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

The roles of Epiphyas postvittana GOBP2 in odour detection 76
namely BmPBP1 and HvPBP2 (Große-Wilde et al., 2006; Große-Wilde et al., 2007).
In these studies, the PBPs were shown to be able to replace the DMSO as a
solubilising agent for the sex pheromone components tested, and obtain functional
data for pheromone receptors when tested in a heterologous expression system.
Despite the different types of odorants GOBP2 binds when compared to these
narrowly tuned PBPs, the EpGOBP2 was able to replace DMSO in EpOR1
characterisation assays, acting as a solubilising agent for methyl salicylate and geranyl
acetate.
When DMSO was replaced by EpGOBP2 in the Sf9 cell assay, the EC50 value
decreased (Table 3.1), indicating that in the presence of EpGOBP2, EpOR1
expressing cells were able to bind more odorants at lower concentrations compared to
when DMSO was used. This was true for both methyl salicylate and geranyl acetate
(Table 3.1). The response was not a spontaneous response to EpGOBP2 as assays
carried out with EpGOBP2 alone did not show any response (Figure 3.7), indicating
that EpGOBP2 without being exposed to its binding components is unresponsive to
EpOR1. A possible mechanism of EpGOBP2 function in the EpOR1 characterisation
assay could be that a complex is formed between the EpGOBP2 and the ligand, and
this complex interact with EpOR1. This is supported by the decrease in EC50 values
for both geranyl acetate and methyl salicylate binding to EpOR1. One hypothesis is
that EpGOBP2 might bind the ligands and release them at the membrane surface to
increase the local concentration of ligand in close vicinity of the OR. It could also be
that the observed decrease in EC50 is a result of EpGOBP2 acting as an activated
ligand, as observed for the Drosophila OBP LUSH, which upon binding its ligand,
11-cis vaccenyl acetate undergoes a conformational change (Laughlin et al., 2008).
LUSH alone in this altered state is able to bind to and activate pheromone sensitive
neurons, indicating the role of a PBP as an activated ligand. EpGOBP2 upon binding
to geranyl acetate and methyl salicylate might become activated and act as a ligand
itself and interact with the OR, hence leading to lower EC50 values.
The combined effect of either ligand binding to EpOR1 in the presence of both
DMSO and EpGOBP2 is similar to each binding EpOR1 in the presence of EpGOBP2
alone. These data indicate that the change in the EC50 values of EpOR1 binding to
methyl salicylate and geranyl acetate could be attributed to the presence of EpGOBP2