RAP80 phase separation at DNA double-strand break promotes BRCA1 recruitment

DNA double-strand breaks (DSBs) are among the most severe forms of DNA damage. Effective repair of DSBs is essential for genome stability and preventing tumorigenesis. The BRCA1 tumor suppressor is a central player in homologous recombination repair, and its recruitment to DSB sites is tightly regulated. RAP80 is a known component of the BRCA1-A complex and has been implicated in recruiting BRCA1 to DSBs. However, the timing of RAP80 and BRCA1 recruitment does not always align, suggesting that additional mechanisms may be involved. Recent studies have highlighted liquid-liquid phase separation (LLPS) as a novel regulatory mechanism in cellular signaling. This raises the question of whether LLPS might play a role in RAP80 function and its ability to recruit BRCA1. Prior research has shown that RAP80 interacts with ubiquitin chains, but the functional significance of this interaction remains unclear.

This study aimed to investigate whether RAP80 undergoes LLPS at DSB sites and whether this phase separation is necessary for BRCA1 recruitment. The researchers sought to determine if LLPS contributes to RAP80's function in DNA repair. They also examined the role of ubiquitin chains in modulating RAP80 phase separation. The motivation for this work stems from the observation that RAP80 and BRCA1 recruitment do not always occur simultaneously, suggesting that physical interactions alone may not fully explain BRCA1 recruitment. The study's goal was to uncover new regulatory mechanisms that govern BRCA1 localization to DSBs. Understanding these mechanisms could provide insight into how phase separation influences DNA repair pathways. The researchers hypothesized that RAP80 phase separation is a key step in BRCA1 recruitment and that ubiquitin chains enhance this process.

The researchers used a combination of cellular and in vitro experiments to study RAP80 phase separation. They induced DSBs in cultured cells and monitored RAP80 recruitment and phase separation using fluorescence microscopy. To investigate the structural basis of phase separation, they analyzed RAP80's N-terminal region for intrinsic disorder. In vitro assays were performed to test whether RAP80 could undergo LLPS in the absence of cellular components. The role of ubiquitin chains was assessed using inhibitors and mutagenesis. The researchers also used fluorescence recovery after photobleaching (FRAP) to study the dynamics of RAP80 condensates. To determine the functional significance of phase separation, they disrupted RAP80 condensates and observed the effect on BRCA1 recruitment. These methods allowed them to evaluate the relationship between RAP80 phase separation and BRCA1 localization.

The study found that RAP80 undergoes LLPS at DSB sites in both cellular and in vitro systems. This phase separation was attributed to an intrinsically disordered region (IDR) at the N-terminal end of RAP80. Lys63-linked poly-ubiquitin chains, which form rapidly after DSBs, strongly enhanced RAP80 phase separation. Disrupting RAP80 condensation significantly reduced the formation of BRCA1 foci at DSB sites. This suggests that RAP80 condensates are necessary for recruiting BRCA1 to DNA damage sites. The study also showed that RAP80 phase separation is dynamic and can be modulated by ubiquitin signaling. These findings indicate that LLPS is a key mechanism in RAP80-mediated BRCA1 recruitment. The results support the idea that phase separation contributes to the spatial organization of DNA repair proteins.

The study concluded that RAP80 phase separation at DSB sites is essential for recruiting BRCA1 to DNA damage. The researchers found that this phase separation is driven by an intrinsically disordered region of RAP80 and is enhanced by Lys63-linked ubiquitin chains. Disrupting RAP80 condensates significantly reduced BRCA1 recruitment, suggesting that phase separation is a critical step in this process. The findings indicate that LLPS is a novel regulatory mechanism in DNA repair. The study supports the idea that phase separation contributes to the spatial organization of DNA repair proteins. The results show that ubiquitin signaling modulates RAP80 phase separation and BRCA1 recruitment. These conclusions suggest that LLPS is a key mechanism in the DNA repair pathway. The study provides new insight into how phase separation influences BRCA1 function in DNA repair.