Structure of a silk moth fructose receptor: an ion channel involved in insect chemosensation

Abstract

To find food and avoid toxins, insects interact with their environment by responding to both chemical and physical cues via chemosensory receptors. These receptors are essential for survival; without them, insects cannot forage for food, mate, or seek and identify hosts. Gustatory receptors (GRs), a family of chemoreceptors vastly expanded in insects, are a divergent group of non-selective cation channels that are mainly expressed in gustatory receptor neurons. In general, little is known about the function of specific GRs at the molecular level. In this thesis, I describe my investigations of Bombyx mori (Bm)Gr9 with structural, biochemical, and computational techniques to uncover the molecular basis of its biological function. BmGr9 is a fructose-gated ion channel that is expressed in major taste organs, including the legs and labial palps, and the central nervous system. In this thesis, I present an overview on how to express GRs using stable cell lines, a thorough structural analysis of BmGr9, and insights into ligand selectivity of specific GR subfamilies. I present two structures of BmGr9 in agonist-free and fructose-bound states, respectively. BmGr9 assembles as a homotetramer with a quadrivial pore. Pore opening in BmGr9 is induced by fructose binding to aromatic and polar sidechains in the ligand-binding pocket which causes a constriction of the pocket that leads to pore opening. Pocket constriction is communicated to the channel gate on helix S7b through tight coupling of helix S5, a helix that contributes to the binding pocket, and helix S7b. Understanding BmGr9 and ligand binding properties across the GR family provides insight into how this class of receptors interacts with stimuli. Conservation of properties is important for ligand binding and varies through the GR family depending on ligand class. Taken together, this research improves our understanding of insect gustation and how insect sense small molecules in the environment. GRs have been implicated in seeking and identifying hosts, and insects that are disease vectors often transfer infections while feeding on hosts. Understanding insect gustation has potential to influence the development of strategies to alter host feeding and disease transmission.