A novel SNAP25-caveolin complex correlates with the onset of persistent synaptic potentiation

We have identified synaptic protein complexes in intact rat hippocampal slices using the rapid chemical cross-linking reagent paraformaldehyde. Cellular proteins were rapidly cross-linked, solubilized, separated electrophoretically by SDS-PAGE, and then identified immunologically. Multiple complexes containing syntaxin, the synaptosomal-associated protein of 25 kDa (SNAP25), and vesicle-associated membrane protein (VAMP) were observed to coexist in a single hippocampal slice including a 100 kDa cross-linked protein complex that exhibited the same electrophoretic migration as a member of the previously identified SDS-resistant soluble N-ethylmaleimide-sensitive fusion attachment protein receptor "core" of the 20 S complex. A VAMP-synaptophysin complex, reported previously in vitro, was also observed in the hippocampal slices. This study links biochemical and physiological studies involving presynaptic proteins implicated in secretion and confirms that these proteins that have been studied extensively previously in the presence of detergent do form "bona fide" cellular complexes. Importantly, we have also detected additional novel protein complexes that do not correspond to complexes identified previously in vitro. After the induction of persistent synaptic potentiation, an abundant 40 kDa SNAP25-caveolin1 complex was observed. The SNAP25-caveolin1 complex was not abundant in control slices and, therefore, represents the first demonstration of a reorganization of protein complexes in intact hippocampal slices during the induction of synaptic potentiation. The interaction between caveolin1 and SNAP25 was confirmed biochemically by demonstration of the association of caveolin with recombinant-immobilized SNAP25 and by the coimmunoprecipitation of SNAP25 using caveolin-specific antisera. Caveolin1, like SNAP25, was observed to be abundant in isolated hippocampal nerve terminals (synaptosomes). Immunofluorescent studies demonstrated that both SNAP25 and caveolin1 are present in neurons and colocalize in axonal varicosities. These results suggest that a shortlasting SNAP25-caveolin interaction may be involved in the early phase of synaptic potentiation.