The tau family of microtubule-asssociated proteins has a microtubule-binding domain which includes three or four conserved sequence repeats. Pelleting assays show that when tubulin and tau are coassembled into microtubules, the presence of taxol reduces the amount of tau incorporated. In the absence of taxol, strong binding sites for tau are ®lled by one repeat motif per tubulin dimer; additional tau molecules bind more weakly. We have labelled a repeat motif with nanogold and used three-dimensional electron cryomicroscopy to compare images of microtubules assembled with labelled or unlabelled tau. With kinesin motor domains bound to the microtubule outer surface to distinguish between a-and b-tubulin, we show that the gold label lies on the inner surface close to the taxol binding site on b-tubulin. Loops within the repeat motifs of tau have sequence similarity to an extended loop which occupies a site in a-tubulin equivalent to the taxol-binding pocket in b-tubulin. We propose that loops in bound tau stabilize microtubules in a similar way to taxol, although with lower af®nity so that assembly is reversible.
We investigated whether discodermolide, a novel antimitotic agent, a¡ects the binding to microtubules of tau protein repeat motifs. Like taxol, the new drug reduces the proportion of tau that pellets with microtubules. Despite their di¡ering structures, discodermolide, taxol and tau repeats all bind to a site on L L-tubulin that lies within the microtubule lumen and is crucial in controlling microtubule assembly. Low concentrations of tau still bind strongly to the outer surfaces of preformed microtubules when the acidic C-terminal regions of at least six tubulin dimers are available for interaction with each tau molecule; otherwise binding is very weak. ß
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