β-Bungarotoxin blocks phorbol ester-stimulated phosphorylation of MARCKS, GAP-43 and synapsin I in rat brain synaptosomes

Abstract

The phospholipase A2 neurotoxin, β-bungarotoxin, presynaptically blocks acetylcholine release. Its mechanism of action is unknown; however, our previous studies suggest that it inhibits phosphorylation of synaptosomal proteins, which might be expected to decrease neurotransmitter release. In our present study, we found that 1 nM β-BuTX blocked phorbol ester-stimulated phosphorylation of GAP-43, MARCKS and synapsin I without affecting their basal phosphorylation. In contrast, a 1 nM concentration of the non-neurotoxic enzyme. Naja naja atra phospholipase A2 did not affect the phorbol ester-stimulated phosphorylation of these proteins but increased the basal phosphorylation of GAP-43 and MARCKS. Although it has been suggested that cytosolic calmodulin is increased by phosphorylation of the protein kinase C substrates, GAP-43 and MARCKS, we found no change in calmodulin levels by phorbol ester or β-bungarotoxin. The stimulation of phosphorylation by Naja naja atra phospholipase A2 may be due to products liberated as a result of its phospholipase A2 activity. In contrast, the inhibition of phosphorylation by β-bungarotoxin appears to be due to an action which may be unrelated its relatively weak phospholipase A2 activity. Inhibition of phosphorylation by β-bungarotoxin is a possible mechanism by which it could block acetylcholine release. Furthermore, β-bungarotoxin may be a useful tool to study the physiological role of phosphorylation of synaptosomal proteins in neurotransmitter release.