In vitro expression of the 15 kDa subunit of the mediatophore and functional reconstitution of acetylcholine release.

The mediatophore is a presynaptic oligomeric protein purified from the presynaptic plasma membrane of Torpedo synaptosomes on the basis of its ability to mediate a calcium-dependent acetylcholine release when solubilized and reconstituted into proteoliposomes. We investigated the ACh translocating activity of the 15 kDa proteolipid subunit of the mediatophore when expressed in Xenopus oocytes and reconstituted into proteoliposomes loaded with ACh. 1. A calcium-dependent ACh translocation was observed when oocytes were injected with polyadenylated mRNAs extracted from the electric lobe of the Torpedo brain or with an in vitro transcribed RNA encoding the 15 kDa subunit. 2. No release response was obtained when oocytes were non-injected or injected with Torpedo liver mRNAs. 3. This ACh translocation mechanism showed calcium-dependent activation and desensitisation and was inhibited by cetiedil, sharing these properties with the release of ACh observed at the synapse. 4. The ACh translocating activity of an N terminal deleted mediatophore 15 kDa subunit was strongly reduced and the deleted proteolipid appeared less sensitive to the action of cetiedil (alpha-cyclohexyl-alpha-(3-thienyl)-acetate of perhydroazepinyl-alpha-ethyl citrate monohydrate). 5. A significant ACh release response was observed when the 15 kDa proteolipid of the H(+)-ATPase from bovine chromaffin granules was tested. 6. These results show that this ACh translocating activity could be induced in the oocyte membranes by the expression of the 15 kDa subunit alone.