For an insect, the environment contains a great deal of chemical compounds, most physiologically irrelevant (noise), whereas others may be essential because they carry information (semiochemicals) about mate-finding, food-source, oviposition site, and many other features of the environment. Information-carrying small hydrophobic ligands must reach the olfactory receptors in order to be translated into the brain language (neuronal activity). In addition, odor-oriented navigation requires that chemical signals be deactivated (inactivated) in a millisecond timescale. There is growing evidence in the literature that this inordinate temporal precision of the insect olfactory system is determined by the early events (peripheral interactions) rather than by intracellular signaling processes (signal transduction). Three major types of proteins make the insect sense of smell so remarkable, namely, odorant-binding proteins (OBPs), odorant-degrading enzymes (ODEs), and olfactory receptors (ORs). Our recent studies unveiled some features of OBPs, which solubilize ligands and ferry them to their olfactory receptors. Upon interaction with negatively-charged membranes, the C-terminal sequence, which is an unstructured conformation in the pheromone-OBP complex, rearranges to form a regular a-helix that occupies the pheromone-binding pocket, thus "ejecting" the pheromone to the receptor. We are now testing the hypothesis that fast inactivation of chemical signals is achieved by an enzymatic process regulated by odorant-degrading enzymes. Because ODE are expressed in such low amounts that hitherto isolation and protein-based gene cloning has not been possible, the molecular basis of signal inactivation is still terra incognita. Using a bioinformatics approach, we were able to clone the first cDNAs encoding odorant-degrading enzymes, with a putative catalytic site characterized by the sequence Gly-Glu-Ser-Ala-Gly-Ala.
Keywords: odorant-binding, protein
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