A liquid +TIP-network drives microtubule dynamics through tubulin condensation

Tubulin dimers assemble into a dynamic microtubule network throughout the cell. Microtubule dynamics and network organization must be precisely tuned for the microtubule cytoskeleton to regulate a dazzling array of dynamic cell behaviors. Since tubulin concentration determines microtubule growth, we studied here a novel regulatory mechanism of microtubule dynamics: local tubulin condensation. We discovered that two microtubule tip-binding proteins, CLIP-170 and EB3, undergo phase separation and form a liquid-like network at the growing microtubule tip in cells. We prove in vitro that this liquid network has the capacity to locally condense tubulin. This process of tubulin co-condensation is spatially initiated at the microtubule tip and temporally regulated to occur only when there is tip growth. Tubulin condensation at the growing microtubule tip increases growth speeds three-fold and strongly reduces depolymerization events. With this work we establish a new mechanism to regulate microtubule dynamics by enrichment of tubulin at strategically important locations: the growing microtubule tips. ### Competing Interest Statement The authors have declared no competing interest.