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1.  Nucleosome resection at a double-strand break during Non-Homologous Ends Joining in mammalian cells - implications from repressive chromatin organization and the role of ARTEMIS 
BMC Research Notes  2011;4:13.
The S. cerevisiae mating type switch model of double-strand break (DSB) repair, utilizing the HO endonuclease, is one of the best studied systems for both Homologous Recombination Repair (HRR) and direct ends-joining repair (Non-Homologous Ends Joining - NHEJ). We have recently transposed that system to a mammalian cell culture model taking advantage of an adenovirus expressing HO and an integrated genomic target. This made it possible to compare directly the mechanism of repair between yeast and mammalian cells for the same type of induced DSB. Studies of DSB repair have emphasized commonality of features, proteins and machineries between organisms, and differences when conservation is not found. Two proteins that stand out that differ between yeast and mammalian cells are DNA-PK, a protein kinase that is activated by the presence of DSBs, and Artemis, a nuclease whose activity is modulated by DNA-PK and ATM. In this report we describe how these two proteins may be involved in a specific pattern of ends-processing at the DSB, particularly in the context of heterochromatin.
We previously published that the repair of the HO-induced DSB was generally accurate and occurred by simple rejoining of the cohesive 3'-overhangs generated by HO. During continuous passage of those cells in the absence of puromycin selection, the locus appears to have become more heterochromatic and silenced by displaying several features. 1) The site had become less accessible to cleavage by the HO endonuclease; 2) the expression of the puro mRNA, which confers resistance to puromycin, had become reduced; 3) occupancy of nucleosomes at the site (ChIP for histone H3) was increased, an indicator for more condensed chromatin. After reselection of these cells by addition of puromycin, many of these features were reversed. However, even the reselected cells were not identical in the pattern of cleavage and repair as the cells when originally created. Specifically, the pattern of repair revealed discrete deletions at the DSB that indicated unit losses of nucleosomes (or other protein complexes) before religation, represented by a ladder of PCR products reminiscent of an internucleosomal cleavage that is typically observed during apoptosis. This pattern of cleavage suggested to us that perhaps, Artemis, a protein that is believed to generate the internucleosomal fragments during apoptosis and in DSB repair, was involved in that specific pattern of ends-processing. Preliminary evidence indicates that this may be the case, since knock-down of Artemis with siRNA eliminated the laddering pattern and revealed instead an extensive exonucleolytic processing of the ends before religation.
e have generated a system in mammalian cells where the absence of positive selection resulted in chromatin remodeling at the target locus that recapitulates many of the features of the mating-type switching system in yeast. Specifically, just as for yeast HML and HMR, the locus had become transcriptionally repressed; accessibility to cleavage by the HO endonuclease was reduced; and processing of the ends was drastically changed. The switch was from high-fidelity religation of the cohesive ends, to a pattern of release of internucleosomal fragments, perhaps in search of micro-homology stretches for ligation. This is consistent with reports that the involvement of ATM, DNA-PK and Artemis in DSB repair is largely focused to heterochromatic regions, and not required for the majority of IR-induced DSB repair foci in euchromatin.
PMCID: PMC3035584  PMID: 21255428
2.  Tousled kinase TLK1B mediates chromatin assembly in conjunction with Asf1 regardless of its kinase activity 
BMC Research Notes  2010;3:68.
The Tousled Like Kinases (TLKs) are involved in chromatin dynamics, including DNA replication and repair, transcription, and chromosome segregation. Indeed, the first two TLK1 substrates were identified as the histone H3 and Asf1 (a histone H3/H4 chaperone), which immediately suggested a function in chromatin remodeling. However, despite the straightforward assumption that TLK1 acts simply by phosphorylating its substrates and hence modifying their activity, TLK1 also acts as a chaperone. In fact, a kinase-dead (KD) mutant of TLK1B is functional in stimulating chromatin assembly in vitro. However, subtle effects of Asf1 phosphorylation are more difficult to probe in chromatin assembly assays. Not until very recently was the Asf1 site phosphorylated by TLK1 identified. This has allowed for probing directly the functionality of a site-directed mutant of Asf1 in chromatin assembly assays.
Addition of either wt or non-phosphorylatable mutant Asf1 to nuclear extract stimulates chromatin assembly on a plasmid. Similarly, TLK1B-KD stimulates chromatin assembly and it synergizes in reactions with supplemental Asf1 (wt or non-phosphorylatable mutant).
Although the actual function of TLKs as mediators of Asf1 activity cannot be easily studied in vivo, particularly since in mammalian cells there are two TLK genes and two Asf1 genes, we were able to study specifically the stimulation of chromatin assembly in vitro. In such assays, clearly the TLK1 kinase activity was not critical, as neither a non-phosphorylatable Asf1 nor use of the TLK1B-KD impaired the stimulation of nucleosome formation.
PMCID: PMC2845150  PMID: 20222959
3.  Tousled kinase TLK1B counteracts the effect of Asf1 in inhibition of histone H3–H4 tetramer formation 
BMC Research Notes  2009;2:128.
The Tousled-like kinases (TLKs) function in processes of chromatin assembly, including replication, transcription, repair, and chromosome segregation. TLK1 interacts specifically with the chromatin assembly factor Asf1, a histone H3–H4 chaperone, and with Rad9, a protein involved in DNA repair. Asf1 binds to the H3–H4 dimer at the same interface that is used for formation of the core tetramer, and hence Asf1 is implicated in disruption of the tetramer during transcription, although Asf1 also has a function in chromatin assembly during replication and repair.
We have used protein crosslinking with purified components to probe the interaction between H3, H4, Asf1, and TLK1B. We found that TLK1B, by virtue of its binding to Asf1, can restore formation of H3–H4 tetramers that is sterically prevented by adding Asf1.
We suggest that TLK1B binds to Asf1 in a manner that interferes with its binding to the H3–H4 dimer, thereby allowing for H3–H4 tetramerization. A description of the function of TLK1 and Asf1 in chromatin remodeling is presented.
PMCID: PMC2713256  PMID: 19586531

Results 1-3 (3)