On the function of the pheromone binding protein in the olfactory hairs of Antheraea polyphemus

Before a pheromone molecule of Antheraea polyphemus can stimulate a dendrite of a sensillum trichodeum, it has to pass through an aqueous solution, the “sensillum lymph”. Furthermore, after such a stimulation it has to be rapidly inactivated to prevent continuous stimulation of the dendrite. In the sensillum lymph a pheromone binding protein in mM concentrations is present. The literature describes two hypotheses concerning the function of the binding protein: one hypothesis claims that the latter serves to carry the pheromone through the sensillum lymph, while the other assumes a migration of the pheromone to the dendrite through the pore tubuli and an inactivator function of the binding protein. By means of a perfusion technique, the dendrites of sensilla trichodea were bathed in a solution of pheromone [(E,Z)-6,11-hexadecadienyl acetate] in Ringer with and without binding protein or bovine serum albumin for comparison. The responses of the cells were measured electrophysiologically. To elicit nerve impulses, a pheromone concentration of 180 nM in Ringer was necessary. Addition of binding protein or bovine serum albumin decreased the threshold concentration by a factor of 100. It was calculated that with such a pheromone concentration (1.8 nM) plus binding protein less than 1000 pheromone molecules within the hair produced a strong cell response, although in equilibrium 99.96% of these molecules are bound to the binding protein. It was therefore concluded that pheromone binding protein can act as a solubilizer or carrier.