Summary From an shRNA screen, we identified ClpP as a member of the mitochondrial proteome whose knockdown reduced the viability of K562 leukemic cells. Expression of this mitochondrial protease that has structural similarity to the cytoplasmic proteosome is increased in the leukemic cells from approximately half of patients with AML. Genetic or chemical inhibition of ClpP killed cells from both human AML cell lines and primary samples in which the cells showed elevated ClpP expression, but did not affect their normal counterparts. Importantly, Clpp knockout mice were viable with normal hematopoiesis. Mechanistically, we found ClpP interacts with mitochondrial respiratory chain proteins and metabolic enzymes, and knockdown of ClpP in leukemic cells inhibited oxidative phosphorylation and mitochondrial metabolism.

Dynamic changes in histone modification are critical for regulating DNA double-strand break (DSB) repair. Activation of the Tip60 acetyltransferase by DSBs requires interaction of Tip60 with histone H3 methylated on lysine 9 (H3K9me3). However, how H3K9 methylation is regulated during DSB repair is not known. Here, we demonstrate that a complex containing kap-1, HP1, and the H3K9 methyltransferase suv39h1 is rapidly loaded onto the chromatin at DSBs. Suv39h1 methylates H3K9, facilitating loading of additional kap-1/HP1/suv39h1 through binding of HP1's chromodomain to the nascent H3K9me3. This process initiates cycles of kap-1/HP1/ suv39h1 loading and H3K9 methylation that facilitate spreading of H3K9me3 and kap-1/HP1/suv39h1 complexes for tens of kilobases away from the DSB. These domains of H3K9me3 function to activate the Tip60 acetyltransferase, allowing Tip60 to acetylate both ataxia telangiectasia-mutated (ATM) kinase and histone H4. Consequently, cells lacking suv39h1 display defective activation of Tip60 and ATM, decreased DSB repair, and increased radiosensitivity. Importantly, activated ATM rapidly phosphorylates kap-1, leading to release of the repressive kap-1/HP1/suv39h1 complex from the chromatin. ATM activation therefore functions as a negative feedback loop to remove repressive suv39h1 complexes at DSBs, which may limit DSB repair. Recruitment of kap-1/HP1/suv39h1 to DSBs therefore provides a mechanism for transiently increasing the levels of H3K9me3 in open chromatin domains that lack H3K9me3 and thereby promoting efficient activation of Tip60 and ATM in these regions. Further, transient formation of repressive chromatin may be critical for stabilizing the damaged chromatin and for remodeling the chromatin to create an efficient template for the DNA repair machinery.histone methylation | homologous recombination D NA double-strand breaks (DSBs) are toxic and must be repaired to maintain genomic stability. Detection of DSBs requires recruitment of the mre11-rad50-nbs1 (MRN) complex to the DNA ends (1). MRN then recruits and activates the ataxia telangiectasia-mutated (ATM) kinase (2, 3) through a mechanism that also requires the Tip60 acetyltransferase (3). Tip60 directly acetylates and activates ATM's kinase activity (4-6) and functions, in combination with MRN, to promote ATM-dependent phosphorylation of DSB repair proteins (3), including histone H2AX. This process creates domains of phosphorylated H2AX (γH2AX) extending for hundreds of kilobases along the chromatin (7,8). Mdc1 then binds to γH2AX, providing a landing pad for other DSB repair proteins, including the RNF8/RNF168 ubiquitin ligases (1, 3, 9, 10). Tip60 also plays a critical role in regulating chromatin structure at DSBs as part of the NuA4-Tip60 complex (11). NuA4-Tip60 catalyzes histone exchange (via the p400 ATPase subunit) and acetylation of histone H4 (by Tip60) at DSBs (12-15), leading to the formation of open, flexible chromatin domains adjacent to the break (12, 13). These open chromatin structures then facilitate histone ub...

Chromatin remodeling during DNA double-strand break (DSB) repair is required to facilitate access to and repair of DSBs. This remodeling requires increased acetylation of histones and a shift in nucleosome organization to create open, relaxed chromatin domains. However, the underlying mechanism driving changes in nucleosome structure at DSBs is poorly defined. Here, we demonstrate that histone H2A.Z is exchanged onto nucleosomes at DSBs by the p400 remodeling ATPase. H2A.Z exchange at DSBs shifts the chromatin to an open conformation, and is required for acetylation and ubiquitination of histones and for loading of the brca1 complex. H2A.Z exchange also restricts single-stranded DNA production by nucleases and is required for loading of the Ku70/80 DSB repair protein. H2A.Z exchange therefore promotes specific patterns of histone modification and reorganization of the chromatin architecture, leading to the assembly of a chromatin template which is an efficient substrate for the DSB repair machinery.

BLM encodes a member of the highly conserved RecQ DNA helicase family, which is essential for the maintenance of genome stability. Homozygous inactivation of BLM gives rise to the cancer predisposition disorder Bloom's syndrome. A common feature of many RecQ helicase mutants is a hyperrecombination phenotype. In Bloom's syndrome, this phenotype manifests as an elevated frequency of sister chromatid exchanges and interhomologue recombination. We have shown previously that BLM, together with its evolutionarily conserved binding partner topoisomerase III␣ (hTOPO III␣), can process recombination intermediates that contain double Holliday junctions into noncrossover products by a mechanism termed dissolution. Here we show that a recently identified third component of the human BLM͞hTOPO III␣ complex, BLAP75͞ RMI1, promotes dissolution catalyzed by hTOPO III␣. This activity of BLAP75͞RMI1 is specific for dissolution catalyzed by hTOPO III␣ because it has no effect in reactions containing either Escherichia coli Top1 or Top3, both of which can also catalyze dissolution in a BLM-dependent manner. We present evidence that BLAP75͞RMI1 acts by recruiting hTOPO III␣ to double Holliday junctions. Implications of the conserved ability of type IA topoisomerases to catalyze dissolution and how the evolution of factors such as BLAP75͞RMI1 might confer specificity on the execution of this process are discussed.Bloom's syndrome ͉ Holliday junction dissolution ͉ topoisomerase III ͉ sister chromatid exchanges T he RecQ family of DNA helicases is essential for the maintenance of genome stability (1). The human genome contains five RecQ helicase genes. Mutations in three of these genes give rise to clinically defined cancer predisposition disorders (2). One of these disorders is Bloom's syndrome (BS), which is caused by biallelic mutations in the BLM gene (3). The BLM protein is a 3Ј-5Ј DNA helicase that processes a broad range of structurally diverse DNA substrates (4-7). These substrates include DNA structures that arise during homologous recombination, such as D-loops and Holliday junctions (5, 6). These structures are of particular relevance to the BS phenotype because BS cells display elevated levels of homologous recombination (8). This hyperrecombination phenotype is also a feature of Saccharomyces cerevisiae and Schizosaccharomyces pombe mutants defective in their respective BLM orthologs, SGS1 and rqh1 ϩ (9-11). In the case of BS cells, recombination events are particularly apparent between sister chromatids, and such recombination events are termed sister chromatid exchanges (SCEs) (8). These exchanges arise primarily as a consequence of crossing-over during the processing of recombination intermediates (12).BLM exists in a complex with topoisomerase III␣ (hTOPO III␣), a type IA topoisomerase (13,14). This complex is evolutionarily conserved, and functional and͞or physical interactions between RecQ helicases and type IA topoisomerases have also been demonstrated in bacteria and yeast (9, 15-17). Two type IA topoisomerases ar...

Bloom's syndrome (BS) is a rare human genetic disorder characterized by dwarfism, immunodeficiency, genomic instability and cancer predisposition. We have previously purified three complexes containing BLM, the helicase mutated in this disease. Here we demonstrate that BLAP75, a novel protein containing a putative OB-fold nucleic acid binding domain, is an integral component of BLM complexes, and is essential for their stability in vivo. Consistent with a role in BLM-mediated processes, BLAP75 colocalizes with BLM in subnuclear foci in response to DNA damage, and its depletion impairs the recruitment of BLM to these foci. Depletion of BLAP75 by siRNA also results in deficient phosphorylation of BLM during mitosis, as well as defective cell proliferation. Moreover, cells depleted of BLAP75 display an increased level of sisterchromatid exchange, similar to cells depleted of BLM by siRNA. Thus, BLAP75 is an essential component of the BLM-associated cellular machinery that maintains genome integrity.

However, the limited lithium resource in earth is detrimental to the further application due to the possible increasing cost and unstable energy supply. [12][13][14][15] Therefore, there is an urgent demand for developing alternative energy storage devices with low cost while maintaining a comparable performance to LIBs. Among them, sodium-ion batteries (SIBs) have become the worldwide focus owing to abundant resources and low cost. [16][17][18][19] To develop high-performance SIBs, it remains challenging to discover/develop suitable electrode materials (especially cathode) to satisfy the requirement of long-term cycling stability and rate-capability.Owing to larger radius of Na + than that of Li + (0.98 vs 0.69 Å), various cathode materials with large open frameworks, including layered transitionmetal oxides [19][20][21][22][23][24][25] and polyanionic compounds, [2,13,18,[26][27][28][29][30][31][32][33] have been developed for (1) the improved sodium storage capacity, (2) the facilitated Na + diffusion in the lattice, and (3) the restricted structure degradation caused by Na + insertion/extraction. The NASICON (sodium (Na) super ion conductor) Na x M 2 (NO 4 ) 3 (M = transition metal, N = P 5+ , Si 4+ , S 6+ , and Mo 6+ ) structure with 3D large open framework allows for rapid and reversible ion diffusion in the lattice, which is now developed as electrode with promising electrochemical performance. [34] Among these, the Na 3 V 2 (PO 4 ) 3 (NVP) becomes a "shining star" with high sodium diffusion ability and remarkable high energy density (i.e., 400 Wh kg −1 ). [18] However, the high ionic diffusion ability of NVP is accompanied with poor electronic conductivity, [35] which results in the low utilization of active materials even at low rates. In order to obtain remarkable performance of NVP, the hurdles of poor rate capability and cycling stability need to be further addressed. Recently, carbon-coated active nanocrystals embedded in a porous carbon matrix, which demonstrated excellent rate performance and cycling stability for Li 3 V 2 (PO 4 ) 3 cathodes, [36,37] as well as for NVP cathodes. [38][39][40] In general, the porous carbon content is usually high, which may lead to the decrease of tap density and entire cell volumetric energy density. [41,42] Li and co-workers reported adaptive graphene gel films as a highly compact electrode with Na 3 V 2 (PO 4 ) 3 (NVP) is regarded as a promising cathode for advanced sodiumion batteries (SIBs) due to its high theoretical capacity and stable sodium (Na) super ion conductor (NASICON) structure. However, strongly impeded by its low electronic conductivity, the general NVP delivers undesirable rate capacity and fails to meet the demands for quick charge. Herein, a novel and facile synthesis of layer-by-layer NVP@reduced graphene oxide (rGO) nanocomposite is presented through modifying the surface charge of NVP gel precursor. The well-designed layered NVP@rGO with confined NVP nanocrystal in between rGO layers offers high electronic and ionic conductivity as well as sta...

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The high theoretical capacity and natural abundance of SiO 2 make it a promising high-capacity anode material for lithium-ion batteries. However, its widespread application is significantly hampered by the intrinsic poor electronic conductivity and drastic volume variation. Herein, a unique hollow structured Ni/SiO 2 nanocomposite constructed by ultrafine Ni nanoparticle (≈3 nm) functionalized SiO 2 nanosheets is designed. The Ni nanoparticles boost not only the electronic conductivity but also the electrochemical activity of SiO 2 effectively. Meanwhile, the hollow cavity provides sufficient free space to accommodate the volume change of SiO 2 during repeated lithiation/ delithiation; the nanosheet building blocks reduce the diffusion lengths of lithium ions. Due to the synergistic effect between Ni and SiO 2 , the Ni/SiO 2 composite delivers a high reversible capacity of 676 mA h g −1 at 0.1 A g −1 . At a high current density of 10 A g −1 , a capacity of 337 mA h g −1 can be retained after 1000 cycles.