Orco is the co-receptor of insect odorant receptors (ORs), expressed in olfactory sensory neurons (OSNs) from insects antennae [1]. Its structure and function are highly conserved across insect species [2].
Function
Orco is essential for the function, assemble and membrane localization of ORs. This co-receptor binds to a specific odorant receptor (ORx) on the membrane of OSNs, forming a heteromeric complex. Thus, the ORx/Orco complex functions as a non-selective cation channel, with ORx ensuring the specificity of odor and pheromones ligands that activate the channel, contributing to signal transduction which results in behavioral changes [3][4]. In the absence of an ORx, Orco is also capable of forming functional channels activated by VUAA1 [5].
Structural highlights
In the absence of an ORx, Orco is capable of forming a , with four subunits encircling a channel. Each is composed of 7 transmembrane domains (S1-S7), an intracellular N-terminus and an extracellular C-terminus. is divided into a cytoplasmic helix (S7a) and a transmembrane helix (S7b), separated by a 15-residue β-hairpin loop. The majority of the protein is intramembranous, while are exposed to the extracellular environment, and a small intracellular domain, called the anchor domain, extends into the cytoplasm. The anchor domain is named as such because its where the majority of resides and thus 'anchoring' the subunits within the lipid membrane. In the core of the anchor domain is S7a surrounded by S4, S5 and S6. The S1-S6 domains of each subunit are narrowly tethered to their respective S7b helix and are separated from the domains of the other subunits. Each subunit contributes with a that lines the ion-conduction pathway. The pore is narrowest near the extracellular end, where two residues, Leu 473 and Val 469, forms a , preventing the passage of hydrated ions, which characterizes its closed state [6].
