Mediatophore, a protein supporting quantal acetylcholine release

Abstract

After having reconstituted in artificial membranes the calcium-dependent acetylcholine release step, and shown that essential properties of the mechanism were preserved, we purified from Torpedo electric organ nerve terminals a protein, the mediatophore, able to release acetylcholine upon calcium action. A plasmid encoding for Torpedo mediatophore was introduced into cells deficient for acetylcholine release and for the expression of the cholinergic genomic locus defined by the co-regulated choline acetyltransferase and vesicular transporter genes. The transfected cells became able to release acetylcholine in response to a calcium influx in the form of quanta. The cells had to be loaded with acetylcholine since they did not synthesize it, and without transporter they could not concentrate it in vesicles. We may then attribute the observed quanta to mediatophores. We know from previous works that like the release mechanism, mediatophore is activated at high calcium concentrations and desensitized at low calcium concentrations. Therefore only the mediatophores localized within the calcium microdomain would be activated synchronously. Synaptic vesicles have been shown to take up calcium and those of the active zone are well situated to control the diffusion of the calcium microdomain and consequently the synchronization of mediatophores. If this was the case, synchronization of mediatophores would depend on vesicular docking and on proteins ensuring this process.