AbstractsBiology & Animal Science

Chromatin compaction is regulated by different factors, among them histone posttranslational modifications. There are different histone modifications, and among them, acetylation and methylation of lysine residues. Acetylation levels are regulated by histone acetyltransferases (HATs) and histone deacetylases (HDACs), enzymes that catalyze addition and removal, respectively, of acetyl groups from histone lysine residues. Among the four classes of HDACs, the class III which correspond to the members of the Sir2 family or Sirtuins are quite unique. They participate in the response to a wide variety of stress stimuli through their requirement of NAD+ as a cofactor in their enzymatic activity. Mammals harbor seven Sirtuins (SirT1-SirT7). Among them, SirT6 is a nuclear protein involved in both genomic stability and metabolic homeostasis. Interestingly, SirT6 regulates the majority of these processes through an effect on chromatin, based on deacetylation of a histone mark, H3K9ac. However, the mechanism involved has not been fully characterized. In this work, we aim to study the consequences of SirT6 function in chromatin organization. We show that SirT6 overexpression induces gene silencing, which fits with the role of SirT6 as a repressor, shown by previous reports. Our studies show that SirT6 interacts with proteins or multi-protein complexes also involved in gene silencing, such as components of NuRD complex, or the HMTs EZH2, Suv39h1. However, we have focused the project in understanding the functional relationship between SirT6 and a H3K9-specific methyltransferases that we have identified as Suv39h1 and G9a. Suv39h1 trimethylates H3K9 and is essential in the establishment and maintenance of pericentromeric and telomeric constitutive heterochromatin. Interestingly, Suv39h1 was previously found to interact with SirT1 in the context of constitutive and facultative heterochromatin formation. Our work shows that the functional relationship between Suv39h1 and SirT6 is quite different from the already described between SirT1 and Suv39h1. SirT6 mediates a non canonical monoubiquitination in Suv39h1 in three conserved cysteines of the pre-SET domain. We have also identified SKP2 as the E3 ubiquitin ligase responsible of this monoubiquitination. SKP2 has a well-known role in promoting poliubiquitination and degradation of protein involved in G1/S checkpoint such as p21 and p27 and its levels are regulated during cell cycle progression. Our data show that SKP2 levels are also regulated by SirT6 through deacetylation, which in turn induce a double phosphorylation that prevents its degradation. Suv39h1 monoubiquitination is induced by double thymidine block and nocodazole treatments that arrest cells in G1/S phase and early mitosis, respectively. Furthermore, Suv39h1 monoubiquitination is induced by NF-kB pathway activation such as TNFa treatment, the overexpression of the NF-kB transcription factor RelA, or of the activator IKKa. Moreover, the promoter of the NF-kB global repressor, IkBa, is regulated by the interplay between…