Distribution of proteins migrating with fast axonal transport. Their relationship to smooth endoplasmic reticulum

The cross-sectional distribution of 3H-labeled proteins moving with the front and plateau of fast axonal transport was analyzed by quantitative electron microscopic autoradiography in sciatic nerves of rats following injection of a 1:1 mixture of [3H]proline and [3H]lysine into lumbar ventral horns. While in the front the transported proteins were uniformly distributed within the axons excluding the axolemma, which was not labeled, in the plateau 90% of 3H-labeled proteins were uniformly distributed and 10% were exclusively located at a 80-nm annulus with its outer edge at the axolemma. Ultrastructural morphometry showed that the density of smooth endoplasmic reticulum (SER) in a 160 nm wide subaxolemmal annulus was approximately twice that in the remaining axon, where it was uniform. These findings: (a) are consistent with the concept that membranous materials are released from the fast transport, move laterally and are inserted into the axolemma; (b) indicate that a portion of SER, largely located at the periphery of the axon, is not involved in fast transport.