Glycoproteins are modified in the axon of R2, the giant neuron of Aplysia californica, after intra-axonal injection of [3H]N-acetylgalactosamine.

We have found evidence that mechanisms exist in the axon by which proteins, originally synthesized in the cell body, can be modified. Incorporation of [3H]N-acetyl-D-galactosamine into macromolecules was studied in the axon of R2, the giant identified neuron of the abdominal ganglion of Aplysia. The precursor sugar, injected directly into the major axon in the right connective, labeled both glycoproteins and glycolipids. These macromolecules were associated with membranes and at least 90% of the incorporated radioactivity could be sedimented by centrifugation at 105,000 X g. Radiocutography of injected axons with the light microscope showed that most of the silver grains were located over the axon rather than over other tissues in the right connective. Grains appeared over a variety of axonal components, but quantitative electron microscopic radioautography revealed that vesicles were the only organells significantly labeled. Disco polyacrylamide gel electrophoresis in sodium dodecyl sulfate (SDS) of the membrane fraction resolved [5-(3)H]glycoprotein components. These membrane glycoproteins ranged in apparent molecular weight from 20,000 to 160,000 daltons and could be digested by pronase. Only one component had a mobility similar to that of a glycoprotein found in the axon after injection of the cell body. Incorporation of [3H]N-acetylgalactosamine into axonal glycoprotein was unaffected by anisomycin, a potent inhibitor of protein synthesis in Aplysia, and therefore presumably occurred on already existing polypeptide chains. We conclude that these were synthesized in the cell body and exported into the axon where they were modified by the addition of sugar.