A Site-Specific Self-Association of a Protein Hub Drives Its Phase Separation.

Liquid-liquid phase separation (LLPS) is pivotal in generating membraneless organelles and assembling cellular inclusions. Interactions mediated by RNA and intrinsically disordered regions of proteins are ubiquitous mechanisms that drive their LLPS. Here, we identify that a site-specific interaction stimulates the LLPS of WDR5, a chromatin-associated protein hub. Our study proves that WDR5 undergoes self-association between its N-terminal intrinsically disordered region and a multitasking binding site. This mechanism facilitates the formation of liquid droplets in a cell-free environment. Notably, WDR5 undergoes phase separation in mammalian cells, forming nuclear puncta (NP) in response to osmotic stress. Further, nuclear WDR5 condensates encompass a critical oncoprotein transcription factor, MYC, and WDR5-binding RNA under hyperosmotic conditions. Our findings suggest that RNA modulates WDR5 phase separation and influences nuclear puncta formation, potentially serving as a general stress response mechanism. These outcomes illuminate a distinctive mechanochemical signaling process, highlighting the functional interplay among WDR5, RNA, and MYC at the chromatin level, particularly during osmotically induced LLPS.