Fire ant odorant-binding proteins

Odorant-binding proteins (OBPs) are expressed in the antennae and other tissues of many insects and have essential roles in odor and pheromone reception. No OBPs have been previously detected in ant antennae. In the red imported fire ant, Solenopsis invicta, an OBP known as Gp-9 is expressed in the thorax. There are two different alleles: Gp-9B and Gp-9b. Monogyne colonies are homozygous (BB), and polygyne colonies are heterozygous, at Gp-9. We have identified at least ten unique S. invicta OBP sequences in the Lausanne fire ant EST library. Most, including Gp-9 (SiOBP9), have little similarity to any Apis mellifera OBP. Using shotgun proteomics, we analyzed the proteins of worker antennae from polygyne fire ant colonies. We found two OBPs: SiOBP1 and the product of the B allele of SiOBP9 (Gp-9B). Previously Gp-9 had not been detected in fire ant antennae. Keller & Ross (Nature, 394, 573-575, 1998) showed that the Gp-9 genotype is associated with a transferable odor cue that allows Bb workers to distinguish BB queens from Bb queens. The presence of Gp-9B in the antennae of polygyne workers, but not Gp-9b, suggests the possibility that Gp-9B has an olfactory function. We have cloned the coding sequence for Gp-9B, expressed it from E. coli, and purified it by ion exchange and hydrophobic interaction chromatography. Purified Gp-9B binds to the fluorescent dye N-phenyl naphthylamine (NPN) with a dissociation constant of 1.0 μM. In future studies, we will use the NPN fluorescence signal to identify endogenous ligands.