Differences in the abilities of human tau isoforms to promote microtubule assembly

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Abstract
Three isoforms of human tau protein were compared for their abilities to induce microtubule assembly. The three isoforms, τ3 (tau containing three micro‐tubule‐binding domains), τ4 (tau containing four microtubule‐binding domains) and τ4L (tau containing four microtubule binding domains plus a 58‐amino‐acid insert near the N‐terminus) were expressed in E. coli and purified using ammonium sulfate precipitation, ion exchange, and size exclusion chromatography. All three isoforms induced microtubule assembly at micromolar concentrations and showed similar critical concentrations for assembly of 0.4–0.45 μM. However, τ4 induced microtubule formation at a rate five‐ to tenfold faster than either τ3 or τ4L. The rate of microtubule elongation seen with τ4 was twofold greater than with τ3 or τ4L, suggesting that the faster rate of microtubule assembly seen with τ4 was due, at least in part, to faster elongation. τ4 induced a greater number of microtubules to form at steady state than did τ3 or τ4L. The microtubules generated with each tau isoform had similar steady‐state length distributions and were equally susceptible to cold‐induced disassembly. These results indicate that the additional microtubule‐binding domain in τ4 enhances microtubule assembly, while the 58‐amino‐acid insert negates the stimulatory effect of the fourth microtubule‐binding domain.

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