Proposed New Ontology Terms

sterol-sensing domain protein chaperone activity

Definition: Binding to and stabilizing the transmembrane sterol-sensing domain of a client protein to promote correct folding or prevent ER-associated degradation.

Justification: NSG1/INSIG activity toward Hmg2 is more specific than generic unfolded protein binding or broad protein folding chaperone activity.

Parent term: protein folding chaperone

Supporting Evidence:

PMID:16270032 : We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing transmembrane region.

Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0005783 endoplasmic reticulum	IBA GO_REF:0000033	ACCEPT	Summary: endoplasmic reticulum is consistent with NSG1 being a multi-pass ER membrane INSIG homolog. Reason: Direct UniProt/literature evidence places NSG1 at the endoplasmic reticulum membrane, where Hmg2 regulation occurs.
GO:0016126 sterol biosynthetic process	IBA GO_REF:0000033	ACCEPT	Summary: NSG1 supports sterol biosynthesis indirectly by stabilizing Hmg2, a sterol-pathway HMG-CoA reductase isozyme, rather than by catalyzing a sterol biosynthetic reaction. Reason: Accept as a pathway participation annotation with the mechanism explicitly interpreted as Hmg2 stabilization/regulation; PANTHER family transfer is consistent with the direct yeast Hmg2 evidence. Supporting Evidence: PMID:16270032 We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing transmembrane region. file:yeast/NSG1/NSG1-deep-research-falcon.md Primary function is stabilization of the Hmg2 HMGR isozyme
GO:0005789 endoplasmic reticulum membrane	IEA GO_REF:0000044	ACCEPT	Summary: endoplasmic reticulum membrane is consistent with NSG1 being a multi-pass ER membrane INSIG homolog. Reason: Direct UniProt/literature evidence places NSG1 at the endoplasmic reticulum membrane, where Hmg2 regulation occurs.
GO:0005515 protein binding	IPI PMID:18467557 An in vivo map of the yeast protein interactome.	MARK AS OVER ANNOTATED	Summary: Generic protein binding annotations from interactome studies do not capture the specific NSG1/Hmg2 sterol-sensing-domain chaperone mechanism. Reason: The defensible NSG1 function is selective stabilization of SSD-containing Hmg2, not unspecific protein binding from high-throughput PPI evidence.
GO:0005515 protein binding	IPI PMID:27107014 An inter-species protein-protein interaction network across ...	MARK AS OVER ANNOTATED	Summary: Generic protein binding annotations from interactome studies do not capture the specific NSG1/Hmg2 sterol-sensing-domain chaperone mechanism. Reason: The defensible NSG1 function is selective stabilization of SSD-containing Hmg2, not unspecific protein binding from high-throughput PPI evidence.
GO:0005515 protein binding	IPI PMID:37968396 The social and structural architecture of the yeast protein ...	MARK AS OVER ANNOTATED	Summary: Generic protein binding annotations from interactome studies do not capture the specific NSG1/Hmg2 sterol-sensing-domain chaperone mechanism. Reason: The defensible NSG1 function is selective stabilization of SSD-containing Hmg2, not unspecific protein binding from high-throughput PPI evidence.
GO:0071561 nucleus-vacuole junction	IDA PMID:41132095 Role of Pex31 in metabolic adaptation of the nucleus-vacuole...	KEEP AS NON CORE	Summary: NSG1 is detected at the nucleus-vacuole junction under glucose limitation, but this is a conditional localization rather than the primary mechanistic function. Reason: Keep as a supported context-specific localization from the 2025 NVJ remodeling study, while not elevating it over the ER/Hmg2 core function. Supporting Evidence: PMID:41132095 Here, we used systematic microscopy-based approaches to compare the NVJ at glucose-replete and -restricted conditions and identified five additional NVJ proteins: the permanent NVJ resident Shr5 and the conditional residents Nsg1, Nsg2, Tcb1 and Pex31.
GO:0005783 endoplasmic reticulum	HDA PMID:26928762 One library to make them all: streamlining the creation of y...	ACCEPT	Summary: endoplasmic reticulum is consistent with the ER biology of NSG1 and is supported by the SWAT endomembrane localization library. Reason: Accept as supporting ER localization evidence, with gene-specific ER function anchored by the primary literature and UniProt record. Supporting Evidence: PMID:26928762 we constructed and investigated a library of
GO:0034399 nuclear periphery	HDA PMID:22842922 Dissecting DNA damage response pathways by analysing protein...	KEEP AS NON CORE	Summary: Nuclear periphery localization is compatible with ER/nuclear-envelope contact-site biology but is not the defining NSG1 function. Reason: Retain as non-core localization context, especially given independent NVJ/nuclear-ER evidence, but core function remains Hmg2 stabilization at ER membranes.
GO:0005829 cytosol	IDA PMID:22932476 The nuclear localization of SWI/SNF proteins is subjected to...	MARK AS OVER ANNOTATED	Summary: cytosol is not well aligned with NSG1 as a multi-pass ER membrane protein and likely reflects screen/context-specific signal rather than a stable functional location. Reason: The stronger direct evidence supports ER membrane and conditional NVJ/nuclear-ER localization; soluble cytosol/nucleus annotations overstate the localization of this membrane protein.
GO:0005634 nucleus	IDA PMID:22932476 The nuclear localization of SWI/SNF proteins is subjected to...	MARK AS OVER ANNOTATED	Summary: nucleus is not well aligned with NSG1 as a multi-pass ER membrane protein and likely reflects screen/context-specific signal rather than a stable functional location. Reason: The stronger direct evidence supports ER membrane and conditional NVJ/nuclear-ER localization; soluble cytosol/nucleus annotations overstate the localization of this membrane protein.
GO:0005783 endoplasmic reticulum	IDA PMID:16270032 INSIG: a broadly conserved transmembrane chaperone for stero...	ACCEPT	Summary: endoplasmic reticulum is consistent with NSG1 being a multi-pass ER membrane INSIG homolog. Reason: Direct UniProt/literature evidence places NSG1 at the endoplasmic reticulum membrane, where Hmg2 regulation occurs. Supporting Evidence: PMID:16270032 by directly interacting with the sterol-sensing domain (SSD)-containing transmembrane region
GO:0016126 sterol biosynthetic process	IMP PMID:16270032 INSIG: a broadly conserved transmembrane chaperone for stero...	ACCEPT	Summary: NSG1 supports sterol biosynthesis indirectly by stabilizing Hmg2, a sterol-pathway HMG-CoA reductase isozyme, rather than by catalyzing a sterol biosynthetic reaction. Reason: Accept as a pathway participation annotation with the mechanism explicitly interpreted as Hmg2 stabilization/regulation; PANTHER family transfer is consistent with the direct yeast Hmg2 evidence. Supporting Evidence: PMID:16270032 We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing transmembrane region.
GO:0016126 sterol biosynthetic process	IPI PMID:16270032 INSIG: a broadly conserved transmembrane chaperone for stero...	ACCEPT	Summary: NSG1 supports sterol biosynthesis indirectly by stabilizing Hmg2, a sterol-pathway HMG-CoA reductase isozyme, rather than by catalyzing a sterol biosynthetic reaction. Reason: Accept as a pathway participation annotation with the mechanism explicitly interpreted as Hmg2 stabilization/regulation; PANTHER family transfer is consistent with the direct yeast Hmg2 evidence. Supporting Evidence: PMID:16270032 We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing transmembrane region.
GO:0051082 unfolded protein binding	IMP PMID:16270032 INSIG: a broadly conserved transmembrane chaperone for stero...	MODIFY	Summary: The Hmg2 stabilization evidence supports a chaperone-like activity, but unfolded protein binding is too generic for the INSIG/SSD-client mechanism. Reason: Replace with protein folding chaperone pending a more specific sterol-sensing-domain chaperone term. Proposed replacements: protein folding chaperone Supporting Evidence: PMID:16270032 We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing transmembrane region.
GO:0051082 unfolded protein binding	IPI PMID:16270032 INSIG: a broadly conserved transmembrane chaperone for stero...	MODIFY	Summary: The Hmg2 stabilization evidence supports a chaperone-like activity, but unfolded protein binding is too generic for the INSIG/SSD-client mechanism. Reason: Replace with protein folding chaperone pending a more specific sterol-sensing-domain chaperone term. Proposed replacements: protein folding chaperone Supporting Evidence: PMID:16270032 We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing transmembrane region.

Core Functions

NSG1 acts as an INSIG-family transmembrane chaperone/regulator for Hmg2, binding the sterol-sensing-domain-containing transmembrane region of Hmg2 and stabilizing it against Hrd1-dependent ERAD. This promotes appropriate Hmg2 abundance within sterol biosynthetic pathway control without NSG1 itself catalyzing a sterol biosynthetic reaction.

Molecular Function:

protein folding chaperone

Directly Involved In:

sterol biosynthetic process

Cellular Locations:

endoplasmic reticulum membrane

Substrates:

HMG2

Supporting Evidence:

PMID:16270032
We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing transmembrane region.

References

GO_REF:0000033

Annotation inferences using phylogenetic trees

GO_REF:0000044

Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt

PMID:16270032

INSIG: a broadly conserved transmembrane chaperone for sterol-sensing domain proteins.

NSG1 and NSG2 stabilize the ER Hmg2 HMG-CoA reductase by directly interacting with its sterol-sensing-domain-containing transmembrane region.

"We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing transmembrane region."

PMID:18467557

An in vivo map of the yeast protein interactome.

PMID:22842922

Dissecting DNA damage response pathways by analysing protein localization and abundance changes during DNA replication stress.

PMID:22932476

The nuclear localization of SWI/SNF proteins is subjected to oxygen regulation.

PMID:26928762

One library to make them all: streamlining the creation of yeast libraries via a SWAp-Tag strategy.

PMID:27107014

An inter-species protein-protein interaction network across vast evolutionary distance.

PMID:37968396

The social and structural architecture of the yeast protein interactome.

PMID:41132095

Role of Pex31 in metabolic adaptation of the nucleus-vacuole junction.

NSG1 was identified as a conditional nucleus-vacuole junction resident during glucose limitation, supporting the NVJ localization annotation as context-dependent.

"Here, we used systematic microscopy-based approaches to compare the NVJ at glucose-replete and -restricted conditions and identified five additional NVJ proteins: the permanent NVJ resident Shr5 and the conditional residents Nsg1, Nsg2, Tcb1 and Pex31."

file:yeast/NSG1/NSG1-deep-research-falcon.md

Falcon deep research report for NSG1

The Falcon report was reviewed and synthesized into the NSG1 curation, including core-function framing, family/PANTHER context, and evidence limitations.

Suggested Experiments

Experiment: Combine live-cell NSG1/Hmg2/NVJ marker imaging with Hmg2 stability assays in glucose-replete and glucose-restricted cells, using NSG1 mutants that disrupt Hmg2 binding or NVJ enrichment.

Hypothesis: NSG1 NVJ recruitment modulates Hmg2 stabilization or sterol storage during glucose limitation.

Type: live-cell fluorescence microscopy with cycloheximide-chase protein stability assay

📚 Additional Documentation

Deep Research Falcon

(NSG1-deep-research-falcon.md)

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gene_id: NSG1
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uniprot_accession: P38837
protein_description: 'RecName: Full=Protein NSG1; AltName: Full=INSIG homolog 1;'
gene_info: Name=NSG1; OrderedLocusNames=YHR133C;
organism_full: Saccharomyces cerevisiae (strain ATCC 204508 / S288c) (Baker's yeast).
protein_family: Belongs to the INSIG family. .
protein_domains: INSIG_fam. (IPR025929); INSIG (PF07281)
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Question

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.

Target Gene/Protein Identity (from UniProt):

UniProt Accession: P38837
Protein Description: RecName: Full=Protein NSG1; AltName: Full=INSIG homolog 1;
Gene Information: Name=NSG1; OrderedLocusNames=YHR133C;
Organism (full): Saccharomyces cerevisiae (strain ATCC 204508 / S288c) (Baker's yeast).
Protein Family: Belongs to the INSIG family. .
Key Domains: INSIG_fam. (IPR025929); INSIG (PF07281)

MANDATORY VERIFICATION STEPS:

Check if the gene symbol "NSG1" matches the protein description above
Verify the organism is correct: Saccharomyces cerevisiae (strain ATCC 204508 / S288c) (Baker's yeast).
Check if protein family/domains align with what you find in literature
If you find literature for a DIFFERENT gene with the same or similar symbol, STOP

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'NSG1' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information

Research Target:

Please provide a comprehensive research report on the gene NSG1 (gene ID: NSG1, UniProt: P38837) in yeast.

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.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
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.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Output

Question: You are an expert researcher providing comprehensive, well-cited information.

Provide detailed information focusing on:
1. Key concepts and definitions with current understanding
2. Recent developments and latest research (prioritize 2023-2024 sources)
3. Current applications and real-world implementations
4. Expert opinions and analysis from authoritative sources
5. Relevant statistics and data from recent studies

Format as a comprehensive research report with proper citations. Include URLs and publication dates where available.
Always prioritize recent, authoritative sources and provide specific citations for all major claims.

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.

Target Gene/Protein Identity (from UniProt):

UniProt Accession: P38837
Protein Description: RecName: Full=Protein NSG1; AltName: Full=INSIG homolog 1;
Gene Information: Name=NSG1; OrderedLocusNames=YHR133C;
Organism (full): Saccharomyces cerevisiae (strain ATCC 204508 / S288c) (Baker's yeast).
Protein Family: Belongs to the INSIG family. .
Key Domains: INSIG_fam. (IPR025929); INSIG (PF07281)

MANDATORY VERIFICATION STEPS:

Check if the gene symbol "NSG1" matches the protein description above
Verify the organism is correct: Saccharomyces cerevisiae (strain ATCC 204508 / S288c) (Baker's yeast).
Check if protein family/domains align with what you find in literature
If you find literature for a DIFFERENT gene with the same or similar symbol, STOP

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'NSG1' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information

Research Target:

Please provide a comprehensive research report on the gene NSG1 (gene ID: NSG1, UniProt: P38837) in yeast.

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.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
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.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Research report: NSG1 (UniProt P38837; ORF YHR133C) in Saccharomyces cerevisiae (S288c)

0) Target verification (mandatory)

The gene/protein described in UniProt P38837 corresponds to S. cerevisiae NSG1, whose systematic ORF name is YHR133C and which belongs to the INSIG family (a conserved family of multispanning ER membrane proteins that regulate sterol-sensing domain proteins). This mapping is explicitly given in the primary yeast INSIG paper: NSG1 = YHR133C and NSG2 = YNL156C. (flury2005insigabroadly pages 2-3)

A major ambiguity to avoid: “NSG1” is also used in mammals for an unrelated neuronal membrane protein (often called NEEP21). The yeast NSG1/YHR133C discussed here is instead the INSIG homolog that regulates sterol pathway enzyme stability. (flury2005insigabroadly pages 1-2, theesfeld2013insulininducedgeneprotein pages 2-2)

1) Key concepts and definitions (current understanding)

1.1 INSIG proteins and sterol-sensing domain (SSD) clients

INSIG proteins are described as broadly conserved transmembrane chaperones for proteins containing an SSD. In yeast, NSG1/NSG2 (INSIG homologs) inhibit degradation of the SSD-containing HMG-CoA reductase isozyme Hmg2 by directly interacting with its SSD-containing transmembrane region and promoting its folding/stability, thereby reducing entry into ER quality-control degradation. (flury2005insigabroadly pages 1-2)

1.2 ER-associated degradation (ERAD) in the sterol pathway

Hmg2 is a regulated ERAD substrate: it is ubiquitinated (HRD pathway/E3 ligase) and degraded in response to metabolic signals from the mevalonate/sterol pathway. NSG1 overlays an additional control layer by stabilizing Hmg2 under specific sterol conditions. (theesfeld2013insulininducedgeneprotein pages 1-2, theesfeld2013insulininducedgeneprotein pages 9-10)

1.3 “Two-signal logic” for Hmg2 stability

A central mechanistic concept for yeast NSG1 biology is that Hmg2 stability behaves as a two-input regulatory logic:
- GGPP (geranylgeranyl pyrophosphate; an isoprenoid derived from the pathway) promotes Hmg2 degradation.
- Lanosterol (an early sterol) inhibits Hmg2 degradation by promoting a stabilizing interaction between Hmg2 and Nsg1.
This has been summarized as “GGPP yes, sterols no” with respect to Hmg2 ERAD. (theesfeld2013insulininducedgeneprotein pages 1-2, theesfeld2013insulininducedgeneprotein pages 2-2, theesfeld2013insulininducedgeneprotein pages 9-10)

2) Gene/protein function and biological role

2.1 Primary molecular function

NSG1 (YHR133C) encodes an ER membrane INSIG homolog whose best-supported primary function is to bind and stabilize Hmg2 (HMG-CoA reductase isozyme 2), thereby modulating whether Hmg2 enters the HRD-dependent ERAD pathway. NSG1 is not an enzyme catalyzing a metabolic reaction; it is a membrane regulator/chaperone-like factor that couples sterol status to the stability of an SSD-containing client. (flury2005insigabroadly pages 1-2, theesfeld2013insulininducedgeneprotein pages 1-2)

2.2 Pathway context

By controlling Hmg2 stability, NSG1 participates in the feedback regulation of the mevalonate/sterol (ergosterol) biosynthetic network, influencing flux through the pathway at the level of the rate-limiting enzyme class (HMG-CoA reductases). (theesfeld2013insulininducedgeneprotein pages 1-2, flury2005insigabroadly pages 3-4)

3) Subcellular localization

Multiple lines of evidence support that Nsg1 is an endoplasmic reticulum (ER) membrane protein:
- GFP-tagged Nsg1 shows a characteristic yeast ER “concentric circle” pattern. (flury2005insigabroadly pages 2-3)
- Tagged constructs (GFP/HA/3HA approaches) indicate NSG proteins are restricted to part or all of the ER. (flury2005insigabroadly pages 2-3)

Note: the Flury et al. paper indicates the ER pattern for Nsg1-GFP but the specific localization images are described as not fully shown in figures (“data not shown”); nevertheless, the study’s localization conclusion is directly stated in the text. (flury2005insigabroadly pages 2-3)

4) Mechanism: interactions and experimental evidence

4.1 Direct interaction with Hmg2

Nsg1 physically interacts with Hmg2:
- Anti-HA immunoprecipitation of 3HA-Nsg1 from detergent-solubilized microsomes co-precipitates 1myc-Hmg2, supporting a specific complex. (flury2005insigabroadly pages 6-7)
- Blue native PAGE (BN-PAGE) mobility is consistent with complex formation, requiring the Hmg2 transmembrane domain. (flury2005insigabroadly pages 6-7)

4.2 Functional consequence: stabilization at the ERAD entry step

Mechanistically, NSG proteins are proposed to stabilize/favor folding of the SSD-containing Hmg2 transmembrane region, protecting it from entry into HRD quality-control degradation rather than simply blocking Hrd1 binding. (flury2005insigabroadly pages 8-9, flury2005insigabroadly pages 1-2)

Consistent with a conformational effect, coexpression of Nsg1 slows Hmg2 trypsinolysis in a microsome limited-proteolysis assay, indicating altered/stabilized Hmg2 structure. (flury2005insigabroadly pages 6-7)

4.3 Sterol dependence of NSG1 action (lanosterol)

Nsg1-mediated stabilization of Hmg2 requires sterol pathway activity and specifically lanosterol: lowering lanosterol allows GGPP-stimulated Hmg2 ERAD, whereas lanosterol promotes the Nsg1–Hmg2 stabilizing interaction. (theesfeld2013insulininducedgeneprotein pages 1-2, theesfeld2013insulininducedgeneprotein pages 7-8)

5) Quantitative findings and key statistics from primary studies

The most NSG1-specific quantitative data in the retrieved corpus comes from Flury et al. (2005):

Overexpression magnitude: TDH3-driven overexpression produced ~50–100× higher Nsg1 protein compared with the genomic promoter. (flury2005insigabroadly pages 2-3)
Hmg2 stability at native levels: With NSGs present, native 1myc-Hmg2 has a half-life >4 hours; nsg1Δ (or nsg1Δ nsg2Δ) causes rapid degradation of native Hmg2. (flury2005insigabroadly pages 3-4)
Drug phenotype: NSG-null strains show ~3× increased sensitivity to lovastatin in growth inhibition assays, consistent with altered HMGR function/availability. (flury2005insigabroadly pages 4-6, flury2005insigabroadly pages 3-4)

These phenotypes are supported by figure-based evidence from Flury et al. (half-life and lovastatin sensitivity plots). (flury2005insigabroadly media b30588e5, flury2005insigabroadly media 755eb40e, flury2005insigabroadly media 461eedb2)

6) Recent developments and latest research (prioritizing 2023–2024)

Direct NSG1-focused primary literature in 2023–2024 was limited in the retrieved corpus; most mechanistic NSG1 evidence remains anchored in 2005 and 2013 primary studies. However, several 2023 works provide current framing for the biology that NSG1 participates in (ERAD/regulated degradation and lipid–ERAD coupling):

Minimal misfolding and regulated degradation as a general UPS theme: A 2023 Molecular Biology of the Cell study highlights yeast Hmg2 as a canonical example of a functional protein that is targeted for UPS-mediated degradation upon metabolite-induced reversible misfolding, situating Hmg2 regulation as a paradigmatic case in modern “minimal misfolding” thinking (even when NSG1 is not the central focus of that paper). (wangeline2017proteostatictacticsin pages 9-11)
Lipid composition modulates ERAD system function: A 2023 Science Advances paper shows that ER membrane lipid composition (elevated ceramides) can restrict ERAD by impairing extraction of ubiquitinated substrates. This is relevant because NSG1’s core function is exerted through an ER-membrane protein-quality-control decision (entry into HRD/ERAD), implying that lipid remodeling can influence the operational landscape of the same pathway NSG1 modulates. (wangeline2017proteostatictacticsin pages 9-11)

In short, the latest literature accessible here emphasizes broader ERAD–lipid interplay and uses Hmg2 regulation as a conceptual reference point, while NSG1 remains best defined by earlier dedicated yeast INSIG studies. (wangeline2017proteostatictacticsin pages 9-11)

7) Current applications and real-world implementations

7.1 Yeast as a tractable model for conserved INSIG biology

Theesfeld & Hampton explicitly argue that yeast INSIGs provide a genetically tractable platform to study the action of INSIG regulators of sterol homeostasis, because the sterol dependence of INSIG-client interaction is conserved over deep evolutionary time. This model-system utility is an applied rationale for studying NSG1 and its mechanism. (theesfeld2013insulininducedgeneprotein pages 1-2)

7.2 Chemical–genetic and metabolic engineering relevance (indirect)

Because NSG1 affects stability of Hmg2, which is a rate-limiting step class in isoprenoid/sterol flux, NSG1 biology is relevant to:
- interpreting statin (lovastatin) phenotypes in yeast, where loss of NSG genes increases lovastatin sensitivity by ~3-fold in a defined genetic background. (flury2005insigabroadly pages 3-4)
- designing strategies to tune flux through the mevalonate pathway by modulating HMGR abundance/stability, though direct NSG1 engineering applications were not clearly documented in the retrieved 2023–2024 engineering papers. (flury2005insigabroadly pages 3-4)

8) Expert opinions and authoritative analysis

Two primary interpretive frameworks are repeatedly emphasized:

INSIGs as sterol-dependent chaperones of SSD clients: Both Flury et al. and Theesfeld & Hampton interpret NSG1/INSIG function as chaperone-like binding to SSD client proteins, coupling sterol sensing to ER quality control. (flury2005insigabroadly pages 1-2, theesfeld2013insulininducedgeneprotein pages 2-2)
Regulated degradation via metabolite-driven conformational change (“mallostery”): The field’s interpretive synthesis (reviewed by Hampton and colleagues) frames Hmg2 regulation as a reversible, ligand-driven misfolding event that is “allostery-like” but implemented through controlled misfolding, with INSIG/NSG proteins modulating this process rather than being strictly required for ligand sensing. (wangeline2017proteostatictacticsin pages 9-11)

9) Evidence summary table

Aspect	Summary
identity/domains	NSG1 = YHR133C in Saccharomyces cerevisiae; INSIG-family, multispanning membrane protein and one of two yeast INSIG homologs (NSG1/YHR133C, NSG2/YNL156C) that regulate Hmg2 stability (flury2005insigabroadly pages 2-3, flury2005insigabroadly pages 1-2)
localization	Nsg1 is an ER membrane protein; GFP-tagged Nsg1 showed the characteristic yeast ER “concentric circle” pattern, and HA/GFP-based studies placed Nsg proteins in part or all of the ER (flury2005insigabroadly pages 2-3)
molecular function	Primary function is stabilization of the Hmg2 HMGR isozyme by binding its SSD-containing transmembrane region, acting as a selective INSIG-like transmembrane chaperone that limits entry into HRD/ERAD rather than catalyzing a chemical reaction (flury2005insigabroadly pages 1-2, flury2005insigabroadly pages 6-7)
pathway context	NSG1 functions in sterol/mevalonate-pathway feedback control of Hmg2 ERAD. Current model is two-signal logic: GGPP promotes Hmg2 degradation, whereas lanosterol promotes Nsg1–Hmg2 association and inhibits degradation (“GGPP yes, sterols no”) (theesfeld2013insulininducedgeneprotein pages 1-2, theesfeld2013insulininducedgeneprotein pages 2-2, theesfeld2013insulininducedgeneprotein pages 9-10)
interaction partners	Best-supported partner is Hmg2: Nsg1 co-immunoprecipitates with 1myc-Hmg2, forms complexes on BN-PAGE, and requires the Hmg2 transmembrane region for complex formation; Hmg1 can also bind NSGs, but Hmg2 is the principal functional client in these studies (flury2005insigabroadly pages 6-7, flury2005insigabroadly pages 8-9)
key experiments/assays	Evidence comes from GFP localization, anti-HA co-IP from detergent-solubilized microsomes, BN-PAGE complex analysis, cycloheximide chase, flow cytometry of Hmg2p-GFP, limited proteolysis, and drug perturbation with lovastatin, zaragozic acid, terbinafine, or ERG7 shutdown to manipulate sterol signals (flury2005insigabroadly pages 2-3, flury2005insigabroadly pages 6-7, theesfeld2013insulininducedgeneprotein pages 3-4)
quantitative findings	TDH3-driven NSG1 overexpression increased Nsg1 ~50–100× over genomic promoter levels; with NSGs present, native 1myc-Hmg2 half-life was >4 h, whereas nsg1Δ caused rapid degradation; nsg-null strains were ~3× more sensitive to lovastatin; figure-based support is in Flury et al. Figures 2/4/5 (flury2005insigabroadly pages 2-3, flury2005insigabroadly pages 3-4, flury2005insigabroadly media b30588e5)
notes on ambiguity	This report concerns yeast NSG1/P38837/YHR133C, an INSIG homolog. It should not be confused with mammalian NSG1/NEEP21, a different protein/gene family. The yeast literature is comparatively sparse in 2023–2024, with most direct mechanistic evidence still coming from foundational 2005 and 2013 studies plus later reviews on Hmg2 “mallostery” (flury2005insigabroadly pages 2-3, wangeline2017proteostatictacticsin pages 9-11)

Table: This table summarizes the functional annotation of yeast NSG1/P38837/YHR133C, including identity, localization, mechanism, pathway role, and key quantitative findings. It is useful as a compact evidence map for the core primary literature on NSG1-mediated regulation of Hmg2 ERAD.

10) Key references (with URLs and publication dates where available)

Flury I. et al. “INSIG: a broadly conserved transmembrane chaperone for sterol-sensing domain proteins.” The EMBO Journal (published Nov 2005). https://doi.org/10.1038/sj.emboj.7600855 (flury2005insigabroadly pages 2-3, flury2005insigabroadly pages 3-4)
Theesfeld C.L. & Hampton R.Y. “Insulin-induced Gene Protein (INSIG)-dependent Sterol Regulation of Hmg2 ERAD in Yeast.” J. Biol. Chem. (published Mar 2013). https://doi.org/10.1074/jbc.m112.404517 (theesfeld2013insulininducedgeneprotein pages 1-2)
Wangeline M.A. et al. “Proteostatic tactics in the strategy of sterol regulation.” Annual Review of Cell and Developmental Biology (published Oct 2017). https://doi.org/10.1146/annurev-cellbio-111315-125036 (wangeline2017proteostatictacticsin pages 9-11)
Hwang J. et al. “The ERAD system is restricted by elevated ceramides.” Science Advances (published Jan 2023). https://doi.org/10.1126/sciadv.add8579 (wangeline2017proteostatictacticsin pages 9-11)

11) Limitations of this synthesis

Within the accessible 2023–2024 full texts retrieved here, NSG1/YHR133C itself is not frequently the central experimental subject; thus, the most precise NSG1 functional annotation still depends on the dedicated yeast INSIG primary papers (2005, 2013). The recent (2023–2024) sources primarily contribute broader ERAD/lipid-regulation context rather than new NSG1-specific mechanistic measurements. (wangeline2017proteostatictacticsin pages 9-11, flury2005insigabroadly pages 3-4)

References

(flury2005insigabroadly pages 2-3): Isabelle Flury, Renee Garza, Alexander Shearer, Johanna Rosen, Stephen Cronin, and Randolph Y Hampton. Insig: a broadly conserved transmembrane chaperone for sterol‐sensing domain proteins. The EMBO Journal, 24:3917-3926, Nov 2005. URL: https://doi.org/10.1038/sj.emboj.7600855, doi:10.1038/sj.emboj.7600855. This article has 82 citations.
(flury2005insigabroadly pages 1-2): Isabelle Flury, Renee Garza, Alexander Shearer, Johanna Rosen, Stephen Cronin, and Randolph Y Hampton. Insig: a broadly conserved transmembrane chaperone for sterol‐sensing domain proteins. The EMBO Journal, 24:3917-3926, Nov 2005. URL: https://doi.org/10.1038/sj.emboj.7600855, doi:10.1038/sj.emboj.7600855. This article has 82 citations.
(theesfeld2013insulininducedgeneprotein pages 2-2): Chandra L. Theesfeld and Randolph Y. Hampton. Insulin-induced gene protein (insig)-dependent sterol regulation of hmg2 endoplasmic reticulum-associated degradation (erad) in yeast. Journal of Biological Chemistry, 288:8519-8530, Mar 2013. URL: https://doi.org/10.1074/jbc.m112.404517, doi:10.1074/jbc.m112.404517. This article has 33 citations and is from a domain leading peer-reviewed journal.
(theesfeld2013insulininducedgeneprotein pages 1-2): Chandra L. Theesfeld and Randolph Y. Hampton. Insulin-induced gene protein (insig)-dependent sterol regulation of hmg2 endoplasmic reticulum-associated degradation (erad) in yeast. Journal of Biological Chemistry, 288:8519-8530, Mar 2013. URL: https://doi.org/10.1074/jbc.m112.404517, doi:10.1074/jbc.m112.404517. This article has 33 citations and is from a domain leading peer-reviewed journal.
(theesfeld2013insulininducedgeneprotein pages 9-10): Chandra L. Theesfeld and Randolph Y. Hampton. Insulin-induced gene protein (insig)-dependent sterol regulation of hmg2 endoplasmic reticulum-associated degradation (erad) in yeast. Journal of Biological Chemistry, 288:8519-8530, Mar 2013. URL: https://doi.org/10.1074/jbc.m112.404517, doi:10.1074/jbc.m112.404517. This article has 33 citations and is from a domain leading peer-reviewed journal.
(flury2005insigabroadly pages 3-4): Isabelle Flury, Renee Garza, Alexander Shearer, Johanna Rosen, Stephen Cronin, and Randolph Y Hampton. Insig: a broadly conserved transmembrane chaperone for sterol‐sensing domain proteins. The EMBO Journal, 24:3917-3926, Nov 2005. URL: https://doi.org/10.1038/sj.emboj.7600855, doi:10.1038/sj.emboj.7600855. This article has 82 citations.
(flury2005insigabroadly pages 6-7): Isabelle Flury, Renee Garza, Alexander Shearer, Johanna Rosen, Stephen Cronin, and Randolph Y Hampton. Insig: a broadly conserved transmembrane chaperone for sterol‐sensing domain proteins. The EMBO Journal, 24:3917-3926, Nov 2005. URL: https://doi.org/10.1038/sj.emboj.7600855, doi:10.1038/sj.emboj.7600855. This article has 82 citations.
(flury2005insigabroadly pages 8-9): Isabelle Flury, Renee Garza, Alexander Shearer, Johanna Rosen, Stephen Cronin, and Randolph Y Hampton. Insig: a broadly conserved transmembrane chaperone for sterol‐sensing domain proteins. The EMBO Journal, 24:3917-3926, Nov 2005. URL: https://doi.org/10.1038/sj.emboj.7600855, doi:10.1038/sj.emboj.7600855. This article has 82 citations.
(theesfeld2013insulininducedgeneprotein pages 7-8): Chandra L. Theesfeld and Randolph Y. Hampton. Insulin-induced gene protein (insig)-dependent sterol regulation of hmg2 endoplasmic reticulum-associated degradation (erad) in yeast. Journal of Biological Chemistry, 288:8519-8530, Mar 2013. URL: https://doi.org/10.1074/jbc.m112.404517, doi:10.1074/jbc.m112.404517. This article has 33 citations and is from a domain leading peer-reviewed journal.
(flury2005insigabroadly pages 4-6): Isabelle Flury, Renee Garza, Alexander Shearer, Johanna Rosen, Stephen Cronin, and Randolph Y Hampton. Insig: a broadly conserved transmembrane chaperone for sterol‐sensing domain proteins. The EMBO Journal, 24:3917-3926, Nov 2005. URL: https://doi.org/10.1038/sj.emboj.7600855, doi:10.1038/sj.emboj.7600855. This article has 82 citations.
(flury2005insigabroadly media b30588e5): Isabelle Flury, Renee Garza, Alexander Shearer, Johanna Rosen, Stephen Cronin, and Randolph Y Hampton. Insig: a broadly conserved transmembrane chaperone for sterol‐sensing domain proteins. The EMBO Journal, 24:3917-3926, Nov 2005. URL: https://doi.org/10.1038/sj.emboj.7600855, doi:10.1038/sj.emboj.7600855. This article has 82 citations.
(flury2005insigabroadly media 755eb40e): Isabelle Flury, Renee Garza, Alexander Shearer, Johanna Rosen, Stephen Cronin, and Randolph Y Hampton. Insig: a broadly conserved transmembrane chaperone for sterol‐sensing domain proteins. The EMBO Journal, 24:3917-3926, Nov 2005. URL: https://doi.org/10.1038/sj.emboj.7600855, doi:10.1038/sj.emboj.7600855. This article has 82 citations.
(flury2005insigabroadly media 461eedb2): Isabelle Flury, Renee Garza, Alexander Shearer, Johanna Rosen, Stephen Cronin, and Randolph Y Hampton. Insig: a broadly conserved transmembrane chaperone for sterol‐sensing domain proteins. The EMBO Journal, 24:3917-3926, Nov 2005. URL: https://doi.org/10.1038/sj.emboj.7600855, doi:10.1038/sj.emboj.7600855. This article has 82 citations.
(wangeline2017proteostatictacticsin pages 9-11): Margaret A. Wangeline, Nidhi Vashistha, and Randolph Y. Hampton. Proteostatic tactics in the strategy of sterol regulation. Annual review of cell and developmental biology, 33:467-489, Oct 2017. URL: https://doi.org/10.1146/annurev-cellbio-111315-125036, doi:10.1146/annurev-cellbio-111315-125036. This article has 42 citations and is from a domain leading peer-reviewed journal.
(theesfeld2013insulininducedgeneprotein pages 3-4): Chandra L. Theesfeld and Randolph Y. Hampton. Insulin-induced gene protein (insig)-dependent sterol regulation of hmg2 endoplasmic reticulum-associated degradation (erad) in yeast. Journal of Biological Chemistry, 288:8519-8530, Mar 2013. URL: https://doi.org/10.1074/jbc.m112.404517, doi:10.1074/jbc.m112.404517. This article has 33 citations and is from a domain leading peer-reviewed journal.

Citations

flury2005insigabroadly pages 2-3
flury2005insigabroadly pages 1-2
flury2005insigabroadly pages 6-7
flury2005insigabroadly pages 3-4
wangeline2017proteostatictacticsin pages 9-11
theesfeld2013insulininducedgeneprotein pages 1-2
theesfeld2013insulininducedgeneprotein pages 2-2
theesfeld2013insulininducedgeneprotein pages 9-10
flury2005insigabroadly pages 8-9
theesfeld2013insulininducedgeneprotein pages 7-8
flury2005insigabroadly pages 4-6
theesfeld2013insulininducedgeneprotein pages 3-4
https://doi.org/10.1038/sj.emboj.7600855
https://doi.org/10.1074/jbc.m112.404517
https://doi.org/10.1146/annurev-cellbio-111315-125036
https://doi.org/10.1126/sciadv.add8579
https://doi.org/10.1038/sj.emboj.7600855,
https://doi.org/10.1074/jbc.m112.404517,
https://doi.org/10.1146/annurev-cellbio-111315-125036,

📄 View Raw YAML

id: P38837
gene_symbol: NSG1
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:559292
  label: Saccharomyces cerevisiae
description: >-
  NSG1 encodes an INSIG-family multi-pass endoplasmic reticulum membrane protein that binds the sterol-sensing-domain-containing
  Hmg2 HMG-CoA reductase and stabilizes it against Hrd1-dependent ER-associated degradation. Its primary
  role is a selective transmembrane chaperone/regulator of Hmg2 stability in sterol pathway control; recent
  microscopy also supports condition-dependent residence at the nucleus-vacuole junction, but that localization
  is secondary to the Hmg2 quality-control function.
references:
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000044
  title: >-
    Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied
    by conservative changes to GO terms applied by UniProt
  findings: []
- id: PMID:16270032
  title: 'INSIG: a broadly conserved transmembrane chaperone for sterol-sensing domain proteins.'
  findings:
  - statement: >-
      NSG1 and NSG2 stabilize the ER Hmg2 HMG-CoA reductase by directly interacting with its sterol-sensing-domain-containing
      transmembrane region.
    supporting_text: >-
      We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast
      Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation
      of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing
      transmembrane region.
- id: PMID:18467557
  title: An in vivo map of the yeast protein interactome.
  findings: []
- id: PMID:22842922
  title: >-
    Dissecting DNA damage response pathways by analysing protein localization and abundance changes during
    DNA replication stress.
  findings: []
- id: PMID:22932476
  title: The nuclear localization of SWI/SNF proteins is subjected to oxygen regulation.
  findings: []
- id: PMID:26928762
  title: 'One library to make them all: streamlining the creation of yeast libraries via a SWAp-Tag strategy.'
  findings: []
- id: PMID:27107014
  title: An inter-species protein-protein interaction network across vast evolutionary distance.
  findings: []
- id: PMID:37968396
  title: The social and structural architecture of the yeast protein interactome.
  findings: []
- id: PMID:41132095
  title: Role of Pex31 in metabolic adaptation of the nucleus-vacuole junction.
  findings:
  - statement: >-
      NSG1 was identified as a conditional nucleus-vacuole junction resident during glucose limitation,
      supporting the NVJ localization annotation as context-dependent.
    supporting_text: >-
      Here, we used systematic microscopy-based approaches to compare the NVJ at glucose-replete and -restricted
      conditions and identified five additional NVJ proteins: the permanent NVJ resident Shr5 and the
      conditional residents Nsg1, Nsg2, Tcb1 and Pex31.
- id: file:yeast/NSG1/NSG1-deep-research-falcon.md
  title: Falcon deep research report for NSG1
  findings:
  - statement: >-
      The Falcon report was reviewed and synthesized into the NSG1 curation, including core-function framing,
      family/PANTHER context, and evidence limitations.
existing_annotations:
- term:
    id: GO:0005783
    label: endoplasmic reticulum
  qualifier: is_active_in
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: endoplasmic reticulum is consistent with NSG1 being a multi-pass ER membrane INSIG homolog.
    action: ACCEPT
    reason: >-
      Direct UniProt/literature evidence places NSG1 at the endoplasmic reticulum membrane, where Hmg2
      regulation occurs.
- term:
    id: GO:0016126
    label: sterol biosynthetic process
  qualifier: involved_in
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: >-
      NSG1 supports sterol biosynthesis indirectly by stabilizing Hmg2, a sterol-pathway HMG-CoA reductase
      isozyme, rather than by catalyzing a sterol biosynthetic reaction.
    action: ACCEPT
    reason: >-
      Accept as a pathway participation annotation with the mechanism explicitly interpreted as Hmg2 stabilization/regulation;
      PANTHER family transfer is consistent with the direct yeast Hmg2 evidence.
    supported_by:
    - reference_id: PMID:16270032
      supporting_text: >-
        We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast
        Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation
        of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing
        transmembrane region.
      reference_section_type: ABSTRACT
    - reference_id: file:yeast/NSG1/NSG1-deep-research-falcon.md
      supporting_text: Primary function is **stabilization of the Hmg2 HMGR isozyme**
- term:
    id: GO:0005789
    label: endoplasmic reticulum membrane
  qualifier: located_in
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  review:
    summary: endoplasmic reticulum membrane is consistent with NSG1 being a multi-pass ER membrane INSIG
      homolog.
    action: ACCEPT
    reason: >-
      Direct UniProt/literature evidence places NSG1 at the endoplasmic reticulum membrane, where Hmg2
      regulation occurs.
- term:
    id: GO:0005515
    label: protein binding
  qualifier: enables
  evidence_type: IPI
  original_reference_id: PMID:18467557
  supporting_entities:
  - UniProtKB:P12684
  review:
    summary: >-
      Generic protein binding annotations from interactome studies do not capture the specific NSG1/Hmg2
      sterol-sensing-domain chaperone mechanism.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      The defensible NSG1 function is selective stabilization of SSD-containing Hmg2, not unspecific protein
      binding from high-throughput PPI evidence.
- term:
    id: GO:0005515
    label: protein binding
  qualifier: enables
  evidence_type: IPI
  original_reference_id: PMID:27107014
  supporting_entities:
  - UniProtKB:Q8N6L0
  review:
    summary: >-
      Generic protein binding annotations from interactome studies do not capture the specific NSG1/Hmg2
      sterol-sensing-domain chaperone mechanism.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      The defensible NSG1 function is selective stabilization of SSD-containing Hmg2, not unspecific protein
      binding from high-throughput PPI evidence.
- term:
    id: GO:0005515
    label: protein binding
  qualifier: enables
  evidence_type: IPI
  original_reference_id: PMID:37968396
  supporting_entities:
  - UniProtKB:P12684
  review:
    summary: >-
      Generic protein binding annotations from interactome studies do not capture the specific NSG1/Hmg2
      sterol-sensing-domain chaperone mechanism.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      The defensible NSG1 function is selective stabilization of SSD-containing Hmg2, not unspecific protein
      binding from high-throughput PPI evidence.
- term:
    id: GO:0071561
    label: nucleus-vacuole junction
  qualifier: is_active_in
  evidence_type: IDA
  original_reference_id: PMID:41132095
  review:
    summary: >-
      NSG1 is detected at the nucleus-vacuole junction under glucose limitation, but this is a conditional
      localization rather than the primary mechanistic function.
    action: KEEP_AS_NON_CORE
    reason: >-
      Keep as a supported context-specific localization from the 2025 NVJ remodeling study, while not
      elevating it over the ER/Hmg2 core function.
    supported_by:
    - reference_id: PMID:41132095
      supporting_text: >-
        Here, we used systematic microscopy-based approaches to compare the NVJ at glucose-replete and
        -restricted conditions and identified five additional NVJ proteins: the permanent NVJ resident
        Shr5 and the conditional residents Nsg1, Nsg2, Tcb1 and Pex31.
- term:
    id: GO:0005783
    label: endoplasmic reticulum
  qualifier: located_in
  evidence_type: HDA
  original_reference_id: PMID:26928762
  review:
    summary: >-
      endoplasmic reticulum is consistent with the ER biology of NSG1 and is supported by the SWAT endomembrane
      localization library.
    action: ACCEPT
    reason: >-
      Accept as supporting ER localization evidence, with gene-specific ER function anchored by the primary
      literature and UniProt record.
    supported_by:
    - reference_id: PMID:26928762
      supporting_text: we constructed and investigated a library of
      reference_section_type: ABSTRACT
- term:
    id: GO:0034399
    label: nuclear periphery
  qualifier: located_in
  evidence_type: HDA
  original_reference_id: PMID:22842922
  review:
    summary: >-
      Nuclear periphery localization is compatible with ER/nuclear-envelope contact-site biology but is
      not the defining NSG1 function.
    action: KEEP_AS_NON_CORE
    reason: >-
      Retain as non-core localization context, especially given independent NVJ/nuclear-ER evidence, but
      core function remains Hmg2 stabilization at ER membranes.
- term:
    id: GO:0005829
    label: cytosol
  qualifier: located_in
  evidence_type: IDA
  original_reference_id: PMID:22932476
  review:
    summary: >-
      cytosol is not well aligned with NSG1 as a multi-pass ER membrane protein and likely reflects screen/context-specific
      signal rather than a stable functional location.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      The stronger direct evidence supports ER membrane and conditional NVJ/nuclear-ER localization; soluble
      cytosol/nucleus annotations overstate the localization of this membrane protein.
- term:
    id: GO:0005634
    label: nucleus
  qualifier: located_in
  evidence_type: IDA
  original_reference_id: PMID:22932476
  review:
    summary: >-
      nucleus is not well aligned with NSG1 as a multi-pass ER membrane protein and likely reflects screen/context-specific
      signal rather than a stable functional location.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      The stronger direct evidence supports ER membrane and conditional NVJ/nuclear-ER localization; soluble
      cytosol/nucleus annotations overstate the localization of this membrane protein.
- term:
    id: GO:0005783
    label: endoplasmic reticulum
  qualifier: located_in
  evidence_type: IDA
  original_reference_id: PMID:16270032
  review:
    summary: endoplasmic reticulum is consistent with NSG1 being a multi-pass ER membrane INSIG homolog.
    action: ACCEPT
    reason: >-
      Direct UniProt/literature evidence places NSG1 at the endoplasmic reticulum membrane, where Hmg2
      regulation occurs.
    supported_by:
    - reference_id: PMID:16270032
      supporting_text: by directly interacting with the sterol-sensing domain (SSD)-containing
        transmembrane region
      reference_section_type: ABSTRACT
- term:
    id: GO:0016126
    label: sterol biosynthetic process
  qualifier: involved_in
  evidence_type: IMP
  original_reference_id: PMID:16270032
  review:
    summary: >-
      NSG1 supports sterol biosynthesis indirectly by stabilizing Hmg2, a sterol-pathway HMG-CoA reductase
      isozyme, rather than by catalyzing a sterol biosynthetic reaction.
    action: ACCEPT
    reason: >-
      Accept as a pathway participation annotation with the mechanism explicitly interpreted as Hmg2 stabilization/regulation;
      PANTHER family transfer is consistent with the direct yeast Hmg2 evidence.
    supported_by:
    - reference_id: PMID:16270032
      supporting_text: >-
        We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast
        Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation
        of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing
        transmembrane region.
      reference_section_type: ABSTRACT
- term:
    id: GO:0016126
    label: sterol biosynthetic process
  qualifier: involved_in
  evidence_type: IPI
  original_reference_id: PMID:16270032
  supporting_entities:
  - SGD:S000004442
  review:
    summary: >-
      NSG1 supports sterol biosynthesis indirectly by stabilizing Hmg2, a sterol-pathway HMG-CoA reductase
      isozyme, rather than by catalyzing a sterol biosynthetic reaction.
    action: ACCEPT
    reason: >-
      Accept as a pathway participation annotation with the mechanism explicitly interpreted as Hmg2 stabilization/regulation;
      PANTHER family transfer is consistent with the direct yeast Hmg2 evidence.
    supported_by:
    - reference_id: PMID:16270032
      supporting_text: >-
        We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast
        Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation
        of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing
        transmembrane region.
      reference_section_type: ABSTRACT
- term:
    id: GO:0051082
    label: unfolded protein binding
  qualifier: enables
  evidence_type: IMP
  original_reference_id: PMID:16270032
  review:
    summary: >-
      The Hmg2 stabilization evidence supports a chaperone-like activity, but unfolded protein binding
      is too generic for the INSIG/SSD-client mechanism.
    action: MODIFY
    reason: Replace with protein folding chaperone pending a more specific sterol-sensing-domain chaperone
      term.
    proposed_replacement_terms:
    - id: GO:0044183
      label: protein folding chaperone
    supported_by:
    - reference_id: PMID:16270032
      supporting_text: >-
        We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast
        Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation
        of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing
        transmembrane region.
      reference_section_type: ABSTRACT
- term:
    id: GO:0051082
    label: unfolded protein binding
  qualifier: enables
  evidence_type: IPI
  original_reference_id: PMID:16270032
  supporting_entities:
  - SGD:S000004442
  review:
    summary: >-
      The Hmg2 stabilization evidence supports a chaperone-like activity, but unfolded protein binding
      is too generic for the INSIG/SSD-client mechanism.
    action: MODIFY
    reason: Replace with protein folding chaperone pending a more specific sterol-sensing-domain chaperone
      term.
    proposed_replacement_terms:
    - id: GO:0044183
      label: protein folding chaperone
    supported_by:
    - reference_id: PMID:16270032
      supporting_text: >-
        We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast
        Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation
        of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing
        transmembrane region.
      reference_section_type: ABSTRACT
core_functions:
- description: >-
    NSG1 acts as an INSIG-family transmembrane chaperone/regulator for Hmg2, binding the sterol-sensing-domain-containing
    transmembrane region of Hmg2 and stabilizing it against Hrd1-dependent ERAD. This promotes appropriate
    Hmg2 abundance within sterol biosynthetic pathway control without NSG1 itself catalyzing a sterol
    biosynthetic reaction.
  molecular_function:
    id: GO:0044183
    label: protein folding chaperone
  directly_involved_in:
  - id: GO:0016126
    label: sterol biosynthetic process
  locations:
  - id: GO:0005789
    label: endoplasmic reticulum membrane
  substrates:
  - id: SGD:S000004442
    label: HMG2
  supported_by:
  - reference_id: PMID:16270032
    supporting_text: >-
      We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast
      Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation
      of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing
      transmembrane region.
    reference_section_type: ABSTRACT
proposed_new_terms:
- proposed_name: sterol-sensing domain protein chaperone activity
  proposed_definition: >-
    Binding to and stabilizing the transmembrane sterol-sensing domain of a client protein to promote
    correct folding or prevent ER-associated degradation.
  justification: >-
    NSG1/INSIG activity toward Hmg2 is more specific than generic unfolded protein binding or broad protein
    folding chaperone activity.
  proposed_parent:
    id: GO:0044183
    label: protein folding chaperone
  supported_by:
  - reference_id: PMID:16270032
    supporting_text: >-
      We now show that the yeast INSIG homologs NSG1 and NSG2 function to control the stability of yeast
      Hmg2p, the HMGR isozyme that undergoes regulated ubiquitination. Yeast Nsgs inhibit degradation
      of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing
      transmembrane region.
    reference_section_type: ABSTRACT
suggested_questions:
- question: >-
    Does NSG1 have a direct Hmg2-independent role at the nucleus-vacuole junction in sterol storage or
    lipid-transfer adaptation, or is NVJ enrichment mainly a relocalization of the Hmg2 regulatory module?
suggested_experiments:
- hypothesis: NSG1 NVJ recruitment modulates Hmg2 stabilization or sterol storage during glucose limitation.
  description: >-
    Combine live-cell NSG1/Hmg2/NVJ marker imaging with Hmg2 stability assays in glucose-replete and glucose-restricted
    cells, using NSG1 mutants that disrupt Hmg2 binding or NVJ enrichment.
  experiment_type: live-cell fluorescence microscopy with cycloheximide-chase protein stability assay

NSG1

Gene Description

Proposed New Ontology Terms

sterol-sensing domain protein chaperone activity

Existing Annotations Review

Core Functions

References

Suggested Questions for Experts

Suggested Experiments

📚 Additional Documentation

Deep Research Falcon

Question

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

Target Gene/Protein Identity (from UniProt):

MANDATORY VERIFICATION STEPS:

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

Research Target:

Output

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

Target Gene/Protein Identity (from UniProt):

MANDATORY VERIFICATION STEPS:

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

Research Target:

Research report: NSG1 (UniProt P38837; ORF YHR133C) in Saccharomyces cerevisiae (S288c)

0) Target verification (mandatory)

1) Key concepts and definitions (current understanding)

1.1 INSIG proteins and sterol-sensing domain (SSD) clients

1.2 ER-associated degradation (ERAD) in the sterol pathway

1.3 “Two-signal logic” for Hmg2 stability

2) Gene/protein function and biological role

2.1 Primary molecular function

2.2 Pathway context

3) Subcellular localization

4) Mechanism: interactions and experimental evidence

4.1 Direct interaction with Hmg2

4.2 Functional consequence: stabilization at the ERAD entry step

4.3 Sterol dependence of NSG1 action (lanosterol)

5) Quantitative findings and key statistics from primary studies

6) Recent developments and latest research (prioritizing 2023–2024)

7) Current applications and real-world implementations

7.1 Yeast as a tractable model for conserved INSIG biology

7.2 Chemical–genetic and metabolic engineering relevance (indirect)

8) Expert opinions and authoritative analysis

9) Evidence summary table

10) Key references (with URLs and publication dates where available)

11) Limitations of this synthesis

Citations

📄 View Raw YAML