Differences in the distribution of specific glycoproteins among the regions of a single identified neuron.

Neurons are highly differentiated cells whose various regions must differ in macromolecular composition. This is demonstrated in the present study which shows that specific membrane glycoproteins are routed to particular sites in the cell. When [3H]fucose of [3H]N-acetylgalactosamine are injected into R2, the giant neuron of Aplysia, they are incorporated into relatively few glycoproteins, several of which can be readily identified from cell to cell. Because R2's cell body is so large, its surface membrane can be isolated 94% free of cytosol by manual dissection. The purity of the membrane was assessed by checking the distribution of the enzyme choline acetyltransferase. R2's axon can also be analyzed separately from the cell body. At 24 h after injection, SDS polyacrylamide gel electrophoresis shows that glycoprotein-I (mol. wt. 180,000) is the major labeled component of the external membrane where it is enriched relative to the content of the other cytosolic membranes. In contrast, glycoprotein-V (mol. wt. 90,000) predominates among the membranes of the axon. The disposition of glycoprotein-I in the external membrane was indicated by exposing R2 to low concentrations of pronase in situ 24 h after injection. Labeled glycopeptides were released from R2's surface and gel filtration and high voltage electrophoresis indicated that some of these were derived from glycoprotein-I. Examination of the isolated surface membrane after proteolysis showed a reduced amount of labeled glycoprotein-I. Consistent with these findings, a glycoprotein of similar molecular weight as component-I was labeled when R2 was treated with galactose oxidase and potassium borotritide. These results indicate that the carbohydrate moieties of glycoprotein-I extend from R2's surface.