Insects heavily depend on their ability to detect chemical cues from the environment, which are involved in mediating a wide range of behaviors, from communication between conspecific individuals to the recognition of specific features of the environment, such as food sources, oviposition sites, etc. These semiochemicals are normally buried in complex mixture of odorants from a myriad of sources. This has led to the development of remarkably selective and sensitive olfactory system, which approach the theoretical limit for a detector. In order to convey their messages, pheromones and other semiochemicals must reach the dendrites of olfactory receptors neurons, where G protein-coupled olfactory receptors (Clyne et al., 1999; Vosshall et al., 1999; Vosshall et al., 2000) are supposedly located. The last part of the journey towards the receptors is protein-assisted given that dendrites are surrounded by sensillar lymph and pheromones are largely hydrophobic in nature. Although pheromone-binding proteins seem to play a significant part in the selectivity of the olfactory system, it is unlikely that in scarab beetles they are "chiral filters" (Wojtasek et al., 1998). By contrast, pheromone-degrading enzymes (esterases) are probably stereospecific (Leal, unpublished). Our recent structural biology studies (Sandler et al., 2000) suggest that odorant-binding proteins not only ferry the ligands to their receptors, but also protect the chemical signals from odorant-degrading enzymes (Vogt and Riddiford, 1981; Vogt et al., 1985). The ligands (pheromones and other semiochemicals) are released from the PBP-pheromone complexes upon interaction with negatively-charged membrane surfaces, which leads to conformational changes (Leal, 2000; Wojtasek and Leal, 1999) and opening of the binding site (Sandler et al., 2000).
P. J. Clyne, C. G. Warr, M. R. Freeman, D. Lessing, J. Kim, and J. R. Carlson. A novel family of divergent seven-transmembrane proteins: Candidate odorant receptors in Drosophila. Neuron 22, 327-338 (1999).
W. S. Leal. Duality monomer-dimer of the pheromone-binding protein from Bombyx mori. Biochem. Biophysic Res. Commun. 268, 521-529 (2000).
B. H. Sandler, L. Nikonova, W. S. Leal, and J. Clardy. Sexual attraction in the silkworm moth: structure of the pheromone-binding protein-bombykol complex. Chemistry & Biol. 7, 143-151 (2000).
R. G. Vogt, and L. M. Riddiford. Pheromone binding and inactivation by moth antennae. Nature 293, 161-163 (1981).
R. G. Vogt, L. M. Riddiford, and G. D. Prestwich. Kinetic properties of a sex pheromone-degrading enzyme: The sensillar esterase of Antheraea polyphemus. Proc. Natl. Acad. Sci. USA 82, 8827-8831 (1985).
L. B. Vosshall, H. Amrein, P. S. Morozov, A. Rzhetsky, and R. Axel. A spatial map of olfactory receptor expression in the Drosophila antenna. Cell 96, 725-736 (1999).
L. B. Vosshall, A. L. Wong, and R. Axel. An olfactory sensory map in the fly brain. Cell 102, 147-159 (2000).
H. Wojtasek, B. S. Hansson, and W. S. Leal. Attracted or repelled: A matter of two neurons, one pheromone binding protein, and a chiral center. Biochem. Biophys. Res. Commun., 250, 217-222 (1998).
H. Wojtasek, and W. S. Leal. Conformational change in the pheromone-binding protein from Bombyx mori induced by pH and by interaction with membranes. J. Biol. Chem. 274, 30950-30956 (1999).
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA