Mechanisms of Olfaction in Insects - ResearchSpace@Auckland ...

Functional characterisation of Epiphyas postvittana odorant receptor 1 54
odorant binding of EpOR1 can be seen in the Hill slopes of the dose response curves.
The Hill slope for geraniol is 3.2, suggesting a steep increase in binding with
increasing concentrations of the odorant, while the Hill slopes for the other
compounds are all below 1, suggesting shallower curves with a lower level of
cooperativity of the ligand into the receptor binding pocket.
EpOR1 is broadly tuned, recognising a range of compounds with different chemical
properties. Ligand responses at a concentration of 10 -5 M show the broad response
repertoire of EpOR1 (Table 2.1), from a seven carbon ester to a 15 carbon
sesquiterpene; from cyclic to acyclic compounds. This is comparable with results
obtained by Jordan et al. (2009) for EpOR3, which binds fifteen different compounds,
from a six carbon ester to a 15-carbon sesquiterpene. The broad tuning of EpOR1 at
10 -5 M is reduced to only five compounds when EpOR1 expressing Sf9 cells were
challenged with decreasing concentrations of the ten compounds listed in Table 2.1 in
a dose-dependent manner. These compounds, shown in Table 2.2 are 8-12 carbon
aromatics, with the smaller compounds (methyl salicylate, has eight carbons as
compared with geranyl acetate that has 12 carbons) eliciting higher sensitivity.
Perhaps the chemistry of the compound affects the binding specificity of the ORs, as
the results obtained by Hallem and Carlson, (2006) shows the preference for small
aromatic ring compounds by Drosophila OR49a. The activation threshold of EpOR3
by citral from Jordan et al. (2009) is 10 -15 M. This concentration is 10 5 -fold lower than
the threshold of EpOR1 for citral. The recognition of the same compound by the two
E. postvittana ORs suggests that the moth employs these two receptors in a
combinatorial manner that may help to expand the dynamic range of its olfactory
system. This combinatorial approach has also been observed in Drosophila, where a
range of ORs are able to detect the same odorants with different sensitivities, for
example, the compounds isobutyl acetate is detected by seven different ORs, and 2-
pentanone is detected by ten different ORs (Hallem and Carlson, 2006). This broad
tuning of ORs may be a mechanism for the insect to be able to detect more
compounds in various combinations, for example, to find suitable oviposition sites, so
as to lay eggs in close proximity of food source for the larvae, away from infested
plants, with the limited number of ORs that they have [from the genome sequence of
B. mori 68 ORs have been annotated so far, (Wanner et al., 2007; Tanaka et al.,
2009)]. This broad tuning of EpOR1 to its ligands as determined in the Sf9 cell assay