Aplysia attractin: Biophysical characterization and modeling of a water-borne pheromone

Attractin, a 58-residue protein secreted by the mollusk Aplysia californica , stimulates sexually mature animals to approach egg cordons. Attractin fro m five different Aplysia species are similar to 40% identical in sequence. Recombinant attractin, expressed in insect cells and purified by reverse-ph ase high-performance liquid chromatography (RP-HPLC), is active in a bioass ay using A. brasiliana; its circular dichroism (CD) spectrum indicates a pr edominantly a-helical structure. Matrix-assisted laser desorption/ionizatio n mass spectrometry (MALDI-MS) characterization of proteolytic fragments id entified disulfide bonds between the six conserved cysteines (I-VI, Il-V, I ll-IV, where the Roman numeral indicates the order of occurrence in the pri mary sequence). Attractin has no significant similarity to any other sequen ce in the database. The protozoan Euplotes pheromones were selected by fold recognition as possible templates. These diverse proteins have three cr-he lices, with six cysteine residues disulfide-bonded in a different pattern f rom attractin. Model structures with good stereochemical parameters were pr epared using the EXDIS/DIAMOD/FANTOM program suite and constraints based on sequence alignments with the Euplotes templates and the attractin disulfid e bonds. A potential receptor-binding site is suggested based on these data . Future structural characterization of attractin will be needed to confirm these models.