Fig. 3. Model for microtubule assembly by severing and transport of nucleating templates. In this model, a -tubulin ring complex associates with the minus end of a microtubule, while the microtubule extends by the addition of tubulin subunits at the fast-growing, GTP-tubulin-containing plus end (dark green). Severing of the minus end is achieved by the formation of a hexamer of katanin p60 subunits, whose association with the microtubule wall is coordinated by the larger p80 subunit, which may transiently dimerize with the p60 subunits. Microtubule-mediated ATPase activity results in inward movement of the p60 subunits, an action that cleaves the ring complex from the microtubule minus end. Katanin subunits dissociate but the lock-washer-shaped ring complex is transported along the microtubule by a plus-end-directed kinesin. The extent of transport along the microtubule may be regulated by the relative activities of plus- and minus-end-directed kinesins. The ring complex serves as a template for the assembly of additional microtubules. Repeated generation, severing and transport of nucleating templates at the minus end of the original microtubule may explain how the fractal tree complexes shown in Fig. 2A develop.

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