If you cannot reach an item on the top shelf, do you ask for assistance? Like people on their tiptoes, olfactory receptors might need helping hands to access molecules just out of reach. Benton et al. report an insect lipid-binding protein that delivers pheromones to olfactory receptors in a recent article in Nature.
In mammals, olfactory neurons use standard G-protein-coupled receptors for chemosensation. In insects, odorant receptors contain a ligand-binding domain and OR83b coreceptor, which is important in receptor trafficking. How does this complex recognize and respond to odors? The authors did a high-throughput screen to identify new genes important in insect olfaction.
By comparing Drosophila, Anopheles (mosquitos) and 8 non-insect genomes, the authors identified 2135 insect-specific genes. They localized 339 of the genes that lacked known functions. Sensory neuron membrane protein (SNMP) and its species-specific orthologs localized to the antenna, a major olfactory organ, in Drosphila, Anopheles and Antheraea (silk moths). SNMP is related to CD36, an immune receptor for bacteria-derived lipids.
In the Drosophila antenna, SNMP localized to olfactory sensory neurons (OSNs) expressing OR83b and to non-neural support cells in trichoid sensory cilia, which are important in pheromone detection. SNMP expression was similar in OR83b knockout and wild-type Drosophila, suggesting that SNMP expression is not dependent on OR83b or olfactory receptor expression. However, in Drosophila expressing SNMP and OR83b linked to complementary portions of yellow fluorescent protein, several OSNs were fluorescent, suggesting that OR83b and SNMP are closely associated. SNMP-expressing OSNs projected to the glomeruli in the antennal lobe that mediate pheromone detection.
OR67d responds to the Drosophila pheromone cis-vaccenyl acetate (cVA). Wild-type OSNs that express OR67d show little basal activity. In response to cVA, these neurons fire trains of action potentials. In contrast, SNMP knockout OSNs showed increased basal activity relative to wild-type OSNs, but did not respond to cVA. Targeted SNMP expression rescued normal basal activity and cVA response in OR67d OSNs but not in non-neural support cells. OR83b knockout OSNs, which do not respond to cVA, showed low basal activity even in the absence of SNMP, suggesting that SNMP acts upstream of OR67d to mediate cVA detection in OSNs.
OR22a but not OR67d OSNs respond to fruit ester ligands, like ethyl butyrate. However, ethyl butyrate induced trains of action potentials in OR67d OSNs ectopically expressing OR22a, and SNMP deficiency did not affect this response. OR67d OSNs expressing the moth pheromone receptor HR13 responded to the volatile moth pheromone (Z)-11-hexadecenal; however, SNMP deficiency blocked this response. Like cVA, (Z)-11-hexadecenal has a long, hydrophobic hydrocarbon tail, which is missing in ethyl butyrate.
OR67d does not require SNMP to respond to pheromones applied directly to the cuticle covering OSNs. However, like other odorants, pheromones are normally aerosolized in the air, and SNMP knockout OSNs do not respond to puffs of cVA in the air. Therefore, like other CD36 family members, SNMP binds fatty pheromones in the air and delivers them to OR67d. Perhaps other binding proteins assist in the detection of different types of odorant.