Odorant-
binding proteins (OBPs), also named
pheromone-
binding proteins when the odorant is a
pheromone, are essential for insect olfaction. They solubilize odorants that reach the port of entry of the olfactory system, the pore tubules in antennae and other olfactory appendages. Then, OBPs transport these hydrophobic compounds through an aqueous sensillar lymph to receptors embedded on dendritic membranes of olfactory receptor neurons. Structures of OBPs from mosquito species have shed new light on the mechanism of transport, although there is considerable debate on how they deliver odorant to receptors. An OBP from the southern house mosquito, Culex quinquefasciatus, binds the hydrophobic moiety of a mosquito oviposition
pheromone (MOP) on the edge of its binding cavity. Likewise, it has been demonstrated that the orthologous
protein from the
malaria mosquito binds the
insect repellent DEET on a similar edge of its binding pocket. A high school research project was aimed at testing whether the orthologous
protein from the
yellow fever mosquito, AaegOBP1, binds
DEET and other
insect repellents, and MOP was used as a positive control. Binding assays using the fluorescence reporter N-phenyl-1-naphtylamine (NPN) were inconclusive. However, titration of NPN fluorescence emission in AaegOBP1
solution with MOP led to unexpected and intriguing results. Quenching was observed in the initial phase of titration, but addition of higher doses of MOP led to a stepwise increase in fluorescence emission coupled with a blue shift, which can be explained at least in part by formation of MOP
micelles to house stray NPN molecules.