Tau is an abundant microtubule-associated protein in neurons. Tau aggregation into insoluble fibrils is a hallmark of Alzheimer's disease and other dementias 1 , yet the physiological state of tau molecules within cells remains unclear. Using single molecule imaging, we directly observe that the microtubule lattice regulates reversible tau self-association, leading to localized, dynamic condensation of tau molecules on the microtubule surface. Tau condensates form selectively permissible barriers, spatially regulating the activity of microtubule severing enzymes and the movement of molecular motors through their boundaries. We propose that reversible selfassociation of tau molecules, gated by the microtubule lattice, is an important mechanism of tau's biological functions, and that oligomerization of tau is a common property shared between the physiological and disease forms of the molecule.

No abstract

SUMMARY Adenomatous polyposis coli (APC) is a microtubule plus-end scaffolding protein important in biology and disease. APC is implicated in RNA localization, although the mechanisms and functional significance remain unclear. We show that APC is an RNA-binding protein, and identify an RNA interactome by HITS-CLIP. Targets were highly enriched for APC-related functions, including microtubule organization, cell motility, cancer and neurologic disease. Among the targets is β2B-tubulin, known to be required in human neuron and axon migration. We show β2B-tubulin is synthesized in axons and localizes preferentially to dynamic microtubules in the growth cone periphery. APC binds the β2B-tubulin 3'UTR; treatments interfering with this interaction reduced β2B-tubulin mRNA axonal localization and expression, depleted dynamic microtubules and the growth cone periphery, and impaired neuron migration. These results identify APC as a platform binding functionally-related protein and RNA networks, and suggest a self-organizing model for the microtubule to localize synthesis of its own subunits.

Highlights d Microtubule-associated proteins (MAPs) act as modulators of motor movement d Dendrite-localized DCX, DCLK1, and MAP9 inhibit kinesin-1 but not kinesin-3 d MAP9 enhances kinesin-3 motility in vitro and in vivo via the motor's K-loop d MAP9 inhibits the dynein-dynactin complex by blocking the p150-tubulin interaction

Tau is an abundant microtubule-associated protein in neurons. Tau aggregation into insoluble fibrils is a hallmark of Alzheimer's disease and other dementias, yet the physiological state of tau molecules within cells remains unclear. Using single molecule imaging, we directly observe that the microtubule lattice regulates reversible tau selfassociation, leading to dynamic condensation of tau molecules on the microtubule surface. Tau condensates form selectively permissible barriers, spatially regulating the activity of MT severing enzymes and the movement of molecular motors through their boundaries. We propose that reversible self-association of tau molecules, controlled by the microtubule, is an important mechanism of tau's biological functions, and that oligomerization of tau is a common property shared between the physiological and disease forms of the molecule.