Proteins rapidly transported to the synapses of a single identified neuron of Aplysia californica.

The cell body of R2, a giant cholinergic neuron of Aplysia californica, resides in the abdominal ganglion, whereas its synapses are on thousands of unicellular mucus glands located in the skin. Due to the great spatial separation between the site of macromolecular synthesis and the presynaptic terminals, rapid axonal transport can be used to segregate synaptic proteins from those to be used elsewhere in the cell. The proteins of R2 were labeled by incubating the abdominal ganglion in [35S]methionine for 5 hr in a chamber separated from the rest of the isolated central nervous system. After 50 hr, 28 radiolabeled proteins were reproducibly found by one- and two-dimensional polyacrylamide gel electrophoresis to be transported to the distal regions of peripheral nerves P6, P7, and P8 that innervate the parapodia and middle body wall. We are sure that R2 is the source of these proteins since radioautography of sections taken throughout the nervous system, complemented by cobalt tracings, showed that R2 is the only neuron in the abdominal ganglion with axons in these nerves. Nine of the 28 transported proteins are glycoproteins since they were also labeled after injecting R2's cell body with [3H]-L-fucose. There is evidence that the proteins and glycoproteins are destined for R2's presynaptic terminals. For example, in experiments in which the body wall and parapodium remained attached to the nerves, the proteins were transported to the skin region that contains the glands. Moreover, analyses of the distribution of the rapidly transported proteins by qualitative radioautography and by extrusion of axoplasm indicated that none are constituents of the axolemma.(ABSTRACT TRUNCATED AT 250 WORDS)