BIOCH1MIE, 1979, 61, 719-723. Amino acid sequence of a postsynaptic neurotoxin the venom of the Australian tiger snake Notechis scutatus scutatus. t om James HALPERT, Jan FOHLMAN and David E~KER. Institute o[ Biochemistry, University o[ Uppsala, Box 576, S-751 23 Uppsala I, Sweden. R6sum6, Summary. Dans le venin du serpent tigre (IVotechis scutatus scutatus), les phospholipases basiques myotoxiques et neurotoxiques presynaptiques, comme la notexine et Notechis II-5, et d'autres homologues de phospholipase comme IVotechis ILl, forment 60 p. cent des prot6ines. Toutefois, on trouve aussi en faibles quantit6s plusieurs neurotoxines - curaromim6tiques ,, post-synap- tiques. Parrot eUes, la prot6ine majeure est une neurotox/ne ,, longue ,, typique, contenant 73 acides amin6s organis6s en une seule chaine polypeptidique pr6sentant 5 ponts disulfure. Le poids mol6culaire calcul6 d'apr~s la s6quence d'acides amin6s est 8051. La DLso (injection in- traveineuse chez la souris) est de 125 ~g/kq. Although 60 percent of the protein in tiger snake (Notechis scutatus scutatus) venom con- sists of the basic pre-synaptically neurotoxic and myotoxic phospholipases notexin and No- techis II-5 and other phospholipase homoloqs such as Notechis ILl, several post-synaptic ,, curaremimetic ,, neurotoxins are present in small amounts. The major one of these is a ty- pical ,, long ,, neurotoxin containing 73 amino acids in a single peptide chain cross-linked by five disulfide bridges. The formula weight cal- culated from the amino acid sequence is 8,051. The LDs0 for intravenous injection into mice is 125 ~g/kq. Key words : amino acid sequence, snake, venom, neurotoxin. Introduction. The Australian tiger snake Notechis scutatus scutatus is one of lhe world's most poisonous snakes. Our interest in the venom has been direc- ted mainly towards the specialized presynapti- cally neuroloxic phospholipases A, notexin and Notechis II-5, which block neuromuscular trans- mission by interfering with the release of acetyl- choline from motor nerve termin.als [1-4]. N~texin and Notechis II-5 are also potent myotoxins [5], and considerable evidence indicates that the phos- pholipase A activity is implicated in both the neu- rotoxic and myo~toxic actions [6, 7]. We have already reported that the venom also contains small amounts of several postsynaptic neurotoxins [1, 8], and although at least 50 such neurotoxins have been sequenced [9] no one has yet reported the sequence of ,any postsynaptic neurotoxin from an Australian elapid. We have therefore determined the amino acid sequence of the principal postsyn,aptic neurotoxin from tiger snake venom, designated Notechis Ill-4, to see how it compares with the many cobra, krait, and sea snake toxins for which sequences are already available. Notechis Ill-4 is a << tong >) type neuro- toxin containing 73 amino acid residues and 5 di- sulfide bridges. The formula weight is 8 051. Methods. The isolation of the neurotoxin by gel filtration of the crude tiger snake venom (Eric Worrell's Australian Reptile Park, P.O. Box 192, Gosford, N.S.W. 2250, Australia) on Sephadex G-75 followed by gradient ion exchange chromatography of fraction III on Bio-Rex 70 has been described earlier [fl]. Reduction and S-carboxymethylation, digestion with proteolytic en- zymes, purification of peptides, manual Edman degra- dation, identification of phenylthiohydantoin deriva-