Direct interaction between synaptotagmin and the intracellular loop I-II of neuronal voltage-sensitive sodium channels

Synaptotagmin, a synaptic vesicle protein involved in Ca2+-regulated exocyt osis, displayed direct high affinity interaction with neuronal sodium chann els. Monoclonal antibodies directed against synaptotagmins I and II adsorbe d in a concentration-dependent and -specific manner [H-3]saxitoxin prelabel ed sodium channels extracted with detergent from nerve endings. Conversely, co-immunoprecipitation of synaptotagmin was achieved by antibodies against sodium channel subunits. Consistent with the co-immunoprecipitation assays , solubilized [H-3]saxitoxin-prelabeled sodium channels were trapped on imm obilized maltose binding protein (MBP)-synaptotagmin. In vitro recombinant protein assays were employed to identify the interaction site of synaptotag min I, which was located on the cytoplasmic loop between domains I and II o f the sodium channel alpha IIA subunit. The co-immunoprecipitated synaptota gmin-sodium channel complexes were found to be Ca2+-dependent; this effect was mimicked by Ba2+ and Sr2+ but not Mg2+. Finally the complex was shown t o be distinct from the synaptotagmin-SNARE protein complex that can selecti vely interact with presynaptic calcium channels (N and P/Q types). Thus, ou r findings demonstrate an unexpected and direct interaction between sodium channels and synaptotagmin. The Ca2+-regulated association between sodium c hannels and a protein implicated in vesicular fusion may have intriguing co nsequences for the establishment and regulation of neuronal excitability.