Isolation and molecular cloning of novel peptide toxins from the sea anemone Antheopsis maculata

Three peptide toxins (Am I–III) with crab toxicity were isolated from the sea anemone Anthopleura maculata by gel filtration and reverse-phase HPLC. Am I was weakly lethal to crabs (LD ₅₀ 830 μg/kg) and Am III was potently lethal (LD ₅₀ 70 μg/kg), while Am II was only paralytic (ED ₅₀ 420 μg/kg). The complete amino acid sequences of the three toxins were determined by cDNA cloning based on 3‧-Race and 5‧-Race. Although Am III (47 residues) is an analogue of the well-known type 1 sea anemone sodium channel toxins, both Am I (27 residues) and II (46 residues) are structurally novel peptide toxins. Am I is a new toxin having no sequence homologies with any toxins. Am II shares 28–39% identity with the recently characterized sea anemone toxins inhibiting specialized ion channels, BDS-I and II from Anemonia sulcata and APETx1 and 2 from Anthopleura elegantissima. The precursor proteins of the three toxins are commonly composed of a signal peptide, a propart with a pair of basic residues (Lys-Arg) at the end and the remaining portion. Very interestingly, the Am I precursor protein contains as many as six copies of Am I.