A hydrodynamic instability drives TPX2 protein droplet formation on microtubules and leads to branching microtubule nucleation*

Microtubules are protein polymers with a variety of roles in biological cells. During cell division, many microtubules are generated by branching from the surface of preexisting microtubules. Recent work in vitro and ex vivo shows that the protein TPX2 drives the nucleation of branched microtubules by bringing augmin and the gamma-tubulin ring complex (gTuRC) to microtubules. It has also been shown that TPX2’s ability to condense into a liquid phase is important for branching. Using atomic force microscopy, fluorescence imaging, electron microscopy, and hydrodynamic theory, we show that the dynamics of liquid TPX2 are crucial for nucleating branched microtubules. Initially, TPX2 coats the surface of microtubules in seconds, producing a cylindrical, liquid tube around the microtubule with an even thickness of 13-17 nm. Then, this layer loses stability due to surface tension, producing discrete droplets regularly spaced by 140-250 nm via a Rayleigh-Plateau mechanism. These droplets bind augmin and gTuRC to the microtubule surface, thus localizing where branches can form. Together, our work explains how the liquid phase of TPX2 leads to branching microtubule nucleation.