MICROTUBULE TRANSPORT AND ASSEMBLY DURING AXON GROWTH

There is controversy concerning the mechanisms by which the axonal mic rotubule (MT) array is elaborated, with some models focusing on MT ass embly and other models focusing on MT transport. We have proposed a co mposite model in which MT assembly and transport are both important (J oshi, H.C., and P.W. Baas. 1993. J. Cell Biol. 121:1191-1196). In the present study, we have taken a novel approach to evaluate the merits o f this proposal. Biotinylated tubulin was microinjected into cultured neurons that had already grown short axons. The axons were then permit ted to grow longer, after which the cells were prepared for immunoelec tron microscopic analyses. We reasoned that any polymer that assembled or turned over subunits after the introduction of the probe should la bel for biotin, while any polymer that was already assembled but did n ot turnover should not label. Therefore, the presence in the newly gro wn region of the axon of any unlabeled MT polymer is indicative of MT transport. In sampled regions, the majority of the polymer was labeled , indicating that MT assembly events are active during axon growth. Va rying amounts of unlabeled polymer were also present in the newly grow n regions, indicating that MT transport also occurs. Together these fi ndings demonstrate that MT assembly and transport both contribute to t he elaboration of the axonal MT array.