Expression of tau protein in non-neuronal cells: microtubule binding and stabilization.

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The microtubule-associated protein tau is a developmentally regulated family of neuronal phosphoproteins that promotes the assembly and stabilization of microtubules. The carboxy-terminal half of the protein contains three copies of an imperfectly repeated sequence; this region has been found to bind microtubules in vitro. In addition, a fourth copy of the repeat has been found in adult-specific forms of tau protein. To examine the structure and function of tau protein in vivo, we have transiently expressed fetal and adult forms of tau protein and tau protein fragments in tissue culture cells. Biochemical analysis reveals full-length products with heterogeneity in post-translational modification synthesized in the cells. Immunofluorescent staining of transfected cells shows that, under our conditions, sequences on both sides of the repeat region are required for in vivo microtubule co-localization. These additional regions may be required either for enhancing microtubule contacts or for proper protein folding in the cell. In our expression system, the bundling of cellular microtubules occurs only in transfections using four-repeat tau constructs; any four-repeat construct capable of binding is also able to induce bundling. Our data suggest that the presence of bundles is correlated with enhanced microtubule stability; factors that increase stability such as higher levels of tau protein expression or the presence of the fourth repeat, increase the fraction of transfected cells showing bundles. Finally, the presence of tau protein in the cell allows all interphase microtubules to become acetylated, a post-translational modification usually reserved for a subset of stable cellular microtubules.