Competition for microtubule lattice spacing between a microtubule expander and compactor.

Microtubules exist in expanded and compacted states, as defined by the lattice spacing of αβ-tubulin dimers. Changes in lattice spacing have been linked to factors such as GTP-hydrolysis, the binding of microtubule-associated proteins (MAPs), the tubulin code, and microtubule bending. These diverse factors exert opposing molecular driving forces on the microtubule lattice that push lattice spacing toward expanded or compacted states. To better understand how these opposing forces are reconciled, we developed in vitro and cell-based model systems for the competition between a microtubule expander (paclitaxel) and a microtubule compactor (doublecortin or DCX). Using an in vitro reconstitution approach, we show that paclitaxel expands microtubules cooperatively. In cells, high concentrations of paclitaxel cause DCX to relocalize to compacted lattices found at concave bends. When the concentration of DCX is increased, however, we find that DCX re-compacts the previously expanded microtubules in vitro. Consistently, high expression levels of DCX prevent its relocalization in paclitaxel-treated cells. When the competition between paclitaxel and DCX is "balanced," we observe a complex phenotype: DCX simultaneously localizes to both long, straight clusters and concave bends, whereas other regions on the microtubule network remain DCX free. We conclude that multiple lattice spacings can coexist in cells. Our results indicate that competition for microtubule lattice spacing is a critical aspect of microtubule physiology.