Mate attraction in Aplysia involves a long-distance water-borne signal (the protein pheromone attractin), which is released during egg laying. Aplysia californica attractin attracts species that produce closely related attractins, such as Aplysia brasiliana, whose geographic distribution does not overlap that of A. californica. This finding suggests that other mollusks release attractin-related pheromones to form and maintain breeding aggregations. We describe four additional members of the attractin family: A. brasiliana, Aplysia fasciata, Aplysia depilans (which aggregates with A. fasciata aggregations), and Aplysia vaccaria (which aggregates with A. californica aggregations). On the basis of their sequence similarity with A. californica attractin, the attractin proteins fall into two groups: A. californica, A. brasiliana, and A. fasciata (91-95% identity), and A. depilans and A. vaccaria (41-43% identity). The sequence similarity within the attractin family, the conserved six cysteines, and the compact fold of the NMR solution structure of A. californica attractin suggest a common fold for this pheromone family containing two antiparallel helices. The second helix contains the IEECKTS sequence conserved in Aplysia attractins. Mutating surface-exposed charged residues within this heptapeptide sequence abolishes attractin activity, suggesting that the second helix is an essential part of the receptor-binding interface.

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Peptide and protein growth factors play critical roles in the control of proliferation, differentiation and survival of most, if not all, cell types. In this review, we describe a newly isolated growth factor from Aplysia californica, mollusk derived growth factor (MDGF), that is a member of the adenosine deaminase-related growth factor (ADGF) subfamily. Other known subfamily members from a range of invertebrate and vertebrate species include: insect-derived growth factor, Drosophila ADGFs, tsetse salivary growth factors, insect adenosine deaminases (ADAs; Lutzomyia, Culex, Aedes, Anopheles), and cat eye syndrome critical region gene 1 (CECR1) in humans, pigs, and zebrafish. ADGFs from vertebrates and invertebrates contain both an ADA domain and a novel N-terminal region of about 100 amino acids. Catalytic residues involved in ADA activity are conserved in ADGFs, and inhibitors of ADA can block ADGF activity. ADA enzymatic activity has been shown, by inhibitor and site-directed mutagenesis studies, to be related to the ability of ADGFs from many species to stimulate cell proliferation. The available evidence suggests that the conversion of adenosine to inosine (or their analogs) is important for the mitogenic actions of ADGFs. Future investigations of this novel subfamily should lead to the identification of their receptors.

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