"Isogaba Maware": quality control of genome DNA by checkpoints

Cloning and characterization of liver-specific isoform of Chk1 gene from rat

We have isolated and characterized an isoform of protein kinase Chk1 gene from rat liver and a rat liver cDNA library by 5'-rapid amplification of cDNA ends. The gene (Cil) contains the C-terminal region of the Chk1 gene, but the 5'-end is derived from a sequence in the intron of Chk1 preceding the C-terminal domain by differential RNA splicing. The kinase domain of Chk1 gene is absent in this isoform. Tissue RNA and protein blot analyses indicated that Cil was specifically expressed only in rat liver, and its expression increased with liver development. Expression of Cil was found to be reduced in three rat hepatoma cell lines examined. A promoter trap experiment suggested that a promoter was located in the intron preceding the C-terminal domain of Chk1, and transcription from this novel promoter generated the new 5' noncoding exon of Cil. Thus Cil was generated by both alternate promoter usage and differential RNA splicing. UV irradiation induced caffeine-sensitive phosphorylation of both Chk1 and Cil at Ser-345 in Chk1 and its equivalent site in Cil, implying a role for ATR kinase in the phosphorylation of both proteins. We demonstrated the interaction between the kinase domain of Chk1 and Cil using a yeast two-hybrid assay and pull-down technique. In contrast to the effect of Chk1, Cil was found to decrease the transactivating function of p53, and the S63A mutation of Cil abolished this effect. These results suggest that Cil may serve as a dominant negative competitor of Chk1 as suggested previously.

CAF-1 and the inheritance of chromatin states: at the crossroads of DNA replication and repair

Chromatin is no longer considered to be a static structural framework for packaging DNA within the nucleus but is instead believed to be an interactive component of DNA metabolism. The ordered assembly of chromatin produces a nucleoprotein template capable of epigenetically regulating the expression and maintenance of the genome. Factors have been isolated from cell extracts that stimulate early steps in chromatin assembly in vitro. The function of one such factor, chromatin-assembly factor 1 (CAF-1), might extend beyond simply facilitating the progression through an individual assembly reaction to its active participation in a marking system. This marking system could be exploited at the crossroads of DNA replication and repair to monitor genome integrity and to define particular epigenetic states.

Drosophila p53 binds a damage response element at the reaper locus

The tumor suppressor gene p53 regulates multiple cellular responses to DNA damage, but the transcriptional targets that specify these responses are incompletely understood. We describe a Drosophila p53 homolog and demonstrate that it can activate transcription from a promoter containing binding sites for human p53. Dominant-negative forms of Drosophila p53 inhibit both transactivation in cultured cells and radiation-induced apoptosis in developing tissues. The cis-regulatory region of the proapoptotic gene reaper contains a radiation-inducible enhancer that includes a consensus p53 binding site. Drosophila p53 can activate transcription from this site in yeast and a multimer of this site is sufficient for radiation induction in vivo. These results indicate that reaper is a direct transcriptional target of Drosophila p53 following DNA damage.

mus304 encodes a novel DNA damage checkpoint protein required during Drosophila development

Checkpoints block cell cycle progression in eukaryotic cells exposed to DNA damaging agents. We show that several Drosophilahomologs of checkpoint genes, mei-41, grapes, and14-3-3ε, regulate a DNA damage checkpoint in the developing eye. We have used this assay to show that the mutagen-sensitive gene mus304 is also required for this checkpoint. mus304 encodes a novel coiled-coil domain protein, which is targeted to the cytoplasm. Similar to mei-41,mus304 is required for chromosome break repair and for genomic stability. mus304 animals also exhibit three developmental defects, abnormal bristle morphology, decreased meiotic recombination, and arrested embryonic development. We suggest that these phenotypes reflect distinct developmental consequences of a single underlying checkpoint defect. Similar mechanisms may account for the puzzling array of symptoms observed in humans with mutations in the ATM tumor suppressor gene.

A CAF-1-PCNA-mediated chromatin assembly pathway triggered by sensing DNA damage

Sensing DNA damage is crucial for the maintenance of genomic integrity and cell cycle progression. The participation of chromatin in these events is becoming of increasing interest. We show that the presence of single-strand breaks and gaps, formed either directly or during DNA damage processing, can trigger the propagation of nucleosomal arrays. This nucleosome assembly pathway involves the histone chaperone chromatin assembly factor 1 (CAF-1). The largest subunit (p150) of this factor interacts directly with proliferating cell nuclear antigen (PCNA), and critical regions for this interaction on both proteins have been mapped. To isolate proteins specifically recruited during DNA repair, damaged DNA linked to magnetic beads was used. The binding of both PCNA and CAF-1 to this damaged DNA was dependent on the number of DNA lesions and required ATP. Chromatin assembly linked to the repair of single-strand breaks was disrupted by depletion of PCNA from a cell-free system. This defect was rescued by complementation with recombinant PCNA, arguing for role of PCNA in mediating chromatin assembly linked to DNA repair. We discuss the importance of the PCNA-CAF-1 interaction in the context of DNA damage processing and checkpoint control.

HRad17 colocalizes with NHP2L1 in the nucleolus and redistributes after UV irradiation

The rad17 gene of Schizosaccharomyces pombe plays an important role as a checkpoint protein following DNA damage and during DNA replication. The human homologue of S. pombe rad17, Hrad17, was recently identified, but its function has not yet been established. Using the yeast two-hybrid system, we determined that HRad17 can interact with a nucleolar protein, NHP2L1. This interaction was also demonstrated biochemically, in human cells. Immunofluorescence studies revealed that HRad17 and NHP2L1 colocalize to the nucleolus, and immunogold labeling further resolved the location of NHP2L1 to the dense fibrillar component of the nucleolus. Interestingly, the localization of HRad17 in the nucleolus was altered in response to UV irradiation. These results provide some insight into the DNA damage and replication checkpoint mechanisms of HRad17.

Quantitative measurement of mammalian chromosome mitotic loss rates using the green fluorescent protein

We have measured the mitotic loss rates of mammalian chromosomes in cultured cells. The green fluorescent protein (GFP) gene was incorporated into a non-essential chromosome so that cells containing the chromosome fluoresced green, while those lacking it did not. The proportions of fluorescent and non-fluorescent cells were measured by fluorescence activated cell sorter (FACS) analysis. Loss rates ranged from 0.005% to 0.20% per cell division in mouse LA-9 cells, and from 0.02% to 0.40% in human HeLa cells. The rate of loss was elevated by treatment with aneugens, demonstrating that the system rapidly identifies agents which induce chromosome loss in mammalian cells.

Mouse Hus1, a homolog of the Schizosaccharomyces pombe hus1+ cell cycle checkpoint gene

Cell cycle checkpoints are regulatory mechanisms that arrest the cell cycle or initiate programmed cell death when critical events such as DNA replication fail to be completed or when DNA or spindle damage occurs. In fission yeast, cell cycle checkpoint responses to DNA replication blocks and DNA damage require the hus1+ gene. Mammalian homologs of hus1+ were recently identified, and here we report a detailed analysis of mouse Hus1. An approximately 4.2-kb full-length cDNA encoding the 32-kDa mouse Hus1 protein was isolated. The genomic structure and exon-intron boundary sequences of the gene were determined, and mouse Hus1 was found to consist of nine exons. Mouse Hus1 was mapped to the proximal end of chromosome 11 and is therefore a candidate gene for the mouse mutation germ cell deficient, which maps to the same genomic region. Finally, mouse Hus1 was found to be expressed in a variety of adult tissues and at several stages of embryonic development.

DNA damage and replication checkpoints in the fission yeast, Schizosaccharomyces pombe

Eukaryotic organisms have developed an array of mechanisms for minimizing the consequences of damage to their DNA molecules and the consequences of interference with their DNA replication. Among these mechanisms are the DNA damage and replication checkpoints, which inhibit passage from one cell cycle stage to the next when DNA is damaged or replication is incomplete. Studies of these checkpoints in the fission yeast, Schizosaccharomyces pombe, complement studies in other organisms and provide valuable insight into the nature of the proteins responsible for these checkpoints and how such proteins may function.

Faithful anaphase is ensured by Mis4, a sister chromatid cohesion molecule required in S phase and not destroyed in G1 phase

The loss of sister chromatid cohesion triggers anaphase spindle movement. The budding yeast Mcd1/Scc1 protein, called cohesin, is required for associating chromatids, and proteins homologous to it exist in a variety of eukaryotes. Mcd1/Scc1 is removed from chromosomes in anaphase and degrades in G1. We show that the fission yeast protein, Mis4, which is required for equal sister chromatid separation in anaphase is a different chromatid cohesion molecule that behaves independent of cohesin and is conserved from yeast to human. Its inactivation in G1 results in cell lethality in S phase and subsequent premature sister chromatid separation. Inactivation in G2 leads to cell death in subsequent metaphase-anaphase progression but missegregation occurs only in the next round of mitosis. Mis4 is not essential for condensation, nor does it degrade in G1. Rather, it associates with chromosomes in a punctate fashion throughout the cell cycle. mis4 mutants are hypersensitive to hydroxyurea (HU) and UV irradiation but retain the ability to restrain cell cycle progression when damaged or sustaining a block to replication. The mis4 mutation results in synthetic lethality with a DNA ligase mutant. Mis4 may form a stable link between chromatids in S phase that is split rather than removed in anaphase.

Damage and replication checkpoint control in fission yeast is ensured by interactions of Crb2, a protein with BRCT motif, with Cut5 and Chk1

Fission yeast Cut5/Rad4 plays a unique role in the genome maintenance as it is required for replication, replication checkpoint, and normal UV sensitivity. It is unknown, however, how Cut5 protein is linked to other checkpoint proteins, and what part it plays in replication and UV sensitivity. Here we report that Cut5 interacts with a novel checkpoint protein Crb2 and that this interaction is needed for normal genome maintenance. The carboxyl terminus of Crb2 resembles yeast Rad9 and human 53BP1 and BRCA1. Crb2 is required for checkpoint arrests induced by irradiation and polymerase mutations, but not for those induced by inhibited nucleotide supply. Upon UV damage, Crb2 is transiently modified, probably phosphorylated, with a similar timing of phosphorylation in Chk1 kinase, which is reported to restrain Cdc2 activation. Crb2 modification requires other damage-sensing checkpoint proteins but not Chk1, suggesting that Crb2 acts at the upstream of Chk1. The modified Crb2 exists as a slowly sedimenting form, whereas Crb2 in undamaged cells is in a rapidly sedimenting structure. Cut5 and Crb2 interact with Chk1 in a two-hybrid system. Moreover, moderate overexpression of Chk1 suppresses the phenotypes of cut5 and crb2 mutants. Cut5, Crb2, and Chk1 thus may form a checkpoint sensor-transmitter pathway to arrest the cell cycle.