SNAP-25 is the major immunoreactive component of the brain-specific D3 protein

The rat synaptosomal membrane antigen D3, which has been studied for more than 20 years since its first demonstration as a brain-specific protein, was shown to bind monoclonal antibodies to SNAP-25, a presynaptic plasma membrane-associated protein. In addition to SNAP-25, anti-D3 antiserum also precipitated two polypeptides of 34-36 kDa and 14-16 kDa. This suggests that anti-D3 binds all three of the 20S synaptic vesicle fusion particle components, i.e. SNAP-25, syntaxin, and vesicle-associated membrane protein (VAMP)/synaptobrevin. However, the botulinum toxins C and D, which are endoproteases selective for syntaxin and VAMP/synptobrevin, respectively, were unable to remove the D3 immunoreactivity from synaptosomal membranes. Therefore, it is concluded that D3 immunoreactivity is directed only against SNAP-25.