THE HIGH-AFFINITY BINDING OF CLOSTRIDIUM-BOTULINUM TYPE-B NEUROTOXIN TO SYNAPTOTAGMIN-II ASSOCIATED WITH GANGLIOSIDES G(T1B) G(D1A)/

I-125-labeled botulinum type B neurotoxin was shown to bind specifical ly to recombinant rat synaptotagmins I and II. Binding required recons titution of the recombinant proteins with gangliosides G(T1b)/G(D1a). Scatchard plot analyses revealed a single class of binding site with d issociation constants of 0.23 and 2.3 nM for synaptotagmin II and syna ptotagmin I, respectively, values very similar to those of the high- ( 0.4 nM) and low-affinity (4.1 nM) binding sites in synaptosomes. The h igh-affinity binding of neurotoxin to synaptosomes was specifically in hibited by a monoclonal antibody recognizing with the amino-terminal r egion of synaptotagmin II. These results suggest that this region of s ynaptotagmin II participates in the formation of the high-affinity tox in binding site by associating with specific gangliosides.