Researchers at Rockefeller have used an ultra sensitive device to detect the olfactory systems of insects that help them smell out food and danger. The study titled, “Cryo-EM structure of the insect olfactory receptor Orco,” on the ion channels that form the basis of the odour detecting systems of insects was published in the latest issue of the journal Nature.
Researchers Vanessa Ruta and her colleagues have described the structure of an ion channel that is the basis of the smelling mechanism of insects. They write about the different varieties of such channels which have evolved so that insects of different kinds can smell differently. For example while mosquitoes can smell out blood, flies can smell out rotting food and butterflies can smell nectar containing flowers.
Around two decades back Leslie Vosshall, the Robin Chemers Neustein Professor at Rockefeller, had first identified odour detecting receptors in insects. He and his team then had found that these receptors differ from the olfactory receptors in humans and other animals. The exact nature of these receptors and how they functioned was still elusive until this new research. The latest research uses electron microscopy to see how these ion channels look and understand how they function.
The authors of the study speculate that diverse range of insects has tens of millions of receptor varieties that have evolved with time. Each of these receptors can detect specific chemicals with perfect accuracy. Their functions however are similar, the authors explain. These receptors form an ion channel. This is like a passage way that connects the inside of the cell with the outside. The passage allows only charged ions or particles. The channel remains closed. It opens only when the outer gate of the channel meets the specific chemical or the odorant.
Ruta, the Gabrielle H. Reem and Herbert J. Kayden associate professor in a statement explained, “Insect odorant receptors are very likely the largest family of ion channels in nature, and they’re incredibly diverse. So we were faced with a fundamental mystery: How do you get millions of variants of a channel that are so different but that all do the same thing?”
To understand the details of the channel they looked at a subunit of the ion channel called the Orco. Each of these channels were found to be made up of one Orco protein and a odorant receptor protein that would identify the smell.
The receptor protein varied from species to species giving the receptor its specificity (flowers for butterflies and blood for mosquitoes for example).
The Orco however remains same for all species explained Ruta. Thus they focussed on Orco using cryoelectron microscopy and finally could define the structure of a channel made entirely of Orco proteins.
This channel was called the Orco homomer. The channel had a central pore or a hole that allowed ions to flow. The channel itself had four subunits that were bound at a single point using an anchor domain.
Ruta explained that the channel looked like a “bouquet with each flower held together at the center and the stalks splaying apart in different directions.”
Ruta said that once they observed the features of the Orco homomer, they could understand how it worked. Joel Butterwick, lead author on the study said that the Orco was performing two roles and was keeping everything together. It was also helping in the channel function, he explained.
The binding site of the chemical odorant evolved freely because the main structure was held fast by the Orco he said.
Ruta said that this study could pave the way to develop better insect repellents and reduce the insect vector borne diseases that plague mankind.