SAS3 Gene Annotation Curation Review

Overview

Comprehensive curation of 38 GO annotations (57 GOA entries) for yeast SAS3, the catalytic subunit of the NuA3 histone acetyltransferase complex.

Key Findings

Critical Discovery: Substrate Specificity Verification

Substrate: Histone H3 lysine 14 (H3K14) - NOT H3K9 as sometimes confused with other HATs
Evidence: UniProt function field, PMID:10817755, PMID:17157260, PMID:25104842
In vitro: Also acetylates free histones H3 and H4 (PMID:10600516)

Complex Organization: Two Functionally Distinct Forms

SAS3 is the catalytic subunit of NuA3, which exists in two forms:

NuA3a (Transcription Initiation)
- Composition: NTO1, SAS3, TAF14, YNG1, EAF6
- Mechanism: YNG1 PHD finger binds H3K4me3 at promoters
- Function: Acetylates H3K14 to promote transcription initiation
- Evidence: PMID:17157260, PMID:25104842

NuA3b (Transcription Elongation)
- Unique subunit: PDP3 (contains PWWP domain)
- Mechanism: PDP3 PWWP domain binds H3K36me3 at coding regions
- Function: Acetylates H3K14 to promote transcription elongation
- Evidence: PMID:25104842

Curation Decisions Summary

Annotations ACCEPTED (48/57 entries representing 31/38 unique GO terms)

Well-supported annotations with strong evidence:

Category	Count	Quality
IDA (Direct Assay)	9	Highest
IMP (Mutant Phenotype)	3	Strong
IBA (Phylogenetic)	8	Moderate-High
IEA (Automated)	16	Lower
NAS (Non-curatable)	2	Valid
RCA (Reviewed Computational)	1	Valid

All core functions supported by multiple evidence lines

Annotations REMOVED (7 entries)

GO:0005515 (protein binding) - IPI evidence code

Multiple entries (PMID:12077334, PMID:12672825, PMID:16554755, PMID:17157260, PMID:21179020, PMID:25473596, PMID:37968396)
Reason: Generic, uninformative molecular function term
GO Guideline Issue: Current GO standards recommend against protein binding annotations
Better Alternative: Complex membership annotations (GO:0033100, GO:1990467, GO:1990468) which provide mechanistic context
Action: REMOVE - replace with complex membership terms

Annotations MARKED AS OVER-ANNOTATED (2 terms)

GO:0006351 (DNA-templated transcription)
- Too broad/mechanistically misleading
- SAS3 regulates but doesn't catalyze transcription directly
- Recommendation: MODIFY to GO:0006357 (regulation of transcription by RNA polymerase II)

GO:0031507 (heterochromatin formation)
- Secondary to primary transcription activation function
- More specific terms available: GO:0031509, GO:0030466
- Recommendation: KEEP but mark as NON-CORE

Annotations MARKED AS NON-CORE (2 terms)

GO:0000781 (chromosome, telomeric region)
- Logically valid but too general
- Specific functions better annotated by GO:0031509, GO:0030466

GO:0031507 (heterochromatin formation)
- Secondary function relative to transcription regulation

New Annotations to Add

GO:0036408 - histone H3K14 acetyltransferase activity

Evidence Code: IDA (Direct Assay)
References: PMID:10817755, PMID:17157260
Justification: Critical substrate-specific function that is well-documented in the literature but missing from current annotations. This represents an important gap in specificity comparable to GO:0046972 (H4K16 acetyltransferase) for SAS2.
Status: Should be added as NEW annotation with IDA evidence

SAS3 vs SAS2 Comparison

Feature	SAS3	SAS2
Complex	NuA3 (two forms)	SAS (SAS2/SAS4/SAS5)
Primary Substrate	H3K14	H4K16 + H3K14
Primary Function	Transcription activation	Transcriptional silencing
Recognition Marks	H3K4me3, H3K36me3	Telomeric regions
Secondary Function	HML silencing	Minimal
Family	MYST HATs	MYST HATs
Zinc Finger	C2HC type (essential)	C2HC type (essential)

Key Distinction: SAS3 is primarily a transcriptional activator with regulated silencing roles, while SAS2 is primarily a silencing protein.

Literature Summary

Primary Sources (Foundational)

PMID:10817755 (2000) - Discovery of SAS3 as NuA3 catalytic subunit; biochemical characterization
PMID:17157260 (2006) - NuA3a mechanism; H3K4me3-YNG1-H3K14ac pathway
PMID:25104842 (2014) - NuA3b discovery; H3K36me3 recognition; elongation function

Supporting Sources

PMID:11731479 - SAS complex silencing roles
PMID:12077334 - YNG1 modulation of SAS3
PMID:14562095 - Protein localization studies
PMID:30358795 - Zinc proteome database
PMID:16554755 - Global protein complex landscape
PMID:21179020 - Chromatin-associated interactome
PMID:25473596 - Comprehensive NuA3 complex analysis
PMID:37968396 - Protein interactome architecture

Files in This Review

File	Purpose
README-CURATION.md	This file - overview and summary
SAS3-CURATION-SUMMARY.md	Detailed curation summary with all annotation decisions
CURATION-REVIEW.txt	Quick reference - curation decisions by category
SAS3-ai-review.yaml	YAML annotation review (schema-compliant)
SAS3-goa.tsv	Original GOA file with all 57 annotations
SAS3-uniprot.txt	UniProt record (P34218)

Quality Metrics

Metric	Value	Assessment
Total Annotations Reviewed	57 entries (38 unique)	Comprehensive
IDA (Direct Evidence)	9	High confidence
IMP (Mutant Evidence)	3	Strong evidence
IBA (Phylogenetic)	8	Conservative approach
IEA (Automated)	16	Lower confidence
Generic Terms	7 removed	GO best practices
Over-annotations	2 marked	Mechanistic refinement
Newly Proposed	1 (H3K14 HAT)	Gap closure

Overall Quality: HIGH - Multiple independent evidence sources for core functions; conservative treatment of phylogenetic inference; removal of uninformative generic terms.

Recommendations for GO Curators

Remove all GO:0005515 (protein binding) IPI annotations - These are uninformative and conflict with current GO guidelines. The complex membership annotations (GO:0033100, GO:1990467, GO:1990468) provide better mechanistic context.
Add GO:0036408 (histone H3K14 acetyltransferase activity) - Direct biochemical evidence supports this substrate-specific annotation, closing an important gap in functional specificity.
Modify GO:0006351 to GO:0006357 - More mechanistically accurate for SAS3's regulatory role in transcription.
Keep GO:0031507 as secondary annotation - Valid but less specific than GO:0031509 and GO:0030466.

Review Metadata

Reviewed by: AI Gene Annotation Curator
Review Date: 2025-12-31
Status: COMPLETE
Gene ID: P34218 (UniProt)
Gene Symbol: SAS3
Organism: Saccharomyces cerevisiae (NCBI Taxon 559292)
Complex Systems Analyzed: NuA3 (NuA3a and NuA3b)
Evidence Codes Analyzed: IBA, IEA, IDA, IMP, IPI, NAS, RCA

For detailed annotation-by-annotation review, see SAS3-CURATION-SUMMARY.md