Insect odorant receptors function as heteromeric odorant-gated cation channels 
comprising a conventional odorant-sensitive tuning receptor, and a conserved 
co-receptor (Orco). An Orco agonist, VUAA1, is able to activate both heteromeric 
and homomeric Orco-containing channels. Very little is known about specific 
residues in Orco that contribute to cation permeability and gating. We 
investigated the importance of two conserved Asp residues, one in each of 
transmembrane domains 5 and 7, for channel function by mutagenesis. Drosophila 
melanogaster Orco and its substitution mutants were expressed in HEK cells and 
VUAA1-stimulated channel activity was determined by Ca(2+) influx and whole-cell 
patch clamp electrophysiology. Substitution of D466 in transmembrane 7 with 
amino acids other than glutamic acid resulted in a substantial reduction in 
channel activity. The D466E Orco substitution mutant was ~2 times more sensitive 
to VUAA1. The permeability of the D466E Orco mutant to cations was unchanged 
relative to wild-type Orco. When D466E Orco is co-expressed with a conventional 
tuning odorant receptor, the heteromeric complex also shows increased 
sensitivity to an odorant. Thus, the effect of the D466E mutation is not 
specific to VUAA1 agonism or dependent on homomeric Orco assembly. We suggest 
the gain-of-activation characteristic of the D466E mutant identifies an amino 
acid that is likely to be important for activation of both heteromeric and 
homomeric insect odorant receptor channels.