Purification of synaptic ribbons, structural components of the photoreceptor active zone complex

Synaptic ribbons are plasma membrane-associated structural elements in photoreceptor synaptic terminals. They seem to act as high capacity "docking sites" of synaptic vesicles that provide the fusion sites of the photoreceptor synapse ("active zones"), with a large supply of immobilized synaptic vesicles rapidly available for exocytosis. Synaptic ribbons are regarded as a specialized type of presynaptic densities found in virtually all synapses. The molecular composition of presynaptic densities and synaptic ribbons is unknown. The aim of this study was the isolation of synaptic ribbons from photoreceptor synapses. For this purpose, we first isolated a membrane fraction from the bovine retina that was strongly enriched in photoreceptor synapses. From this fraction, a Triton X-100-resistant subfraction was purified that consisted mainly of synaptic ribbons and their disassembly products. The high enrichment of synaptic ribbons was verified by electron microscopy and immunolabeling using an antibody that specifically binds to synaptic ribbons. SDS-PAGE analysis of this synaptic ribbon fraction displayed several major polypeptide bands migrating at approximately 240, 60, 55, 43, and 30 kDa. The purification procedure described here is a first promising step toward the identification of the yet unknown constituents of synaptic ribbons from photoreceptor synapses and possibly also of presynaptic densities from other synapses.