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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General Introduction 9<br />
OBPs have also been implicated <strong>in</strong> protect<strong>in</strong>g the bound ligand from degradation. In<br />
A. polyphemus, a pheromone degrad<strong>in</strong>g enzyme (PDE) has been isolated and shown<br />
to effectively degrade the pheromone <strong>in</strong> vitro (Vogt et al., 1985). However, this<br />
activity is reduced <strong>in</strong> the presence <strong>of</strong> PBP, an <strong>in</strong>dication that the PBP is protect<strong>in</strong>g the<br />
pheromone from the PDE (Vogt and Riddiford, 1986). This is also supported by <strong>in</strong><br />
vivo experiments <strong>in</strong> B. mori and A. polyphemus that show two different rates <strong>of</strong><br />
pheromone degradation, with 17% <strong>of</strong> the pheromone degraded <strong>in</strong>itially and the<br />
rema<strong>in</strong><strong>in</strong>g 83% degraded at a slower rate. One explanation for this might be the<br />
formation <strong>of</strong> PBP/pheromone complex as the pheromone enters the sensillum lymph<br />
and its consequent protection from PDE (Kasang, 1971; Kasang and Kaissl<strong>in</strong>g, 1972;<br />
Kasang, 1973; Kasang et al., 1988; Kasang et al., 1989a; Kasang et al., 1989b).<br />
In vivo b<strong>in</strong>d<strong>in</strong>g assays provide evidence that the pheromone/PBP complex <strong>in</strong>teracts<br />
with the receptor molecules. Syed et al. (2006) expressed BmOR1 <strong>in</strong> the empty-<br />
neuron system, a mutant fly stra<strong>in</strong> that is devoid <strong>of</strong> its native receptors <strong>in</strong> the ab3A<br />
neuron, and showed that receptor activity was achieved when the neuron was<br />
stimulated with bombykol, albeit with low sensitivity. This sensitivity was enhanced<br />
by the co-expression <strong>of</strong> the B. mori PBP, <strong>in</strong>dicat<strong>in</strong>g that the PBP is <strong>in</strong>volved <strong>in</strong> the<br />
pheromone/receptor <strong>in</strong>teraction. Selective response <strong>of</strong> receptor neurons to pheromone<br />
components have been observed <strong>in</strong> the presence <strong>of</strong> different PBPs. A. polyphemus<br />
sensilla trichodea express<strong>in</strong>g the ORN tuned to the pheromone component (E,Z)-6,11-<br />
hexadecadienyl acetate elicited a response upon stimulation with the pheromone<br />
solubilised <strong>in</strong> DMSO, ApolPBP1 and ApolPBP3 (Poph<strong>of</strong>, 2004). However, no<br />
response was elicited when the pheromone was solubilised <strong>in</strong> ApolPBP2, giv<strong>in</strong>g<br />
further evidence <strong>of</strong> pheromone/PBP complex <strong>in</strong>teraction with PR. Evidence for such<br />
pheromone/PBP complex <strong>in</strong>teractions have also been shown <strong>in</strong> vivo and <strong>in</strong> vitro <strong>in</strong> B.<br />
mori. When solubilised <strong>in</strong> DMSO, both bombykol and bombykal b<strong>in</strong>d BmOR1 but<br />
when DMSO is replaced with BmPBP, only the bombykol/BmPBP complex is able to<br />
activate BmOR1 (Große-Wilde et al., 2006).<br />
Interaction <strong>of</strong> the pheromone/PBP complex with receptor molecules has also been<br />
shown <strong>in</strong> flies. The Drosophila OBP LUSH has been suggested to <strong>in</strong>teract directly<br />
with ORs (Kim et al., 1998). Flies mutant for LUSH were previously shown to have<br />
odour defects <strong>in</strong> that they were unable to avoid high concentrations <strong>of</strong> alcohol, as