Both A chain and B chain of β-bungarotoxin are functionally involved in the facilitation of spontaneous transmitter release in Xenopus nerve–muscle cultures

Abstract

In the present study, Xenopus nerve–muscle cultures were used to explore the functional roles of A chain (a phospholipase A 2 subunit) and B chain (a non-phospholipase A 2 subunit) of Bungarus multicinctus β-bungarotoxin. It was found that β-bungarotoxin induced an increment of the frequency of spontaneous synaptic currents (SSCs) in the nerve–muscle cultures. Modification of β-bungarotoxin with pyridoxal-5′-phosphate or substitution of Ca 2+ with Ba 2+ in buffer abolished the phospholipase A 2 activity of β-bungarotoxin and the facilitatory phase of SSC as well. Antibodies that were directed specifically against A chain or B chain effectively inhibited phospholipase A 2 activity, and as a consequence the SSC frequency was not greatly different from the control rate. These results suggest that both A and B chains are indispensable parts of β-bungarotoxin for inducing the facilitation of SSC frequency with Xenopus nerve–muscle cultures.