The three-dimensional structure of erabutoxin b, a neurotoxin in the venom of 
the sea snake Laticauda semifasciata, has been determined from a 2.75 A 
resolution electron density map. Erabutoxin b is one of a family of snake venom 
neurotoxins, all low-molecular-weight proteins, which block neuromuscular 
transmission at the postsynaptic membrane. They specifically inhibit the 
acetylcholine receptor. The molecular shape is that of a shallow elongated 
saucer with a footed stand formed by the six-membered ring at the COOH-terminal 
end. The central core of the molecule is an assembly of four disulfide bridges. 
Three long chain loops emerge as broad fronds from the core region. 
Approximately 40% of the main chain is organized into a twisted antiparallel 
beta-pleated sheet of five short strands. In 28 snake venom neurotoxins of 
established sequence which inhibit the acetylcholine receptor, the four 
disulfide bridges and seven other residues remain invariant. Three substitution 
positions conserve residue type. In one wing of the molecule, there is a broad 
shallow depression which may characterize the reactive site. It is populated by 
the sevent invariant residues and two of the three type conserved residues. This 
region is "anchored" on the undersurface of the molecule by the hydroxyl group 
of Ser-9, the remaining conservatively substituted residue.