Chemical Communication: A Visit with Insects

Fundamental to the behavioral biology, organisms have the ability to detect and respond to chemical stimuli. Olfactory signal transduction and information processing in insects (e.g., moths) is a prime example of chemical communication found in nature for its exquisite sensitivity and selectivity. Although not completely understood yet, extensive research on the biology and chemistry of this complex event has revealed many facets of olfaction where donors, recipients, enormous pool of chemicals/stimulators, binding/carrier proteins and cellular receptors play their respective role with high precision, selectivity and sensitivity. Pheromone-binding proteins (PBPs), present in the antenna of male moth and other insect species, bind the volatile hydrophobic pheromone molecules and transport them across the aqueous sensillar lymph to the membrane-bound G protein-coupled receptor proteins. Recent structural studies on the PBP and PBP-pheromone complex have advanced our knowledge about the likely mode of ligand release and activation of pheromone receptors/odorant receptors. The pH-dependent conformational changes of the PBP play the key role in binding to the selective ligand, then shuttling and ultimately releasing the ligand to the receptor at the target cell membrane. Both the relatively higher pH of the sensillar lymph and the lower pH at the dendritic membrane are physiologically very important to foster the binding to and release of the ligand from PBP respectively. The NMR structures of the PBP at high and low pH provide evidence in support of this mechanism. However, the pH-induced structural change of pheromone-binding protein is quite different between two moths (B. mori and A. polyphemus) thus far studied.