This thesis reports the experimental results of my research in the field of insect chemoreception. The focus is on biochemical events occurring at the periphery of the olfactory system, where odorants and pheromones are detected and discriminated. In particular, I have studied a class of soluble proteins that very recently have been shown to represent the key elements in insect chemosensing. As suggested by their name, Odorant-binding proteins (OBPs) bind chemicals present in the environment and convey the information to membrane-bound receptors, where the chemical messages are translaed into electric signals to be processed by the brain.
For my experimental work I chose species very different in their biology, chemical ecology and phylogenetic position, aphids and mosquitoes. Both are of great economical importance, aphids represent one of the major pests in agriculture, not only for their direct damage to plants, but also as carriers of deadly microorganisms. Mosquitoes are vectors of serious diseases worldwide, causing enormous numbers of victims particularly among the populations of developing countries.
In aphids, we have cloned and characterised for the first time 4 OBPs in different aphid species. Three of them have been expressed in bacterial system and functionally studied. Unlike OBPs in other insects, those of aphids are well conserved across even very distant species. Ligand-binding experiments have shown similar behaviour of the three proteins towards several organic compounds, but also some signiﬁcant selectivities. In particular, (E)-β-farnesene, the aphids alarm pheromone and its related compound farnesol exhibited good afﬁnity to OBP3, but did not bind the other two proteins. On the basis of such results we can suggest that OBP3 mediates the typical response of aphids to the alarm pheromone and propose a strategy for the design and synthesis of new repellents for aphids.
In the malaria mosquito, Anopheles gambiae, seven OBPs have been expressed, purified and characterised for their ligand-binding properties. Binding experiments performed with mixtures of two OBPs of An. gambiae, suggested that OBP4 could form heterodimers with OBP1 and OBP3, thus generating new protein species with different properties. Such phenomenon could also occur in vivo, based on the observation that the genes encoding OBP1 and OBP4 are co-expressed in the same sensillum. Western blot has revealed that OBP47, an anomalous member of this class of proteins, being longer and more complex, is glycosylated in the mosquito. This is the first report of glycosylation in insect OBPs. The same protein was also successfully expressed both in bacterial and yeast systems. A highly purified samples has been crystallised by our collaborator, Christian Cambillau, Marseille, and its folding has been solved, being the first three-dimensional structure of a C-plus OBP. Moreover, the expression of OBPs has been mapped in the antennae of An. gambiae using a proteomic approach in collaboration with the group of CISM, University of Firenze, revealing that only two OBPs are expressed at relatively high levels in male antennae of An. gambiae, while at least a dozen are expressed in the female antennae.
A parallel work investigated the expression of the most abundant OBP in the sex organs of An. gambiae, as well as of another mosquito species, Aedes albopictus. In both species this protein (OBP9 in An. gambiae, OBP22 in Ae. albopictus) is produced in the male sex organs and transferred to the female during mating. In the latter species, OBP22 is complexed with nonanal, a new putative male pheromone. This work supports the idea that, as in vertebrates, also in insects OBPs may play a double role in the detection and in the release of pheromones.
The results reported in this thesis represent a major contribution to the characterisation of OBPs in insect and provide strong evidence that OBPs are directly and specifically involved in insect chemical communication. In particular, controlling the populations of dangerous insects can be best achieved after the fine and specific interactions between semiochemicals and their binding proteins are completely understood and clarified.