SIR2

Summary: SIR2 localizes to nucleus where it executes core silencing functions at HML, HMR, and telomeres.

Reason: SIR2 is definitively nuclear-localized and essential for transcriptional silencing at mating-type loci and telomeres. IBA inference from conserved orthologs is appropriate.

Summary: SIR2 functions as core component of silencing complexes that repress transcription at specific loci.

Reason: SIR2 is a true transcriptional corepressor that represses transcription at silent mating-type loci, telomeres, and rDNA through heterochromatin formation. The mechanism involves histone deacetylation and chromatin compaction. IBA appropriately reflects this conserved function.

Summary: SIR2 participates in DNA damage response through suppression of recombination in rDNA and telomeres.

Reason: SIR2 suppresses recombination at rDNA repeats and telomeres, which could be considered part of genome stability/DNA damage response. However, this is not a primary function - the primary role is transcriptional silencing. Recombination suppression is a consequence of heterochromatin formation. IBA is reasonable but function is better captured by more specific terms like negative regulation of recombination.

Summary: SIR2 is an essential component of subtelomeric silencing complexes.

Reason: SIR2 is a core component of the Sir2-Sir3-Sir4 complex that specifically maintains subtelomeric heterochromatin. This is a well-established, experimentally demonstrated function. IBA appropriately reflects this conserved activity across sirtuin orthologs.

Summary: SIR2 catalyzes NAD-dependent deacetylation of histone H3 lysine 14.

Reason: This is the canonical histone deacetylase activity of SIR2. Direct experimental evidence demonstrates H3K14 deacetylation is essential for silencing function. IBA appropriately captures this conserved enzymatic activity.

Summary: SIR2 catalyzes NAD-dependent deacetylation of histone H3 lysine 9.

Reason: Direct experimental evidence demonstrates SIR2 deacetylates H3K9 in vitro and this activity is functionally important for silencing. IBA appropriately reflects this conserved enzymatic specificity.

Summary: SIR2 catalyzes NAD-dependent deacetylation of histone H4 lysine 16.

Reason: H4K16 deacetylation is the most critical substrate for SIR2 function in vivo. Mutational studies demonstrate H4K16 acetylation is essential for Sir3 binding and silencing. This is a core, well-established activity. IBA appropriately reflects this.

Summary: SIR2 is a chromatin-associated protein that functions as integral component of silencing chromatin complexes.

Reason: SIR2 localizes to chromatin and is a core component of heterochromatin-forming complexes. This is a broad but accurate cellular component term reflecting SIR2s association with chromatin structure.

Summary: SIR2 localizes to nucleolus as component of RENT complex that silences rDNA.

Reason: SIR2 is a well-characterized component of the RENT (regulator of nucleolar silencing and telophase exit) complex. Nucleolar localization is specifically required for rDNA silencing. This is a core, essential localization for one of SIR2s major functions.

Summary: Correct but uninformatively generic term; SIR2's experimentally grounded repair involvement is the specific child GO:0006303 (NHEJ), and it is a secondary consequence of silencing rather than a core function.

Reason: Not an over-annotation. SIR2 carries an IMP annotation to GO:0006303 (double-strand break repair via nonhomologous end joining), and GO:0006303 is a descendant of GO:0006302 which is a descendant of this term (ancestry checked against QuickGO), so by the true-path rule DNA repair involvement follows from evidence already accepted in this review. What is wrong with the term is only its altitude: it is the generic ancestor and says far less than the NHEJ child, and SIR2 is not a repair enzyme - its requirement for end joining is a downstream consequence of its silencing function, whose mechanism is not established by the cached evidence and is not asserted here. Retained as non-core rather than removed or flagged as over-annotated.

Summary: SIR2 organizes chromatin into heterochromatin structures through histone deacetylation.

Reason: SIR2 functions as a chromatin organizer that compacts chromatin fiber into condensed heterochromatin form. This process involves deacetylating histones and recruiting other silencing factors to establish repressed chromatin domains.

Summary: SIR2 regulates transcription by maintaining repressive chromatin at specific loci.

Reason: While SIR2 affects transcription through chromatin remodeling, it does not directly catalyze transcription. Its role is transcriptional regulation/repression. The broader term "transcription" can be retained but is less informative than negative regulation terms. Treating as non-core process.

Summary: SIR2 participates in genome stability through recombination suppression.

Reason: Duplicate of an IBA annotation; same reasoning applies. SIR2 suppresses rDNA recombination through heterochromatin formation, contributing to genome stability. However, this is a secondary effect of its primary silencing function.

Summary: Generic term, but correctly applied - the sirtuin reaction is formally an acyl transfer, and UniProt classifies SIR2 under EC 2.3.1.286 (EC class 2.3, acyltransferases).

Reason: Not an error. SIR2 does not simply hydrolyse the acetyl-lysine bond; it consumes NAD+ and transfers the acetyl group onto ADP-ribose, and the UniProt record for P06700 records this explicitly - EC 2.3.1.286 and the reaction "N(6)-acetyl-L-lysyl-[protein] + NAD(+) + H2O = 2''-O-acetyl-ADP-D-ribose + nicotinamide + L-lysyl-[protein]". The GOA row derives from UniProt keyword KW-0808 (Transferase), which UniProt assigns on exactly that basis. The term is therefore correct but uninformatively generic, so it is kept as non-core in favour of the specific NAD-dependent deacetylase terms rather than removed.

Summary: SIR2 is the founding member of NAD-dependent histone deacetylases. The crystal structure of Sir2 in complex with a Sir4 fragment (PDB 4IAO; Sir2 captured with its catalytic Zn site and an ADP-ribose product) resolves the conserved NAD-dependent deacetylase catalytic domain and shows that Sir4 binding allosterically stimulates this deacetylase activity.

Reason: This is the primary catalytic activity of SIR2. Extensive biochemical evidence establishes this as the core molecular function. The NAD-dependence is a defining and essential characteristic of SIR2 activity.

Summary: SIR2 is essential for forming and maintaining silent chromatin at HML and HMR mating-type loci.

Reason: SIR2 was originally identified as a silencer of mating-type loci through its role in heterochromatin formation at HML and HMR. This is a core, conserved function with extensive experimental support.

Summary: SIR2 localizes to nuclear lumen where it functions in transcriptional silencing complexes.

Reason: SIR2 is a soluble nuclear protein that functions in the nuclear lumen as component of silencing complexes. This is an appropriate subcellular compartment designation.

Summary: SIR2 catalyzes NAD-dependent deacetylation of lysine residues in protein substrates.

Reason: This is a more general term capturing SIR2s core catalytic activity as an NAD-dependent deacetylase that acts on protein lysine residues. Both histones and non-histone proteins can be substrates. This term appropriately generalizes the specific H3K14, H3K9, and H4K16 deacetylase activities.

Summary: SIR2 represses transcription at silent mating loci, telomeres, and rDNA through heterochromatin formation.

Reason: SIR2 function is fundamentally about transcriptional repression at specific genomic loci. This is a primary biological process activity. The annotation appropriately captures the mechanism by which SIR2 regulates transcription.

Summary: SIR2 suppresses recombination at rDNA repeats and telomeres through heterochromatin formation.

Reason: SIR2 prevents aberrant recombination at rDNA hotspots and telomeres. This suppression is a direct consequence of heterochromatin formation and is well-documented. This is a core function distinct from, but related to, silencing.

Summary: SIR2 binds a zinc ion as an essential cofactor for deacetylase catalysis.

Reason: SIR2 contains a zinc-binding pocket that is structurally essential for catalytic activity. Crystal structures confirm zinc coordination by Cys and His residues. This is a validated, necessary cofactor interaction.

Summary: SIR2 binds NAD+ as essential cofactor for histone deacetylase activity. The crystal structure of Sir2 in complex with a Sir4 fragment (PDB 4IAO, with the catalytic Zn and an ADP-ribose product bound) resolves the conserved NAD-dependent deacetylase catalytic domain, consistent with the obligate NAD+ requirement.

Reason: NAD+ is the obligate substrate/cofactor for SIR2 catalytic activity. Crystal structures show extensive NAD+ binding interactions. The NAD-dependence is a defining feature distinguishing sirtuins from other deacetylases.

Summary: Protein binding is too vague and non-informative for SIR2 curation.

Reason: GO:0005515 "protein binding" is discouraged by GO curators as it lacks biological specificity. SIR2 interacts with specific proteins (Sir3, Sir4, CDC14, NET1, etc.) but these interactions are better captured through protein complex and localization terms. Multiple IPI entries for the same vague term should be consolidated and replaced with more specific molecular function or complex membership terms.

Summary: SIR2 is essential for forming and maintaining heterochromatin at ribosomal DNA repeats.

Reason: SIR2 is a core component of the RENT complex that establishes and maintains rDNA heterochromatin. This is a well-established, experimentally demonstrated function with multiple supporting studies.

Summary: SIR2 localizes to the nucleolus as component of RENT complex.

Reason: Duplicate of IEA annotation with different evidence code. NAS (Non-traceable Author Statement) appropriately reflects localization demonstrated in the referenced paper. Both annotations are valid and complementary.

Summary: SIR2 is essential for heterochromatin formation and maintenance at multiple genomic loci.

Reason: Heterochromatin formation is a core function of SIR2. Through histone deacetylation and interactions with Sir3/Sir4, SIR2 establishes repressive chromatin states. This is a primary biological process.

Summary: SIR2 contains a structurally essential zinc-binding pocket.

Reason: RCA (Reviewed Computational Analysis) is appropriate for this annotation as zinc binding is computationally predictable and structurally validated. The zinc ion is essential for catalytic activity and properly confirmed.

Summary: SIR2 organizes chromatin through histone modifications and compaction.

Reason: Direct experimental evidence from chromosome stretching analysis demonstrates SIR2s role in chromatin compaction. This is a core function.

Summary: SIR2 is a core component of the Sir2-Sir3-Sir4 silencing complex.

Reason: Direct biochemical evidence demonstrates SIR2 forms stable complexes with Sir3 and Sir4. This complex membership is a validated, core feature of SIR2 function.

Summary: Direct biochemical assays confirm SIR2 histone deacetylase activity requires NAD. The crystal structure of Sir2 bound to a Sir4 fragment (PDB 4IAO, with the catalytic Zn and an ADP-ribose product) provides the structural basis for this NAD-dependent deacetylase activity and shows it is allosterically stimulated by Sir4 binding.

Reason: Duplicate of IEA annotation with stronger IDA evidence from direct enzyme assays. Both annotations are valid and complement each other.

Summary: SIR2 regulates mating-type switching through heterochromatin maintenance at HM loci.

Reason: SIR2 maintains silent chromatin at the mating-type loci HML and HMR, which is essential for preventing mating-type switching. Silencing ensures that only one mating type is expressed.

Summary: SIR2 binds DNA replication origins as part of heterochromatin formation.

Reason: Direct evidence shows SIR2 and Sir3 associate with euchromatic DNA replication origins. This binding is likely related to chromatin organization and transcriptional regulation at these origins.

Summary: SIR2 negatively regulates DNA replication at certain chromosomal regions.

Reason: SIR2 inhibits DNA replication presumably through heterochromatin formation and blocking replication fork progression. However, this is likely a secondary effect of silencing, not a primary function. Treating as non-core.

Summary: SIR2 is essential for establishing and maintaining subtelomeric heterochromatin.

Reason: Multiple strong IMP citations demonstrate SIR2s requirement for subtelomeric silencing. This is a core function of the Sir2-Sir3-Sir4 complex.

Summary: SIR2 function at subtelomeric regions is conserved from mating-type loci.

Reason: Demonstrates that silencing factors function at both silent mating-type loci and telomeric regions using conserved mechanisms. Duplicate annotation with different reference - both valid.

Summary: SIR2 and Ku proteins maintain telomeric silencing through coordinated mechanisms.

Reason: Third SGD annotation for the same core function, and the underlying function is not in doubt. Noting the limits of what we can check - the cached abstract refers to SIR2's role in telomere position effect as previously established rather than as a new result of this paper, and full_text_available is false, so the SGD curator saw the experiment supporting this IMP and we did not. Per CLAUDE.md the curator's call is not second-guessed on that basis.

Summary: SIR2 localizes to telomeric regions and is required for telomere clustering at nuclear periphery.

Reason: Direct evidence of SIR2 localization to telomeres as part of silent chromatin maintenance and nuclear organization.

Summary: SIR2 binds telomeric DNA as a core component of silencing complexes.

Reason: Direct binding assays confirm SIR2 protein associates with telomeric DNA in vivo. Duplicate with IDA evidence.

Summary: SIR2 localizes to and is required for heterochromatin structure.

Reason: Direct evidence of SIR2 localization within heterochromatic regions. This is a core structural feature of SIR2 function.

Summary: SIR2 contributes to genome stability through rDNA recombination suppression.

Reason: SIR2 suppresses rDNA recombination and promotes genomic stability. However, this is a downstream effect of heterochromatin formation, not a primary function. Secondary effect of core silencing.

Summary: SIR2 is genetically required for Ku-dependent NHEJ-type DSB repair, demonstrated directly by an in vivo plasmid rejoining assay, though this is a secondary consequence of its silencing function rather than a catalytic NHEJ function.

Reason: The cited paper directly assayed this: "using an in vivo plasmid rejoining assay, we demonstrate that SIR2, SIR3 and SIR4...are essential for Ku-dependent DSB repair" (PMID:9501103 abstract). SIR2 is not part of the core NHEJ catalytic machinery (Ku70/80, Lig4/Dnl4, Xrs2), but the IMP evidence shows it is genuinely required for efficient Ku-dependent end-joining repair in vivo. This is a real, experimentally demonstrated, secondary function - not one to remove. The mechanism by which loss of SIR2 impairs end-joining is not established by the cached abstract and is not asserted here.

Summary: SIR2 was originally identified as essential for silencing mating-type loci.

Reason: Original identification of SIR2 function - this is a core, foundational role. Multiple IMP citations demonstrate requirement for silencing at HML and HMR.

Summary: SIR2 is required for recruitment and localization of Ku/telomerase complexes to telomeres.

Reason: SIR2 acts as a docking site or facilitates localization of other protein complexes to telomeric chromatin. This is a valid functional role in telomere maintenance.

Summary: SIR2 affects sister chromatid cohesion through chromatin effects.

Reason: SIR2 influences cohesion presumably through chromatin organization effects, but direct cohesin function is not its primary role. This is a secondary/pleiotropic effect of SIR2s chromatin regulatory activity.

Summary: SIR2 facilitates nuclear periphery localization and clustering of telomeres.

Reason: SIR2 is required for telomere tethering and nuclear organization. This is a validated function in telomere biology and nuclear architecture.

Summary: SIR2 genetically interacts in chromatin organization functions.

Reason: IGI (Inferred from Genetic Interaction) appropriately captures genetic interactions affecting chromatin organization. Multiple genetic interactions support this annotation.

Summary: SIR2 suppresses DNA recombination at rDNA repeats.

Reason: Additional IMP evidence for recombination suppression. Core function.

Summary: SIR2 participates in silencing of transposons like Ty1 elements.

Reason: SIR2 silences transposable elements partially through ncRNA (transcripts from Ty1 loci). However, the primary mechanism is heterochromatin formation, not ncRNA pathway control. This is a secondary function.

Summary: SIR2 suppresses amplification of rDNA.

Reason: SIR2 prevents overamplification of rDNA repeats through silencing and recombination suppression. However, this is a downstream effect of rDNA heterochromatin maintenance, not a primary core function.

Summary: SIR2 is essential for rDNA heterochromatin formation.

Reason: Early, foundational evidence establishing SIR2 role in rDNA silencing. Core function.

Summary: SIR2 directly localizes to nucleolus.

Reason: Duplicate localization annotation with IDA evidence from microscopy. Valid and complementary to other nucleolus annotations.

Summary: SIR2 maintains mating-type silencing.

Reason: Multiple IMP citations establish this core function. This is a second citation for the same process.

Summary: SIR2 is a core component of the RENT (regulator of nucleolar silencing and telophase exit) complex.

Reason: Direct biochemical evidence of SIR2 complex membership. RENT complex is essential for rDNA silencing.

Summary: SIR2 binds nucleosomes as substrate for deacetylation.

Reason: Direct binding assays demonstrate SIR2 interaction with nucleosomes. The ADP-ribose product binds nucleosomes facilitating complex loading.

Summary: Direct enzymatic assay demonstrates SIR2 deacetylates H3K14.

Reason: Duplicate with stronger IDA evidence. Multiple complementary citations are appropriate.

Summary: SIR2 genetically interacts in recombination suppression.

Reason: IGI evidence from genetic interactions in recombination control. Complements IMP and IEA annotations.

Summary: Direct enzymatic assay demonstrates SIR2 deacetylates H3K9.

Reason: Duplicate with IDA evidence. Multiple citations are appropriate for critical core activities.

Summary: Direct enzymatic assay demonstrates SIR2 deacetylates H4K16.

Reason: Duplicate with IDA evidence. H4K16 is the most critical substrate for SIR2 silencing function. Multiple citations appropriate.

Summary: Protein binding term is non-specific and discouraged by GO curators.

Reason: GO:0005515 'protein binding' is a generic term that lacks biological specificity. While SIR2 does interact with specific proteins (Sir3, Sir4, Net1, etc.), these interactions are better captured through protein complex membership terms (GO:0005677, GO:0030869), protein-protein interaction annotations with specific partners, or more informative molecular function terms describing the nature of the interaction. Multiple IPI entries citing the same vague term should be consolidated and replaced with more specific annotations.

Summary: Protein binding term is non-specific and discouraged by GO curators.

Reason: GO:0005515 'protein binding' is a generic term that lacks biological specificity. Proteome survey studies identify many protein interactions through high-throughput methods, but such broad 'protein binding' annotations are discouraged in favor of complex membership or specific interaction annotations. SIR2's known protein interactions are better represented by existing GO terms for its complexes.

Summary: Protein binding term is non-specific and discouraged by GO curators.

Reason: GO:0005515 'protein binding' is a generic term lacking biological specificity. Global landscape studies of protein complexes identify many interactions through high-throughput methods, but such broad 'protein binding' annotations are discouraged. SIR2 complex memberships and specific interactions are captured by existing GO annotations.

Summary: Protein binding term is non-specific and discouraged by GO curators.

Reason: GO:0005515 'protein binding' is a generic term lacking biological specificity. The study examines recombination control at rDNA through chromatin-associated protein interactions, but general 'protein binding' terms should be replaced with more informative annotations. SIR2's functional interactions are better captured through complex membership and specific biological process terms.

Summary: Protein binding term is non-specific and discouraged by GO curators.

Reason: GO:0005515 'protein binding' is a generic term lacking biological specificity. The study demonstrates that the glucanosyltransferase Gas1 functions in transcriptional silencing, likely through interaction with Sir2, but this interaction is better captured by specific biological process terms rather than generic protein binding.

Summary: Protein binding term is non-specific and discouraged by GO curators.

Reason: GO:0005515 'protein binding' is a generic term lacking biological specificity. Global protein kinase and phosphatase interaction networks from high-throughput studies identify many interactions but generic 'protein binding' annotations should not be used. SIR2 functional interactions are better represented by existing complex membership and biological process terms.

Summary: Protein binding term is non-specific and discouraged by GO curators.

Reason: GO:0005515 'protein binding' is a generic term lacking biological specificity. The chromatin-associated interactome study identifies SIR2 protein interactions through high-throughput methods, but generic 'protein binding' annotations are discouraged. SIR2 chromatin-associated interactions are better captured through complex membership terms.

Summary: Protein binding term is non-specific and discouraged by GO curators.

Reason: GO:0005515 'protein binding' is a generic term lacking biological specificity. Recent studies of yeast protein interactome architecture identify many interactions but generic 'protein binding' annotations should not be used. SIR2 functional protein interactions are better represented by existing GO annotations for its protein complexes and biological processes.

Summary: SIR2 localizes to nucleus during DNA replication stress, consistent with its role in transcriptional silencing.

Reason: SIR2 is a well-established nuclear protein that is essential for transcriptional silencing at mating-type loci and telomeres, and is required for rDNA silencing in the nucleolus. While the original HDA annotation references DNA damage response pathway analysis, SIR2's nuclear localization during DNA replication stress is consistent with its known functions in heterochromatin formation and silencing. This annotation is valid, though there is substantial prior experimental evidence for SIR2 nucleolar and nuclear localization (GO:0005730 and GO:0031981).