The role of the neuronal microtubule-associated protein tau has been studied by generating a series of tau constructs differing in one or several of its subdomains: length and composition of the repeat domains, extensions of the repeats in the N- or C-terminal direction, constructs without repeats, assembly vs projection domain, and number of N-terminal inserts. The interaction of the mutant tau proteins with microtubules was judged by several independent methods. (i) Direct binding assays between tau and taxol-stabilized microtubules yield dissociation constants and stoichiometries. (ii) Light scattering and X-ray scattering of assembling microtubule solutions reflect the capacity of tau to promote microtubule nucleation, elongation, and bundling in bulk solution. (iii) Dark field microscopy of assembling microtubules allows one to assess the efficiency of nucleation and bundling separately. The repeat region alone, the N-terminal domains alone, or the C-terminal tail alone binds only weakly to microtubules. However, binding is strongly enhanced by combinations such as the repeat region plus one or both of the flanking regions which could be viewed as "jaws" for tau on the microtubule surface (the proline-rich domain P upstream of the repeats and the "fifth" repeat R' downstream). Such combinations make tau's binding productive in terms of microtubule assembly and stabilization, while the combination of the flanking regions without repeats binds only unproductively. Efficient nucleation parallels strong binding in most cases, i.e., when a construct binds tightly to microtubules, it also nucleates them efficiently and vice versa. In addition, the proline-rich domain P in combination with the repeats R or the flanking domain R' causes pronounced bundling. This effect disappears when the N-terminal domains (acidic or basic) are added on, suggesting that the tau isoforms are not "bundling proteins" in the proper sense. In spite of the wide range of binding strength and nucleation efficiency, the stoichiometries of binding are rather reproducible (around 0.5 tau/tubulin dimer); this is in remarkable contrast to the effect of certain types of phosphorylation which can strongly reduce the stoichiometry.

The neuronal microtubule-associated protein tau plays an important role in establishing cell polarity by stabilizing axonal microtubules that serve as tracks for motor-protein–driven transport processes. To investigate the role of tau in intracellular transport, we studied the effects of tau expression in stably transfected CHO cells and differentiated neuroblastoma N2a cells. Tau causes a change in cell shape, retards cell growth, and dramatically alters the distribution of various organelles, known to be transported via microtubule-dependent motor proteins. Mitochondria fail to be transported to peripheral cell compartments and cluster in the vicinity of the microtubule-organizing center. The endoplasmic reticulum becomes less dense and no longer extends to the cell periphery. In differentiated N2a cells, the overexpression of tau leads to the disappearance of mitochondria from the neurites. These effects are caused by tau's binding to microtubules and slowing down intracellular transport by preferential impairment of plus-end–directed transport mediated by kinesin-like motor proteins. Since in Alzheimer's disease tau protein is elevated and mislocalized, these observations point to a possible cause for the gradual degeneration of neurons.

The dynamic instability of microtubules is thought to be regulated by MAPs and phosphorylation. Here we describe the effect of the neuronal microtubule-associated protein tau by observing the dynamics of single microtubules by video microscopy. We used recombinant tau isoforms and tau mutants, and we phosphorylated tau by the neuronal kinases MARK (affecting the KXGS motifs within tau's repeat domain) and cdk5 (phosphorylating Ser-Pro motifs in the regions flanking the repeats). The variants of tau can be broadly classified into three categories, depending on their potency to affect microtubule dynamics. "Strong" tau variants have four repeats and both flanking regions. "Medium" variants have one to three repeats and both flanking regions. "Weak" variants lack one or both of the flanking regions, or have no repeats; with such constructs, microtubule dynamics is not significantly different from that of pure tubulin. N- or C-terminal tails of tau have no influence on dynamic instability. The two ends of microtubules (plus and minus) showed different activities but analogous behavior. These results are consistent with the "jaws" model of tau where the flanking regions are considered as targeting domains whereas the addition of repeats makes them catalytically active in terms of microtubule stabilization. The dominant changes in the parameters of dynamic instability induced by tau are those in the dissociation rate and in the catastrophe rate (up to 30-fold). Other rates change only moderately or not at all (association rate increased up to twofold, rates of rescue or rapid shrinkage decreased up to approximately twofold). The order of repeats has little influence on microtubule dynamics (i.e., repeats can be re-arranged or interchanged), arguing in favor of the "distributed weak binding" model proposed by Butner and Kirschner (1991); however, we confirmed the presence of a "hotspot" of binding potential involving Lys274 and Lys281 observed by Goode and Feinstein, 1994. Phosphorylation of Ser-Pro motifs by cdk5 (mainly Ser 202, 235, and 404) in the flanking regions had a moderate effect on microtubule dynamics while phosphorylation at the "Alzheimer"-site Ser262 MARK eliminated tau's interactions with microtubules. In both cases the predominant effects of phosphorylation are on the rates of tubulin dissociation and catastrophe whereas the effects on the rates of association or rescue are comparatively small.

In Alzheimer's disease the neuronal microtubule-associated protein tau becomes highly phosphorylated, loses its binding properties, and aggregates into paired helical filaments. There is increasing evidence that the events leading to this hyperphosphorylation are related to mitotic mechanisms. Hence, we have analyzed the physiological phosphorylation of endogenous tau protein in metabolically labeled human neuroblastoma cells and in Chinese hamster ovary cells stably transfected with tau. In nonsynchronized cultures the phosphorylation pattern was remarkably similar in both cell lines, suggesting a similar balance of kinases and phosphatases with respect to tau. Using phosphopeptide mapping and sequencing we identified 17 phosphorylation sites comprising 80-90% of the total phosphate incorporated. Most of these are in SP or TP motifs, except S214 and S262. Since phosphorylation of microtubule-associated proteins increases during mitosis, concomitant with increased microtubule dynamics, we analyzed cells mitotically arrested with nocodazole. This revealed that S214 is a prominent phosphorylation site in metaphase, but not in interphase. Phosphorylation of this residue strongly decreases the tau-microtubule interaction in vitro, suppresses microtubule assembly, and may be a key factor in the observed detachment of tau from microtubules during mitosis. Since S214 is also phosphorylated in Alzheimer's disease tau, our results support the view that reactivation of the cell cycle machinery is involved in tau hyperphosphorylation.

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