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. |
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Download this section (compressed HTML)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
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