Association of syntaxin with SNAP-25 and VAMP (synaptobrevin) during axonal transport

Two proteins of the presynaptic plasma membrane, syntaxin and SNAP-25, and a synaptic vesicle membrane protein, VAMP/synaptobrevin, form stable protein complexes that are involved in the docking and fusion of synaptic vesicles at the presynaptic membrane. These protein complexes have also been described in a homogeneous population of cholinergic synaptosomes purified from Torpedo electric organ. In the present study, we performed similar experiments combining velocity sedimentation and immunoprecipitation on control or ligated electric nerves and found it was possible to distinguish syntaxin that is in the axonal plasma membrane from syntaxin that is transported by the fast anterograde axonal flow. Although syntaxin that is resident in axonal membranes is associated with SNAP-25 but not with VAMP, syntaxin that accumulates proximally to a ligature is associated with both SNAP-25 and VAMP in a stoichiometry very similar to that of the nerve terminal complex. In control nerves, lower amounts of syntaxin form a complex with VAMP, in proportion to syntaxin, which is conveyed by the axonal flow. Because added VAMP was unable to associate with syntaxin in solubilized control nerves and because neither solubilization by SDS nor dilution to the nanomolar range of syntaxin and VAMP concentrations before solubilization change the stoiechiometry between the immunoprecipitated proteins, this complex appears to be both formed prior to solubilization and stable thereafter. Hence, heterotrimeric complexes containing syntaxin, SNAP-25, and VAMP are already formed during fast anterograde axonal transport, before reaching the nerve endings.