Phase Separation of YAP Reprograms Cells for Long-term YAP Target Gene Expression

Yes-associated Protein (YAP) regulates cell proliferation and survival, and is over-expressed in most malignant tumors. As a transcriptional co-activator, its nuclear localization is the key determinant of its function. Recent work has revealed that hyperosmotic stress leads to YAP nuclear translocation and target gene expression, but how this overrides canonical YAP regulation by the Hippo pathway is unclear. Here we showed that YAP has an intrinsic ability to form liquid-like condensates when macromolecular crowding inside a cell increases, and this caused YAP to translocate to the nucleus, initiating downstream YAP signaling. Within seconds of inducing macromolecular crowding by hyperosmotic stress, YAP partitioned into cytoplasmic and nuclear droplets. Cytoplasmic YAP droplets concentrated YAP into an environment where it could be phosphorylated by NLK for nuclear translocation, whereas YAP nuclear droplets (enriched in TEAD1 and TAZ) functioned to re-organize gene enhancer elements, readying transcription of proliferation genes once the cell had either adapted to or been relieved of hyperosmotic stress. Thus, principles of liquid phase partitioning are used for reprogramming cells into a YAP-dependent, proliferative state in response to macromolecular crowding.