Binding of beta-bungarotoxin to synaptic membrane fractions of chick brain

beta-Bungarotoxin (beta-BuTx) is a presynaptically active snake venom phospholipase A2 which, in the chick, is cytotoxic for certain subclasses of central and peripheral neurons. In order to elucidate whether the toxin's specificity is due to the existence of specific neuronal binding sites, the binding of 125I-labeled beta-BuTx to synaptic membrane fractions of chick brain was investigated. These experiments defined a single class of saturable high affinity binding sites (KD = 0.47 +/- 0.14 nM, k+1 = 4.3 x 10(6) M-1 s-1, k-1 = 1.08 x 10(-4) s-1) whose pharmacological properties correlated with that of the cytotoxic action of beta-BuTx on cholinergic neurons in chick retina cultures. The density of 125I-beta-BuTx binding sites in synaptic membrane fractions was low (50 fmol/mg of protein). Also specific toxin binding occurred only to membrane fractions of chick organs known to contain beta-BuTx-sensitive cells or nerve endings, i.e. brain, retina, and muscle, but not liver and heart. The binding of 125I-beta-BuTx to synaptic membrane fractions was dependent on Ca2+; Co2+ and Sr2+, but not Mg2+, could replace Ca2+ in the binding reaction. The membrane binding site for 125I-beta-BuTx was sensitive to heat and high concentrations of pronase, and thus most likely is a protein.