The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
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this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
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The evidence retrieved and synthesized here concerns budding yeast Sir4, a “silent information regulator” protein that functions as the scaffold of the Sir2/Sir3/Sir4 (SIR) silencing complex at telomeres and the silent mating type loci (HML/HMR), consistent with UniProt P11978 (SIR4; YDR227W) and the Sir4 SID/PAD domain architecture described for S. cerevisiae Sir4. Sir4 is discussed specifically in the context of yeast heterochromatin, telomere clustering/anchoring at the nuclear periphery, and SIR-dependent transcriptional silencing (ponce2023theroleof pages 46-50, ponce2023theroleof pages 50-56, dhillon2024transcriptionalsilencingin pages 2-4).
Sir4 is best understood as a non-enzymatic regulatory/scaffold protein whose primary molecular function is to assemble and organize a multivalent silencing apparatus by bringing together:
- Sir2, an NAD+-dependent histone deacetylase, and
- Sir3, a nucleosome-binding structural silencing factor,
- plus telomere/silencer-bound recruiters and nuclear-envelope tethers.
This scaffold role is emphasized in mechanistic descriptions of the SIR complex, where Sir4 links Sir2 catalytic activity to chromatin binding/spreading mediated by Sir3 (ponce2023theroleof pages 46-50, ponce2023theroleof pages 50-56, ruault2021sir3mediateslongrange pages 1-2).
A recurring mechanistic definition of SIR-mediated silencing in budding yeast is: (i) nucleation of Sir binding at silencers/telomeres, followed by (ii) iterative spreading/propagation of Sir binding across nucleosomes via Sir2-driven deacetylation that creates higher-affinity binding sites for Sir3/Sir4, producing a stable repressed chromatin domain (dhillon2024transcriptionalsilencingin pages 2-4, ponce2023theroleof pages 50-56).
Silent chromatin in budding yeast forms repressive subcompartments concentrated near the nuclear periphery, where telomeres cluster and concentrate the SIR complex (ruault2021sir3mediateslongrange pages 1-2). A canonical quantitative description is that 32 telomeres cluster into ~3–5 foci in exponentially growing haploid cells (ponce2023sirtelomeresilencing pages 8-11, ruault2021sir3mediateslongrange pages 1-2).
A key emergent concept in modern models is that silencing is robust at the domain level despite constant molecular turnover (“constant flux”) of Sir proteins and nucleosomes, achieved by many weak multivalent interactions that collectively stabilize a silent state (dhillon2024transcriptionalsilencingin pages 10-12).
Sir4’s functional annotation is best captured by its interaction-enabling domains that connect telomere/silencer recruitment to Sir2 activity and nuclear-envelope tethering.
| Sir4 region / motif | Residues | Experimentally supported partner(s) | Functional role in Sir4 biology | Key evidence |
|---|---|---|---|---|
| Ku-binding motif | 100–115 | yKu80 | Contributes to telomere tethering/positioning at the nuclear periphery and links Sir4 to Ku-dependent telomere functions | (ponce2023theroleofa pages 46-50, ponce2023theroleof pages 46-50) |
| TOC motif (second of two clusters) | 172–180 | Not clearly assigned | Conserved N-terminal motif in Sir4; specific function remains unresolved in the cited sources | (ponce2023theroleofa pages 46-50, ponce2023theroleof pages 46-50) |
| N-terminal Rap1-binding region | 142–591 | Rap1 | Helps recruit the SIR complex to telomeric repeats and supports nucleation of subtelomeric heterochromatin | (ponce2023theroleofa pages 46-50, ponce2023theroleof pages 46-50, ponce2023theroleof pages 50-56) |
| N-terminal linker DNA protection region | N-terminal; precise subregion not fully delimited here | Chromatin/linker DNA | Enhances silencing by protecting linker DNA and supporting heterochromatin architecture; N-terminus is also phosphorylated by Cdc28 | (ponce2023theroleofa pages 46-50, ponce2023theroleof pages 46-50) |
| Sir2-interaction domain (SID) | 737–893 | Sir2 | Forms the Sir2–Sir4 core scaffold; recruits/allosterically supports Sir2 and couples deacetylation to SIR spreading | (ponce2023theroleof pages 46-50, ponce2023theroleof pages 50-56) |
| C-terminal Rap1-binding region | 839–1358 | Rap1 | Additional Rap1 contact surface that strengthens telomeric recruitment/nucleation of SIR chromatin | (ponce2023theroleof pages 46-50) |
| Partitioning and anchoring domain (PAD) | 950–1262 | Esc1 | Anchors telomeric SIR domains to the inner nuclear membrane/nuclear periphery and contributes to telomere partitioning | (ponce2023theroleof pages 46-50, ponce2023theroleof pages 50-56) |
| H-BRCT-like region within PAD | 961–1085 | Phosphorylated targets including Esc1 | Phospho-target recognition module implicated in perinuclear anchoring of silent chromatin | (ponce2023theroleof pages 46-50) |
| C-terminal coiled-coil (CC) | 1271–1347 | Sir4 (homodimer), Sir3, yKu70 | Mediates Sir4 homodimerization, generates two Sir3-binding sites, supports effective silencing, and also contacts yKu70 | (ponce2023theroleof pages 46-50) |
| Full-length Sir4 as SIR scaffold | Full protein | Sir2, Sir3, Rap1, Esc1, yKu70/yKu80 | Structural scaffold of the SIR complex; assembles telomeric/HM heterochromatin, supports spreading after Sir2-dependent H4K16 deacetylation, and helps tether/cluster telomeres at the nuclear envelope | (ponce2023theroleof pages 46-50, dhillon2024transcriptionalsilencingin pages 2-4, ponce2023theroleof pages 50-56) |
Table: This table summarizes the mapped domains and motifs of budding yeast Sir4 (UniProt P11978/YDR227W), their major interaction partners, and the core functional roles those regions play in silencing, telomere anchoring, and SIR complex assembly. It is useful as a compact functional annotation map grounded in the available cited evidence.
Key points supported by the retrieved evidence:
- Sir2 interaction: Sir4 contains a Sir2-interaction domain (SID; aa 737–893) and Sir2–Sir4 interaction can allosterically stimulate Sir2 activity and stabilize the complex, coupling deacetylation to silencing (ponce2023theroleof pages 46-50, ponce2023theroleof pages 50-56).
- Rap1 recruitment: Rap1’s C-terminus binds Sir4 (and Sir3), providing a direct telomeric recruitment mechanism (ruault2021sir3mediateslongrange pages 1-2, ponce2023theroleof pages 42-46).
- Nuclear envelope anchoring: Sir4 contains a PAD that binds Esc1 and includes an H-BRCT-like module that recognizes phosphorylated ligands (including Esc1), supporting perinuclear anchoring and repression (ponce2023theroleof pages 46-50, deshpande2020thesir4h‐brct pages 2-4).
- Ku pathway coupling: Sir4 contains a Ku-binding motif and additional interactions with yKu factors, tying silencing/tethering to telomere biology (ponce2023theroleof pages 46-50).
Silencers recruit Sir2/Sir3/Sir4 (and may channel remodelers to create evenly spaced nucleosomes), establishing “directionality” for spreading (dhillon2024transcriptionalsilencingin pages 2-4). At telomeres, the SIR complex is recruited to TG1–3 repeats via Rap1, with Sir4–Rap1 being central to nucleation models (ruault2021sir3mediateslongrange pages 1-2, ponce2023theroleof pages 50-56).
A widely used mechanistic loop is: Sir2 deacetylates H4K16 on adjacent nucleosomes → Sir3/Sir4 bind better to the resulting hypoacetylated chromatin → repeated cycles spread the complex and increase domain-wide avidity (dhillon2024transcriptionalsilencingin pages 2-4, ponce2023theroleof pages 50-56). Dhillon & Kamakaka (2024) emphasize that stability emerges from a domain-wide web/mesh of interactions; a single nucleosome losing Sir occupancy is unlikely to immediately permit durable transcription (dhillon2024transcriptionalsilencingin pages 10-12).
Dhillon & Kamakaka (2024) synthesize an updated view in which Sir proteins stably silence weak regulatory elements by changing transcription bursting and nucleosome mobility, but fail to robustly repress strong housekeeping regulatory elements (dhillon2024transcriptionalsilencingin pages 10-12). They describe hysteresis in transitions between active and silent states, with thresholds around ~75% acetylation/deacetylation of nucleosomes across the domain (dhillon2024transcriptionalsilencingin pages 10-12).
| Paper | Publication date | Main Sir4-related finding | Quantitative data | URL / DOI | Evidence |
|---|---|---|---|---|---|
| Dhillon & Kamakaka 2024, Epigenetics & Chromatin | Sep 2024 | Updated review/model: Sir4 acts within multivalent Sir–nucleosome networks that stabilize silent domains despite flux; Sir-mediated silencing is probabilistic and domain-wide rather than a rigid static block. | Silent-state hysteresis at HM loci requires ~75% nucleosome acetylation to lose silencing and >75% unacetylated histones to re-establish it; HML silencing loss occurs in ~1/1000 cells in wild type. | https://doi.org/10.1186/s13072-024-00553-7 | (dhillon2024transcriptionalsilencingin pages 10-12) |
| Yuan & Moazed 2024, PNAS | Jan 2024 | Engineered epigenetic inheritance system leverages the native Sir3–Sir4/Sir2 interaction logic, highlighting how reduced-complexity silent chromatin can be built from positive-feedback design principles derived from the SIR system. | Study is conceptually quantitative but the retrieved excerpt mainly supports that Sir3 naturally interacts with Sir2-bound Sir4; no Sir4-specific numerical metric extracted in the available text. | https://doi.org/10.1073/pnas.2318455121 | (hamali2023regulationofthe pages 12-13) |
| Ponce et al. 2023, Journal of Cell Biology | Jan 2023 | Nuclear-envelope lipid perturbation by edelfosine disperses Sir4 from telomeres without abolishing telomere anchoring, linking membrane state to Sir4-dependent telomere silencing/clustering. | Rap1 telomere foci increased from ~3–5 to ~6–7 after edelfosine; Sir4 ChIP recovery decreased at three tested telomeres; 224 genes were differentially expressed (119 up, 105 down); 12.6% of >2-fold upregulated genes were subtelomeric vs 4.2% genome-wide; 29/120 genes in the 0–10 kb subtelomeric zone were upregulated; PAU and COS expression increased by 2.5 ± 0.49 and 1.5 ± 0.82 ln-fold, respectively. | https://doi.org/10.1101/2022.07.08.499406 | (ponce2023sirtelomeresilencing pages 8-11, ponce2023sirtelomeresilencing pages 11-15) |
| Bondra & Rine 2023, PNAS | Sep 2023 | Context-specific silencing analysis of Rap1 reinforces the importance of Rap1-dependent Sir4 recruitment in determining whether promoter-bound Rap1 behaves as an activator or a silencing factor. | No Sir4-specific numerical estimate retrieved from the available excerpts, but the paper narrows silencing to a step after activator recruitment and before productive transcription in Sir-silenced chromatin. | https://doi.org/10.1073/pnas.2304343120 | (ponce2023sirtelomeresilencing pages 50-52) |
| Miangolarra et al. 2023, bioRxiv | Aug 2023 | Modeling plus experiment support two-way cooperativity between silencer occupancy and Sir–nucleosome binding; Sir4 titration experiments support the idea that Sir4 dosage affects occupancy and bistable silencing behavior. | Silencing loss rates were ~1%; silencing establishment dropped ~20-fold when locus size increased from 6 to 16 nucleosomes; more than half of the drop in E-silencer binding occurred when nucleosomal Sir binding fell below 20% of wild type. | https://doi.org/10.1101/2023.08.12.552948 | (miangolarra2023twowayfeedbackbetween pages 6-8) |
| Deshpande et al. 2020, EMBO Journal | Sep 2020 | Structural/mechanistic basis for a Sir4 phospho-interaction hub: the H-BRCT region in the PAD binds phosphorylated Esc1/Ubp10/Ty5 peptides and is required for proper Sir4 localization, telomere clustering, and repression. | Sir4 H-BRCT structure solved at 1.1 Å; Kd values included ~0.07 µM for Esc1/Ubp10 phosphopeptides and 5.57 µM for Ty5; sir4 RKR mutants showed 1–3 Sir4 foci per nucleus vs 4–6 in wild type and ~30% lower nuclear Sir4-GFP intensity; overexpressing isolated wt H-BRCT caused ~40% reduction in Sir4-containing telomere clusters and abolished URA3 silencing. | https://doi.org/10.15252/embj.2019101744 | (deshpande2020thesir4h‐brct pages 6-7, deshpande2020thesir4h‐brct pages 2-4) |
| Wu et al. 2021, PNAS | Dec 2021 | Quantitative buffering analysis identifies Sir4 as the limiting SIR component for silencing robustness, more sensitive to dosage reduction than Sir2 or Sir3. | Reducing SIR4 dosage by ~2–3-fold significantly weakened silencing, whereas similar reductions of Sir2 or Sir3 did not; loss of silencing required 50–75% acetyl-mimic histones. | https://doi.org/10.1073/pnas.2111841118 | (wu2021measuringthebuffering pages 1-2) |
| Larin et al. 2015, PLOS Genetics | Nov 2015 | Sir4 abundance and availability regulate de novo heterochromatin assembly; telomeres compete with HM loci for a limiting Sir4 pool. | Sir4 levels decreased ~4–5-fold after 5 h of alpha-factor G1 arrest and recovered after two cell cycles; de novo silencing required 1–2 divisions; halving Sir4 slowed establishment whereas increased Sir4 accelerated establishment. | https://doi.org/10.1371/journal.pgen.1005425 | (larin2015competitionbetweenheterochromatic pages 1-2, larin2015competitionbetweenheterochromatic pages 2-4) |
Table: This table compiles recent and foundational quantitative findings for budding yeast Sir4/SIR4, emphasizing 2023–2024 developments while retaining older landmark studies needed to interpret Sir4 dosage, localization, and silencing mechanisms.
A 2023 study perturbed nuclear-envelope lipids using the lysolipid analog edelfosine and observed major architectural consequences: telomere anchoring remained intact, but telomere clustering was impaired (Rap1 foci increased) and Sir4 association with telomeres decreased by ChIP, with loss of punctate Sir4 foci by microscopy (ponce2023sirtelomeresilencing pages 8-11). Transcriptomically, 224 genes were differentially expressed (119 up, 105 down), and subtelomeric genes were overrepresented among >2-fold upregulated targets (12.6% vs 4.2% genome background; p = 0.0001) (ponce2023sirtelomeresilencing pages 11-15). These results connect Sir4’s telomeric function to membrane composition and nuclear architecture rather than only DNA sequence and histone marks (ponce2023sirtelomeresilencing pages 8-11, ponce2023sirtelomeresilencing pages 11-15).
A 2023 preprint and a 2024 PNAS paper on heterochromatin bistability model two-way coupling between chromatin compaction and histone modification state, and explicitly consider Sir complex/Sir4 titration effects on binding and switching (miangolarra2023twowayfeedbackbetween pages 6-8, miangolarra2024twowayfeedbackbetween pages 6-7). A key mechanistic advance emphasized in the 2023 modeling/evidence is two-way cooperativity: reduced Sir binding on nucleosomes decreases silencer occupancy, not only vice versa (miangolarra2023twowayfeedbackbetween pages 6-8). The 2024 PNAS work also frames bistability as dependent on Sir4 concentration windows that shift with silencer strength (e.g., sir1Δ) (miangolarra2024twowayfeedbackbetween pages 6-7).
Yuan & Moazed engineered reduced-complexity silencing systems (H3K9 methylation transplanted into S. cerevisiae) and explicitly position their designs relative to the native SIR system: in the natural pathway, Sir2 deacetylates H4K16, Sir3 recognizes unmodified H4K16, and Sir3 associates with Sir2 via Sir4 (yuan2023asimplemechanism pages 1-7, yuan2024minimalrequirementsfor pages 1-2). These studies demonstrate that the SIR system’s architecture (read–write feedback plus multivalency) can inspire synthetic epigenetic memory circuits (yuan2024minimalrequirementsfor pages 2-3).
A concrete real-world implementation is the use of Sir-inspired interaction logic to construct engineered silent chromatin with heritable behavior. Yuan & Moazed report that engineered H3K9me2 marks can be rapidly lost by dilution after recruiter release (TetR-SET untethers in ~30 min; H3K9me2 becomes undetectable after ~6 h) (yuan2024minimalrequirementsfor pages 2-3). Their engineered systems allow testing minimal requirements for inheritance and show how positive feedback loops can transmit silent information over generations (yuan2024minimalrequirementsfor pages 1-2, yuan2023asimplemechanism pages 11-14). While these systems are not native Sir4-dependent heterochromatin per se, they are a practical “application” of Sir4-centered mechanistic principles (yuan2023asimplemechanism pages 1-7, yuan2024minimalrequirementsfor pages 1-2).
The edelfosine study demonstrates a tractable experimental “implementation” in which altering nuclear envelope lipid composition changes telomere clustering, Sir4 telomeric association, and subtelomeric transcription, linking silencing phenotypes to membrane state (ponce2023sirtelomeresilencing pages 8-11, ponce2023sirtelomeresilencing pages 11-15). This supports use of the SIR4 system as a model for mechanochemical coupling between nuclear membranes and chromatin regulation.
Sir4 abundance is not merely permissive; it can be an experimental control knob. Sir4 levels drop strongly in prolonged G1 arrest and its abundance controls de novo heterochromatin establishment speed (larin2015competitionbetweenheterochromatic pages 2-4). Such dosage dependence makes SIR4 a practical lever for dissecting establishment vs maintenance phases of silencing (larin2015competitionbetweenheterochromatic pages 1-2).
Dhillon & Kamakaka (2024, Epigenetics & Chromatin, Sep 2024; URL in table above) provide an expert synthesis that is particularly relevant for functional annotation because it reframes Sir4/SIR silencing as:
- probabilistic at the level of transcription bursts and nucleosome configurations,
- stabilized by hysteresis and domain-wide cooperativity,
- robust through sub-optimization of multiple nodes rather than a single deterministic “off switch.”
This view explicitly incorporates Sir4 as part of a multivalent Sir mesh that stabilizes silent domains even though individual Sir–nucleosome contacts are weak/transient (dhillon2024transcriptionalsilencingin pages 10-12).
| Quantitative data point | Value | Experimental context | Citation |
|---|---|---|---|
| Telomere cluster number in untreated cells | ~3–5 foci | Rap1-GFP live-cell imaging of 32 telomeres in haploid cells; baseline telomere clustering at the nuclear periphery | (ponce2023sirtelomeresilencing pages 8-11, ruault2021sir3mediateslongrange pages 1-2) |
| Telomere cluster number after edelfosine | ~6–7 foci | Nuclear-envelope lipid perturbation disperses Sir4-associated telomere clusters without abolishing anchoring | (ponce2023sirtelomeresilencing pages 8-11) |
| Differentially expressed genes after edelfosine | 224 total | RNA-seq after NE deformation by edelfosine | (ponce2023sirtelomeresilencing pages 11-15) |
| Upregulated vs downregulated genes after edelfosine | 119 up, 105 down | Same RNA-seq dataset; stringent cutoff > ln(2)-fold and FDR p < 0.01 | (ponce2023sirtelomeresilencing pages 11-15) |
| Subtelomeric fraction among strongly upregulated genes | 12.6% | Genes upregulated >2-fold after edelfosine were enriched in subtelomeric regions | (ponce2023sirtelomeresilencing pages 11-15) |
| Genome-wide subtelomeric baseline | 4.2% | Background fraction of yeast genes in the same subtelomeric interval | (ponce2023sirtelomeresilencing pages 11-15) |
| Subtelomeric enrichment significance | p = 0.0001 | Fisher’s exact test for enrichment of subtelomeric genes among edelfosine-upregulated targets | (ponce2023sirtelomeresilencing pages 11-15) |
| Upregulated genes in the 0–10 kb subtelomeric zone | 29 of 120 genes | Sir2/Sir4-regulated subtelomeric interval queried after edelfosine treatment | (ponce2023sirtelomeresilencing pages 11-15) |
| PAU-family induction after edelfosine | 2.5 ± 0.49 ln-fold | qPCR validation of known Sir-regulated subtelomeric targets | (ponce2023sirtelomeresilencing pages 11-15) |
| COS-family induction after edelfosine | 1.5 ± 0.82 ln-fold | qPCR validation of known Sir-regulated subtelomeric targets | (ponce2023sirtelomeresilencing pages 11-15) |
| Sir4 level drop during prolonged G1 arrest | ~4–5-fold decrease after 5 h | Cell-cycle regulation of Sir4 abundance; de novo heterochromatin establishment studies | (larin2015competitionbetweenheterochromatic pages 2-4) |
| Time for Sir4 level recovery after G1 arrest | 2 cell cycles | Recovery of Sir4 protein abundance after release from arrest | (larin2015competitionbetweenheterochromatic pages 2-4) |
| De novo silencing assembly time | 1–2 divisions | Full silent chromatin formation/repression requires more than one cell cycle | (larin2015competitionbetweenheterochromatic pages 1-2) |
| Silencing weakened by reduced SIR4 dosage | ~2–3-fold reduction in SIR4 dosage | Quantitative buffering analysis identifies Sir4 as the limiting SIR component | (wu2021measuringthebuffering pages 1-2) |
| Acetyl-mimic threshold for loss of silencing | 50–75% of nucleosomes | H4K16 acetyl-mimic threshold measured for stability of silent chromatin | (wu2021measuringthebuffering pages 1-2) |
| Acetylation threshold to lose silencing in updated review model | ~75% of nucleosomes | Review synthesis of hysteresis in silent-domain switching | (dhillon2024transcriptionalsilencingin pages 10-12) |
| Deacetylation threshold to re-establish silencing | >75% unacetylated nucleosomes | Hysteresis model for transition from active to silent state | (dhillon2024transcriptionalsilencingin pages 10-12) |
| HML silencing loss frequency | ~1 in 1000 cells | Wild-type stability of HM-locus silencing | (dhillon2024transcriptionalsilencingin pages 10-12) |
| Modeled/observed silencing loss rate at HMR | ~1% | Bistability model with dynamic chromatin compaction and Sir feedback | (miangolarra2023twowayfeedbackbetween pages 6-8) |
| Drop in establishment rate with larger locus size | ~20-fold decrease | HMR establishment falls as locus expands from 6 to 16 nucleosomes | (miangolarra2023twowayfeedbackbetween pages 6-8) |
| Silencer-binding nonlinearity threshold | >50% of E-silencer binding loss when nucleosome Sir binding <20% of WT | Cooperative link between nucleosome-bound Sir complex and silencer occupancy | (miangolarra2023twowayfeedbackbetween pages 6-8, miangolarra2024twowayfeedbackbetween pages 6-7) |
| Sir4 titration concentration window | ~0–0.4 µM | PNAS 2024 modeling/fit of Sir4 concentration versus bistability coefficient | (miangolarra2024twowayfeedbackbetween pages 8-9) |
| Scaling factor between estradiol-induced and native Sir4 promoter regimes | ~2-fold | Model–experiment normalization for Sir4 titration/switching analyses | (miangolarra2024twowayfeedbackbetween pages 6-7) |
| Sir4 H-BRCT affinity for Esc1/Ubp10 phosphopeptides | ~0.07 µM KD | MST measurements of phosphopeptide binding by Sir4 H-BRCT | (deshpande2020thesir4h‐brct pages 6-7) |
| Alternate reported Sir4 H-BRCT affinity value | 2.96 µM KD | Figure-reported Ubp10-related binding value in Deshpande et al. | (deshpande2020thesir4h‐brct pages 6-7) |
| Sir4 H-BRCT affinity for Ty5 phosphopeptide | 5.57 µM KD | Weaker phosphopeptide interaction than Esc1/Ubp10 | (deshpande2020thesir4h‐brct pages 6-7) |
| Sir4 H-BRCT crystal structure resolution | 1.1 Å | Structural definition of the Sir4 phosphopeptide-binding module | (deshpande2020thesir4h‐brct pages 2-4) |
| Wild-type Sir4 nuclear foci | 4–6 foci per nucleus | Sir4-GFP localization with intact H-BRCT phosphobinding | (deshpande2020thesir4h‐brct pages 6-7) |
| sir4 RKR mutant nuclear foci | 1–3 foci per nucleus | H-BRCT phosphobinding-defective mutant reduces Sir4 clustering | (deshpande2020thesir4h‐brct pages 6-7) |
| Nuclear Sir4-GFP reduction in sir4 RKR | ~30% lower | Total nuclear Sir4 signal reduced by H-BRCT phosphobinding mutation | (deshpande2020thesir4h‐brct pages 6-7) |
| Reduction in Sir4-containing telomere clusters upon isolated H-BRCT overexpression | ~40% reduction | Dominant competition for phospho-interactors disrupts clustering and silencing | (deshpande2020thesir4h‐brct pages 6-7) |
| Rap1 occupancy density at telomeres | ~1 Rap1 every ~20 bp | Recruitment platform for Sir4 at telomeric repeats | (ponce2023theroleof pages 42-46) |
| Rap1 molecules per telomere | ~15–20 | Estimated telomere-bound Rap1 copy number supporting Sir4 recruitment | (ponce2023theroleof pages 42-46) |
| Fraction of total cellular Rap1 at telomeres | ~10% | Indicates strong enrichment of Rap1 at telomeres relative to rest of genome | (ponce2023theroleof pages 42-46) |
| SIR complex stoichiometry on nucleosomes | 1:1:1 Sir2:Sir3:Sir4 | Telomeric SIR complex composition | (ponce2023theroleof pages 42-46, ponce2023theroleof pages 46-50) |
| In vitro SIR:nucleosome stoichiometry in spreading model | 2:1 | Biochemical model for SIR complex engagement with nucleosomes | (ponce2023theroleof pages 50-56) |
| Distance of SIR spreading from nucleation sites | 3–20 kb | Extent of telomeric heterochromatin spreading | (ponce2023theroleof pages 50-56) |
Table: This table compiles the main numerical findings relevant to S. cerevisiae Sir4 across localization, silencing, structural binding, and transcriptomic studies. It is useful as a compact evidence map for comparing Sir4 dosage effects, telomere organization, and recent 2023–2024 mechanistic results.
A few high-value quantitative takeaways for Sir4 functional annotation:
- Telomere organization: baseline ~3–5 telomere foci can shift to ~6–7 under nuclear-envelope lipid perturbation, concomitant with reduced Sir4 telomeric ChIP recovery and Sir4 focus dispersion (ponce2023sirtelomeresilencing pages 8-11).
- Sir4 as limiting component: silencing sensitivity to Sir4 dosage is measurable; reducing SIR4 gene dosage by ~2–3-fold significantly weakens silencing (wu2021measuringthebuffering pages 1-2).
- Silent-domain robustness thresholds: loss of silencing requires large-scale acetylation perturbation (50–75% acetyl-mimic histones; review synthesis emphasizes ~75% thresholds and hysteresis) (wu2021measuringthebuffering pages 1-2, dhillon2024transcriptionalsilencingin pages 10-12).
- Phospho-dependent perinuclear silencing module: Sir4 H-BRCT binds Esc1/Ubp10/Ty5 phosphopeptides with Kd values in the ~0.07–5.57 µM range; disrupting phospho-binding reduces Sir4 foci number and silencing (deshpande2020thesir4h‐brct pages 6-7).
Sir4 (SIR4; UniProt P11978; YDR227W) is a nuclear silencing scaffold protein that organizes SIR heterochromatin by coupling sequence-specific recruitment (e.g., Rap1 at telomeres; silencer factors at HM loci) with enzymatic histone deacetylation by Sir2 and nucleosome binding/spreading by Sir3, and by enabling nuclear-envelope tethering and clustering through PAD/H-BRCT-mediated phospho-interactions (notably with Esc1) and Ku-linked telomere pathways (ponce2023theroleof pages 46-50, ponce2023theroleof pages 50-56, deshpande2020thesir4h‐brct pages 2-4). Recent work emphasizes that this system is quantitatively tunable (Sir4 is limiting), architecturally sensitive (nuclear envelope lipid state affects Sir4 telomere association and subtelomeric gene regulation), and best described by probabilistic, domain-wide, hysteretic models rather than static occupancy alone (wu2021measuringthebuffering pages 1-2, ponce2023sirtelomeresilencing pages 8-11, dhillon2024transcriptionalsilencingin pages 10-12).
References
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(ponce2023theroleof pages 50-56): ML Sosa Ponce. The role of nuclear envelope lipids in nuclear shape and transcription. Unknown journal, 2023.
(dhillon2024transcriptionalsilencingin pages 2-4): Namrita Dhillon and Rohinton T. Kamakaka. Transcriptional silencing in saccharomyces cerevisiae: known unknowns. Epigenetics & Chromatin, Sep 2024. URL: https://doi.org/10.1186/s13072-024-00553-7, doi:10.1186/s13072-024-00553-7. This article has 4 citations and is from a peer-reviewed journal.
(ruault2021sir3mediateslongrange pages 1-2): Myriam Ruault, Vittore F. Scolari, Luciana Lazar-Stefanita, Antoine Hocher, Isabelle Loïodice, Romain Koszul, and Angela Taddei. Sir3 mediates long-range chromosome interactions in budding yeast. Genome Research, 31:411-425, Feb 2021. URL: https://doi.org/10.1101/gr.267872.120, doi:10.1101/gr.267872.120. This article has 35 citations and is from a highest quality peer-reviewed journal.
(ponce2023sirtelomeresilencing pages 8-11): Maria Laura Sosa Ponce, Mayrene Horta Remedios, Sarah Moradi-Fard, Jennifer A Cobb, and Vanina Zaremberg. Sir telomere silencing depends on nuclear envelope lipids and modulates sensitivity to a lysolipid. The Journal of Cell Biology, Jan 2023. URL: https://doi.org/10.1101/2022.07.08.499406, doi:10.1101/2022.07.08.499406. This article has 10 citations.
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(yuan2024minimalrequirementsfor pages 2-3): Andy H. Yuan and Danesh Moazed. Minimal requirements for the epigenetic inheritance of engineered silent chromatin domains. Proceedings of the National Academy of Sciences of the United States of America, Jan 2024. URL: https://doi.org/10.1073/pnas.2318455121, doi:10.1073/pnas.2318455121. This article has 7 citations and is from a highest quality peer-reviewed journal.
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