NSG2 encodes a multi-pass endoplasmic-reticulum membrane INSIG-family protein. Together with the paralog Nsg1, Nsg2 regulates sterol homeostasis by interacting with sterol-sensing-domain-containing HMG-CoA reductase proteins, especially Hmg2, and limiting HRD-dependent ER-associated degradation. Nsg1 is the dominant native stabilizer in the classic Hmg2 assays, but Nsg2 can stabilize Hmg2 when overexpressed and contributes in Nsg1-deficient contexts. Recent localization work also places Nsg2 at the nucleus-vacuole junction during glucose starvation, consistent with context-dependent sterol-regulatory remodeling at ER-vacuole contact sites.
Definition: Binding to sterol-sensing-domain-containing membrane protein clients in the endoplasmic reticulum to stabilize them by limiting ubiquitination and ER-associated degradation.
Justification: Nsg2 and related Nsg/INSIG-family proteins interact directly with the sterol-sensing-domain-containing transmembrane region of Hmg2 and inhibit its HRD-dependent degradation. GO:0044183 protein folding chaperone is the closest available GO term in this review, but it overemphasizes de novo folding relative to SSD-client stabilization and ERAD protection.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0005783
endoplasmic reticulum
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: PANTHER/IBA transfer to endoplasmic reticulum is consistent with the INSIG family context and with direct experimental localization of Nsg proteins to ER membranes.
Reason: NSG2 is an INSIG-family multi-pass ER membrane protein. The family-transfer annotation is appropriate and agrees with focused evidence from the yeast NSG/Hmg2 literature.
Supporting Evidence:
PMID:16270032
Yeast Nsgs inhibit degradation of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing transmembrane region.
UniProt:P53898
SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Multi-pass membrane protein.
|
|
GO:0016126
sterol biosynthetic process
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: The PANTHER/IBA sterol biosynthetic process annotation is consistent with the conserved INSIG-family role in regulating sterol-pathway HMG-CoA reductase stability.
Reason: NSG2 should not be interpreted as a sterol biosynthetic enzyme. Its role is regulatory/chaperone-like, acting through Hmg2 stability and sterol-sensing domain client interactions. The process annotation remains appropriate because this regulatory function affects sterol biosynthesis.
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.
file:yeast/NSG2/NSG2-deep-research-falcon.md
NSG2 functions in sterol/ergosterol homeostasis and regulated ER-associated degradation of Hmg2.
|
|
GO:0005789
endoplasmic reticulum membrane
|
IEA
GO_REF:0000044 |
ACCEPT |
Summary: UniProt subcellular-location mapping to ER membrane is consistent with NSG2's experimentally supported INSIG-family membrane protein identity.
Reason: ER membrane is the core compartment for Nsg2 regulation of Hmg2 stability.
Supporting Evidence:
UniProt:P53898
SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Multi-pass membrane protein.
|
|
GO:0071561
nucleus-vacuole junction
|
IDA
PMID:41132095 Role of Pex31 in metabolic adaptation of the nucleus-vacuole... |
KEEP AS NON CORE |
Summary: Direct microscopy evidence identifies Nsg2 as a nucleus-vacuole junction resident under low-glucose conditions.
Reason: The NVJ annotation is valid but context-dependent. It should be retained as a starvation/contact-site localization without replacing ER membrane as the core site for the classic Hmg2 stability function. PMID:41132095 places Nsg2 at the NVJ together with Nsg1 and other conditional residents, so the NVJ signal is best treated as a paralog-shared starvation response rather than evidence for an Nsg2-specific core activity.
Supporting Evidence:
PMID:41132095
We identified Pex31, Nsg1, Nsg2, Shr5, and Tcb1 as NVJ residents.
PMID:41132095
Nsg1, Nsg2 and Tcb1 belong to the large group of conditional NVJ residents that accumulate at low-glucose conditions.
|
|
GO:0005783
endoplasmic reticulum
|
HDA
PMID:26928762 One library to make them all: streamlining the creation of y... |
ACCEPT |
Summary: The high-throughput ER localization is consistent with focused evidence and the UniProt ER membrane annotation.
Reason: ER localization is well aligned with NSG2's INSIG-family role in regulating ER membrane Hmg2 stability.
Supporting Evidence:
UniProt:P53898
SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Multi-pass membrane protein.
|
|
GO:0005783
endoplasmic reticulum
|
HDA
PMID:14562095 Global analysis of protein localization in budding yeast |
ACCEPT |
Summary: The Huh et al. high-throughput localization is consistent with NSG2's ER membrane protein identity.
Reason: This broad ER annotation is correct, although ER membrane is more specific.
Supporting Evidence:
UniProt:P53898
SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Multi-pass membrane protein.
|
|
GO:0016126
sterol biosynthetic process
|
IGI
PMID:16270032 INSIG: a broadly conserved transmembrane chaperone for stero... |
ACCEPT |
Summary: Genetic evidence supports NSG2 participation in sterol biosynthesis through regulation of Hmg2 stability, with partial redundancy and unequal contribution relative to NSG1.
Reason: The annotation is best interpreted as a regulatory contribution to sterol biosynthesis, not as direct catalysis. The IGI evidence with NSG1/Hmg2 context is biologically coherent.
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.
|
|
GO:0051082
unfolded protein binding
|
IMP
PMID:16270032 INSIG: a broadly conserved transmembrane chaperone for stero... |
MODIFY |
Summary: The evidence supports a dedicated transmembrane chaperone function for an SSD-containing client rather than broad unfolded protein binding.
Reason: Nsg2 and Nsg1 interact with the Hmg2 sterol-sensing-domain-containing transmembrane region and promote Hmg2 stability/folding. GO:0044183 protein folding chaperone better captures this client-specific chaperone role than generic GO:0051082 unfolded protein binding.
Proposed replacements:
protein folding chaperone
Supporting Evidence:
PMID:16270032
Yeast Nsgs inhibit degradation of Hmg2p in a highly specific manner, by directly interacting with the sterol-sensing domain (SSD)-containing transmembrane region.
PMID:16270032
One way to unify the known, disparate actions of INSIGs is to view them as known adaptations of a chaperone dedicated to SSD-containing client proteins.
|
Q: What client specificity distinguishes Nsg2 from Nsg1 at native expression levels, and which HMG-CoA reductase isozyme or sterol-sensing-domain clients require Nsg2 specifically?
Q: Does low-glucose recruitment of Nsg2 to the nucleus-vacuole junction alter Hmg1/Hmg2 stability, enzyme activity, or sterol flux directly?
Q: How reliable are the PANTHER/IBA NSG2 annotations across INSIG subfamilies, given the unequal contributions of Nsg1 and Nsg2 in yeast?
Experiment: Compare Hmg1 and Hmg2 stability, ERAD flux, and sterol intermediate levels in nsg1, nsg2, and nsg1 nsg2 mutants under sterol-replete, sterol-depleted, and low-glucose conditions.
Type: genetics
Experiment: Map Nsg2 client contacts by crosslinking or co-immunoprecipitation of functional tagged Nsg2 with Hmg1/Hmg2 SSD-domain mutants.
Type: biochemistry
Experiment: Quantify Nsg2 recruitment to the NVJ during glucose starvation and test whether disrupting NVJ tethering changes sterol ester, squalene, or HMG-CoA reductase activity phenotypes.
Type: microscopy
provider: falcon
model: Edison Scientific Literature
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template_file: templates/gene_research_go_focused.md
template_variables:
organism: yeast
gene_id: NSG2
gene_symbol: NSG2
uniprot_accession: P53898
protein_description: 'RecName: Full=Protein NSG2; AltName: Full=INSIG homolog 2;'
gene_info: Name=NSG2; OrderedLocusNames=YNL156C; ORFNames=N1747;
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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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.
DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'NSG2' 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
Please provide a comprehensive research report on the gene NSG2 (gene ID: NSG2, UniProt: P53898) 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.
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.
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.
DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'NSG2' 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
Please provide a comprehensive research report on the gene NSG2 (gene ID: NSG2, UniProt: P53898) 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.
NSG2 (YNL156C) in S. cerevisiae encodes a yeast INSIG-family multi-pass membrane protein originally identified in a screen for genes that stabilize the regulated-degradation HMG-CoA reductase isozyme Hmg2. It is one of two yeast INSIG orthologs (NSG1 and NSG2) and is substantially diverged from mammalian INSIG proteins, but functionally related. (flury2005insigabroadly pages 1-2, flury2005insigabroadly pages 2-3)
Important ambiguity note: “NSG2” is also used in other fungi (e.g., Candida albicans ORF19.273) with distinct phenotypes and mechanistic context; those results should not be conflated with S. cerevisiae YNL156C. (lv2018nsg2(orf19.273)encoding pages 1-2)
INSIG proteins are broadly conserved regulators of sterol metabolism that act by binding SSD-containing membrane proteins and modulating their behavior in response to sterol-pathway signals. In S. cerevisiae, NSG1/NSG2 represent a variant of INSIG action: they act as dedicated transmembrane chaperones that stabilize (rather than promote degradation of) the SSD-containing target Hmg2, limiting its entry into ER-associated degradation (ERAD). (flury2005insigabroadly pages 1-2)
The regulated degradation of yeast Hmg2 (an HMG-CoA reductase isozyme) integrates multiple lipid signals. Hmg2 degradation is promoted by a mevalonate-pathway signal derived from farnesyl pyrophosphate (FPP)—with geranylgeranyl pyrophosphate (GGPP) identified as a likely endogenous degradation-promoting signal—while lanosterol supports INSIG (Nsg)-dependent stabilization via Nsg–Hmg2 interaction. This yields a “two-signal” logic in which sterol state (lanosterol) gates the ability of the GGPP-derived signal to drive ERAD entry. (theesfeld2013insulininducedgeneprotein pages 2-2)
NSG2 corresponds to ORF YNL156C and encodes a multispanning membrane protein classified as a yeast homolog of mammalian INSIG proteins. (flury2005insigabroadly pages 2-3, flury2005insigabroadly pages 1-2)
Multiple lines of evidence place Nsg proteins as endoplasmic reticulum (ER) membrane residents; GFP-tagging and immunostaining show ER-like “concentric circle” morphology, and large-scale GFP-fusion localization resources also place them in part or all of the ER. (flury2005insigabroadly pages 2-3)
A more recent model extends localization to membrane contact sites: during glucose starvation, Nsg2 (with Nsg1, Hmg1, and Hmg2) accumulates at the nucleus–vacuole junction (NVJ) in a manner promoted by Ypf1. (fujimoto2025glucosestarvationsensing pages 7-9)
The core experimentally supported molecular function of NSG2 in yeast is regulation of the stability of Hmg2, by INSIG-like interaction with the Hmg2 SSD-containing transmembrane region. NSG proteins directly interact with the Hmg2 transmembrane region and promote folding/stability, sparing Hmg2 from ERAD. (flury2005insigabroadly pages 1-2)
Nsg1 vs Nsg2 specificity: At native levels, Nsg1 is the principal stabilizer for Hmg2; deletion of NSG1 (and the double deletion) causes rapid Hmg2 degradation, while nsg2Δ alone did not measurably destabilize native Hmg2 in the same assays. However, Nsg2 can stabilize Hmg2 when overexpressed, and genetic evidence suggests Nsg2 contributes to residual stabilization when Nsg1 is absent. (flury2005insigabroadly pages 3-4, theesfeld2013insulininducedgeneprotein pages 7-8)
In the Hampton lab’s model, Nsg proteins modulate whether Hmg2 enters the HRD ERAD pathway. When sterol synthesis supports INSIG-client binding (lanosterol present), Nsg proteins stabilize Hmg2; when sterol support is reduced, Hmg2 becomes unstable and is degraded via ERAD. (theesfeld2013insulininducedgeneprotein pages 2-2, theesfeld2013insulininducedgeneprotein pages 7-8)
A 2025 preprint proposes that glucose starvation induces NVJ remodeling that recruits sterol-pathway regulators. In this model, Nsg2 is stabilized during glucose starvation (opposite to Nsg1), and Nsg2 contributes to a negative feedback mechanism that limits HMG-CoA reductase activity and helps maintain sterol homeostasis during starvation. (fujimoto2025glucosestarvationsensing pages 7-9)
Evidence gap for 2023–2024: In the retrieved corpus, there were no direct 2023–2024 primary studies focused specifically on S. cerevisiae NSG2/YNL156C mechanistic function.
Most recent retrieved mechanistic advance: A 2025 bioRxiv preprint links Nsg2 to glucose-starvation-dependent NVJ recruitment and to repression of excessive sterol synthesis (ergosterol ester and squalene accumulation in double mutants), expanding NSG2 function from classical ERAD-linked Hmg2 stabilization to broader metabolic-state sensing via membrane contact sites. (fujimoto2025glucosestarvationsensing pages 7-9)
Loss of NSG genes caused a measurable phenotype relevant to sterol-pathway inhibition: strains expressing only Hmg2p showed approximately three-fold greater sensitivity to lovastatin when NSG genes were absent, consistent with NSG proteins supporting Hmg2 stability and thereby impacting flux through the mevalonate/sterol pathway under drug challenge. (flury2005insigabroadly pages 3-4)
The NVJ/glucose-starvation study explicitly states that its findings provide a foundation for developing yeast-based strategies to enhance industrial production of commercially valuable lipids such as ergosterol and squalene, by manipulating the regulatory module involving Nsg1/Nsg2 and HMG-CoA reductase activity. (fujimoto2025glucosestarvationsensing pages 7-9)
A central interpretive contribution of the foundational EMBO Journal work is the unifying model that INSIG proteins can be viewed as dedicated chaperones for SSD-containing membrane proteins, and that the diverse regulatory outcomes (ER retention, ERAD modulation, etc.) can be understood as adaptations of this chaperone-like interaction. This framing is explicitly proposed in the yeast NSG context. (flury2005insigabroadly pages 1-2)
The 2013 JBC study further supports deep evolutionary conservation of the sterol dependence of INSIG-client interactions, emphasizing lanosterol’s role in stabilizing the INSIG–HMGR relationship in yeast. (theesfeld2013insulininducedgeneprotein pages 2-2, theesfeld2013insulininducedgeneprotein pages 7-8)
Flury et al. provide figure panels showing that overexpression of NSG1 or NSG2 stabilizes Hmg2p-GFP (flow cytometry histograms) and stabilizes full-length 1myc-Hmg2p (CHX-chase immunoblots). (flury2005insigabroadly media edede554, flury2005insigabroadly media a8738ede)
| Topic | Evidence summary | Key experimental approach | Primary source (author/year) and URL |
|---|---|---|---|
| Family/domain | NSG2 corresponds to YNL156C in S. cerevisiae and encodes a multispanning membrane protein identified as a yeast INSIG homolog; yeast has two INSIG orthologs, NSG1 and NSG2. The proteins are substantially diverged from mammalian INSIGs but retain a conserved sterol-regulatory role (flury2005insigabroadly pages 1-2, flury2005insigabroadly pages 2-3). | Sequence comparison/homology analysis; genetic identification in Hmg2 stabilizer screen | Flury et al., 2005 — https://doi.org/10.1038/sj.emboj.7600855 |
| Localization | Both Nsg1 and Nsg2 are reported as ER membrane proteins; GFP/immunostaining gave a typical yeast ER “concentric circle” pattern, and external GFP-localization resources also placed both proteins in part or all of the ER. More recent work additionally places Nsg2 at the nucleus–vacuole junction (NVJ) during glucose starvation, with Ypf1 promoting NVJ recruitment (flury2005insigabroadly pages 2-3, fujimoto2025glucosestarvationsensing pages 7-9). | GFP tagging; HA immunostaining; microscopy/localization analysis | Flury et al., 2005 — https://doi.org/10.1038/sj.emboj.7600855; Fujimoto & Tamura, 2025 — https://doi.org/10.1101/2025.05.29.656913 |
| Molecular function | NSG proteins act as INSIG-like transmembrane chaperones for the SSD-containing HMG-CoA reductase isozyme Hmg2. NSG1/NSG2 overexpression stabilizes Hmg2, and NSGs directly interact with the Hmg2 transmembrane/SSD-containing region, promoting folding and sparing it from ERAD. At native levels, Nsg1 is the dominant stabilizer, but Nsg2 can stabilize Hmg2 when overexpressed and contributes in the absence of Nsg1 (flury2005insigabroadly pages 1-2, flury2005insigabroadly pages 2-3, flury2005insigabroadly pages 3-4, theesfeld2013insulininducedgeneprotein pages 7-8). | High-copy overexpression; CHX chase; flow cytometry; co-precipitation/interaction assays | Flury et al., 2005 — https://doi.org/10.1038/sj.emboj.7600855; Theesfeld & Hampton, 2013 — https://doi.org/10.1074/jbc.m112.404517 |
| Pathway role | NSG2 functions in sterol/ergosterol homeostasis and regulated ER-associated degradation (ERAD) of Hmg2. Nsg-mediated stabilization is linked to lanosterol-dependent INSIG-client interaction; when sterol support is low, Hmg2 is more rapidly degraded through the HRD ERAD pathway. Recent work suggests Nsg2, together with Nsg1, also acts as a negative regulator of Hmg1 activity during glucose starvation to restrain excess sterol synthesis (theesfeld2013insulininducedgeneprotein pages 2-2, theesfeld2013insulininducedgeneprotein pages 7-8, fujimoto2025glucosestarvationsensing pages 7-9). | Sterol depletion/inhibitor experiments (terbinafine); mutant analysis; CHX chase; metabolic labeling | Theesfeld & Hampton, 2013 — https://doi.org/10.1074/jbc.m112.404517; Fujimoto & Tamura, 2025 — https://doi.org/10.1101/2025.05.29.656913 |
| Key phenotypes / quantitative data | Overexpression from the TDH3 promoter raised NSG protein abundance by about 50–100-fold and stabilized Hmg2. Native Hmg2 was reported as highly stable (half-life >4 h) when NSG genes were present, whereas nsg1Δ or nsg1Δ nsg2Δ caused rapid degradation; nsg2Δ alone did not measurably destabilize native Hmg2 in that setting. Loss of NSGs caused roughly 3-fold greater lovastatin sensitivity in strains relying on Hmg2. In double-mutant nsg1Δ nsg2Δ cells under glucose starvation, ergosterol esters and squalene drastically accumulated, and Nsg2 re-expression reduced abnormal lipid-droplet accumulation (flury2005insigabroadly pages 2-3, flury2005insigabroadly pages 3-4, fujimoto2025glucosestarvationsensing pages 7-9). | Flow cytometry; CHX chase/immunoblotting; drug-sensitivity assay; ^14C-acetate metabolic labeling and TLC; lipid-droplet imaging | Flury et al., 2005 — https://doi.org/10.1038/sj.emboj.7600855; Fujimoto & Tamura, 2025 — https://doi.org/10.1101/2025.05.29.656913 |
| Recent developments / applications | No direct 2023–2024 NSG2-specific yeast primary study was retrieved in context. The newest available mechanistic evidence is a 2025 preprint showing GS-dependent stabilization/NVJ recruitment of Nsg2 and proposing that manipulation of the Nsg1/Nsg2 module could support yeast-based production strategies for ergosterol and squalene. This gives NSG2 relevance for metabolic engineering of lipid outputs, although the application remains prospective (fujimoto2025glucosestarvationsensing pages 7-9). | Glucose-starvation remodeling studies; NVJ localization; sterol metabolic labeling/TLC | Fujimoto & Tamura, 2025 — https://doi.org/10.1101/2025.05.29.656913 |
Table: This table summarizes the main experimentally supported functional-annotation facts for yeast NSG2/YNL156C, including its INSIG-family identity, ER/NVJ localization, role in Hmg2 sterol-regulated stability, and key quantitative phenotypes. It is useful as a compact evidence map linking function, pathway context, methods, and primary sources.
NSG2/YNL156C (UniProt P53898) is best annotated as an ER (and starvation-induced NVJ) multi-pass INSIG-family membrane protein that participates in sterol homeostasis primarily by modulating the stability and/or activity of HMG-CoA reductase isozymes, especially by stabilizing the SSD-containing isozyme Hmg2 (thereby limiting its HRD-dependent ERAD), and—under glucose starvation—contributing with Nsg1 to negative regulation of Hmg1 to prevent excessive sterol synthesis. (flury2005insigabroadly pages 1-2, flury2005insigabroadly pages 3-4, theesfeld2013insulininducedgeneprotein pages 2-2, fujimoto2025glucosestarvationsensing pages 7-9)
References
(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.
(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.
(lv2018nsg2(orf19.273)encoding pages 1-2): Quan-Zhen Lv, Yu-Lin Qin, Lan Yan, Liang Wang, Chuyue Zhang, and Yuan-Ying Jiang. Nsg2 (orf19.273) encoding protein controls sensitivity of candida albicans to azoles through regulating the synthesis of c14-methylated sterols. Frontiers in Microbiology, Feb 2018. URL: https://doi.org/10.3389/fmicb.2018.00218, doi:10.3389/fmicb.2018.00218. This article has 15 citations and is from a peer-reviewed journal.
(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.
(fujimoto2025glucosestarvationsensing pages 7-9): Shintaro Fujimoto and Yasushi Tamura. Glucose starvation sensing through membrane remodeling at the nucleus-vacuole junction coordinates ergosterol biosynthesis. bioRxiv, May 2025. URL: https://doi.org/10.1101/2025.05.29.656913, doi:10.1101/2025.05.29.656913. This article has 2 citations.
(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.
(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 media edede554): 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 a8738ede): 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.
id: P53898
gene_symbol: NSG2
product_type: PROTEIN
status: COMPLETE
taxon:
id: NCBITaxon:559292
label: Saccharomyces cerevisiae
description: >-
NSG2 encodes a multi-pass endoplasmic-reticulum membrane INSIG-family protein.
Together with the paralog Nsg1, Nsg2 regulates sterol homeostasis by
interacting with sterol-sensing-domain-containing HMG-CoA reductase proteins,
especially Hmg2, and limiting HRD-dependent ER-associated degradation. Nsg1 is
the dominant native stabilizer in the classic Hmg2 assays, but Nsg2 can
stabilize Hmg2 when overexpressed and contributes in Nsg1-deficient contexts.
Recent localization work also places Nsg2 at the nucleus-vacuole junction
during glucose starvation, consistent with context-dependent sterol-regulatory
remodeling at ER-vacuole contact sites.
existing_annotations:
- term:
id: GO:0005783
label: endoplasmic reticulum
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: >-
PANTHER/IBA transfer to endoplasmic reticulum is consistent with the INSIG
family context and with direct experimental localization of Nsg proteins to
ER membranes.
action: ACCEPT
reason: >-
NSG2 is an INSIG-family multi-pass ER membrane protein. The family-transfer
annotation is appropriate and agrees with focused evidence from the yeast
NSG/Hmg2 literature.
supported_by:
- reference_id: PMID:16270032
supporting_text: >-
Yeast Nsgs inhibit degradation of Hmg2p in a highly specific manner, by
directly interacting with the sterol-sensing domain (SSD)-containing
transmembrane region.
- reference_id: UniProt:P53898
supporting_text: >-
SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Multi-pass
membrane protein.
- term:
id: GO:0016126
label: sterol biosynthetic process
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: >-
The PANTHER/IBA sterol biosynthetic process annotation is consistent with
the conserved INSIG-family role in regulating sterol-pathway HMG-CoA
reductase stability.
action: ACCEPT
reason: >-
NSG2 should not be interpreted as a sterol biosynthetic enzyme. Its role is
regulatory/chaperone-like, acting through Hmg2 stability and sterol-sensing
domain client interactions. The process annotation remains appropriate
because this regulatory function affects sterol biosynthesis.
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.
- reference_id: file:yeast/NSG2/NSG2-deep-research-falcon.md
supporting_text: >-
NSG2 functions in sterol/ergosterol homeostasis and regulated
ER-associated degradation of Hmg2.
- term:
id: GO:0005789
label: endoplasmic reticulum membrane
evidence_type: IEA
original_reference_id: GO_REF:0000044
review:
summary: >-
UniProt subcellular-location mapping to ER membrane is consistent with
NSG2's experimentally supported INSIG-family membrane protein identity.
action: ACCEPT
reason: >-
ER membrane is the core compartment for Nsg2 regulation of Hmg2 stability.
supported_by:
- reference_id: UniProt:P53898
supporting_text: >-
SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Multi-pass membrane
protein.
- term:
id: GO:0071561
label: nucleus-vacuole junction
evidence_type: IDA
original_reference_id: PMID:41132095
review:
summary: >-
Direct microscopy evidence identifies Nsg2 as a nucleus-vacuole junction
resident under low-glucose conditions.
action: KEEP_AS_NON_CORE
reason: >-
The NVJ annotation is valid but context-dependent. It should be retained as
a starvation/contact-site localization without replacing ER membrane as the
core site for the classic Hmg2 stability function. PMID:41132095 places
Nsg2 at the NVJ together with Nsg1 and other conditional residents, so the
NVJ signal is best treated as a paralog-shared starvation response rather
than evidence for an Nsg2-specific core activity.
supported_by:
- reference_id: PMID:41132095
supporting_text: >-
We identified Pex31, Nsg1, Nsg2, Shr5, and Tcb1 as NVJ residents.
- reference_id: PMID:41132095
supporting_text: >-
Nsg1, Nsg2 and Tcb1 belong to the large group of conditional NVJ
residents that accumulate at low-glucose conditions.
- term:
id: GO:0005783
label: endoplasmic reticulum
evidence_type: HDA
original_reference_id: PMID:26928762
review:
summary: >-
The high-throughput ER localization is consistent with focused evidence and
the UniProt ER membrane annotation.
action: ACCEPT
reason: >-
ER localization is well aligned with NSG2's INSIG-family role in regulating
ER membrane Hmg2 stability.
supported_by:
- reference_id: UniProt:P53898
supporting_text: >-
SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Multi-pass membrane
protein.
- term:
id: GO:0005783
label: endoplasmic reticulum
evidence_type: HDA
original_reference_id: PMID:14562095
review:
summary: >-
The Huh et al. high-throughput localization is consistent with NSG2's ER
membrane protein identity.
action: ACCEPT
reason: >-
This broad ER annotation is correct, although ER membrane is more specific.
supported_by:
- reference_id: UniProt:P53898
supporting_text: >-
SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Multi-pass membrane
protein.
- term:
id: GO:0016126
label: sterol biosynthetic process
evidence_type: IGI
original_reference_id: PMID:16270032
review:
summary: >-
Genetic evidence supports NSG2 participation in sterol biosynthesis through
regulation of Hmg2 stability, with partial redundancy and unequal
contribution relative to NSG1.
action: ACCEPT
reason: >-
The annotation is best interpreted as a regulatory contribution to sterol
biosynthesis, not as direct catalysis. The IGI evidence with NSG1/Hmg2
context is biologically coherent.
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.
- term:
id: GO:0051082
label: unfolded protein binding
evidence_type: IMP
original_reference_id: PMID:16270032
review:
summary: >-
The evidence supports a dedicated transmembrane chaperone function for an
SSD-containing client rather than broad unfolded protein binding.
action: MODIFY
reason: >-
Nsg2 and Nsg1 interact with the Hmg2 sterol-sensing-domain-containing
transmembrane region and promote Hmg2 stability/folding. GO:0044183
protein folding chaperone better captures this client-specific chaperone
role than generic GO:0051082 unfolded protein binding.
proposed_replacement_terms:
- id: GO:0044183
label: protein folding chaperone
supported_by:
- reference_id: PMID:16270032
supporting_text: >-
Yeast Nsgs inhibit degradation of Hmg2p in a highly specific manner, by
directly interacting with the sterol-sensing domain (SSD)-containing
transmembrane region.
- reference_id: PMID:16270032
supporting_text: >-
One way to unify the known, disparate actions of INSIGs is to view them
as known adaptations of a chaperone dedicated to SSD-containing client
proteins.
references:
- id: GO_REF:0000033
title: Annotation inferences using phylogenetic trees
findings:
- statement: >-
NSG2 IBA annotations are family-transfer annotations from the INSIG/PANTHER
context and should be interpreted as conserved regulatory/chaperone
functions, not sterol-enzyme activity.
supporting_text: >-
GOA WITH/FROM includes PANTHER:PTN000393022 for ER and sterol biosynthetic
process annotations.
- 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: UniProt:P53898
title: UniProtKB entry for NSG2_YEAST
findings:
- statement: >-
Nsg2 is an ER membrane INSIG-family protein.
supporting_text: >-
FUNCTION: Stabilizes the HMG-CoA reductase HMG2 by preventing its
HRD1-dependent degradation. SUBCELLULAR LOCATION: Endoplasmic reticulum
membrane; Multi-pass membrane protein. SIMILARITY: Belongs to the INSIG
family.
- id: PMID:14562095
title: Global analysis of protein localization in budding yeast
findings: []
- id: PMID:16270032
title: "INSIG: a broadly conserved transmembrane chaperone for sterol-sensing domain proteins"
findings:
- statement: >-
Nsg1 and Nsg2 are yeast INSIG homologs that regulate Hmg2 stability through
direct interaction with its SSD-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.
- statement: >-
The NSG proteins are best interpreted as dedicated chaperones for
SSD-containing client proteins.
supporting_text: >-
One way to unify the known, disparate actions of INSIGs is to view them as
known adaptations of a chaperone dedicated to SSD-containing client
proteins.
- id: PMID:26928762
title: >-
One library to make them all: streamlining the creation of yeast libraries
via a SWAp-Tag strategy
findings: []
- id: PMID:41132095
title: Role of Pex31 in metabolic adaptation of the nucleus-vacuole junction
findings:
- statement: >-
Nsg2 is a conditional nucleus-vacuole junction resident during low-glucose
conditions.
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/NSG2/NSG2-deep-research-falcon.md
title: Falcon deep research report for NSG2
findings:
- statement: >-
Falcon supports the interpretation of NSG2 as an INSIG-family ER membrane
chaperone/regulator of Hmg2 stability with conditional NVJ localization.
supporting_text: >-
NSG2/YNL156C is best annotated as an ER and starvation-induced NVJ
multi-pass INSIG-family membrane protein that participates in sterol
homeostasis primarily by modulating the stability and/or activity of
HMG-CoA reductase isozymes.
core_functions:
- description: >-
Nsg2 is a dedicated INSIG-family ER membrane chaperone/regulator for
sterol-sensing-domain-containing HMG-CoA reductase clients, especially Hmg2.
It contributes to sterol biosynthesis/homeostasis by stabilizing Hmg2 and
limiting HRD-dependent ERAD, with functional overlap but not complete
equivalence to Nsg1.
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
supported_by:
- reference_id: PMID:16270032
supporting_text: >-
Yeast Nsgs inhibit degradation of Hmg2p in a highly specific manner, by
directly interacting with the sterol-sensing domain (SSD)-containing
transmembrane region.
- reference_id: file:yeast/NSG2/NSG2-deep-research-falcon.md
supporting_text: >-
At native levels, Nsg1 is the principal stabilizer for Hmg2; however, Nsg2
can stabilize Hmg2 when overexpressed and contributes to residual
stabilization when Nsg1 is absent.
proposed_new_terms:
- proposed_name: INSIG-family sterol-sensing-domain client ERAD protection activity
proposed_definition: >-
Binding to sterol-sensing-domain-containing membrane protein clients in the
endoplasmic reticulum to stabilize them by limiting ubiquitination and
ER-associated degradation.
justification: >-
Nsg2 and related Nsg/INSIG-family proteins interact directly with the
sterol-sensing-domain-containing transmembrane region of Hmg2 and inhibit
its HRD-dependent degradation. GO:0044183 protein folding chaperone is the
closest available GO term in this review, but it overemphasizes de novo
folding relative to SSD-client stabilization and ERAD protection.
suggested_questions:
- question: >-
What client specificity distinguishes Nsg2 from Nsg1 at native expression
levels, and which HMG-CoA reductase isozyme or sterol-sensing-domain clients
require Nsg2 specifically?
- question: >-
Does low-glucose recruitment of Nsg2 to the nucleus-vacuole junction alter
Hmg1/Hmg2 stability, enzyme activity, or sterol flux directly?
- question: >-
How reliable are the PANTHER/IBA NSG2 annotations across INSIG subfamilies,
given the unequal contributions of Nsg1 and Nsg2 in yeast?
suggested_experiments:
- description: >-
Compare Hmg1 and Hmg2 stability, ERAD flux, and sterol intermediate levels
in nsg1, nsg2, and nsg1 nsg2 mutants under sterol-replete, sterol-depleted,
and low-glucose conditions.
experiment_type: genetics
- description: >-
Map Nsg2 client contacts by crosslinking or co-immunoprecipitation of
functional tagged Nsg2 with Hmg1/Hmg2 SSD-domain mutants.
experiment_type: biochemistry
- description: >-
Quantify Nsg2 recruitment to the NVJ during glucose starvation and test
whether disrupting NVJ tethering changes sterol ester, squalene, or HMG-CoA
reductase activity phenotypes.
experiment_type: microscopy