Synthesis of β-Tubulin, Actin, and Other Proteins in Axons of Sympathetic Neurons in Compartmented Cultures

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Abstract
The proteins needed for growth and maintenance of the axon are generally believed to be synthesized in the cell bodies and delivered to the axons by anterograde transport. However, recent reports suggest that some proteins can also be synthesized within axons. We used [35S]methionine metabolic labeling to investigate axonal protein synthesis in compartmented cultures of sympathetic neurons from newborn rats. Incubation of distal axons for 4 hr with [35S]methionine resulted in a highly specific pattern of labeled axonal proteins on SDS-PAGE, with 4 prominent bands in the 43–55 kDa range. The labeled proteins in axons were not synthesized in the cell bodies, because they were also produced by axons after the cell bodies had been removed. Two of the proteins were identified by immunoprecipitation as actin and β-tubulin. Axons synthesized in vivoin which losses from turnover during axonal transport may limit the availability of cell body synthesized proteins to the distal axons. In this case, a significant fraction of axonal proteins might be supplied by axonal synthesis, which could, therefore, play important roles in axonal maintenance, regeneration, and sprouting.