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 compa red the proximal and distal regions of the axon and the axon shaft wit h regard to their content of newly stabilized MT polymer, using the fo llowing criteria. Stable polymer was identified by its resistance to n ocodazole, and newly stabilized polymer was distinguished from older s table polymer by the staining of the former but not the latter for tyr osinated 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 t he axon and the axon shaft is newly stabilized, respectively. Thus, wh ile MT stabilization occurs throughout the axon, the proximal region i s by far the most active with regard to this process.