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

James A. Galbraith, Thomas S. Reese, Michelle L. Schlief, Paul E. Gallant

Research output: Contribution to journalArticlepeer-review

66 Scopus citations


A major issue in the slow transport of cytoskeletal proteins is the form in which they are transported. We have investigated the possibility that unpolymerized as well as polymerized cytoskeletal proteins can be actively transported in axons. We report the active transport of highly diffusible tubulin oligomers, as well as transport of the less diffusible neurofilament polymers. After injection into the squid giant axon, tubulin was transported in an anterograde direction at an average rate of 2.3 mm/day, whereas neurofilament 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 ≃20% of control levels. Passive diffusion of these proteins occurred in parallel with transport. The diffusion coefficient of the moving tubulin in axoplasm was 8.6 μ2/s compared with only 0.43 μm2/s for neurofilament. These results suggest that the tubulin was transported in the unpolymerized state and that the neurofilament was transported in the polymerized state by an energy-dependent nocodazole/cold- sensitive transport mechanism.

Original languageEnglish (US)
Pages (from-to)11589-11594
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number20
StatePublished - Sep 28 1999
Externally publishedYes


  • Axoplasmic transport
  • Cytoskeleton
  • Diffusion
  • Slow axonal transport

ASJC Scopus subject areas

  • General


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