The structural logic of insect olfactory receptor assembly and gating
Insects detect the chemical world using a large family of odorant-gated ion channels, each formed from a variable odorant-binding subunit (OR) and a single conserved co-receptor Orco. This modular organization is thought to allow tuning ORs to diversify their chemical recognition while Orco provides structural stability to the heteromer. Yet Orco can be autonomously activated by synthetic agonists, suggesting that it may contribute to channel gating rather than serving solely as a structural scaffold. Here, we combine cryo-electron microscopy with analyses of receptor stoichiometry and function to define the structural logic underlying Orco-OR assembly and gating. We show that Orco retains the canonical ligand-binding pocket of ORs but is chemically insulated from environmental odorants by a phospholipid that occupies this site. The Orco agonist VUAA4 instead binds a membrane-accessible crevice adjacent to the gate, defining a distinct site of allosteric modulation. We further demonstrate that Orco-OR heteromers can assemble in multiple stoichiometries through shape complementarity within the intracellular anchor domain and resolve structures with both a 3:1 and 2:2 architecture. Receptors constrained to a 2:2 stoichiometry are functional but productive gating requires cooperative engagement of multiple subunits within the heteromer. Comparison with the distinct gating states of a basal homomeric olfactory receptor suggests that existing Orco-OR structures capture nonconductive intermediates within the broader conformational landscape of this receptor family. Together, these findings suggest how Orco can flexibly assemble and function with highly divergent ORs, acting not simply as a structural scaffold but as an integral partner in cooperative channel gating, thereby enabling the extraordinary diversification of insect olfactory receptors.