Analysis of tubulin transport in nerve processes.

In neurons, the molecular machinery for axonal growth and navigation is localized to the growth cone region, whereas tubulin is synthesized primarily in the cell body. Because diffusion serves as an efficient transport mechanism only for very short distances, tubulin has to be actively transported from the cell body down the axon. Two mechanistically distinct models for tubulin transport have been proposed. "Polymer model" postulates that tubulin moves in the form of microtubules preassembled in the cell body, whereas "subunit model" assumes that axonal microtubules are stationary, and that tubulin is delivered from the cell body in unassembled form. We used three independent quantitative approaches (photobleaching, fluorescence speckle microscopy, and microtubule plus end tracking) to demonstrate that axonal microtubules are stationary in rapidly growing axons produced by Xenopus spinal cord neurons in culture. These experiments strongly support subunit model for tubulin delivery.