Capillary forces from biomolecular condensates on microtubules*

The microtubule-based spindle organizes and segregates chromosomes during cell division. Thousands of microtubules nucleate rapidly and exhibit complex cooperative behavior, which is facilitated by numerous microtubule-associated proteins. A biomolecular condensate of the microtubule-associated protein TPX2 has been found to wet microtubule surfaces, in order to recruit and concentrate factors important for microtubule nucleation. Here, using atomic force microscopy, we directly measure the attractive capillary interactions on TPX2-coated microtubule surfaces. We find that the capillary interactions are significant and can therefore underly the functional role of TPX2 in microtubule nucleation and organization. The capillary interactions are influenced by the concentration of salt, which can affect the phase behavior of TPX2. We construct thermodynamically-consistent mathematical models based on a modified Voorn-Overbeek theory to explain our observations.