Mechanisms of Olfaction in Insects - ResearchSpace@Auckland ...
Mechanisms of Olfaction in Insects - ResearchSpace@Auckland ...
Mechanisms of Olfaction in Insects - ResearchSpace@Auckland ...
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The roles <strong>of</strong> Epiphyas postvittana GOBP2 <strong>in</strong> odour detection 75<br />
phenol, then the terpenes and lastly the ester (Figure 3.5). However, the difference <strong>in</strong><br />
the degree <strong>of</strong> b<strong>in</strong>d<strong>in</strong>g <strong>of</strong> these ligands is small, with only a ten-fold difference<br />
between the highest octanol, and the lowest, pentyl acetate. Nevertheless, all these<br />
compounds are plant volatiles and may play a role <strong>in</strong> E. postvittana olfaction (as<br />
discussed <strong>in</strong> chapter 2).<br />
The moderate b<strong>in</strong>d<strong>in</strong>g constants <strong>of</strong> EpGOBP2 for the range <strong>of</strong> plant volatiles tested<br />
reflect low aff<strong>in</strong>ity which supports the conclusion that GOBPs function to b<strong>in</strong>d a<br />
range <strong>of</strong> odorants. These moderate b<strong>in</strong>d<strong>in</strong>g constants <strong>of</strong> EpGOBP2 for a range <strong>of</strong><br />
volatiles are <strong>in</strong>dicative <strong>of</strong> a role <strong>in</strong> b<strong>in</strong>d<strong>in</strong>g <strong>of</strong> a range <strong>of</strong> odorants by OBPs. Previous<br />
studies <strong>of</strong> GOBPs <strong>in</strong> other lepidopterans have shown their ability to recognise plant<br />
volatiles. M. sexta GOBP2 b<strong>in</strong>ds plant odorants such as (Z)-3-hexen-1-ol, geraniol,<br />
geranyl acetate and limonene (Feng and Prestwich, 1997) with moderate aff<strong>in</strong>ities (10<br />
to 40 µM <strong>of</strong> these odorants were required <strong>in</strong> displacement assays to displace 50 % <strong>of</strong><br />
the bound pheromone analog), comparable to those obta<strong>in</strong>ed for EpGOBP2 <strong>in</strong> this<br />
study. In the honeybee Apis mellifera, the OBP called ASP2 b<strong>in</strong>ds its ligands, mostly<br />
plant scents, with high b<strong>in</strong>d<strong>in</strong>g aff<strong>in</strong>ities <strong>in</strong> the micromolar range (Briand et al., 2001).<br />
This shows that perhaps the Hymenoptera and Lepidoptera OBPs differ <strong>in</strong> their<br />
mechanisms <strong>of</strong> b<strong>in</strong>d<strong>in</strong>g their ligands. S<strong>in</strong>ce EpGOBP2 b<strong>in</strong>ds a range <strong>of</strong> compounds<br />
with different shapes and chemical structures, it can be deduced that the b<strong>in</strong>d<strong>in</strong>g<br />
pocket <strong>of</strong> EpGOBP2 must be spacious and flexible enough to harbor this varied range<br />
<strong>of</strong> compounds. The range <strong>of</strong> compounds tested <strong>in</strong> this study is limited; hence there<br />
might be other stronger b<strong>in</strong>d<strong>in</strong>g ligand(s) that have not been tested yet. Increas<strong>in</strong>g the<br />
test compound range will perhaps give <strong>in</strong>sights <strong>in</strong>to an even wider range <strong>of</strong> ligands for<br />
EpGOBP2, add<strong>in</strong>g to the observation that while the PBP recognise species specific<br />
pheromones, the GOBPs are broadly tuned to recognise a range <strong>of</strong> compounds that<br />
<strong>in</strong>cludes (and might not be limited to) plant volatiles.<br />
When EpGOBP2 was pre-<strong>in</strong>cubated with geranyl acetate or methyl salicylate, and<br />
added to Sf9 cells express<strong>in</strong>g EpOR1, a receptor activation response was observed<br />
(Figures 3.8 and 3.9), <strong>in</strong>dicat<strong>in</strong>g that EpGOBP2 was able to solubilise the odorants<br />
enabl<strong>in</strong>g it to <strong>in</strong>teract with the membrane bound EpOR1. This is the first documented<br />
role <strong>of</strong> a lepidopteran GOBP <strong>in</strong> odorant b<strong>in</strong>d<strong>in</strong>g <strong>in</strong> an <strong>in</strong> vitro OR characterisation<br />
assay. Similar reconstitution assay studies have been done with lepidopteran PBPs,