Structure and action of heteronemertine polypeptide toxins. Specific cross-linking of Cerebratulus lacteus toxin B-IV to lobster axon membrane vesicles

The binding of the crustacean-selective protein neurotoxin, toxin B-IV, from the heteronemertine Cerebratulus lacteus, to lobster axonal and muscle membranes has been studied. Synthesis of a radioactive bifunctional cross-linking reagent, ¹²⁵I-azidosalicylic acid (ASA) B-IV, has allowed these studies as well as experiments that show cross-linking of toxin B-IV to its receptor in axonal membranes. In the absence of photolysis ¹²⁵I-ASA-B-IV binds to vesicles with an apparent K_d of 30 nM and maximal binding of 7.5 pmol per mg membrane protein. Photolysis of the toxin-receptor complex at 366 nm greatly diminishes the rate of dissociation of bound toxin B-IV. Photolysis also results in the specific cross-linking to axonal proteins of molecular masses 38 and 40 kDa. This cross-linking is not observed in the presence of micromolar unlabeled toxin, in the absence of photolysis or in the presence of 150 mM K⁺. There is no evidence of cross-linking to proteins of higher molecular weight. The radiolabeled toxin B-IV was also found to bind to lobster muscle membranes with a dissociation constant of 500 nM and a maximum binding of approx. 4.50 pmol per mg membrane protein.