Treacle’s ability to form liquid phase condensates is essential for nucleolar fibrillar center assembly, efficient rRNA transcription and processing, and rRNA gene repair

Abstract

It is known that major nuclear processes occur differently in the nucleolus when compared to the remainder of the nucleus. This variation could be associated with the nature of the nucleolus, which is considered a multicomponent liquid environment. Among the numerous nucleolar proteins, Treacle holds a special place due to a wealth of experimental evidence highlighting its significance for transcription as well as DNA repair in the nucleolus. However, it remains unclear how Treacle exerts its multifaceted influence on nucleolar processes from a mechanistic perspective. In this study, we demonstrate that Treacle can undergo phase separation in living cells and characterize in detail the structural basis of its condensation and generation of cooperative Treacle-TOPBP1 liquid assemblies. Our data clearly show that the phase separation of Treacle serves a key role in ribosomal DNA transcription and DNA repair. In these processes, Treacle biomolecular condensates serve as a platform for attracting and retaining transcription machinery (UBF1, Pol I) or repair factors (TOPBP1, ATR, and others) on ribosomal genes. Moreover, Treacle and TOPBP1 can form cooperative nuclear biomolecular condensates in the absence of DNA damage. These damage-independent Treacle-TOPBP1 liquid droplets act as fully functional molecular platforms for DNA damage response activation. Our data indicate that by forming biomolecular condensates, Treacle can facilitate intracellular processes and target them to the required intranuclear locations.