title: Locus specificity determinants in the multifunctional yeast silencing protein Sir2.
abstract: Yeast SIR2, the founding member of a conserved gene family, acts to modulate chromatin structure in three different contexts: silent (HM) mating-type loci, telomeres and rDNA. At HM loci and telomeres, Sir2p forms a complex with Sir3p and Sir4p. However, Sir2p's role in rDNA silencing is Sir3/4 independent, requiring instead an essential nucleolar protein, Net1p. We describe two novel classes of SIR2 mutations specific to either HM/telomere or rDNA silencing. Despite their opposite effects, both classes of mutations cluster in the same two regions of Sir2p, each of which borders on a conserved core domain. A surprising number of these mutations are dominant. Several rDNA silencing mutants display a Sir2p nucleolar localization defect that correlates with reduced Net1p binding. Although the molecular defect in HM/telomere-specific mutants is unclear, they mimic an age-related phenotype where Sir3p and Sir4p relocalize to the nucleolus. Artificial targeting can circumvent the silencing defect in a subset of mutants from both classes. These results define distinct functional domains of Sir2p and provide evidence for additional Sir2p-interacting factors with locus-specific silencing functions.
authors.affiliation: Department of Molecular Biology, University of Geneva, Geneva 4, CH-1211 Switzerland.
authors.forename: G
authors.initials: G
authors.lastname: Cuperus
authors.forename: R
authors.initials: R
authors.lastname: Shafaatian
authors.forename: D
authors.initials: D
authors.lastname: Shore
mesh_terms: D002843:Chromatin / Q000378:metabolism / Q000648:ultrastructure*; D015825:Chromosomes, Fungal / Q000378:metabolism / Q000648:ultrastructure*; D004271:DNA, Fungal / Q000235:genetics; D004275:DNA, Ribosomal / Q000235:genetics; D005656:Fungal Proteins / Q000737:chemistry* / Q000378:metabolism; D020868:Gene Silencing; D018390:Gene Targeting; D005799:Genes, Dominant; D005800:Genes, Fungal / Q000235:genetics; D049770:Genes, Mating Type, Fungal; D006655:Histone Deacetylases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D046911:Macromolecular Substances; D016296:Mutagenesis; D015513:Oncogene Proteins / Q000378:metabolism; D010641:Phenotype; D016133:Polymerase Chain Reaction; D017434:Protein Structure, Tertiary; D012441:Saccharomyces cerevisiae / Q000235:genetics*; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae; D056565:Sirtuin 2; D037761:Sirtuins; D013379:Substrate Specificity; D016615:Telomere / Q000235:genetics; D015534:Trans-Activators / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
chemical_list: D002843:Chromatin; D004271:DNA, Fungal; D004275:DNA, Ribosomal; D005656:Fungal Proteins; D046911:Macromolecular Substances; D015513:Oncogene Proteins; C084122:SIR3 protein, S cerevisiae; C061573:SIR4 protein, S cerevisiae; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae; D015534:Trans-Activators; C087778:SIR2 protein, S cerevisiae; D056565:Sirtuin 2; D037761:Sirtuins; D006655:Histone Deacetylases