Involvement of coated vesicles in the initiation of fast axonal transport.

The present study examines whether coated vesicles play a role in the intrasomal transit of newly synthesized fast-transported proteins. Coated vesicles isolated from bullfrog brain were shown to have a protein composition and ultrastructure similar to purified bovine brain coated vesicles. Bullfrog brain was then used as unlabeled carrier for the isolation of coated vesicles from dorsal root ganglia labeled with [3H]leucine. Fast-transported [35S]methionine-labeled proteins were generated in separate preparations from sciatic nerve, and co-electrophoresed on two-dimensional gels with [3H]proteins of the coated vesicle fraction. The [35S]Met fluorographic X-ray film pattern was used as a guide to remove gel regions which were tested for the presence of 3H. By this means, 45 of 67 individual fast-transported proteins examined were found to contain significant levels of 3H. The fact that these proteins have similar net charge and molecular weight characteristics to the mature fast-transported proteins with which they co-migrated, suggests that such species have already undergone post-translational modifications prior to becoming associated with coated vesicles. Since most modifications of this type occur in the Golgi apparatus, it appears that the majority of fast-transported proteins are isolated in association with a population of post-Golgi coated vesicles. The role of coated vesicles is incorporated into a model describing the pathway taken by fast-transported proteins during the initiation of fast axonal transport.