Nuclear Organization and Silencing: Trafficking of Sir Proteins

Gasser, Susan M.; Gotta, Monica; Renauld, Hubert; Laroche, Thierry; Cockell, Moira

doi:10.1002/9780470515501.ch7

Cited by 7 publications

(6 citation statements)

References 40 publications

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“…Cytological markers of silencing are telomeric foci, where telomeres cluster at the nuclear periphery (19). To determine if these were intact in slx5⌬ mutants, immunofluorescence was performed for Sir2 (not shown), and both Sir3GFP (Fig.…”

Section: Fig 3 Epitope-taggedmentioning

confidence: 99%

Slx5 Promotes Transcriptional Silencing and Is Required for Robust Growth in the Absence of Sir2

Darst

Garcia

Koch

et al. 2008

Molecular and Cellular Biology

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The broadly conserved Sir2 NAD ؉ -dependent deacetylase is required for chromatin silencing. Here we report the discovery of physical and functional links between Sir2 and Slx5 (Hex3), a RING domain protein and subunit of the Slx5/8 complex column, which is a ubiquitin E3 ligase that targets sumoylated proteins. Slx5 interacted with Sir2 by two-hybrid and glutathione S-transferase-binding assays and was found to promote silencing of genes at telomeric or ribosomal DNA (rDNA) loci. However, deletion of SLX5 had no detectable effect on the distribution of silent chromatin components and only slightly altered the deacetylation of histone H4 lysine 16 at the telomere. In vivo assays indicated that Sir2-dependent silencing was functionally intact in the absence of Slx5. Although no previous reports suggest that Sir2 contributes to the fitness of yeast populations, we found that Sir2 was required for maximal growth in slx5⌬ mutant cells. A similar requirement was observed for mutants of the SUMO isopeptidase Ulp2/Smt4. The contribution of Sir2 to optimal growth was not due to known Sir2 roles in mating-type determination or rDNA maintenance but was connected to a role of sumoylation in transcriptional silencing. These results indicate that Sir2 and Slx5 jointly contribute to transcriptional silencing and robust cellular growth.In the budding yeast Saccharomyces cerevisiae, transcriptional silencing is defined as constitutive, chromatin-mediated repression of transcription that occurs at HM loci, ribosomal DNA (rDNA), and telomeres. The Sir2 protein, essential for S. cerevisiae transcriptional silencing, is of particular interest as the founding member of the sirtuin family of NAD-dependent protein deacetylases that is conserved throughout eukaryotes. Increased expression of SIR2 or its closest homologs increases the life span in yeast and animals, respectively (reviewed in references 31 and 44). Although these studies suggest the conservation of an aging pathway, molecular mechanisms underlying the conserved function of sirtuins in aging remain unclear. In yeast, the only reported targets of Sir2 activity are histones. By contrast, the mammalian sirtuin SIRT1 deacetylates many different targets, including p53 (reviewed in reference 55). In yeast, Sir2 binds the telomeres and HM loci in complex with Sir3 and Sir4. Deacetylation of histone H4 lysine 16 (H4K16) promotes binding of Sir3 to chromatin and spreading of transcriptional silencing (reviewed in reference 54). However, Sir3 and Sir4 have no known metazoan homologs. Thus, it seems likely that some Sir2 molecular interactions remain undiscovered. Indeed, molecular mechanisms that contribute to sir2⌬ phenotypes, such as suppression of replication onset (47) and unequal inheritance of oxidative damage between mother and daughter cells (1), are undefined. To identify novel Sir2-interacting proteins that might participate in these and other uncharacterized molecular pathways, we performed a screen for Sir2 physical interactions.We identified Slx5 as a novel Sir2 intera...

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Section: Fig 3 Epitope-taggedmentioning

confidence: 99%

Slx5 Promotes Transcriptional Silencing and Is Required for Robust Growth in the Absence of Sir2

Darst

Garcia

Koch

et al. 2008

Molecular and Cellular Biology

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“…It is well known that the efficiency and precision of transcription and replication depend on the positioning of nuclear domains in the eukaryotic nucleus (Gasser et al 1998, Cremer & Cremer 2001, Gasser & Cockell 2001, Cremer et al 2006. One of the best-characterized examples of long-range organization in the nucleus concerns telomeres, which are best studied in lower eukaryotes.…”

Section: Discussionmentioning

confidence: 99%

“…It is well established that in eukaryotes the telomeres are attached to the nuclear envelope and co-purify with components of the sub-nuclear structure, the nuclear matrix/scaffold, and such are thought to have an influence on global regulation of gene expression (de Lange 2002, Gasser 2002, Zakian 1995. Localization of telomeres to the nuclear periphery is capable of contributing to heterochromatin formation and has been reported for yeast (Gasser et al 1998, Andrulis et al 1998, Taddei & Gasser 2004, Drosophila (Mathog et al 1984, Hochstrasser & Sedat 1987 and the pathogenic protozoa Trypanosoma brucei and Plasmodium falciparum (Perez-Morga et al 2001, Scherf et al 2001, Marty et al 2006. In mammalian cells, telomeres are not found at the nuclear periphery but co-purify in a nuclear scaffold preparation (de Lange 1992).…”

Section: Introductionmentioning

confidence: 92%

Cell cycle-dependent regulation of telomere tethering in the nucleus

Paeschke¹,

Juranek²,

Rhodes

et al. 2008

Chromosome Res

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It is well established that telomeres are tethered in the eukaryotic nucleus, but a detailed analysis of the regulation of telomere attachment throughout the cell cycle is still lacking. We show here that the telomeres in the macronucleus of the ciliate Stylonychia lemnae are bound to a sub-nuclear structure by an interaction of the telomere end-binding protein TEBPalpha with three SNS proteins that are integral parts of this structure. In the course of replication, the interaction of TEBPalpha with the SNS proteins is resolved and this process is regulated by cell cycle-specific phosphorylation of the SNS proteins. Our data can be incorporated into a mechanistic model for the regulation of telomere conformation and localization throughout the cell cycle.

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“…In the presence of Sir2, silenced rDNA chromatin adopts a more closed conformation as proved by in vitro micrococcal nuclease sensitivity assays and in vivo dam methyltransferase sensitivity assays (Fritze et al, 1997). Also, cross-linking and immunostaining experiments show that Sir2 binds to a subdomain of the nucleolus (Gasser et al, 1998). In further support of a Sir2-dependent model for rDNA silencing, sir2Δ strains exhibit a substantial increase in the fraction of firing ARSs during DNA replication (Pasero et al, 2002).…”

Section: Ii: Sir2 and Silencing At The Rdna Repeatsmentioning

confidence: 83%

Tdh3 (a GAPDH isozyme) is a novel regulator of Sir2-mediated transcriptional silencing in yeast

Ringel¹

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Nuclear Organization and Silencing: Trafficking of Sir Proteins

Cited by 7 publications

References 40 publications

Slx5 Promotes Transcriptional Silencing and Is Required for Robust Growth in the Absence of Sir2

Slx5 Promotes Transcriptional Silencing and Is Required for Robust Growth in the Absence of Sir2

Cell cycle-dependent regulation of telomere tethering in the nucleus

Tdh3 (a GAPDH isozyme) is a novel regulator of Sir2-mediated transcriptional silencing in yeast

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