DIFFERENTIAL TARGETING TO THE PLASMA-MEMBRANE OF THE TORPEDO 15-KDA PROTEOLIPID EXPRESSED IN OOCYTES

Xenopus laevis oocytes were injected with poly(A)(+) RNAs extracted fr om the electric lobes of Torpedo marmorata, which contain a homogeneou s population of cholinergic neurons. These primed oocytes were able to synthesize acetylcholine and to release the neurotransmitter in a cal cium-dependent manner. Fractionation of oocyte membranes as well as im munofluorescence experiments showed that the 15-kDa proteolipid, a com mon subunit of the vacuolar H+-ATPase and of a presynaptic membrane pr otein capable of calcium-dependent acetylcholine translocation called the mediatophore, was located at the oocyte plasma membrane, In contra st, oocytes injected with separate transcripts encoding the 15-kDa pro teolipid and choline acetyltransferase were unable to release acetylch oline in spite of an equivalent acetylcholine content and a higher lev el of 15-kDa proteolipid expression. We observed by immunofluorescence that under these conditions, the 15-kDa proteolipid was expressed in granular cytoplasmic membranes, which were then identified as being Go lgi vesicles by cell fractionation. The striking difference in the dis tribution of the 15-kDa proteolipid expressed in oocytes primed with T orpedo electric lobe mRNA as compared with that seen in oocytes inject ed with the cRNA alone suggests that another protein endogenous to the electric lobe may be implicated in the localization of the 15-kDa pro teolipid at the plasma membrane. Moreover, such a targeting mechanism could contribute to the capacity of electric lobe mRNA-injected oocyte s to release acetylcholine.