Hierarchical Assembly of Microtubule Matter

The fundamental organizational principles used by the cell to organize its interior space is still not fully elucidated. Two important mechanisms for cellular self-organization include liquid-liquid phase separation of protein species to form membraneless organelles and self-organization of cytoskeletal filaments into larger-scale arrangements such as the actin cortex and microtubule mitotic spindle. We have observed that the microtubule-crosslinking proteins, MAP65 and PRC1, can self-assemble into liquid-like condensates that can catalyze spatially controlled nucleation and growth of microtubule asters with control over the aster organization. Microtubules grown from gel-like condensates can reverse the gelation, controlling the material properties. This interplay between protein condensates and the cytoskeleton could allow for dynamic reorganization of the cell interior in space and time.