Molecule of the Month: Insect Odorant Receptors

Our ability to understand the world around us depends on sensory proteins specialized for detecting stimuli such as light, temperature, and touch. By far the largest group of sensory proteins are chemoreceptors, which allow us to perceive flavors and odors. In mammals, the sense of smell begins in the nose, where airborne molecules encounter specialized cells called olfactory neurons. Each of these neurons expresses just one type of odorant receptor (out of an estimated total of about 400 receptors in humans). Mammalian odorant receptors all belong to the G protein-coupled receptor (GPCR) superfamily. A given odorant binds to a distinct complement of receptors, resulting in the activation of a subset of olfactory neurons that ultimately results in the sensation of a distinct smell.

Insects also rely heavily on smell to navigate their environment. It turns out, however, that insects use a very different mechanism to sense airborne molecules. The primary olfactory organ of many insects are their antennae, which are covered in tiny hair-like structures called sensilla. Odor molecules can enter olfactory sensilla through pores, where they can then interact with odorant receptors on olfactory neurons.

Unlike mammalian odorant receptors, insect receptors are not GPCRs. Rather, insect odorant receptors are ligand-gated ion channels made up of four subunits. Odorant binding opens the channel opens and allows for the entry of ions into the cell, leading to neuronal activation. In most insects, odorant receptors are tetrameric complexes composed of two kinds of subunits: a variable odorant-binding subunit (called OR) and a conserved co-receptor known as Orco. ORs have rapidly expanded and diversified across different insect species to represent one of the largest and most divergent family of ion channels in nature, with potentially millions of different variants. This rapid evolution is thought to contribute to the ability of insects to adapt to very different ecologies.

Although it was originally suspected that olfactory receptors would be built as symmetric tetramers with two ORs and two Orcos, recent structural studies have revealed asymmetric receptors, consisting of one OR subunit paired with three Orco subunits (shown on the right are pdb_00008v00 and pdb_00008v02, and similar asymmetric structures can also be seen in pdb_00008z9a, pdb_00008z9z, pdb_00008v3c, and pdb_00008v3d).

When an odorant binds, it fits into a deep, hydrophobic pocket within the membrane-spanning region of the OR subunit. This binding event causes the OR subunit to undergo a conformational change that opens the channel’s pore and allows ions through. Notably, the Orco subunits remain largely stationary during this process, highlighting the OR subunit’s central role in detecting specific odor molecules.

While many familiar insects, such as flies and bees, express the co-receptor Orco and assemble their odorant receptors as heterotetrameric complexes, some basal insect lineages lack Orco genes entirely. The jumping bristletail (Machilis hrabei), a silverfish-like arthropod considered among the most evolutionarily primitive living insects, encodes only five OR subunits and lacks an Orco gene. Compared to other insect model organisms, such as the fruit fly Drosophila melanogaster, which encodes over 60 OR genes in its genome, the jumping bristletail offers a simplified system to understand how different odorants can be recognized by receptors.

Researchers found that olfactory receptors in M. hrabei could be built from four identical subunits, and that different channels varied widely in sensitivity to odorants. While a receptor built from four OR5 subunits (shown on the left, pdb_00007lic, pdb_00007lid, and pdb_00007lig) activated in response to more than half of 54 odorant molecules tested, including eugenol (the primary component of clove oil) and DEET (an effective insect repellent), homomeric channels assembled from other OR subunits responded to a much narrower set of odorants. Structural analyses suggest that the ability to recognize diverse molecules arises from the nature of the odorant-binding pocket. Rather than relying on a single highly specific interaction, the pocket can accommodate chemically distinct molecules through many weak hydrophobic interactions.

These research studies highlight the flexibility of the Orco/OR system to form complexes containing a variable number of OR subunits across the insect clade. Whether insect odorant receptors are found in other subunit ratios of Orco:OR within an in vivo context is an area of active research.

Understanding how insects detect a diversity of chemical signals has practical implications. Insights into these mechanisms can guide the development of safer and more effective repellents to reduce the spread of insect-borne diseases, as well as strategies to protect crops from destructive insect pests.