Slow transport of unpolymerized tubulin and polymerized neurofilament in the squid giant axon

A major issue in the slow transport of cytoskeletal proteins is the form in which they are transported. We have investigated the possibility that unpo lymerized as well as polymerized cytoskeletal proteins can be actively tran sported in axons. We report the active transport of highly diffusible tubul in oligomers, as well as transport of the less diffusible neurofilament pol ymers. After injection into the squid giant axon, tubulin was transported i n an anterograde direction at an average rate of 2.3 mm/day, whereas neurof ilament was moved at 1.1 mm/day. Addition of the metabolic poisons cyanide or dinitrophenol reduced the active transport of both proteins to less than 10% of control values, whereas disruption of microtubules by treatment of the axon with cold in the presence of nocodazole reduced transport of both proteins to approximate to 20% of control levels, Passive diffusion of thes e proteins occurred in parallel with transport. The diffusion coefficient o f the moving tubulin in axoplasm was 8.6 mu m(2)/s compared with only 0.43 mu m(2)/s for neurofilament. These results suggest that the tubulin,vas tra nsported in the unpolymerized state and that the neurofilament was transpor ted in the polymerized state by an energy-dependent nocodazole/cold-sensiti ve transport mechanism.