The mammalian cytoplasmic protein SirT2 is a member of the Sir2 family of NAD + -dependent protein deacetylases involved in caloric restriction-dependent life span extension. We found that SirT2 and its yeast counterpart Hst2 have a strong preference for histone H4K16Ac in their deacetylation activity in vitro and in vivo. We have pinpointed the decrease in global levels of H4K16Ac during the mammalian cell cycle to the G 2 /M transition that coincides with SirT2 localization on chromatin. Mouse embryonic fibroblasts (MEFs) deficient for SirT2 show higher levels of H4K16Ac in mitosis, in contrast to the normal levels exhibited by SirT1-deficient MEFs. The enzymatic conversion of H4K16Ac to its deacetylated form may be pivotal to the formation of condensed chromatin. Thus, SirT2 is a major contributor to this enzymatic conversion at the time in the cell's life cycle when condensed chromatin must be generated anew. Yeast Hst2 belongs to the Sir2 family of NAD + -dependent protein deacetylases. This family is defined by the presence of an ∼200-amino-acid sequence responsible for the NAD + -dependent deacetylase activity, also found in a wide variety of proteins from yeast to humans. Sir2 is required for the maintenance of silenced chromatin at the mating type loci, telomeres, and rDNA in yeast (Imai et al. 2000;Landry et al. 2000), and is associated with life span extension in both yeast and worms (Kennedy et al. 1997;Tissenbaum and Guarente 2001). The yeast Sir2 family includes four homologs termed Hst1-4, whose functions are still unclear, with the exception of Hst1, which regulates the expression of middle sporulation genes (Xie et al. 1999). Hst2p localizes to the cytoplasm and can affect nuclear silencing by an unknown mechanism (Perrod et al. 2001). Additionally, Hst2p is involved in caloric restriction (CR) dependent life span extension by a Sir2-independent mechanism (Lamming et al. 2005).Among the seven Sir2 mammalian homologs (SirT1-7), SirT2 is the Hst2p counterpart and also localizes to the cytoplasm (Perrod et al. 2001). SirT2 seems to be overexpressed during mitosis, affecting mitotic exit (Dryden et al. 2003), and has been shown to deacetylate ␣-tubulin (North et al. 2003). However, since tubulin does not seem to be acetylated in yeast (Polevoda and Sherman 2002), the functional conservation of Hst2 (from yeast to human) is intriguing and strongly suggests another, more general function. Here we report that the general cytoplasmic localization of SirT2 is singularly exceptional during the G 2 /M phase when SirT2 is now present in the nucleus on chromatin. This correlates with a global decrease in the levels of acetylated H4K16 in the G 2 /M phase of the cell cycle. Cells deficient in SirT2 reflect this association, exhibiting increased levels as well as mislocalization of H4K16Ac and aberrancies in the integrity of their S phase. Results and Discussion SirT2 and Hst2p are NAD + -dependent histone deacetylases with preference for H4K16Ac in vitro and in vivoThe purification of human SirT2 rendered a h...

In contrast to stably repressive, constitutive heterochromatin and stably active, euchromatin, facultative heterochromatin has the capacity to alternate between repressive and activated states of transcription. As such, it is an instructive source to understand the molecular basis for changes in chromatin structure that correlate with transcriptional status. Sirtuin 1 (SIRT1) and suppressor of variegation 3-9 homologue 1 (SUV39H1) are amongst the enzymes responsible for chromatin modulations associated with facultative heterochromatin formation. SUV39H1 is the principal enzyme responsible for the accumulation of histone H3 containing a tri-methyl group at its lysine 9 position (H3K9me3) in regions of heterochromatin. SIRT1 is an NAD+-dependent deacetylase that targets histone H4 at lysine 16 (refs 3 and 4), and through an unknown mechanism facilitates increased levels of H3K9me3 (ref. 3). Here we show that the mammalian histone methyltransferase SUV39H1 is itself targeted by the histone deacetylase SIRT1 and that SUV39H1 activity is regulated by acetylation at lysine residue 266 in its catalytic SET domain. SIRT1 interacts directly with, recruits and deacetylates SUV39H1, and these activities independently contribute to elevated levels of SUV39H1 activity resulting in increased levels of the H3K9me3 modification. Loss of SIRT1 greatly affects SUV39H1-dependent H3K9me3 and impairs localization of heterochromatin protein 1. These findings demonstrate a functional link between the heterochromatin-related histone methyltransferase SUV39H1 and the histone deacetylase SIRT1.

Sirtuins, a family of protein deacetylases, promote cellular homeostasis by mediating communication between cells and environment. The enzymatic activity of the mammalian sirtuin SIRT7 targets acetylated lysine in the N‐terminal tail of histone H3 (H3K18Ac), thus modulating chromatin structure and transcriptional competency. SIRT7 deletion is associated with reduced lifespan in mice through unknown mechanisms. Here, we show that SirT7‐knockout mice suffer from partial embryonic lethality and a progeroid‐like phenotype. Consistently, SIRT7‐deficient cells display increased replication stress and impaired DNA repair. SIRT7 is recruited in a PARP1‐dependent manner to sites of DNA damage, where it modulates H3K18Ac levels. H3K18Ac in turn affects recruitment of the damage response factor 53BP1 to DNA double‐strand breaks (DSBs), thereby influencing the efficiency of non‐homologous end joining (NHEJ). These results reveal a direct role for SIRT7 in DSB repair and establish a functional link between SIRT7‐mediated H3K18 deacetylation and the maintenance of genome integrity.

The establishment of the epigenetic mark H4K20me1 (monomethylation of H4K20) by PR-Set7 during G 2 /M directly impacts S-phase progression and genome stability. However, the mechanisms involved in the regulation of this event are not well understood. Here we show that SirT2 regulates H4K20me1 deposition through the deacetylation of H4K16Ac (acetylation of H4K16) and determines the levels of H4K20me2/3 throughout the cell cycle. SirT2 binds and deacetylates PR-Set7 at K90, modulating its chromatin localization. Consistently, SirT2 depletion significantly reduces PR-Set7 chromatin levels, alters the size and number of PR-Set7 foci, and decreases the overall mitotic deposition of H4K20me1. Upon stress, the interaction between SirT2 and PR-Set7 increases along with the H4K20me1 levels, suggesting a novel mitotic checkpoint mechanism. SirT2 loss in mice induces significant defects associated with defective H4K20me1-3 levels. Accordingly, SirT2-deficient animals exhibit genomic instability and chromosomal aberrations and are prone to tumorigenesis. Our studies suggest that the dynamic cross-talk between the environment and the genome during mitosis determines the fate of the subsequent cell cycle.

Sirtuins are NAD-dependent deacetylases that sense oxidative stress conditions and promote a protective cellular response. The Sirtuin SirT1 is involved in facultative heterochromatin formation through an intimate functional relationship with the H3K9me3 methyltransferase Suv39h1, a chromatin organization protein. However, SirT1 also regulates Suv39h1-dependent constitutive heterochromatin (CH) through an unknown mechanism; interestingly, SirT1 does not significantly localize in these regions. Herein, we report that SirT1 controls global levels of Suv39h1 by increasing its half-life through inhibition of Suv39h1 lysine 87 polyubiquitination by the E3-ubiquitin ligase MDM2. This in turn increases Suv39h1 turnover in CH and ensures genome integrity. Stress conditions that lead to SirT1 upregulation, such as calorie restriction, also induce higher levels of Suv39h1 in a SirT1-dependent manner in vivo. These observations reflect a direct link between oxidative stress response and Suv39h1 and support a dynamic view of heterochromatin, in which its structure adapts to cell physiology.