VAT‐1 from Torpedo electric organ forms a high‐molecular‐mass protein complex within the synaptic vesicle membrane

VAT‐1 is an abundant 41‐kDa protein from Torpedo cholinergic synaptic vesicles. Most of VAT‐1 immunoreactivity (70%) is localized to the synaptic vesicle membrane while the rest (30%) copurifies with larger membranous fragments. VAT‐1 forms a high‐molecular‐mass complex within the synaptic vesicle membrane. The Stokes radius of the VAT‐1 complex is 4.85 nm and the sedimentation coefficient is 8.0 × 10⁻¹³ S. Using these values, the calculated apparent mass of the VAT‐1 complex is 176 kDa and the friction coefficient is consistent with that for a globular protein. Electrophoresis of solubilized synaptic vesicle proteins following cross‐linking resulted in a 40‐kDa ladder which was detected by VAT‐1 antibodies. This is in accord with VAT‐1 protein complex being composed primarily of VAT‐1 subunits. The hydrodynamic characteristics of the VAT‐1 protein complex suggest that it is composed of three or four VAT‐1 subunits. Synaptophysin, an abundant component of Torpedo synaptic vesicle membranes, which has a similar apparent size as VAT‐1, is not part of the VAT‐1 protein complex. Interactions between the subunits within the protein complex do not depend on disulfide bonds or on lowering the ionic strength. However, partial dissociation of VAT‐1 subunits from the complex occurs by chelating calcium ions.