The balance between tau protein's microtubule growth and nucleation activities: implications for the formation of axonal microtubules.

The microtubule-associated protein tau is found primarily in neuronal tissues and is highly enriched in the axon. It promotes microtubule assembly in vitro and stabilizes microtubules in cells. To study how tau protein might be involved in the unique features of axonal microtubules, we have analyzed the effect of E. coli-synthesized tau protein using an in vitro centrosome-mediated microtubule regrowth assay over a wide range of tau/tubulin ratios. We report that microtubule assembly promoted by tau protein exhibits characteristic changes dependent on the tau/tubulin ratio. Above a threshold level, nucleation of new microtubules is favored over growth of existing ones. tau isoform variation does not change this phase transition in microtubule assembly. We discuss how tau might participate in the elaboration of axonal morphology based on our results and present evidence that the phase transition from microtubule growth to nucleation is critical for axonal development.