BRCC36-deubiquitinating enzyme (DUB) forms two different complexes through interactions with two different adaptor proteins Abraxas and ABRO1 in cells.Abraxas mainly localizes in the nucleus, mediating the interaction of BRCC36 with BRCA1. ABRO1 is mainly localized in the cytoplasm. Because it lacks the BRCA1-interacting motif, the ABRO1 complex does not interact with BRCA1. Both BRCC36-containing complexes contain common components including BRE and NBA1/MERIT40. Here, we found that the two complexes are assembled in a similar manner and NBA1 and BRE interaction is critical for maintaining the integrity of both of the complexes. Knockdown of NBA1 or BRE leads to decreased levels of components of the two BRCC36-containing complexes. We provided evidence that NBA1 interacts with BRE through a C-terminal conserved motif of the NBA1 protein and a C-terminal UEV domain of the BRE protein. Furthermore, the NBA1-BRE interaction is required for cellular resistance to ionizing irradiation and NBA1's role in recruiting BRCA1 to DNA damage sites. Together, these studies reveal critical interactions required for the formation and function of BRCC36-containing DUB complexes.Modification of proteins by the covalent attachment of ubiquitin (Ub) 2 to the lysine (Lys) residue of a target protein is a key regulatory mechanism of many cellular processes including the DNA damage response (1, 2). In response to ionizing irradiation, ubiquitination occurs at DNA damage sites in a central DNA damage kinases ATM/ATR-dependent manner (3). Ubiquitin Lys63 (K63) polyubiquitin chain linkages play important roles in the recruitment of repair factors in the DNA damage response (3). K63-linked polyubiquitin chains are assembled through a conserved heterodimer of an E2 complex composed of a catalytically active Ubc13 subunit and its inactive sequence homolog Mms2 that lacks the enzymatic active site (4, 5). The formation of K63-polyubiquitins at DNA damage sites depends on two E3 ligases RNF8 and RNF168 (3,6). In addition, a HECT domain-containing E3 ligase HERC2 is also involved in facilitating the formation of K63-linked ubiquitin chains (7).Deubiquitinating enzymes (DUBs) catalyze the removal of Ub from Ub-conjugated substrate proteins and it has become increasingly obvious that Ub deconjugation plays important roles in regulating Ub-dependent pathways (8, 9). BRCC36 is a member of a small family of DUBs containing JAMM/MPNϩ domain proteins, which are Zn 2ϩ -binding metalloproteases (10 -12). It selectively cleaves K63-linked polyubiquitin (12-14), and is required for proper checkpoint regulation in response to DNA damage (13,(15)(16)(17). It plays an important role in recruiting BRCA1 to DNA damage sites, as BRCC36 deficiency leads to decreased BRCA1 accumulation at DNA damage sites (3).BRCC36 is a component of the BRCA1 A complex (13, 17, 18). The BRCA1 A complex contains at least five different components, Abraxas, NBA1/MERIT40, BRE, Rap80, and BRCC36. It associates with BRCA1 through an interaction of Abraxas with the BRCA1 C-termina...
Summary Germline mutations of BRCA1 confer hereditary susceptibility to breast and ovarian cancer. However, somatic mutation of BRCA1 is infrequent in sporadic breast cancers. The BRCA1 protein C-terminus BRCT domains interact with multiple proteins and are required for BRCA1's tumor suppressor function. In this study, we demonstrated that Abraxas, a BRCA1 BRCT domain-interacting protein, plays a role in tumor suppression. Abraxas exerts its function through binding to BRCA1 to regulate DNA repair and maintain genome stability. Both homozygous and heterozygous Abraxas knockout mice exhibited decreased survival and increased tumor incidence. The gene encoding Abraxas suffers from gene copy loss and somatic mutations in multiple human cancers including breast, ovarian, and endometrial cancers, suggesting that mutation and loss of function of Abraxas may contribute to tumor development in human patients.
Protection of the stalled replication fork is crucial for responding to replication stress and minimizing its impact on chromosome instability, thus preventing diseases, including cancer. We found a new component, Abro1, in the protection of stalled replication fork integrity. Abro1 deficiency results in increased chromosome instability, and Abro1-null mice are tumor-prone. We show that Abro1 protects stalled replication fork stability by inhibiting DNA2 nuclease/WRN helicase-mediated degradation of stalled forks. Depletion of RAD51 prevents the DNA2/WRN-dependent degradation of stalled forks in Abro1-deficient cells. This mechanism is distinct from the BRCA2-dependent fork protection pathway, in which stable RAD51 filament formation prevents MRE11-dependent degradation of the newly synthesized DNA at stalled forks. Thus, our data reveal a new aspect of regulated protection of stalled replication forks that involves Abro1.
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