Redox-shift of the pheromone-binding protein in the silkmoth Antheraea polyphemus

In pheromone-sensitive hairs of the male silkmoth Antheraea polyphemus, two electrophoretically distinct pheromone-binding proteins (PBPs) are present. They indicate no amino acid sequence diversity according to peptide mapping, but differ in their redox state, as shown by free-sulfhydryl-group-specific cleavage at cysteine residues with 2-nitro-5-thiocyanobenzoic acid. In kinetic studies, the pheromone was initially bound mainly by the reduced PBP but later by the oxidized PBP, where all six cysteine residues form disulfide bonds. This redox shift was observed only in the homogenate of isolated olfactory hairs, where proteins of the sensillum lymph and receptive dendrites are present. In control experiments with purified binding proteins, the proportion of pheromone bound to the oxidized PBP did not increase with increasing incubation time, suggesting that disulfide formation does not occur spontaneously but is mediated by the sensory hairs, possibly by interaction with the receptor cell membrane. These data suggest that arriving hydrophobic pheromone molecules are first bound by the reduced PBP and transported through the aqueous sensillum lymph towards the receptor molecules of the dendritic membrane. The oxidized complex might not be able to activate further receptors and, thus, effectively deactivate the pheromone molecules within the sensillum lymph.