Analysis of the assembly, stabilization and maturation of multiphasic TAZ biomolecular condensates

Phase separation is an important mechanism ensuring efficient regulation and function in Hippo signaling. Particularly, phase separation of nuclear TAZ has been demonstrated to be essential for its activity. However, the mechanisms of TAZ condensate assembly and maturation are yet undefined. Here we explored these mechanisms using FRAP with two laser beam sizes complemented by microscopy and cell biology approaches. We show that TAZ condensates are multiphasic, with a more stable core and labile periphery. TAZ initially forms small nascent clusters, likely via self-nucleation through the CC domain. These gradually mature into larger condensates through interaction with additional proteins via the WW domain. The condensates are further stabilized/activated by interaction with transcription factors and complexes including TEAD4 and P-TEFb. Of note, the ability of TAZ to form mature condensates is essential for its activities in cellular morphogenesis and tumorigenesis. Our study presents detailed mechanistic analysis of TAZ phase separation, revealing a highly dynamic nature of TAZ condensate maturation and activation.