence similarity with moth OBPs. We show that the OS-E and OS-F transcription units are located <1 kb apart. They are oriented in the same direction and display a similar intron-exon organization. Expression of both OS-E and OS-F proteins is restricted spatially to the ventrolateral region of the Drosophila antenna. Within this region both OS-E and OS-F proteins are expressed within two different types of sensory hairs: in most, if not all, sensilla trichodea and in approximately 40% of the interspersed small sensilla basiconica. We consistently observe that OS-E and OS-F are coexpressed, indicating that an individual sensillum can contain more than one odorant-binding protein. The functional significance of the observed expression pattern and its implications for olfactory coding are discussed.

Mechanisms of olfactory discrimination: converging evidence for common principles across phyla

Olfaction begins with the transduction of the information carried by odor molecules into electrical signals in sensory neurons. The activation of different subsets of sensory neurons to different degrees is the basis for neural encoding and further processing of the odor information by higher centers in the olfactory pathway. Recent evidence has converged on a set of transduction mechanisms, involving G-protein-coupled second-messenger systems, and neural processing mechanisms, involving modules called glomeruli, that appear to be adapted for the requirements of different species. The evidence is highlighted in this review by focusing on studies in selected vertebrates and in insects and crustaceans among invertebrates. The findings support the hypothesis that olfactory transduction and neural processing in the peripheral olfactory pathway involve basic mechanisms that are universal across most species in most phyla.

Expression of an olfactory receptor in Escherichia coli: purification, reconstitution, and ligand binding

An olfactory receptor has been expressed in bacterial cells as a fusion protein with glutathione S-transferase (GST). Overexpression of receptor protein yielding about 10% of the cell protein was achieved with mutants lacking the N-terminus and the first transmembrane region or with mutants carrying three positively charged residues in the first intracellular loop. The overexpressed fusion protein accumulated in inclusion bodies and could be solubilized in detergent. It was purified by metal chelation chromatography based on a C-terminal 6-histidine tag, and the GST portion was removed after proteolytic cleavage. The purified receptor was reconstituted into lipid vesicles and specific binding of odor ligands was shown by photoaffinity labeling and tryptophan fluorescence measurements. Thus, for the first time, an odorant receptor/ligand pair becomes available in large amounts for biophysical and screening studies.

When is a butterfly like an elephant?

The behaviours of organisms as diverse as elephants and butterflies are affected by pheromones with identical or similar structures. Recent developments in the molecular biology of pheromone detection suggest why.

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.

Ligand specificity of pheromone-binding proteins of the processionary moth

Photoaffinity labeling of proteins extracted from sensory hairs and antennal branches of the processionary moth Thaumetopoea pityocampa with a tritium-labeled diazoacetate analogue of the sex pheromone (Z)-13-hexadecen-11-ynyl acetate revealed a 15-kDa pheromone-binding protein in male moth sensory hairs (SH-15). A different 15-kDa protein in male antennal branches (B-15) was not photolabeled. All extracts except male sensory hairs showed a photolabeled 20-kDa protein; a photolabeled male 30-kDa protein in the branches (B-30) was also observed. The 20-kDa proteins in the sensory hairs (SH-20) and branches (B-20) showed differing affinities for the photoaffinity analogues; moreover, SH-15 exhibits higher affinity for the natural pheromone, (Z)-13-hexadecen-11-ynyl acetate, than for its alcohol metabolite and other analogues in competitive displacement experiments. The affinity shown by the pheromone-binding protein for the metabolic product suggests that the alcohol may be also transported by the binding protein. Interestingly, a shift in labeling from SH-15 to SH-20 was produced in the presence of an excess of the natural pheromone, its alcohol and other analogues. The binding showed little discrimination among structurally similar analogues of the pheromone, while saturated and aromatic molecules showed little affinity for the proteins of either sensory hairs or antennal branches.