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Odorant-binding proteins (OBPs) are small (10 to 30 kDa) soluble
protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing m ...
s secreted by auxiliary cells surrounding olfactory receptor neurons, including the nasal
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It i ...
of many vertebrate species and in the sensillar lymph of chemosensory sensilla of insects. OBPs are characterized by a specific protein domain that comprises six
α-helices
α-helices
joined by three disulfide bonds. Although the function of the OBPs as a whole is not well established, it is believed that they act as odorant transporters, delivering the odorant molecules to
olfactory receptor Olfactory receptors (ORs), also known as odorant receptors, are expressed in the cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the ...
s in the
cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates the interior of all cells from the out ...
of sensory neurons. The olfactory receptors of terrestrial animals exist in an aqueous environment, yet detect odorants that are primarily hydrophobic. The aqueous solubility of hydrophobic odorants is greatly enhanced via odorant-binding proteins, which exist in the extracellular fluid surrounding the odorant receptors. This family is composed of pheromone binding proteins (PBP), which are male-specific and associate with pheromone-sensitive neurons and general-odorant-binding proteins (GOBP). These proteins were initially identified on the basis of their ability to bind with moderate-affinity radioactively labeled odorants.


Structure

OBPs are small proteins on the order of 14 kDa in size. All odorant binding proteins are believed to have a common structure despite their genetic diversity and highly variable primary structures. In vertebrates, OBPs are a part of the lipocalin family. They are structurally characterized by a β-barrel motif composed of antiparallel β-sheets. Insect OBPs share very little amino acid sequence similarity to vertebrate OBPs as they mainly contain domains. OBPs are divergent across and within species. The percentage of conserved residues between species has been shown to be as low as 8%. OBPs' have a characteristic signature that is recognized by a conserved pattern of six cysteines that are connected in the protein by three disulfide bridges. Their structures have been investigated to explore new bio-inspired repellents against mosquitoes, with potentially improved OBP binding affinity, selectivity, and reduced volatility.


Function

The functions of odorant binding proteins as a whole is not well understood. They are generally believed to increase the solubility of hydrophobic odorants by binding them and transporting them across the aqueous sensillum lymph to receptors in the dendrites, and several studies support a role for OBPs in olfactory perception ''in vivo''. Some odorant binding proteins are hypothesized to hasten odor response termination by extracting odorant molecules from the sensillar lymph or from receptors themselves. Presently, just one OBP, Obp76a, has been thoroughly investigated in the olfactory system of ''Drosophila'' and has a known physiological role. Obp76a, better known as LUSH, is located trichoid sensilla and is necessary for normal response of the odor receptor Or67d to its pheromone ligand Vaccenyl acetate, cis-vaccenyl acetate (cVA), although responses of Or67d to cVA have been detected in the absence of Obp76a LUSH has also been found to bind cVA ''in vitro'' and is known to bind other insect pheromones, short-chain alcohols, and phthalates. In 2016, Larter et al. found that the deletion of the sole abundant OBP, Obp28a, in ab8 sensilla of ''Drosophila'' does not reduce the magnitude of their olfactory responses, suggesting that Obp28a is not required for odorant transport and that ab8 sensilla do not require an abundant OBP. Their results further suggest Obp28a may be buffering changes in the odor environment, possibly as molecular gain control, which has not been previously reported for OBPs. OBPs are thought to have multiple roles besides olfaction, including reproduction, egg laying and antiinflammatory responses.


Expression

OBPs are numerous and diverse. In ''Drosophila'', they are encoded by 52 genes of the same family yet only share 20% amino acid similarity between themselves. Some are encoded by the most abundant Messenger RNA, mRNAs of the antennae. Within and between species, OBPs are expressed in several different tissues, including the antennal sensilla, the taste system, and chemosensory organs. They are also known to be Ectopic expression, ectopically expressed in tissues such as the gut. Genomic analysis of ''Drosophila'' and other insect species (''Anopheles gambiae'', ''Western honey bee, Apis mellifera,'' ''Bombyx mori'', and ''Red flour beetle, Triboliumcastaneum'') has revealed that the OBP genes significantly differ between species. The OBP family contains 21 (in A. ''mellifera'') to 66 genes (in ''A. gambiae''), whereas it ranges from 52 members in ''Drosophila'' to 20 in ''T. castaneum''. Generally these genes are irregularly scattered across the genome. Most (69% of the OBP genes in ''Drosophila'') are arranged in small clusters from 2 to 6 OBP genes. The ''Drosophila'' OBP gene family has been classified into several subfamilies based on structural features, functional information, and phylogenetic relationships: the Classic, Minus-C, Plus-C, Dimer, PBP/GOBP, ABPI and ABPII, CRLBP, and D7 subfamilies. These subfamilies are unequally distributed across arthropods, even among the Fly, dipterans and are totally absent in some species.


See also

* Insect pheromone-binding protein * Aroma compound, Odorant * Olfactory receptor * Olfactory receptor neuron


References

{{Reflist Olfactory system Lipocalins