Phosphorylation-mediated interactions with TOPBP1 couple 53BP1 and 9-1-1 to control the G1 DNA damage checkpoint.

Coordination of the cellular response to DNA damage is organised by multi-domain 'scaffold' proteins, including 53BP1 and TOPBP1, which recognise post-translational modifications such as phosphorylation, methylation and ubiquitylation on other proteins, and are themselves carriers of such regulatory signals. Here we show that the DNA damage checkpoint regulating S-phase entry is controlled by a phosphorylation-dependent interaction of 53BP1 and TOPBP1. BRCT domains of TOPBP1 selectively bind conserved phosphorylation sites in the N-terminus of 53BP1. Mutation of these sites does not affect formation of 53BP1 or ATM foci following DNA damage, but abolishes recruitment of TOPBP1, ATR and CHK1 to 53BP1 damage foci, abrogating cell cycle arrest and permitting progression into S-phase. TOPBP1 interaction with 53BP1 is structurally complimentary to its interaction with RAD9-RAD1-HUS1, allowing these damage recognition factors to bind simultaneously to the same TOPBP1 molecule and cooperate in ATR activation in the G1 DNA damage checkpoint.

Drosha drives the formation of DNA:RNA hybrids around DNA break sites to facilitate DNA repair.

The error-free and efficient repair of DNA double-stranded breaks (DSBs) is extremely important for cell survival. RNA has been implicated in the resolution of DNA damage but the mechanism remains poorly understood. Here, we show that miRNA biogenesis enzymes, Drosha and Dicer, control the recruitment of repair factors from multiple pathways to sites of damage. Depletion of Drosha significantly reduces DNA repair by both homologous recombination (HR) and non-homologous end joining (NHEJ). Drosha is required within minutes of break induction, suggesting a central and early role for RNA processing in DNA repair. Sequencing of DNA:RNA hybrids reveals RNA invasion around DNA break sites in a Drosha-dependent manner. Removal of the RNA component of these structures results in impaired repair. These results show how RNA can be a direct and critical mediator of DNA damage repair in human cells.

Repair of DNA Double-Strand Breaks in Heterochromatin.

DNA double-strand breaks (DSBs) are among the most damaging lesions in DNA, since, if not identified and repaired, they can lead to insertions, deletions or chromosomal rearrangements. DSBs can be in the form of simple or complex breaks, and may be repaired by one of a number of processes, the nature of which depends on the complexity of the break or the position of the break within the chromatin. In eukaryotic cells, nuclear DNA is maintained as either euchromatin (EC) which is loosely packed, or in a denser form, much of which is heterochromatin (HC). Due to the less accessible nature of the DNA in HC as compared to that in EC, repair of damage in HC is not as straightforward as repair in EC. Here we review the literature on how cells deal with DSBs in HC.

Human BRCA1-BARD1 ubiquitin ligase activity counteracts chromatin barriers to DNA resection.

The opposing activities of 53BP1 and BRCA1 influence pathway choice in DNA double-strand-break repair. How BRCA1 counteracts the inhibitory effect of 53BP1 on DNA resection and homologous recombination is unknown. Here we identify the site of BRCA1-BARD1 required for priming ubiquitin transfer from E2∼ubiquitin and demonstrate that BRCA1-BARD1's ubiquitin ligase activity is required for repositioning 53BP1 on damaged chromatin. We confirm H2A ubiquitination by BRCA1-BARD1 and show that an H2A-ubiquitin fusion protein promotes DNA resection and repair in BARD1-deficient cells. BRCA1-BARD1's function in homologous recombination requires the chromatin remodeler SMARCAD1. SMARCAD1 binding to H2A-ubiquitin and optimal localization to sites of damage and activity in DNA repair requires its ubiquitin-binding CUE domains. SMARCAD1 is required for 53BP1 repositioning, and the need for SMARCAD1 in olaparib or camptothecin resistance is alleviated by 53BP1 loss. Thus, BRCA1-BARD1 ligase activity and subsequent SMARCAD1-dependent chromatin remodeling are critical regulators of DNA repair.

Sumoylation of eIF4A2 affects stress granule formation.

Regulation of protein synthesis is crucial for cells to maintain viability and to prevent unscheduled proliferation that could lead to tumorigenesis. Exposure to stress results in stalling of translation, with many translation initiation factors, ribosomal subunits and mRNAs being sequestered into stress granules or P bodies. This allows the re-programming of the translation machinery. Many aspects of translation are regulated by post-translational modification. Several proteomic screens have identified translation initiation factors as targets for sumoylation, although in many cases the role of this modification has not been determined. We show here that eIF4A2 is modified by SUMO, with sumoylation occurring on a single residue (K226). We demonstrate that sumoylation of eIF4A2 is modestly increased in response to arsenite and ionising radiation, but decreases in response to heat shock or hippuristanol. In arsenite-treated cells, but not in hippuristanol-treated cells, eIF4A2 is recruited to stress granules, suggesting sumoylation of eIF4A2 correlates with its recruitment to stress granules. Furthermore, we demonstrate that the inability to sumoylate eIF4A2 results in impaired stress granule formation, indicating a new role for sumoylation in the stress response.

ATM Localization and Heterochromatin Repair Depend on Direct Interaction of the 53BP1-BRCT2 Domain with γH2AX.

53BP1 plays multiple roles in mammalian DNA damage repair, mediating pathway choice and facilitating DNA double-strand break repair in heterochromatin. Although it possesses a C-terminal BRCT2 domain, commonly involved in phospho-peptide binding in other proteins, initial recruitment of 53BP1 to sites of DNA damage depends on interaction with histone post-translational modifications--H4K20me2 and H2AK13/K15ub--downstream of the early γH2AX phosphorylation mark of DNA damage. We now show that, contrary to current models, the 53BP1-BRCT2 domain binds γH2AX directly, providing a third post-translational mark regulating 53BP1 function. We find that the interaction of 53BP1 with γH2AX is required for sustaining the 53BP1-dependent focal concentration of activated ATM that facilitates repair of DNA double-strand breaks in heterochromatin in G1.

Weighing up the possibilities: Controlling translation by ubiquitylation and sumoylation.

Regulation of protein synthesis is of fundamental importance to cells. It has a critical role in the control of gene expression, and consequently cell growth and proliferation. The importance of this control is supported by the fact that protein synthesis is frequently upregulated in tumor cells. The major point at which regulation occurs is the initiation stage. Initiation of translation involves the interaction of several proteins to form the eIF4F complex, the recognition of the mRNA by this complex, and the subsequent recruitment of the 40S ribosomal subunit to the mRNA. This results in the formation of the 48S complex that then scans the mRNA for the start codon, engages the methionyl-tRNA and eventually forms the mature 80S ribosome which is elongation-competent. Formation of the 48S complex is regulated by the availability of individual initiation factors and through specific protein-protein interactions. Both of these events can be regulated by post-translational modification by ubiquitin or Ubls (ubiquitin-like modifiers) such as SUMO or ISG15. We provide here a summary of translation initiation factors that are modified by ubiquitin or Ubls and, where they have been studied in detail, describe the role of these modifications and their effects on regulating protein synthesis.

Linking up and interacting with BRCT domains.

BRCT domains are present in an ever expanding family of proteins that includes many DNA repair and checkpoint proteins. The most prominent member of the BRCT family is BRCA1, mutations in which are responsible for a high proportion of breast and ovarian cancers. BRCT domains act as protein-protein interaction modules and facilitate the formation of hetero- and homo-oligomers. The domains occur either singly or in pairs, with up to eight domains in a single protein. When in pairs the domains are separated by a short inter-BRCT linker. Numerous crystal structures have been determined for BRCT domains from a range of different proteins, which indicate that the overall structure of the BRCT domains is generally well conserved. In contrast, the positions and structures of the linker regions are more varied, as are the roles of the linkers. Here, we describe the protein-protein interactions involving three different inter-BRCT linker regions, those of DNA ligase IV (LigIV), Schizosaccharomyces pombe Crb2 and human 53BP1.

Nep1, a Schizosaccharomyces pombe deneddylating enzyme.

Nedd8 is a ubiquitin-like modifier that is attached to the cullin components of E3 ubiquitin ligases. More recently, p53 has also been shown to be Nedd8-modified. Nedd8 attachment occurs in a manner similar to that observed for other ubiquitin-like modifiers. In the present study, we report on the characterization of Nep1, a deneddylating enzyme in fission yeast (Schizosaccharomyces pombe). Unlike loss of ned8, deletion of the nep1 gene is not lethal, although nep1.d cells are heterogeneous in length, suggesting a defect in cell-cycle progression. Viability of nep1.d cells is dependent on a functional spindle checkpoint but not on the DNA integrity checkpoint. Deletion of a related gene (nep2), either alone or in combination with nep1.d, also has little effect on cell viability. We show that Nep1 can deneddylate the Pcu1, Pcu3 and Pcu4 cullins in vitro and that its activity is sensitive to N-ethylmaleimide, consistent with the idea that it is a member of the cysteine protease family. nep1.d cells accumulate Nedd8-modified proteins, although these do not correspond to modified forms of the cullins, suggesting that, although Nep1 can deneddylate cullins in vitro, this is not its main function in vivo. Nep1 can be co-precipitated with the signalosome subunit Csn5. Nep1 itself is present in a high-molecular-mass complex, but the presence of this complex is not dependent on the production of intact signalosomes. Our results suggest that, in vivo, Nep1 may be responsible for deneddylating proteins other than cullins.

Composition and architecture of the Schizosaccharomyces pombe Rad18 (Smc5-6) complex.

The rad18 gene of Schizosaccharomyces pombe is an essential gene that is involved in several different DNA repair processes. Rad18 (Smc6) is a member of the structural maintenance of chromosomes (SMC) family and, together with its SMC partner Spr18 (Smc5), forms the core of a high-molecular-weight complex. We show here that both S. pombe and human Smc5 and -6 interact through their hinge domains and that four independent temperature-sensitive mutants of Rad18 (Smc6) are all mutated at the same glycine residue in the hinge region. This mutation abolishes the interactions between the hinge regions of Rad18 (Smc6) and Spr18 (Smc5), as does mutation of a conserved glycine in the hinge region of Spr18 (Smc5). We purified the Smc5-6 complex from S. pombe and identified four non-SMC components, Nse1, Nse2, Nse3, and Rad62. Nse3 is a novel protein which is related to the mammalian MAGE protein family, many members of which are specifically expressed in cancer tissue. In initial steps to understand the architecture of the complex, we identified two subcomplexes containing Rad18-Spr18-Nse2 and Nse1-Nse3-Rad62. The subcomplexes are probably bridged by a weaker interaction between Nse2 and Nse3.

Cell-cycle-dependent localisation of Ulp1, a Schizosaccharomyces pombe Pmt3 (SUMO)-specific protease.

We report here on the characterisation of Ulp1, a component of the SUMO modification process in S. pombe. Recombinant S. pombe Ulp1 has de-sumoylating activity; it is involved in the processing of Pmt3 (S. pombe SUMO) and can, to a limited extent, remove Pmt3 from modified targets in S. pombe cell extracts. ulp1 is not essential for cell viability, but cells lacking the gene display severe cell and nuclear abnormalities. ulp1-null (ulp1.d) cells are sensitive to ultraviolet radiation in a manner similar to rad31.d and hus5.62, which have mutations in one subunit of the activator and the conjugator for the ubiquitin-like protein SUMO respectively. However ulp1.d cells are less sensitive to ionising radiation and hydroxyurea (HU) than are rad31.d and hus5.62. ulp1-null cells are defective in processing precursor Pmt3 and display reduced levels of Pmt3 conjugates compared with wild-type cells. The slow growth phenotype of ulp1 null cells is not substantially rescued by over-expression of the mature form of Pmt3 (Pmt3-GG), suggesting that the de-conjugating activity of Ulp1 is required for normal cell cycle progression. During the S and G2 phases of the cell cycle the Ulp1 protein is localised to the nuclear periphery. However, during mitosis the pattern of staining alters, and during anaphase, Ulp1 is observed within the nucleus. Ulp1 localisation at the nuclear periphery is generally re-established by the time of septation (S phase).