Halcurin, a polypeptide toxin from the sea anemone Halcurias sp., with a structural resemblance to type 1 and 2 toxins.

The aqueous extract of the sea anemone Halcurias sp. belonging to the suborder Endocoelantheae was found to be potently lethal to crabs, although it showed neither lethal activity in mice nor hemolytic activity. A polypeptide toxin (named halcurin) with a LD50 of 5.8 micrograms/kg against crabs was isolated by gel filtration on Sephadex G-50 and reverse-phase high-performance liquid chromatography on TSKgel ODS-120T. The complete amino acid sequence of halcurin comprising 47 residues was elucidated by sequence analysis of the native molecule and its enzymatic fragment. Comparison with the known sea anemone polypeptide toxins (types 1-3), which are all from members of the suborder Nynantheae, revealed a high sequence homology (49-74%) of halcurin with type 2 toxins. Also, halcurin has several residues conserved for only type 1 toxins. These results, together with the fact the Halcurias sp. is a more primitive species than members of Nynantheae, suggest that type 1 and 2 toxins have evolved from a common ancestor with a sequence more similar to halcurin.