1.A.69 The Heteromeric Odorant Receptor Channel (HORC) Family
In insects, each olfactory sensory neuron expresses between one and three ligand-binding members of the olfactory receptor (OR) gene family, along with the highly conserved and broadly expressed Or83b co-receptor. The functional insect OR consists of a heteromeric complex of unknown stoichiometry but comprising at least one variable odorant-binding subunit and one constant Or83b family subunit. Insect ORs lack homology to G-protein-coupled chemosensory receptors in vertebrates and possess a distinct seven-transmembrane topology with the amino terminus located intracellularly. Sato et al. (2008) and Touhara (2009) showed that heteromeric insect ORs comprise a new class of ligand-activated non-selective cation channels. Heterologous cells expressing silkmoth, fruitfly or mosquito heteromeric OR complexes show extracellular Ca2+ influx and cation-non-selective ion conductance on stimulation with odorant or pheromone. Odour-evoked OR currents are independent of known G-protein-coupled second messenger pathways. The fast response kinetics and OR-subunit-dependent K+ ion selectivity of the insect OR complex support the hypothesis that the complex between OR and Or83b itself confers channel activity. The ligand (odorant)-gated ion channels formed by an insect OR complex seem to be the basis for a unique strategy that insects have acquired to respond to the olfactory environment (Sato et al., 2008). These odorant receptors have been reviewed (Wicher 2015).
Insect odorant receptors are composed of conventional odorant receptors (for example, Or22a), dimerized with a ubiquitously expressed chaperone protein, such as Or83b in Drosophila. Or83b has a structure akin to GPCRs, but has an inverted orientation in the plasma membrane. However, G proteins are expressed in insect olfactory receptor neurons, and olfactory perception is modified by mutations affecting the cAMP transduction pathway. Application of odorants to mammalian cells co-expressing Or22a and Or83b results in non-selective cation currents activated by means of ionotropic and metabolotropic pathways, and a subsequent increase in the intracellular Ca2+ concentration (Wicher et al., 2008). Expression of Or83b alone leads to functional ion channels not directly responding to odorants, but being directly activated by intracellular cAMP or cGMP. Insect odorant receptors thus form ligand-gated channels as well as complexes of odorant-sensing units and cyclic-nucleotide-activated non-selective cation channels. They, thereby, provide rapid and transient as well as sensitive and prolonged odorant signalling (Wicher et al., 2008). Their evolution, development, gene expression and funtion have been discussed by Yan et al. 2020.
ORs have been identified from four insect orders (Coleoptera, Lepidoptera, Diptera, and Hymenoptera). Although all ORs share the same G-protein coupled receptor structure with seven transmembrane domains, they present poor sequence homologies within and between species. D. melanogaster is the only insect species where Ors have been extensively studied from expression pattern establishment to functional investigations (Jacquin-Joly and Merlin, 2004). One OR type is selectively expressed in a subtype of olfactory receptor neurons, and one olfactory neuron expresses only one type of OR. In addition, all olfactory neurons expressing one OR type converge to the same glomerulus in the antennal lobe. The olfactory mechanism, thus, appears to be conserved between insects and vertebrates (Jacquin-Joly and Merlin, 2004).
After the discovery of the complete repertoire of D. melanogaster Olfactory Receptors (ORs), candidate ORs have been identified from at least 12 insect species from four orders (Coleoptera, Lepidoptera, Diptera, and Hymenoptera). Although all ORs share the same G-protein coupled receptor structure with seven TMSs, they share poor sequence identity. One OR type is selectively expressed in a subtype of olfactory receptor neurons, and one olfactory neuron expresses only one type of OR. The olfactory mechanism, further, appears to be conserved between insects and vertebrates. The C-terminal region (TMSs4-7) of OR83b is involved in homodimer and heterodimer formation (with OR22a) which suggests why the C-termini of insect ORs are highly conserved. There may be two possible ion channel pathways, one formed by the TMS4-5 region with the intracellular pore-forming domain and the other formed by TM5-6 with the extracellular pore forming domain. Odorant receptors generally comprise the obligate co-receptor, Orco, and one of a family of highly divergent odorant 'tuning' receptors. The two subunits are thought to come together at some as-yet unknown stoichiometry to form a functional complex that is capable of both ionotropic and metabotropic signalling. Segments and residues involved in this interaction have been identified (Carraher et al. 2015).
Olfactory systems must detect and discriminate among an enormous variety of odorants. To contend with this challenge, diverse species have converged on a common strategy in which odorant identity is encoded through the combinatorial activation of large families of olfactory receptors, thus allowing a finite number of receptors to detect a vast chemical world. Del Mármol et al. 2021 offered structural and mechanistic insight into how an individual olfactory receptor can flexibly recognize diverse odorants. They found that the olfactory receptor MhOR5 from the jumping bristletail, Machilis hrabei, assembles as a homotetrameric odorant-gated ion channel with broad chemical tuning. Using cryo-EM, they elucidated the structure of MhOR5 in multiple gating states, alone and in complex with two of its agonists, the odorant eugenol and the insect repellent DEET. Both ligands are recognized through distributed hydrophobic interactions within the same geometrically simple binding pocket located in the transmembrane region of each subunit, suggesting a structural logic for the promiscuous chemical sensitivity of this receptor. Mutation of individual residues lining the binding pocket predictably altered the sensitivity of MhOR5 to eugenol and DEET and broadly reconfigured the receptor's tuning. Thus, diverse odorants share the same structural determinants for binding (Del Mármol et al. 2021).
The generalized reaction catalyzed by HORC is:
cations (in) cations (out)
Heteromeric odorant receptor, OR (Sato et al., 2008). OR22a senses fruit-derived esters. These olfactory receptors may have 3-d structures resembling animal rhodopsins, human citronellic terpenoid receptors, OR1A1 and OA1A2 and the mouse eugenol receptor, OR-EG (Ramdya and Benton, 2010). Molecular modelling of oligomeric states of DmOR83b has been reported (Harini and Sowdhamini, 2012). Recombinant receptor together with the co-receptor, Orco, has been overproduced, purified and reconstituted in a lipid bilayer (Carraher et al. 2013). Orco (Or83b) forms a dimer that is fully functional for Ca2+ transport, is regulated by calmodulin and interacts normally with Or22a. The native Orco is therefore probably a dimer (Mukunda et al. 2014).
Heterometic odorant receptor (OR) of Drosophila melanogaster:
Odorant receptor, OR2 (Carraher et al., 2012).
OR2 of Anopheles gambiae (Q8WTE6)
Odorant receptor 56a. Mediates aversive responses to harmful microbial (bacterial and fungal) products such as geosmin (trans-1,10-dimetnyl-trans-9-decalol). (Stensmyr et al. 2012).
OR56a of Drosophila melanogaster
Ordorant receptor 67b of 421 aas and 8 TMSs (Identical to Or67b of D. melanogaster)
Or67b of Drosophila simulans
Odorant receptor 10b of 406 aas and 7 TMSs
Or10b of Drosophila melanogaster
The insect heteromeric CO2 receptor: GR21a (Olfactory receptor 21a; 454 aas) GR63a (Olfactory receptor 63a; 512 aas) are coexpressed in antennal neurons of insects and together comprise the peripheral sensory receptor for CO2 (Ramdya and Benton, 2010). These proteins are members of the 7Tm-7 superfamily of putative 7TMS proteins.
The gustatory receptor for CO2, GR21a/GRG3a of Drosophila melanogaster
Uncharacterized protein of 382 aas and 9 TMSs.
UP of Frankliniella occidentalis (western flower thrips)
Gustatory receptor for sugar taste 64e-like protein, GR64e, of 486 aas and 8 TMSs.
GR64e protein of Atta cephalotes (Leafcutter ant)
Uncharacterized protein of 416 aas and 7 TMSs.
UP of Aphis gossypii (cotton aphid)
Uncharacterized protein of 425 aas and 7 TMSs.
UP of Amphibalanus amphitrite
Fructose-regulated Ca2+/cation channel, Gustatory (fructose) receptor-9, Gr9 (Sato et al., 2011). Gr9 is widely expressed in the central nervous system (CNS), as well as oral sensory organs and is involved in the promotion of feeding behaviors (Mang et al. 2016).
GR-9 of Bombyx mori (B3GTD7)
Gustatory receptor 43a isoform A. Functions as a narrowly tuned fructose receptor in taste neurons (Miyamoto et al. 2012), being both necessary and sufficient to sense hemolymph fructose.
GR43a of Drosophila melanogaster (Q9V4K2)
Gustatory receptor 28b isoform D of 470 aas and 8 TMSs. It mediates acceptance or avoidance behavior, depending on its substrates. Its atypical expression suggests additional nongustatory roles in the nervous system and tissues involved in proprioception (warmth receptor), hygroreception, and other sensory modalities. It is also possible that it has chemosensory roles in the detection of internal ligands (Thorne and Amrein 2008). Saponins function in natural self-defense for plants to deter various insects due to their unpleasant taste and toxicity. Sang et al. 2019 provided evidence that saponin from Quillaja saponaria functions as an antifeedant as well as an insecticide to ward off insects in both the larval and the adult stages.
GR28b of Drosophila melanogaster (Q9VM08)
Gustatory receptor 2a isoform B
GR2a of Drosophila melanogaster (Q9W594)
High energy light unresponsive protein 1, Lite1; chemoreceptor GUR-2 of 439 aas and 8 TMSs. It is a photoreceptor for short wavelength (UV) light that mediates UV-light-induced avoidance behavior (Edwards et al. 2008, Liu et al. 2010, Gong et al. 2016). It directly senses and absorbs both UV-A and UV-B light with very high efficiency (Gong et al. 2016). Absorption of UV-B but not UV-A light shows resistance to photobleaching. In contrast to other photoreceptors, it does not use a prosthetic chromophore to capture photons and only depends on its protein conformation. It may play a role in response to white light exposure (De Magalhaes Filho et al. 2018) as well as color detection (Ghosh et al. 2021).
GUR-2 of Caenorhabditis elegans
Gustatory receptor, GPRGR53, of 430 aas and 7 TMSs. It mediates acceptance or avoidance behavior, depending on its substrates.
GPRGR53 of Anopheles gambiae (African malaria mosquito)
Putative gustatory receptor 98b of 402 aas and 7 TMSs.
GR98b of Bactrocera latifrons
Gustatory receptor family protein 3, Gur-3, of 447 aas and 8 - 9 TMSs. It is a chemoreceptor involved in light-induced avoidance behavior (Bhatla and Horvitz 2015) and probably acts as a molecular sensor in I2 pharyngeal neurons, required for the inhibition of feeding in response to light and hydrogen peroxide. It may be involved in circadian rhythms, probably by acting as a light sensor (Goya et al. 2016). Although it acts with Lite-1 in color detection, it does not act as a photoreceptor (Ghosh et al. 2021).
Gur-3 of Caenorhabditis elegans
The pheromone receptor, Or-1 (Nakagawa et al., 2012)
Or-1 of Bombyx mori (Q5WA61)
Sex pheromone receptor of 416 aas and 7 TMSs (Miura et al. 2010).
pheromone receptor of Ostrinia nubilalis (European corn borer) (Pyralis nubilalis)
Odorant receptor 3, Or3 of 410 aas and 7 TMSs.
Or3 of Epiphyas postvittana (Light brown apple moth)
Odorant (pheromone) receptor, OR-3, BmOR3, Or-3, PR-3, of 439 aas and 7 TMSs in a 2 + 1 + 2 + 2 TMS arrangement. The activation of PRs is coupled to the calcium permeability of their coreceptor (Orco (see TC# 1.A.69.1.1)) or putatively with G proteins (Lin et al. 2021). Using the PR BmOR3 from the silk moth B. mori and its coreceptor BmOrco as a template, Lin et al. 2021 showed that an agonist-induced conformational change of BmOR3 is transmitted to BmOrco through TMS7s of both receptors, resulting in the activation of BmOrco. Key interactions, including an ionic lock and a hydrophobic zipper, are essential for mediating the functional coupling between BmOR3 and BmOrco. BmOR3 also selectively coupled with Gi proteins, which is dispensable for BmOrco coupling. Moreover, trans-7TM BmOR3 recruited arrestin (see TC# 8.A.136) in an agonist-dependent manner, which indicated an important role for BmOR3-BmOrco complex formation in ionotropic functions. Thus, the coupling of G protein and arrestin to a prototype trans-7TMS PR, BmOR3, has been demonstrated (Lin et al. 2021).
OR3 of Bombyx mori (Silk moth)
Odorant receptor 85b (or85b) of 302 aas and 5 putative TMSs. Binds the odorant, heptanone, for activation; 2-nananone is a competitive antagonist. The second half of TMS3 is involved in odorant binding and activation (Nichols and Luetje 2010).
animals (Invertebrates; insects)
Or85b of Drosophila melanogaster
Odorant receptor 22 of 312 aas and 6 TMSs
Or22 of Bombyx mori
Odorant receptor 17 of 401 aas and 8 TMSs
Or17 of Bombyx mori (Silk moth)
Odorant recpetor 278 if 385 aas and 8 TMSs
Or278 of Tribolium castaneum (Red flour beetle)
Odorant receptor 205 of 406 aas and 9 putative TMSs.
Or205 of Tribolium castaneum (Red flour beetle)