Cerebratulus lacteus

The giant of the flatworm world is the milky ribbon worm (Cerebratulus lacteus), which grows to 3 to 4 feet (0.9 to 1.2 m)... 

Nemertines are carnivorous and feed on annelids, crustaceans, molluscs, and occasionally fish. Certain nemertines paralyze their prey by ejecting a tenacious toxic fluid from the proboscis. Cerebratulus lacteus has been found to contain several polypeptides which were believed to be used for defensive purposes, but not for predation. Blumenthal... 

Blumenthal, K.M., and W.R. Kem Structure and Action of Heteronemertine Polypeptide Toxins. Primary Structure of Cerebratulus lacteus Toxin A-III. J. Biol. Chem. 255, 8266 (1980). 

The peptide neurotoxins of nemerteans have been studied in detail only for Cerebratulus lacteus. These toxins can be divided into two groups toxins A, of approximately 11000 Da, and the lighter toxins B, with a relative molecular mass of about 6000 Da. The first group are toxic to most marine animals, whereas the latter are toxic only to mollusks. The primary structures of some of them (A-IIl, B-II, B-IV) have been determined. As an example, the sequence of the 55 amino acids of toxin B-IV is shown below. The tryptophan at position 30 has a primary role in the toxicity of this compotmd (Kem, 1973, 1976 ... 

Barnham, K.J., Dyke, T.R., Kem, W.R., and Norton, R.S. (1997) Stmcture of neurotoxin B-IV from the marine worm Cerebratulus lacteus a helical hairpin cross-linked by disulfide bonding. J. Mol. Biol, 268,886-902. 

Kem, W.R. (1976) Purification and characterization of a new family of polypeptide neurotoxins from the heteronemertine Cerebratulus lacteus (Leidy)./. Biol, Chem, 251,4184-4192. 

---