Protein and glycoprotein composition of subsynaptosomal fractions. Implications for Exocytosis and recycling of synaptic vesicles.

Highly enriched fractions of synaptic junctional complexes, synaptic vesicles, and coated vesicles were isolated from rat forebrains and compared, along with synaptosomal plasma membrane and its nonjunctional components, by discontinuous sodium dodecylsulfate-polyacrylamide gel electrophoresis. When stained for proteins with Coomassie blue, the gels all contained the same protein bands, only in different relative amounts. Mixing experiments revealed no additional bands. However, gel patterns of each fraction were not only quantitatively but also qualitatively different when stained for carbohydrate. These observations suggested that some of the protein bands may vary in their degree of glycosylation among the various synaptic fractions. Within the limits of resolution of the methods used here, these results are consistent with the morphological process of synaptic vesicle exocytosis and recycling but suggest the possibility of a reversible modification of certain membrane glycoproteins as they pass through the various membrane compartments.