The antenna of the male B. mori silkworm moth is exquisitely sensitive to the female sex pheromone bombykol; as little as 3,000 molecules of bombykol per mL of air can evoke a behavioral response. This sensitivity requires the use of the pheromone binding protein (PBP) to carry the highly hydrophobic pheromone through the aqueous environment of the sensillary lymph to odorant receptors on the olfactory neuron.
Function
The B. mori PBP contains an interior, hydrophobic pocket into which the pheromone ligand binds. A conserved residue,
forms a hydrogen bond with the hydroxyl functional group of the pheromone; this is mutated to alanine in lepidopteran species which use acetyl esters as pheromones. Some species with multiple pheromone components possess multiple PBP isoforms, suggesting a role in odorant discrimination as well.
Once the PBP reaches the odorant receptor at the neuronal membrane, it ejects the bound pheromone molecule via a pH-dependent conformational change between the ligand-binding form at high (>6) pH and a nonbinding form at low pH. In the low-pH structure, the ligand binding cavity is filled by a C-terminal alpha helix.
