Tubulin and neurofilament proteins are transported differently in axons of chicken motoneurons.

1. We previously showed that actin is transported in an unassembled form with its associated proteins actin depolymerizing factor, cofilin, and profilin. Here we examine the specific activities of radioactively labeled tubulin and neurofilament proteins in subcellular fractions of the chicken sciatic nerve following injection of L-[35S]methionine into the lumbar spinal cord. 2. At intervals of 12 and 20 days after injection, nerves were cut into 1-cm segments and separated into Triton X-100-soluble and particulate fractions. Analysis of the fractions by high-resolution two-dimensional gel electrophoresis, immunoblotting, fluorography, and computer densitometry showed that tubulin was transported as a unimodal wave at a slower average rate (2-2.5 mm/day) than actin (4-5 mm/day). Moreover, the specific activity of soluble tubulin was five times that of its particulate form, indicating that tubulin is transported in a dimeric or small oligomeric form and is assembled into stationary microtubules. 3. Neurofilament triplet proteins were detected only in the particulate fractions and transported at a slower average rate (1 mm/day) than either actin or tubulin. 4. Our results indicate that the tubulin was transported in an unpolymerized form and that the neurofilament proteins were transported in an insoluble, presumably polymerized form.