2D31 , 2DYP , 3SHV , 3SQD , 3SZM , 1YDP , 2AZM , 3U3Z
72-514: 3014 15270 ENSG00000188486 ENSMUSG00000049932 P16104 P27661 NM_002105 NM_010436 NP_002096 NP_034566 H2A histone family member X (usually abbreviated as H2AX ) is a type of histone protein from the H2A family encoded by the H2AFX gene. An important phosphorylated form is γH2AX (S139), which forms when double-strand breaks appear. In humans and other eukaryotes ,
144-404: A ' helix turn helix turn helix' motif (DNA-binding protein motif that recognize specific DNA sequence). They also share the feature of long 'tails' on one end of the amino acid structure - this being the location of post-translational modification (see below). Archaeal histone only contains a H3-H4 like dimeric structure made out of a single type of unit. Such dimeric structures can stack into
216-1020: A 3-fold increase in the level of p16INK4a tumor suppressor protein, which is capable of inducing cellular senescence and subsequently halting tumor cell proliferation. This is thought primarily to be the result of methylation of the p16INK4 promotor gene by MRE11 . These data suggest maintaining the integrity and normal expression levels of MRN provides a protective effect against tumorigenesis. Suppression of MRE11 expression in genetically engineered human breast ( MCF7 ) and bone (U2OS) cancer cell lines has resulted in decreased migratory capacity of these cells, indicating MRN may facilitate metastatic spread of cancer. Decreased expression of MMP-2 and MMP-3 matrix metalloproteinases , which are known to facilitate invasion and metastasis, occurred concomitantly in these MRE11 knockdown cells. Similarly, overexpression of Nbs1 in human head and neck squamous cell carcinoma (HNSCC) samples has been shown to induce epithelial–mesenchymal transition (EMT), which itself plays
288-551: A barrier to all DNA-based processes that require recruitment of enzymes to their sites of action. To allow DNA repair, the chromatin must be remodeled . γH2AX, the phosphorylated form of H2AX, is involved in the steps leading to chromatin decondensation after DNA double-strand breaks. γH2AX does not, itself, cause chromatin decondensation, but within 30 seconds of ionizing radiation , RNF8 protein can be detected in association with γH2AX. RNF8 mediates extensive chromatin decondensation, through its subsequent interaction with CHD4 ,
360-526: A component of the nucleosome remodeling and deacetylase complex NuRD . An assay for γH2AX generally reflects the presence of double-strand breaks in DNA, though the assay may indicate other minor phenomena as well. On the one hand, overwhelming evidence supports a strong, quantitative correlation between γH2AX foci formation and DNA double-strand break induction following ionizing radiation exposure, based on absolute yields and distributions induced per unit dose. On
432-470: A core promoter prevents the initiation of transcription in vitro, and Michael Grunstein demonstrated that histones repress transcription in vivo, leading to the idea of the nucleosome as a general gene repressor. Relief from repression is believed to involve both histone modification and the action of chromatin-remodeling complexes. Vincent Allfrey and Alfred Mirsky had earlier proposed a role of histone modification in transcriptional activation, regarded as
504-858: A critical role in cancer metastasis. In this same study, Nbs1 levels were significantly higher in secondary tumor samples than in samples from the primary tumor, providing evidence of a positive correlation between metastatic spread of tumor cells and levels of MRN expression. Taken together, these data suggest at least two of the three subunits of MRN play a role in mediating tumor metastasis, likely via an association between overexpressed MRN and both endogenous (EMT transition) and exogenous (ECM structure) cell migratory mechanisms. Cancer cells almost universally possess upregulated telomere maintenance mechanisms which allows for their limitless replicative potential . The MRN complex's biological role in telomere maintenance has prompted research linking MRN to cancer cell immortality. In human HNSCC cell lines, disruption of
576-569: A higher number of histone variants providing a variety of different functions. Recent data are accumulating about the roles of diverse histone variants highlighting the functional links between variants and the delicate regulation of organism development. Histone variants proteins from different organisms, their classification and variant specific features can be found in "HistoneDB 2.0 - Variants" database. Several pseudogenes have also been discovered and identified in very close sequences of their respective functional ortholog genes. The following
648-547: A left-handed super-helical turn to give a particle of around 100 Angstroms across. The linker histone H1 binds the nucleosome at the entry and exit sites of the DNA, thus locking the DNA into place and allowing the formation of higher order structure. The most basic such formation is the 10 nm fiber or beads on a string conformation. This involves the wrapping of DNA around nucleosomes with approximately 50 base pairs of DNA separating each pair of nucleosomes (also referred to as linker DNA ). Higher-order structures include
720-1167: A marker for double strand breaks, it is important to recognize that it is a down-stream proxy that can be useful for representing DNA damage repair. It does not represent double strand breaks themselves and this needs careful consideration when interpreting data from such assays. The γH2AX-assay has several disadvantages, therefore new assays have been created. H2AX has been shown to interact with: Histone In biology , histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei and in most Archaeal phyla . They act as spools around which DNA winds to create structural units called nucleosomes . Nucleosomes in turn are wrapped into 30- nanometer fibers that form tightly packed chromatin . Histones prevent DNA from becoming tangled and protect it from DNA damage . In addition, histones play important roles in gene regulation and DNA replication . Without histones, unwound DNA in chromosomes would be very long. For example, each human cell has about 1.8 meters of DNA if completely stretched out; however, when wound about histones, this length
792-402: A million base pairs on each side of a DNA double-strand break. MDC1 (mediator of DNA damage checkpoint protein 1) then binds to γH2AX and the γH2AX/MDC1 complex then orchestrates further interactions in double-strand break repair. The ubiquitin ligases RNF8 and RNF168 bind to the γH2AX/MDC1 complex, ubiquitylating other chromatin components. This allows the recruitment of BRCA1 and 53BP1 to
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#1732891395017864-503: A molecular manifestation of epigenetics. Michael Grunstein and David Allis found support for this proposal, in the importance of histone acetylation for transcription in yeast and the activity of the transcriptional activator Gcn5 as a histone acetyltransferase. The discovery of the H5 histone appears to date back to the 1970s, and it is now considered an isoform of Histone H1 . MRN complex The MRN complex (MRX complex in yeast)
936-782: A post-translational modification, and binding domains such as BRCT have been characterised. Most well-studied histone modifications are involved in control of transcription. Two histone modifications are particularly associated with active transcription: Three histone modifications are particularly associated with repressed genes: Analysis of histone modifications in embryonic stem cells (and other stem cells) revealed many gene promoters carrying both H3K4Me3 and H3K27Me3 , in other words these promoters display both activating and repressing marks simultaneously. This peculiar combination of modifications marks genes that are poised for transcription; they are not required in stem cells, but are rapidly required after differentiation into some lineages. Once
1008-526: A process that likely involves homologous recombination. In eukaryotes, the MRN complex (through cooperation of its subunits) has been identified as a crucial player in many stages of the repair process of double-strand DNA breaks: initial detection of a lesion, halting of the cell cycle to allow for repair, selection of a specific repair pathway (i.e., via homologous recombination or non-homologous end joining ) and providing mechanisms for initiating reconstruction of
1080-411: A specific class of major histones but also have their own feature that is distinct from the major histones. These minor histones usually carry out specific functions of the chromatin metabolism. For example, histone H3-like CENPA is associated with only the centromere region of the chromosome. Histone H2A variant H2A.Z is associated with the promoters of actively transcribed genes and also involved in
1152-964: A tall superhelix ("hypernucleosome") onto which DNA coils in a manner similar to nucleosome spools. Only some archaeal histones have tails. The distance between the spools around which eukaryotic cells wind their DNA has been determined to range from 59 to 70 Å. In all, histones make five types of interactions with DNA: The highly basic nature of histones, aside from facilitating DNA-histone interactions, contributes to their water solubility. Histones are subject to post translational modification by enzymes primarily on their N-terminal tails, but also in their globular domains. Such modifications include methylation , citrullination , acetylation , phosphorylation , SUMOylation , ubiquitination , and ADP-ribosylation . This affects their function of gene regulation. In general, genes that are active have less bound histone, while inactive genes are highly associated with histones during interphase . It also appears that
1224-467: A very informative mark and dominates the known histone modification functions. Recently it has been shown, that the addition of a serotonin group to the position 5 glutamine of H3, happens in serotonergic cells such as neurons. This is part of the differentiation of the serotonergic cells. This post-translational modification happens in conjunction with the H3K4me3 modification. The serotonylation potentiates
1296-653: Is a protein complex consisting of Mre11 , Rad50 and Nbs1 (also known as Nibrin in humans and as Xrs2 in yeast). In eukaryotes, the MRN/X complex plays an important role in the initial processing of double-strand DNA breaks prior to repair by homologous recombination or non-homologous end joining . The MRN complex binds avidly to double-strand breaks both in vitro and in vivo and may serve to tether broken ends prior to repair by non-homologous end joining or to initiate DNA end resection prior to repair by homologous recombination. The MRN complex also participates in activating
1368-485: Is a list of human histone proteins, genes and pseudogenes: The nucleosome core is formed of two H2A-H2B dimers and a H3-H4 tetramer, forming two nearly symmetrical halves by tertiary structure ( C2 symmetry; one macromolecule is the mirror image of the other). The H2A-H2B dimers and H3-H4 tetramer also show pseudodyad symmetry. The 4 'core' histones (H2A, H2B, H3 and H4) are relatively similar in structure and are highly conserved through evolution , all featuring
1440-514: Is a necessary component protein for telomere elongation by telomerase. Additionally, knockdown of MRN has been shown to significantly reduce the length of the G-overhang at human telomere ends, which could inhibit the proper formation of the so-called T-loop , destabilizing the telomere as a whole. Telomere lengthening in cancer cells by the alternative lengthening of telomeres ( ALT ) mechanism has also been shown to be dependent on MRN, especially on
1512-621: Is a transcription factor which activates histone gene transcription on chromosomes 1 and 6 of human cells. NPAT is also a substrate of cyclin E-Cdk2, which is required for the transition between G1 phase and S phase. NPAT activates histone gene expression only after it has been phosphorylated by the G1/S-Cdk cyclin E-Cdk2 in early S phase. This shows an important regulatory link between cell-cycle control and histone synthesis. Histones were discovered in 1884 by Albrecht Kossel . The word "histone" dates from
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#17328913950171584-572: Is not entirely unexpected, as it is plausible that an increased rate of DNA replication necessitates higher nuclear levels of the MRN complex. However, there is mounting evidence that MRN is itself a component of carcinogenesis , metastasis and overall cancer aggression. In mice models, mutations in the Nbs1 subunit of MRN alone (producing the phenotypic analog of Nijmegen Breakage Syndrome in humans) have failed to produce tumorigenesis. However, double knockout mice with mutated Nbs1 which were also null of
1656-472: Is not required for the recruitment of proteins to the site of DSBs. The histone variant H2AX constitutes about 2-25% of the H2A histones in mammalian chromatin. When a double-strand break occurs in DNA, a sequence of events occurs in which H2AX is altered. Very early after a double-strand break, a specific protein that interacts with and affects the architecture of chromatin is phosphorylated and then released from
1728-509: Is not the evidence of the DSBs. The role of the phosphorylated form of the histone in DNA repair is under discussion but it is known that because of the modification the DNA becomes less condensed, potentially allowing space for the recruitment of proteins necessary during repair of DSBs. Mutagenesis experiments have shown that the modification is necessary for the proper formation of ionizing radiation induced foci in response to double strand breaks, but
1800-525: Is reduced to about 9 micrometers (0.09 mm) of 30 nm diameter chromatin fibers. There are five families of histones, which are designated H1/H5 (linker histones), H2, H3, and H4 (core histones). The nucleosome core is formed of two H2A-H2B dimers and a H3-H4 tetramer . The tight wrapping of DNA around histones, is to a large degree, a result of electrostatic attraction between the positively charged histones and negatively charged phosphate backbone of DNA. Histones may be chemically modified through
1872-499: Is the synthesis of histone proteins: H1, H2A, H2B, H3, H4. These proteins are synthesized during S phase of the cell cycle. There are different mechanisms which contribute to the increase of histone synthesis. Yeast carry one or two copies of each histone gene, which are not clustered but rather scattered throughout chromosomes. Histone gene transcription is controlled by multiple gene regulatory proteins such as transcription factors which bind to histone promoter regions. In budding yeast,
1944-401: Is this helical structure that allows for interaction between distinct dimers, particularly in a head-tail fashion (also called the handshake motif). The resulting four distinct dimers then come together to form one octameric nucleosome core, approximately 63 Angstroms in diameter (a solenoid (DNA) -like particle). Around 146 base pairs (bp) of DNA wrap around this core particle 1.65 times in
2016-460: The DNA is wrapped around histone octamers, consisting of core histones H2A, H2B , H3 and H4 , to form chromatin . H2AX contributes to nucleosome -formation, chromatin-remodeling and DNA repair , and is also used in vitro as an assay for double-strand breaks in dsDNA . H2AX becomes phosphorylated on serine 139, then called γH2AX, as a reaction on DNA double-strand breaks (DSB) . The kinases of
2088-732: The p53 tumor suppressor gene displayed tumor onset significantly earlier than their p53 wildtype controls. This implies that Nbs1 mutations are themselves sufficient for tumorigenesis; a lack of malignancy in the control seems attributable to the activity of p53, not of the benignity of Nbs1 mutations. Extension studies have confirmed an increase in B and T-cell lymphomas in Nbs1-mutated mice in conjunction with p53 suppression, indicating potential p53 inactivation in lymphomagenesis, which occurs more often in NBS patients. Knockdown of MRE11 in various human cancer cell lines has also been associated with
2160-751: The 3'hExo nuclease. SLBP levels are controlled by cell-cycle proteins, causing SLBP to accumulate as cells enter S phase and degrade as cells leave S phase. SLBP are marked for degradation by phosphorylation at two threonine residues by cyclin dependent kinases, possibly cyclin A/ cdk2, at the end of S phase. Metazoans also have multiple copies of histone genes clustered on chromosomes which are localized in structures called Cajal bodies as determined by genome-wide chromosome conformation capture analysis (4C-Seq). Nuclear protein Ataxia-Telangiectasia (NPAT), also known as nuclear protein coactivator of histone transcription,
2232-519: The 30 nm fiber (forming an irregular zigzag) and 100 nm fiber, these being the structures found in normal cells. During mitosis and meiosis, the condensed chromosomes are assembled through interactions between nucleosomes and other regulatory proteins. Histones are subdivided into canonical replication-dependent histones, whose genes are expressed during the S-phase of the cell cycle and replication-independent histone variants , expressed during
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2304-549: The C-domain, and to the N-terminal substrate recognition domain of Clp/Hsp100 proteins. Despite the differences in their topology, these three folds share a homologous helix-strand-helix (HSH) motif. It's also proposed that they may have evolved from ribosomal proteins ( RPS6 / RPS15 ), both being short and basic proteins. Archaeal histones may well resemble the evolutionary precursors to eukaryotic histones. Histone proteins are among
2376-471: The DNA in the nucleus of higher organisms. Bonner and his postdoctoral fellow Ru Chih C. Huang showed that isolated chromatin would not support RNA transcription in the test tube, but if the histones were extracted from the chromatin, RNA could be transcribed from the remaining DNA. Their paper became a citation classic. Paul T'so and James Bonner had called together a World Congress on Histone Chemistry and Biology in 1964, in which it became clear that there
2448-591: The DNA molecule (primarily via spatial juxtaposition of the ends of broken chromosomes). Initial detection is thought to be controlled by both Nbs1 and MRE11. Likewise, cell cycle checkpoint regulation is ultimately controlled by phosphorylation activity of the ATM kinase, which is pathway dependent on both Nbs1 and MRE11. MRE11 alone is known to contribute to repair pathway selection, while MRE11 and Rad50 work together to spatially align DNA molecules: Rad50 tethers two linear DNA molecules together while MRE11 fine-tunes
2520-433: The DNA, making it more accessible for gene expression. Five major families of histone proteins exist: H1/H5 , H2A , H2B , H3 , and H4 . Histones H2A, H2B, H3 and H4 are known as the core or nucleosomal histones, while histones H1/H5 are known as the linker histones. The core histones all exist as dimers , which are similar in that they all possess the histone fold domain: three alpha helices linked by two loops. It
2592-545: The FosB promoter in the nucleus accumbens of the brain, causing 61% increase in FosB expression. This would also increase expression of the splice variant Delta FosB . In the nucleus accumbens of the brain, Delta FosB functions as a "sustained molecular switch" and "master control protein" in the development of an addiction . About 7% of the US population is addicted to alcohol . In rats exposed to alcohol for up to 5 days, there
2664-429: The MRN complex, are causal for Nijmegen Breakage Syndrome . All three disorders belong to a group of chromosomal instability syndromes that are associated with impaired DNA damage response and increased cellular sensitivity to ionising radiation. The MRN complex's roles in cancer development are as varied as its biological functions. Double-strand DNA breaks, which it monitors and signals for repair, may themselves be
2736-920: The Nbs1 gene (which downregulates expression of the entire MRN complex), has resulted in reduced telomere length and persistent lethal DNA damage in these cells. When combined with treatment of PARP (poly (ADP-ribose) polymerase) inhibitor (known as PARPi), these cells showed an even greater reduction in telomere length, arresting tumor cell proliferation both in vitro and in vivo via mouse models grafted with various HNSCC cell lines. While treatment with PARPi alone has been known to induce apoptosis in BRCA mutated cancer cell lines, this study shows that MRN downregulation can sensitize BRCA-proficient cells (those not possessing BRCA mutations) to treatment with PARPi, offering an alternative way to control tumor aggression. The MRN complex has also been implicated in several pathways contributing to
2808-545: The Nbs1 subunit. Taken together, these studies suggest MRN plays a crucial role in maintenance of both length and integrity of telomeres. Mutations in MRE11 have been identified in patients with an ataxia-telangiectasia-like disorder (ATLD). Mutations in RAD50 have been linked to a Nijmegen Breakage Syndrome-like disorder (NBSLD). Mutations in the NBN gene, encoding the human Nbs1 subunit of
2880-474: The PI3-family ( Ataxia telangiectasia mutated , ATR and DNA-PKcs) are responsible for this phosphorylation, especially ATM. The modification can happen accidentally during replication fork collapse or in the response to ionizing radiation but also during controlled physiological processes such as V(D)J recombination. γH2AX is a sensitive target for looking at DSBs in cells. The presence of γH2AX by itself, however,
2952-437: The action of enzymes to regulate gene transcription. The most common modifications are the methylation of arginine or lysine residues or the acetylation of lysine. Methylation can affect how other proteins such as transcription factors interact with the nucleosomes. Lysine acetylation eliminates a positive charge on lysine thereby weakening the electrostatic attraction between histone and DNA, resulting in partial unwinding of
H2AFX - Misplaced Pages Continue
3024-460: The alignment by binding to the ends of the broken chromosomes. Telomeres maintain the integrity of the ends of linear chromosomes during replication and protect them from being recognized as double-strand breaks by the DNA repair machinery. MRN participates in telomere maintenance primarily via association with the TERF2 protein of the shelterin complex. Additional studies have suggested that Nbs1
3096-524: The binding of the general transcription factor TFIID to the TATA box . What was said above of the chemistry of lysine methylation also applies to arginine methylation, and some protein domains—e.g., Tudor domains—can be specific for methyl arginine instead of methyl lysine. Arginine is known to be mono- or di-methylated, and methylation can be symmetric or asymmetric, potentially with different meanings. Enzymes called peptidylarginine deiminases (PADs) hydrolyze
3168-528: The biochemical characteristics of individual histones did not reveal how the histones interacted with each other or with DNA to which they were tightly bound. Also in the 1960s, Vincent Allfrey and Alfred Mirsky had suggested, based on their analyses of histones, that acetylation and methylation of histones could provide a transcriptional control mechanism, but did not have available the kind of detailed analysis that later investigators were able to conduct to show how such regulation could be gene-specific. Until
3240-402: The brain are of central importance in addictions. Once particular epigenetic alterations occur, they appear to be long lasting "molecular scars" that may account for the persistence of addictions. Cigarette smokers (about 15% of the US population) are usually addicted to nicotine . After 7 days of nicotine treatment of mice, acetylation of both histone H3 and histone H4 was increased at
3312-482: The candidate gene for activation of histone gene expression is SBF. SBF is a transcription factor that is activated in late G1 phase, when it dissociates from its repressor Whi5 . This occurs when Whi5 is phosphorylated by Cdc8 which is a G1/S Cdk. Suppression of histone gene expression outside of S phases is dependent on Hir proteins which form inactive chromatin structure at the locus of histone genes, causing transcriptional activators to be blocked. In metazoans
3384-429: The cause of carcinogenic genetic alteration, suggesting MRN provides a protective effect during normal cell homeostasis. However, upregulation of MRN complex sub-units has been documented in certain cancer cell lines when compared to non-malignant somatic cells, suggesting some cancer cells have developed a reliance on MRN overexpression. Since tumor cells have increased mitotic rates compared to non-malignant cells this
3456-424: The cell starts to differentiate, these bivalent promoters are resolved to either active or repressive states depending on the chosen lineage. Marking sites of DNA damage is an important function for histone modifications. Without a repair marker, DNA would get destroyed by damage accumulated from sources such as the ultraviolet radiation of the sun. Epigenetic modifications of histone tails in specific regions of
3528-455: The checkpoint kinase ATM in response to DNA damage. Production of short single-strand oligonucleotides by Mre11 endonuclease activity has been implicated in ATM activation by the MRN complex. The MRN complex has been mainly studied in eukaryotes. However, recent work shows that two of the three protein components of this complex, Mre11 and Rad50, are also conserved in extant prokaryotic archaea. This finding suggests that key components of
3600-509: The chemistry of the histone; methylation leaves the charge of the lysine intact and adds a minimal number of atoms so steric interactions are mostly unaffected. However, proteins containing Tudor, chromo or PHD domains, amongst others, can recognise lysine methylation with exquisite sensitivity and differentiate mono, di and tri-methyl lysine, to the extent that, for some lysines (e.g.: H4K20) mono, di and tri-methylation appear to have different meanings. Because of this, lysine methylation tends to be
3672-687: The chromatin. This protein, heterochromatin protein 1 (HP1)-beta ( CBX1 ), is bound to histone H3 methylated on lysine 9 (H3K9me). Half-maximum release of HP1-beta from damaged DNA occurs within one second. A dynamic alteration in chromatin structure is triggered by HP1-beta release. This alteration in chromatin structure promotes H2AX phosphorylation by ATM , ATR and DNA-PK , allowing formation of γH2AX (H2AX phosphorylated on serine 139). γH2AX can be detected as soon as 20 seconds after irradiation of cells (with DNA double-strand break formation), and half maximum accumulation of γH2AX occurs in one minute. Chromatin with phosphorylated γH2AX extends to about
SECTION 50
#17328913950173744-403: The early 1990s, histones were dismissed by most as inert packing material for eukaryotic nuclear DNA, a view based in part on the models of Mark Ptashne and others, who believed that transcription was activated by protein-DNA and protein-protein interactions on largely naked DNA templates, as is the case in bacteria. During the 1980s, Yahli Lorch and Roger Kornberg showed that a nucleosome on
3816-537: The eukaryotic MRN complex are derived by evolutionary descent from the archaea. In the archaeon Sulfolobus acidocaldarius , the Mre11 protein interacts with the Rad50 protein and appears to have an active role in the repair of DNA damages experimentally introduced by gamma radiation. Similarly, during meiosis in the eukaryotic protist Tetrahymena Mre11 is required for repair of DNA damages, in this case double-strand breaks, by
3888-420: The histones H2A and H2B can also be modified. Combinations of modifications, known as histone marks , are thought to constitute a code, the so-called " histone code ". Histone modifications act in diverse biological processes such as gene regulation , DNA repair , chromosome condensation ( mitosis ) and spermatogenesis ( meiosis ). The common nomenclature of histone modifications is: So H3K4me1 denotes
3960-436: The imine group of arginines and attach a keto group, so that there is one less positive charge on the amino acid residue. This process has been involved in the activation of gene expression by making the modified histones less tightly bound to DNA and thus making the chromatin more accessible. PADs can also produce the opposite effect by removing or inhibiting mono-methylation of arginine residues on histones and thus antagonizing
4032-499: The increase in the rate of histone synthesis is due to the increase in processing of pre-mRNA to its mature form as well as decrease in mRNA degradation; this results in an increase of active mRNA for translation of histone proteins. The mechanism for mRNA activation has been found to be the removal of a segment of the 3' end of the mRNA strand, and is dependent on association with stem-loop binding protein ( SLBP ). SLBP also stabilizes histone mRNAs during S phase by blocking degradation by
4104-727: The insensitivity of cancer stem cells to the DNA damaging effects of chemotherapy and radiation treatment , which is a source of overall tumor aggression. Specifically, the MRN inhibitor Mirin (inhibiting MRE11) has been shown to disrupt the ability of ATM kinase to control the G2-M DNA damage checkpoint , which is required for repair of double-strand DNA breaks. The loss of this checkpoint strips cancer stem cells' ability to repair lethal genetic lesions, making them vulnerable to DNA damaging therapeutic agents. Likewise, overexpression of Nbs1 in HNSCC cells has been correlated with increased activation of
4176-537: The large genomes of eukaryotes inside cell nuclei: the compacted molecule is 40,000 times shorter than an unpacked molecule. Histones undergo posttranslational modifications that alter their interaction with DNA and nuclear proteins. The H3 and H4 histones have long tails protruding from the nucleosome , which can be covalently modified at several places. Modifications of the tail include methylation , acetylation , phosphorylation , ubiquitination , SUMOylation , citrullination , and ADP-ribosylation. The core of
4248-516: The late 19th century and is derived from the German word "Histon" , a word itself of uncertain origin, perhaps from Ancient Greek ἵστημι (hístēmi, “make stand”) or ἱστός (histós, “loom”). In the early 1960s, before the types of histones were known and before histones were known to be highly conserved across taxonomically diverse organisms, James F. Bonner and his collaborators began a study of these proteins that were known to be tightly associated with
4320-729: The long, modified γH2AX/MDC1 chromatin. Other proteins that stably assemble on the extensive γH2AX-modified chromatin are the MRN complex (a protein complex consisting of Mre11 , Rad50 and Nbs1 ), RAD51 and the ATM kinase . Further DNA repair components, such as RAD52 and RAD54, rapidly and reversibly interact with the core components stably associated with γH2AX-modified chromatin. The constitutive level of γH2AX expression in live cells, untreated by exogenous agents, likely represents DNA damage by endogenous oxidants generated during cellular respiration. The packaging of eukaryotic DNA into chromatin presents
4392-451: The lysine in position 4 of histone 3 located at the promoters of the c-fos and the C-C chemokine receptor 2 (ccr2) genes, activating those genes in the nucleus accumbens (NAc). c-fos is well known to be important in addiction . The ccr2 gene is also important in addiction, since mutational inactivation of this gene impairs addiction. The first step of chromatin structure duplication
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#17328913950174464-697: The monomethylation of the 4th residue (a lysine) from the start (i.e., the N-terminal ) of the H3 protein. A huge catalogue of histone modifications have been described, but a functional understanding of most is still lacking. Collectively, it is thought that histone modifications may underlie a histone code , whereby combinations of histone modifications have specific meanings. However, most functional data concerns individual prominent histone modifications that are biochemically amenable to detailed study. The addition of one, two, or many methyl groups to lysine has little effect on
4536-419: The most highly conserved proteins in eukaryotes, emphasizing their important role in the biology of the nucleus. In contrast mature sperm cells largely use protamines to package their genomic DNA, most likely because this allows them to achieve an even higher packaging ratio. There are some variant forms in some of the major classes. They share amino acid sequence homology and core structural similarity to
4608-441: The need to alter multiple lysines to have a significant effect on chromatin structure. The modification includes H3K27ac . Addition of a negatively charged phosphate group can lead to major changes in protein structure, leading to the well-characterised role of phosphorylation in controlling protein function. It is not clear what structural implications histone phosphorylation has, but histone phosphorylation has clear functions as
4680-453: The only eukaryotes that completely lack histones, but later studies showed that their DNA still encodes histone genes. Unlike the core histones, homologs of the lysine-rich linker histone (H1) proteins are found in bacteria, otherwise known as nucleoprotein HC1/HC2. It has been proposed that core histone proteins are evolutionarily related to the helical part of the extended AAA+ ATPase domain,
4752-485: The other hand, not only the formation of distinct γH2AX foci but also the induction of pan-nuclear γH2AX signals have been reported as a cellular reaction to various stressors other than ionizing radiation. The γH2AX signal is always stronger at DNA double-strand breaks than in undamaged chromatin. γH2AX in undamaged chromatin is thought to possibly be generated via direct phosphorylation of H2AX by activated kinases, most likely diffusing from DNA damage sites. In using γH2AX as
4824-598: The positive effect arginine methylation has on transcriptional activity. Addition of an acetyl group has a major chemical effect on lysine as it neutralises the positive charge. This reduces electrostatic attraction between the histone and the negatively charged DNA backbone, loosening the chromatin structure; highly acetylated histones form more accessible chromatin and tend to be associated with active transcription. Lysine acetylation appears to be less precise in meaning than methylation, in that histone acetyltransferases tend to act on more than one lysine; presumably this reflects
4896-430: The prevention of the spread of silent heterochromatin . Furthermore, H2A.Z has roles in chromatin for genome stability. Another H2A variant H2A.X is phosphorylated at S139 in regions around double-strand breaks and marks the region undergoing DNA repair . Histone H3.3 is associated with the body of actively transcribed genes. Histones act as spools around which DNA winds. This enables the compaction necessary to fit
4968-424: The structure of histones has been evolutionarily conserved, as any deleterious mutations would be severely maladaptive. All histones have a highly positively charged N-terminus with many lysine and arginine residues. Core histones are found in the nuclei of eukaryotic cells and in most Archaeal phyla, but not in bacteria . The unicellular algae known as dinoflagellates were previously thought to be
5040-405: The whole cell cycle. In mammals, genes encoding canonical histones are typically clustered along chromosomes in 4 different highly- conserved loci, lack introns and use a stem loop structure at the 3' end instead of a polyA tail . Genes encoding histone variants are usually not clustered, have introns and their mRNAs are regulated with polyA tails. Complex multicellular organisms typically have
5112-415: Was an increase in histone 3 lysine 9 acetylation in the pronociceptin promoter in the brain amygdala complex. This acetylation is an activating mark for pronociceptin. The nociceptin/nociceptin opioid receptor system is involved in the reinforcing or conditioning effects of alcohol. Methamphetamine addiction occurs in about 0.2% of the US population. Chronic methamphetamine use causes methylation of
5184-586: Was no consensus on the number of kinds of histone and that no one knew how they would compare when isolated from different organisms. Bonner and his collaborators then developed methods to separate each type of histone, purified individual histones, compared amino acid compositions in the same histone from different organisms, and compared amino acid sequences of the same histone from different organisms in collaboration with Emil Smith from UCLA. For example, they found Histone IV sequence to be highly conserved between peas and calf thymus. However, their work on
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