Sites of microtubule stabilization for the axon.

We have sought to determine the principal site(s) in the neuron where axonal microtubules (MTs) are stabilized. To accomplish this, we compared the proximal and distal regions of the axon and the axon shaft with regard to their content of newly stabilized MT polymer, using the following criteria. Stable polymer was identified by its resistance to nocodazole, and newly stabilized polymer was distinguished from older stable polymer by the staining of the former but not the latter for tyrosinated alpha-tubulin. Our results indicate that roughly 36.4%, 5.4%, and 2.4% of the total MT mass in the proximal and distal regions of the axon and the axon shaft is newly stabilized, respectively. Thus, while MT stabilization occurs throughout the axon, the proximal region is by far the most active with regard to this process.