Phase separation of a microtubule plus-end tracking protein into a fluid fractal network

Microtubule plus-end tracking proteins (+TIPs) are involved in virtually all microtubule-based cellular processes, and it has been recently proposed that they function as liquid condensates. However, the formation process and internal organization of +TIP condensates are poorly understood. Here, we have investigated the phase separation of the CLIP-170 family member Bik1, a key +TIP implicated in budding yeast cell division. We found that Bik1 is a rod-shaped dimer whose conformation is dominated by its central coiled-coil domain. Liquid condensation is accompanied by Bik1 conformational rearrangements, leading to a 2-3-fold rise in interactions between the proteins folded and disordered domains. In contrast to classical liquids, the supramolecular structure of the Bik1 condensate is heterogeneous, with a fractal structure of protein-rich and protein-free domains. This observation provides structural evidence in support of recent models of biomolecular condensates based on percolation. More broadly, our results provide insights into the structure, dynamic rearrangement, and organization of a complex, multidomain protein in its dilute and condensed phases. Our experimental framework can be extended to other biomolecular condensates, including more intricate +TIP networks.