The ubiquitin E3 ligase activity of BRCA1 and its biological functions

Abstract

The basal-like breast cancer, a new category of breast cancer associated with poor prognosis and possibly unique chemosensitivity, is a current topic in the breast cancer field. Evidence from multiple sources strongly indicate that impairment of BRCA1 pathways is responsible for this phenotype, implying the importance of BRCA1 not only in familial breast cancers but also in sporadic cancers. BRCA1 acts as a hub protein that coordinates a diverse range of cellular pathways to maintain genomic stability. BRCA1 participates in multiple cellular supercomplexes to execute its tasks and, in most of the complexes, BRCA1 exists as a RING heterodimer with BARD1 to provide ubiquitin E3 ligase activity that is required for its tumor suppressor function. It was revealed recently that the BRCA1 RING finger is capable of catalyzing multiple types of ubiquitination depending upon the interacting E2, the ubiquitin carrier protein. BRCA1 may catalyze distinct ubiquitination on different substrates as the situation demands. On the other hand, in response to DNA double-strand breaks where BRCA1 plays its major role for homologous recombination repair, recent evidence showed that ubiquitination is a critical step to recruit BRCA1 to the damaged site through UIM (ubiquitin interacting motif) containing protein RAP80. Thus, ubiquitin and BRCA1 likely affect each other in many ways to perform cellular functions. Elucidation of this mechanism in relation to cell survival is now much anticipated because it could be a key to predict chemosensitivity of basal-like breast cancer.

Figure 1

BRCA1 supercomplex formation in response to DNA damage. BRCA1 acts in many cellular supercomplexes and executes distinct roles. BARD1 stoichiometrically interacts with BRCA1 and exists as a RING heterodimer in all the complexes shown, indicating the importance of E3 ligase activity in each complex. (a) BRCA1-BARD1 in the Pol II-holoenzyme may act as a transcriptional co-activator. In addition, BRCA1 that interacts with hyperphosphorylated Pol II acts as a sensor for DNA damage. After some types of DNA damage, BRCA1 dissociates with Pol II. BRCA1-BARD1 may polyubiquitinate subunits of this complex at this time, including the hyperphosphorylated largest subunit (RPB1) and the common small subunit (RPB8). (b) BRCA1 that interacts with BACH1/BRIP1/FANCJ at its C-terminus locates the replication point with BARD1. This complex interacts with TopBP1 in an ATM dependent manner after DNA damage and causes TopBP1 dissociation from the replication point. This results in the failure to recruit the replication initiation factor Cdc45, likely engaging the intra S-phase checkpoint. (c) Mre11-Rad50-Nbs1 also forms a complex with BRCA1-BARD1 at sites of DNA damage. This BRCA1 complex interacts with CtIP via the BRCA1 C-terminus. Complex formation depends on ATM- and Chk2-mediated phosphorylation. This complex signals the G2/M checkpoint and bridges the two DNA ends of the DSB, an intermediate of both non-homologous end joining (NHEJ) and homologous recombination (HR). (d) BRCA1 constitutes S-phase nuclear foci with BARD1, Rad51 and BRCA2. After DNA damage, these once disperse foci again localize to damaged DNA sites. BRCA1 is necessary for recruiting these proteins. This complex is important for homologous recombination repair of DSB. (e) BRCA1-BARD1 is targeted to polyubiquitin chains at DNA damaged sites through BRCT domain-interacting phosphorylated ABRA1 that interacts with UIM containing protein RAP80.

Figure 2

Models for ubiquitin accumulation at DSB. Upon DSBs, γH2AX could be monoubiquitinated (a) so that RNF8-Ubc13-Mms2 can attach additional Lys63-linked polyubiquitin chains on the ubiquitin (b). The E2 and E3 for monoubiquitination are unknown. BRCA1-BARD1 is then targeted to the polyubiquitin chains through RAP80-ABRA1 in a manner dependent on ABRA1 phosphorylation. BRCA1-BARD1 interacts with UbcH5c and mediates further polyubiquitin chains, including Lys6-linked chains (c). The BRCA1-BARD1-induced polyubiquitin chains further recruit additional BRCA1-BARD1 complexes to damaged sites, resulting in focal accumulation of polyubiquitin chains (d). The polyubiquitin chain assembly is likely required for BRCA2-Rad51 recruitment to damaged sites, allowing execution of homologous recombination repair. Recent studies suggest that the BRCA1-BARD1 heterodimer could also catalyze substrate monoubiquitination with substrate-specific E2s, such as UbcH6, Ube2e2, UbcM2, and Ube2w, as well as additional Lys63-linked polyubiquitin chains with Ubc13-Mms2. The substrate(s) and the timing of this action remain to be determined.