Figure 2.

Olfactory receptors. Model of a hypothetical heterotetrameric complex of D. melanogaster Or22a and the co-receptor Orco (two subunits each). The approximate position of the plasma membrane is indicated in the side view. In Or22a subunits, the residue highlighted in white (M93) is a major contributor to defining behaviourally relevant odour response differences between D. melanogaster and D. sechellia Or22a orthologues [34]; this residue is located within the putative odour-binding site [35]. The ion channel pore is formed at the interface of all four subunits [36]. Models of protein monomers were predicted by AlphaFold2 [37,38]; these exhibit very strong similarity to cryo-electron microscopic (cryoEM) structures of Ors from other insects [35,36]. Models were aligned to the cryoEM structure of the Orco homotetramer from the fig wasp (Apocrypta bakeri) [36] using Coot [39] and visualized in PyMol v. 2.5.4. Although the stoichiometry of Or/Orco complexes is unknown, evidence suggests that they contain at least two tuning Or subunits [32,40].