Newly identified water-borne protein pheromones interact with attractin to stimulate mate attraction in Aplysia

The water-borne protein attractin is a potent sex pheromone involved in forming and maintaining mating and egg-laying aggregations in the marine mollusk Aplysia. Binary blends of attractin and either enticin, temptin, or seductin, three other Aplysia protein pheromones, stimulate mate attraction. The four pheromones are thought to act in concert during egg-laying. The new data presented here show that: (1) the water-borne odor of non-laying Aplysia brasiliana further increases the attractiveness of attractin and of eggs in T-maze bioassays. This suggests that individual Aplysia release additional factors that enhance the effects of attractin, enticin, temptin, and seductin during egg-laying; (2) the N-terminal region of enticin aligns well with the conserved epidermal growth factor (EGF)-like domain of mammalian reproductive proteins known as fertilins, which may mediate intercellular adhesion interactions between eggs and sperm; (3) temptin, according to fold recognition servers, may also have an EGF-like fold. Enticin and temptin also have conserved metal binding sequences that may play a role in their signaling behavior. These results suggest that aspects of mammalian egg-sperm interactions (fertilins) may have evolved from pheromonal signaling mechanisms. We also review the structure, expression, localization, release, and behavioral actions of attractin, enticin, temptin, and seductin.