Here we show that p150Glued promotes microtubule formation in vitro by catalyzing nucleation, increasing the polymerization rate, and inhibiting catastrophe. These activities require dimerization and are dependent on the ability of p150Glued to form a stable complex with tubulin through interactions with both the N-terminal CAP-Gly and basic domains. In primary neurons, we observe that the dominant effect of p150Glued on microtubule dynamics is the suppression of catastrophe (). Finally, we determine that a single point mutation within the CAP-Gly domain of p150Glued causative for a fatal familial form of Parkinson disease, known as Perry Syndrome, leaves p150Glued unable to promote microtubule assembly either in vitro or in neurons.
p150Glued is a neuron-specific microtubule anti-catastrophe factor.
Dynactin was originally identified as a large protein complex that supported dynein-mediated vesicle transport 
. The N-terminus of the 150 kDa subunit binds microtubules independently of dynein 
, and increases the processivity of dynein in vitro
. Recently, it has been demonstrated that the microtubule-binding N-terminus of p150Glued
is dispensable for organelle localization and vesicular motility in nonneuronal cells 
. However, the microtubule-binding CAP-Gly domain of dynactin is required for efficient transport initiation from the distal axon in neurons 
. A plus end-localized pool of p150Glued
may serve to load dynein onto the microtubule 
. However, biochemical analyses and immunolocalization suggest that a large proportion of dynactin may in fact not be in complex with dynein 
. Enriched at the plus end, this population of dynactin would be perfectly poised to affect microtubule dynamics.
Our data suggest a mechanism whereby p150Glued
could modify microtubule dynamics. Recently, it has been suggested that the kinetics of tubulin association and dissociation with the microtubule plus end may be much faster than previously appreciated 
. This makes it increasingly plausible that one mode whereby MAPs alter microtubule dynamics is by modulating the off-rate of tubulin subunits from microtubule plus ends. Since p150Glued
can bind both to microtubules and to soluble tubulin, and because dimerization appears necessary for p150Glued
to robustly modify dynamics, we speculate that p150Glued
may be acting in this capacity by binding to both microtubules and tubulin at the same time, decreasing the off-rate and inhibiting catastrophe, enabling efficient initiation of dynein-mediated retrograde runs (). Interestingly, this mechanism is distinct from the mode by which cytoplasmic dynein independently functions to inhibit catastrophe 
. Areas of the distal neuron where both cytoplasmic and dynactin are localized could be sites of particularly robust microtubule stabilization.
The regulation of these microtubule-modifying abilities of p150Glued
may be multifactorial. We have shown that the basic region is necessary for the modification of microtubule dynamics by p150Glued
, likely by ensuring a stable complex with the distributed acidic nature of tubulin. The basic region is also serine- and threonine-rich, and has been shown to be the target of phosphorylation by regulatory kinases 
, which might further modulate the p150-tubulin interaction during mitosis, or during development. p150Glued
also binds to CLIP-170 
, which could further modify the behavior of p150Glued
in the cell.
, only the p150Glued
isoform expressed in neurons includes both the full CAP-Gly and basic domains that we have shown are necessary to modify microtubule assembly dynamics. We have recently shown that young, developing neurons depleted of p150Glued
are morphologically normal 
. In fact, profound depletion of both EB1 and EB3, which should effectively disrupt plus-end targeting, has no gross effects on neurite outgrowth 
. It may be that, as recent evidence suggests, only as neurons age, and their processes lengthen and elaborate, does the centrosome lose its function as a microtubule organizing center and microtubule dynamics become particularly reliant on plus-end regulation 
. It is perhaps telling that human patients with the Q74P p150Glued
mutation do not show disease onset until the fifth decade of life 
. More broadly, microtubule dynamics may alter in aging or degenerating neurons, as suggested from studies of cells from patients with sporadic Parkinson's and Alzheimer's disease 
In summary, we have identified and characterized a novel role for p150Glued in the tissue-specific stabilization of microtubules, and implicated defects in neurodegeneration. Further studies to disentangle the effects of the mutation on axonal transport and microtubule stability in neurons will be required to clarify the pathogenesis involved.