Molluscan attractins, a family of water-borne protein pheromones with interspecific attractiveness

The marine mollusk Aplysia releases the water-borne pheromone attractin during egg laying. This small protein stimulates the formation and maintenance of mating and egg-laying aggregations. Attractin has been characterized from five Aplysia species: A. californica, A. brasiliana, A. fasciata, A. vaccaria, and A. depilans. We describe here the isolation of attractin from Bursatella leachii, and show that it belongs to the same protein family. The pattern of residue conservation, especially the six invariant cysteines, suggests that all of these attractins have a common fold. The nuclear magnetic resonance solution structure of A. californica attractin contains two antiparallel α-helices, the second of which contains the heptapeptide sequence IEECKTS that has been implicated in attractin function. Synthetic peptides containing this IEECKTS region are attractive, and mutating surface exposed charged residues within this region of attractin abolishes attractin activity. This suggests that the second helix is an essential part of the receptor-binding interface. In contrast to the peptide pheromonal attractants in amphibians, which are species specific, the attractins are, to our knowledge, the first water-borne peptide or protein pheromone family in invertebrates and vertebrates that are not species specific.