High impact information on SIR4

• Unlike SIR3 and SIR4, the SIR2 gene is highly conserved in organisms ranging from archaea to humans [1].
• Moreover, AAR itself promotes the association of multiple copies of Sir3 with Sir2/Sir4 and induces a dramatic structural rearrangement in the SIR complex [2].
• Surprisingly, the 110 kDa SIR4-binding protein is identical to UBP3, one of several previously described yeast enzymes that deubiquitinate target proteins [3].
• A deubiquitinating enzyme interacts with SIR4 and regulates silencing in S. cerevisiae [3].
• Consistent with this view, expression of only the carboxyl terminus of SIR4 interferes with silencing at HM loci and telomeres, which also extends life span [4].

Biological context of SIR4

• SIR3 and SIR4 proteins are required for the positioning and integrity of yeast telomeres [5].
• Furthermore, using an in vivo plasmid rejoining assay, we demonstrate that SIR2, SIR3 and SIR4, three genes shown previously to function in TPE, are essential for Ku-dependent DSB repair [6].
• Within the resolution of these immunodetection techniques, we show that proteins encoded by the SIR3, SIR4, and RAP1 genes colocalize in a statistically significant manner with Y' telomere-associated DNA sequences [7].
• We have identified a null allele of the silent information regulator gene SIR4 as a host mutant that allows for transposition at high temperature [8].
• Nonetheless, the Y' FISH is altered in a qualitative manner in sir3 and sir4 mutant strains, consistent with the previously reported phenotypes of shortened telomeric repeats and loss of telomeric silencing [7].

Anatomical context of SIR4

• We show here that the SIR3 and SIR4 gene products have a sub-nuclear localization similar to the telomere-associated RAP1 protein, which is found primarily in foci at the nuclear periphery of fixed yeast spheroplasts [5].
• The positioning of yeast telomeres depends on SIR3, SIR4, and the integrity of the nuclear membrane [9].

Associations of SIR4 with chemical compounds

• Finally, we provide evidence that the different nicotinamide sensitivities of Sir2/Sir4 and RENT in vitro could contribute to locus-specific differences in how Sir2 activity is regulated in vivo [10].
• K. lactis sir2 mutants are more sensitive than the wild type to ethidium bromide, and K. lactis sir4 mutants are more resistant phenotypes that are not observed for the corresponding mutants of S. cerevisiae [11].

Physical interactions of SIR4

• We used the yeast two-hybrid assay to show that Hdf1 interacts with Sir4, which is involved in transcriptional silencing at telomeres and HM loci [12].
• By two-hybrid analysis, the amino-terminal third of Dot4p interacts with the silencing protein Sir4p [13].
• Sif2p interacts with Sir4p amino-terminal domain and antagonizes telomeric silencing in yeast [14].
• The properties of SIR4 dosage suppression suggest that SIR4 protein may interact directly with RAP1 at silencers [15].
• Here we show that SIR3 co-immunoprecipitates SIR4, RAP1 and histones from cellular extracts, suggesting the presence of large chromatin-associated protein complexes [16].

Regulatory relationships of SIR4

• Together, these findings suggest that Dot4p regulates silencing by acting on Sir4p [13].
• Distribution of a limited Sir2 protein pool regulates the strength of yeast rDNA silencing and is modulated by Sir4p [17].
• SAN1 may act posttranslationally to control the stability or activity of the SIR4 protein [18].
• Using the cloned genes, we showed that SIR3 at a high copy number is able to suppress mutations of SIR4 [19].
• The Yak1p kinase controls expression of adhesins and biofilm formation in Candida glabrata in a Sir4p-dependent pathway [20].

Other interactions of SIR4

• Analyses of sir4 mutants showed that Sir4 is required for deletion by illegitimate recombination and DNA end-joining in the pathway involving Hdf1 [12].
• Repression is restored by creation of a new telomere 13 kb from the integrated reporter or by elevated expression of SIR1, SIR3, and/or SIR4 [21].
• Cells lacking DOT4 exhibited reduced silencing and a corresponding decrease in the level of Sir4p [13].
• In a screen for factors that interact with Sir4p amino terminus, we have cloned SIF2, which encodes a WD40-repeat-containing factor that disrupts telomeric silencing when overexpressed [14].
• We show that the apparent increase in transposition activity in sir4 mutant strains at high temperature is dependent on the RAD52 gene and is thus likely resulting from an increase in Ty1 cDNA recombination, rather than in IN-mediated integration [8].

Analytical, diagnostic and therapeutic context of SIR4

• Using protein affinity chromatography, we show that SIR2, SIR3, and two proteins of 69 and 110 kDa tightly associate with SIR4 [3].
• We show by indirect immunofluorescence that Sir3p and Sir4p are redirected to the nucleolus in the SIR4-42 mutant [22].
• Reactivation of unstably repressed rings was blocked by overexpression of silencing proteins Sir3p and Sir4p, and chromatin immunoprecipitation studies showed that overexpressed Sir3p was incorporated into silent chromatin [23].

References