Binding of botulinum type B neurotoxin to Chinese hamster ovary cells transfected with rat synaptotagmin II cDNA

We have previously identified synaptotagmin, a synaptic vesicle membrane protein from rat brain, as a binding protein for Clostridium botulinum type B neurotoxin. In this report, rat synaptotagmin II was expressed by transfection in Chinese hamster ovary cells and interaction with the neurotoxin was studied. In stable transfectants, the NH₂-terminal region of synaptotagmin was exposed to the extracellular medium. Synaptotagmin-expressing cells were shown to possess an extremely low binding activity for the radioiodinated toxin. However, toxin-binding was markedly increased to cells which had been treated with gangliosides G(T1b) or G(D1a). In synapses, the intravesicular NH₂-terminus of synaptotagmin becomes exposed at the cell surface after following exocytosis. These findings suggest that the NH₂-terminal domain of synaptotagmin II forms the binding site for type B neurotoxin by associating with specific gangliosides in presynaptic plasma membranes.