id: MODULE:srebp_signaling title: SREBP signaling — sterol-regulated ER-to-Golgi escort and two-step cleavage of SREBP; SCAP/INSIG/MBTPS1/MBTPS2/SREBF1/SREBF2 description: >- The SREBP pathway couples membrane sterol levels to transcription of the genes for cholesterol and fatty-acid synthesis and LDL uptake. SREBP precursors (SREBF1 and SREBF2) are ER-membrane proteins whose N-terminal basic helix-loop-helix leucine-zipper transcription factor domain is held on the cytosolic face of the ER. Their C-terminal regulatory domain binds the WD-repeat domain of SCAP, which carries a sterol-sensing domain and escorts SREBP from the ER to the Golgi in COPII vesicles. When ER cholesterol is high, cholesterol binding to SCAP and oxysterol (25-hydroxycholesterol) binding to the ER-resident INSIG proteins (INSIG1, INSIG2) lock SCAP-SREBP into an INSIG-bound complex that the COPII coat cannot recognise, so SREBP stays in the ER. When sterols fall, SCAP is released, SCAP-SREBP is packaged into COPII vesicles and moves to the Golgi. There SREBP is cleaved sequentially: site-1 protease (MBTPS1, a subtilisin-like serine protease) cuts in the luminal loop, and site-2 protease (MBTPS2, an intramembrane zinc metalloprotease) then cuts within the first transmembrane segment. The released N-terminal fragment enters the nucleus and activates genes bearing sterol regulatory elements, including HMGCR, LDLR and INSIG1 itself, closing a negative feedback loop. In mammals SREBF2 preferentially drives cholesterol synthesis, the SREBF1c transcript fatty-acid synthesis, and SREBF1a both. status: DRAFT evidence: - source_id: PW:0000753 title: sterol regulatory element-binding protein signaling pathway statement: >- The module boundary and its seven gene products follow the RGD interactive diagram for this Pathway Ontology term. The diagram draws SCAP associated with Srebf1/2, Insig2 associated with Scap and with 25-hydroxycholesterol, cholesterol associated with Scap, Mbtps1 and Mbtps2 acting on "Srebf at Golgi", and "translocated Srebf" acting on SREBF targets and the cholesterol and fatty-acid biosynthetic pathways. Derived through projects/RGD_PATHWAY_ONTOLOGY (pathways/PW_0000753.yaml). - source_id: GO:0032933 title: SREBP signaling pathway statement: >- The module is grounded in GO SREBP signaling pathway, which is also the lexical exact match for PW:0000753 in the project's PW->GO candidate mappings. - source_id: GO:0032937 title: SREBP-SCAP-Insig complex statement: >- The sterol-retained state is the INSIG-bound SCAP-SREBP complex. - source_id: GO:0031293 title: membrane protein intracellular domain proteolysis statement: >- Release of the SREBP transcription factor domain by site-1 then site-2 cleavage is regulated intramembrane proteolysis. - source_id: gomodel:6205c24300000880 title: Regulation of cholesterol biosynthesis by SREBP (SREBF1 and SREBF2) and retention by the SREBP-SCAP complex (Human) statement: >- An independent, curator-built human GO-CAM (cached under gocams/6205c24300000880/) models six of this module's seven gene products with matching molecular functions: INSIG1 and INSIG2 oxysterol binding and protein sequestering activity in SREBP-SCAP complex retention in ER (GO:0036316), MBTPS1 serine-type endopeptidase activity causally upstream of MBTPS2 metalloendopeptidase activity, and SREBF1 and SREBF2 as RNA polymerase II-specific DNA-binding transcription factors (the model uses the parent GO:0000981 where this module uses the activator child GO:0001228). The model has no SCAP activity, and adds KPNB1 (nuclear import signal receptor activity) for SREBP nuclear import. - source_id: PMID:12202038 title: "Crucial step in cholesterol homeostasis: sterols promote binding of SCAP to INSIG-1, a membrane protein that facilitates retention of SREBPs in ER." statement: >- Establishes the core switch: SCAP escorts SREBP to the Golgi when sterols are low, and sterols make SCAP bind INSIG-1, which keeps it in the ER. supporting_text: In sterol-depleted cells, SCAP escorts SREBPs from ER to Golgi for proteolytic processing, thereby allowing SREBPs to stimulate cholesterol synthesis. - source_id: PMID:17428920 title: "Sterol-regulated transport of SREBPs from endoplasmic reticulum to Golgi: oxysterols block transport by binding to Insig." statement: >- Two sterol receptors converge on the same block: cholesterol acts through SCAP and oxysterols through INSIG. supporting_text: Cholesterol acts by binding to the SREBP-escort protein Scap, thereby causing Scap to bind to anchor proteins called Insigs. Here, we show that oxysterols act by binding to Insigs, causing Insigs to bind to Scap. - source_id: PMID:8674110 title: Sterol-regulated release of SREBP-2 from cell membranes requires two sequential cleavages, one within a transmembrane segment. statement: >- Defines the order and topology of the two proteolytic steps (parts 3 and 4) and the nuclear output (part 5). supporting_text: The first, regulated by sterols, occurs in the lumenal loop. The second, not regulated by sterols, occurs within the first transmembrane domain. The liberated NH2-segment enters the nucleus and activates genes controlling cholesterol synthesis and uptake. - source_id: PMID:16399501 title: Sterol-regulated ubiquitination and degradation of Insig-1 creates a convergent mechanism for feedback control of cholesterol synthesis and uptake. statement: >- Supports the whole sequence and the feedback edge from nuclear SREBP back to INSIG1. supporting_text: Scap/SREBP moves to the Golgi, where SREBP is processed to liberate a nuclear fragment that activates genes for cholesterol synthesis and uptake and the gene for Insig-1. notes: >- Derived from the RGD Pathway Ontology diagram PW:0000753 as the pilot for the RGD_PATHWAY_ONTOLOGY project's diagram->module route. The RGD diagram supplied the boundary, members and topology; GO terms were then chosen against each protein's experimental GO annotations in QuickGO and, for SCAP, against its gene review (sterol sensor activity GO:0032935 for sensing, with cholesterol as the sensed sterol, PMID:17428920, and cargo receptor activity GO:0038024 for the COPII escort; INSIG1 and INSIG2 oxysterol binding GO:0008142 IDA and protein sequestering activity GO:0140311 IDA; MBTPS1 serine-type endopeptidase activity GO:0004252 IDA/EXP; MBTPS2 metalloendopeptidase activity GO:0004222 EXP; SREBF1 DNA-binding transcription activator activity, RNA polymerase II-specific GO:0001228 IDA). Each step is backed by the primary papers that defined it (all abstract-only in the publications cache) and cross-checked against the human GO-CAM gomodel:6205c24300000880, which models six of the seven gene products with matching functions (each module annoton links the model activity it corresponds to under gocam_associations); the INSIG process term (GO:0036316) was taken from that model. The model also contains AKT1 (GO:0004674) and PCK1 (GO:0106264): AKT1 activates PCK1, which phosphorylates INSIG1/2 and lowers their sterol binding (PMID:32322062, one study in hepatocellular carcinoma cells). They are left out of this module because their reported effect opposes retention rather than carrying it out, and because the evidence is a single cancer-cell report. That GO-CAM omits SCAP, which is therefore the one step here without curated GO-CAM support, and includes KPNB1 for SREBP nuclear import, a step this module leaves out until it is reviewed. Gene reviews of all seven members are in a separate PR, and PANTHER family ids are deliberately omitted (gene products are named directly) until their families are checked against the PANTHER member index. Downstream SREBP target genes (HMGCR, LDLR, FASN and others) are outputs, not parts, of this module; the cholesterol and fatty-acid biosynthesis modules are its downstream neighbours. module: id: srebp_signaling label: SREBP signaling pathway module_type: SIGNALING_PATHWAY concepts: - preferred_term: SREBP signaling pathway term: id: GO:0032933 label: SREBP signaling pathway description: >- Sterol-regulated ER-to-Golgi transport and two-step proteolytic activation of SREBP transcription factors. context: cellular_components: - preferred_term: endoplasmic reticulum membrane term: id: GO:0005789 label: endoplasmic reticulum membrane - preferred_term: Golgi membrane term: id: GO:0000139 label: Golgi membrane - preferred_term: nucleus term: id: GO:0005634 label: nucleus parts: - order: 1 role: sterol sensing and ER retention node: id: srebp_sterol_retention label: Sterol-dependent retention of SCAP-SREBP by INSIG in the ER module_type: REGULATORY_STEP description: >- Cholesterol bound to SCAP's sterol-sensing domain and 25-hydroxycholesterol bound to INSIG promote formation of the INSIG-SCAP-SREBP complex, which hides SCAP's COPII sorting signal and holds SREBP in the ER. annotons: - id: scap_sterol_sensing label: SCAP sterol sensing participant: selector_type: GENE_PRODUCT gene_product: preferred_term: SCAP term: id: UniProtKB:Q12770 label: Sterol regulatory element-binding protein cleavage-activating protein function: preferred_term: sterol sensor activity term: id: GO:0032935 label: sterol sensor activity locations: - preferred_term: endoplasmic reticulum membrane term: id: GO:0005789 label: endoplasmic reticulum membrane role_description: >- Senses ER cholesterol; cholesterol binding switches SCAP into the INSIG-binding conformation. evidence: - source_id: PMID:8898195 statement: >- SCAP was cloned as the sterol-regulated activator of SREBP cleavage; part of its membrane domain resembles a sterol-sensing domain. supporting_text: SCAP has multiple membrane-spanning regions, five of which resemble the sterol-sensing domain of HMG CoA reductase - source_id: PMID:17428920 statement: Cholesterol binds SCAP directly. supporting_text: Cholesterol acts by binding to the SREBP-escort protein Scap - source_id: PMID:12963821 statement: Cholesterol changes SCAP conformation, and INSIG lowers the cholesterol needed. supporting_text: Insig proteins reduce the concentration of cholesterol needed in vitro to produce the conformational change in SCAP. - id: insig1_oxysterol_binding label: INSIG1 oxysterol binding participant: selector_type: GENE_PRODUCT gene_product: preferred_term: INSIG1 term: id: UniProtKB:O15503 label: Insulin-induced gene 1 protein function: preferred_term: oxysterol binding term: id: GO:0008142 label: oxysterol binding processes: - preferred_term: SREBP-SCAP complex retention in endoplasmic reticulum term: id: GO:0036316 label: SREBP-SCAP complex retention in endoplasmic reticulum locations: - preferred_term: endoplasmic reticulum membrane term: id: GO:0005789 label: endoplasmic reticulum membrane role_description: >- Binding of 25-hydroxycholesterol switches INSIG1 into its SCAP-binding state. evidence: - source_id: PMID:17428920 statement: INSIGs are oxysterol-binding proteins. supporting_text: These studies define Insigs as oxysterol-binding proteins gocam_associations: - model: gomodel:6205c24300000880 activity: gomodel:6205c24300000880/6205c24300000901 title: INSIG1 oxysterol binding (GO:0008142), part of GO:0036316 - id: insig1_retention label: INSIG1 retention of SCAP-SREBP in the ER participant: selector_type: GENE_PRODUCT gene_product: preferred_term: INSIG1 term: id: UniProtKB:O15503 label: Insulin-induced gene 1 protein function: preferred_term: protein sequestering activity term: id: GO:0140311 label: protein sequestering activity targets: - preferred_term: SREBP-SCAP complex term: id: GO:0032936 label: SREBP-SCAP complex processes: - preferred_term: SREBP-SCAP complex retention in endoplasmic reticulum term: id: GO:0036316 label: SREBP-SCAP complex retention in endoplasmic reticulum locations: - preferred_term: endoplasmic reticulum membrane term: id: GO:0005789 label: endoplasmic reticulum membrane role_description: >- ER-resident brake; sterol-bound INSIG1 binds SCAP and keeps SCAP-SREBP in the ER. INSIG1 is itself an SREBP target gene (see the feedback connection). evidence: - source_id: PMID:12202038 statement: INSIG-1 binds SCAP's sterol-sensing domain and holds SCAP-SREBP in the ER. supporting_text: we identify INSIG-1 as an ER protein that binds the sterol-sensing domain of SREBP cleavage-activating protein (SCAP) and facilitates retention of the SCAP/SREBP complex in the ER. - source_id: PMID:12202038 statement: A SCAP mutant that cannot bind INSIG-1 escapes sterol-mediated ER retention. supporting_text: Mutant SCAP(Y298C) fails to bind INSIG-1 and is resistant to sterol-mediated inhibition of ER exit. gocam_associations: - model: gomodel:6205c24300000880 activity: gomodel:6205c24300000880/6205c24300000917 title: INSIG1 protein sequestering activity (GO:0140311), part of GO:0036316 - id: insig2_oxysterol_binding label: INSIG2 oxysterol binding participant: selector_type: GENE_PRODUCT gene_product: preferred_term: INSIG2 term: id: UniProtKB:Q9Y5U4 label: Insulin-induced gene 2 protein function: preferred_term: oxysterol binding term: id: GO:0008142 label: oxysterol binding processes: - preferred_term: SREBP-SCAP complex retention in endoplasmic reticulum term: id: GO:0036316 label: SREBP-SCAP complex retention in endoplasmic reticulum locations: - preferred_term: endoplasmic reticulum membrane term: id: GO:0005789 label: endoplasmic reticulum membrane role_description: >- Binding of 25-hydroxycholesterol switches INSIG2 into its SCAP-binding state (the Insig2-25-hydroxycholesterol edge on the RGD diagram). evidence: - source_id: PMID:17428920 statement: INSIGs are oxysterol-binding proteins. supporting_text: These studies define Insigs as oxysterol-binding proteins gocam_associations: - model: gomodel:6205c24300000880 activity: gomodel:6205c24300000880/6205c24300000909 title: INSIG2 oxysterol binding (GO:0008142), part of GO:0036316 - id: insig2_retention label: INSIG2 retention of SCAP-SREBP in the ER participant: selector_type: GENE_PRODUCT gene_product: preferred_term: INSIG2 term: id: UniProtKB:Q9Y5U4 label: Insulin-induced gene 2 protein function: preferred_term: protein sequestering activity term: id: GO:0140311 label: protein sequestering activity targets: - preferred_term: SREBP-SCAP complex term: id: GO:0032936 label: SREBP-SCAP complex processes: - preferred_term: SREBP-SCAP complex retention in endoplasmic reticulum term: id: GO:0036316 label: SREBP-SCAP complex retention in endoplasmic reticulum locations: - preferred_term: endoplasmic reticulum membrane term: id: GO:0005789 label: endoplasmic reticulum membrane role_description: >- Paralog of INSIG1 with the same retention activity; unlike INSIG1, its expression does not depend on nuclear SREBP, so it is not on the feedback loop. evidence: - source_id: PMID:12242332 statement: INSIG-2 binds SCAP in a sterol-regulated way and blocks SREBP export. supporting_text: insig-2, a second protein of the endoplasmic reticulum that blocks the processing of sterol regulatory element-binding proteins (SREBPs) by binding to SCAP (SREBP cleavage-activating protein) in a sterol-regulated fashion - source_id: PMID:12242332 statement: Only INSIG-1 production needs nuclear SREBP, so only INSIG1 sits on the feedback loop. supporting_text: production of insig-1, but not insig-2, in cultured mammalian cells requires nuclear SREBPs gocam_associations: - model: gomodel:6205c24300000880 activity: gomodel:6205c24300000880/6205c24300000929 title: INSIG2 protein sequestering activity (GO:0140311), part of GO:0036316 - order: 2 role: ER-to-Golgi escort node: id: srebp_er_golgi_transport label: SCAP-escorted COPII transport of SREBP from ER to Golgi module_type: TRANSPORT_STEP description: >- With sterols low, SCAP is released from INSIG; its sorting signal is recognised by the COPII coat and SCAP-SREBP is carried to the Golgi. annotons: - id: scap_escort label: SCAP escort of SREBP to the Golgi participant: selector_type: GENE_PRODUCT gene_product: preferred_term: SCAP term: id: UniProtKB:Q12770 label: Sterol regulatory element-binding protein cleavage-activating protein function: preferred_term: cargo receptor activity term: id: GO:0038024 label: cargo receptor activity processes: - preferred_term: endoplasmic reticulum to Golgi vesicle-mediated transport term: id: GO:0006888 label: endoplasmic reticulum to Golgi vesicle-mediated transport role_description: >- Binds the SREBP C-terminal regulatory domain through its WD-repeat domain and carries it into COPII vesicles. evidence: - source_id: PMID:15899885 statement: SCAP is loaded into COPII vesicles through Sar1 and Sec23/Sec24. supporting_text: Sorting of SREBPs into COPII vesicles is mediated by Sar1 and the coat proteins Sec23 and Sec24. - source_id: PMID:15899885 statement: The COPII-binding signal is a hexapeptide in a cytoplasmic loop of SCAP. supporting_text: Sec23/24 binding to Scap requires the hexapeptide sequence MELADL located in a cytoplasmic loop of Scap. - source_id: PMID:17428919 statement: Sterol-bound SCAP and INSIG hide that signal from COPII, linking parts 1 and 2. supporting_text: Cholesterol and oxysterols block COPII binding to MELADL by binding to different intracellular receptors, cholesterol to Scap and oxysterols to Insig. - order: 3 role: site-1 cleavage node: id: srebp_site1_cleavage label: Site-1 protease (MBTPS1) cleavage of SREBP in the Golgi lumen module_type: REACTION description: >- MBTPS1, a subtilisin-like serine protease, cuts SREBP in its luminal loop, separating the two transmembrane halves. annotons: - id: mbtps1_s1p label: MBTPS1 site-1 protease activity participant: selector_type: GENE_PRODUCT gene_product: preferred_term: MBTPS1 term: id: UniProtKB:Q14703 label: Membrane-bound transcription factor site-1 protease function: preferred_term: serine-type endopeptidase activity term: id: GO:0004252 label: serine-type endopeptidase activity targets: - preferred_term: SREBP-1 precursor (luminal loop) term: id: UniProtKB:P36956 label: Sterol regulatory element-binding protein 1 - preferred_term: SREBP-2 precursor (luminal loop) term: id: UniProtKB:Q12772 label: Sterol regulatory element-binding protein 2 locations: - preferred_term: Golgi membrane term: id: GO:0000139 label: Golgi membrane role_description: First, luminal cleavage of SREBP. evidence: - source_id: PMID:9809072 statement: S1P makes the first, luminal cut. supporting_text: Release is initiated by Site-1 protease (S1P), which cleaves SREBPs in the ER luminal loop between two membrane-spanning regions. - source_id: PMID:9809072 statement: S1P is a membrane-bound subtilisin-like (serine) protease. supporting_text: The cDNA encodes S1P, an intraluminal 1052-amino-acid membrane-bound subtilisin-like protease. - source_id: gomodel:6205c24300000880 statement: >- The human GO-CAM places MBTPS1 serine-type endopeptidase activity directly upstream of MBTPS2 metalloendopeptidase activity. gocam_associations: - model: gomodel:6205c24300000880 activity: gomodel:6205c24300000880/6205c24300001010 title: MBTPS1 serine-type endopeptidase activity (SREBP branch) - model: gomodel:6205c24300000880 activity: gomodel:6205c24300000880/6205c24300001290 title: MBTPS1 serine-type endopeptidase activity (second SREBP branch) - order: 4 role: site-2 cleavage node: id: srebp_site2_cleavage label: Site-2 protease (MBTPS2) intramembrane cleavage of SREBP module_type: REACTION description: >- MBTPS2, an intramembrane zinc metalloprotease, cuts the N-terminal half within its transmembrane segment and releases the transcription factor domain to the cytosol. annotons: - id: mbtps2_s2p label: MBTPS2 site-2 protease activity participant: selector_type: GENE_PRODUCT gene_product: preferred_term: MBTPS2 term: id: UniProtKB:O43462 label: Membrane-bound transcription factor site-2 protease function: preferred_term: metalloendopeptidase activity term: id: GO:0004222 label: metalloendopeptidase activity targets: - preferred_term: SREBP-1 intermediate (first transmembrane segment) term: id: UniProtKB:P36956 label: Sterol regulatory element-binding protein 1 - preferred_term: SREBP-2 intermediate (first transmembrane segment) term: id: UniProtKB:Q12772 label: Sterol regulatory element-binding protein 2 processes: - preferred_term: membrane protein intracellular domain proteolysis term: id: GO:0031293 label: membrane protein intracellular domain proteolysis locations: - preferred_term: Golgi membrane term: id: GO:0000139 label: Golgi membrane role_description: Second, intramembrane cleavage that frees the SREBP N-terminus. evidence: - source_id: PMID:9659902 statement: S2P is a zinc metalloprotease needed for the intramembrane Site-2 cut. supporting_text: S2P defines a new family of polytopic membrane proteins that contain an HEXXH sequence characteristic of zinc metalloproteases. Mutation of the putative zinc-binding residues abolishes S2P activity. - source_id: PMID:8674110 statement: Cells unable to make the second cut keep the SREBP N-terminus on membranes. supporting_text: Certain mutant Chinese hamster ovary cells are auxotrophic for cholesterol because they fail to carry out the second cleavage gocam_associations: - model: gomodel:6205c24300000880 activity: gomodel:6205c24300000880/6205c24300001020 title: MBTPS2 metalloendopeptidase activity (SREBP branch) - model: gomodel:6205c24300000880 activity: gomodel:6205c24300000880/6205c24300001240 title: MBTPS2 metalloendopeptidase activity (second SREBP branch) - order: 5 role: nuclear transcriptional output node: id: srebp_transcription label: Nuclear SREBP activation of sterol-regulatory-element genes module_type: REGULATORY_STEP description: >- The released N-terminal bHLH-LZ fragment of SREBF1 or SREBF2 enters the nucleus and activates SRE-containing genes of cholesterol and fatty-acid synthesis and LDL uptake, including INSIG1. annotons: - id: srebf1_tf label: SREBF1 transcription activation participant: selector_type: GENE_PRODUCT gene_product: preferred_term: SREBF1 term: id: UniProtKB:P36956 label: Sterol regulatory element-binding protein 1 function: preferred_term: DNA-binding transcription activator activity, RNA polymerase II-specific term: id: GO:0001228 label: DNA-binding transcription activator activity, RNA polymerase II-specific locations: - preferred_term: nucleus term: id: GO:0005634 label: nucleus role_description: >- Mainly fatty-acid synthesis genes (SREBF1c transcript) and both arms (SREBF1a). evidence: - source_id: PMID:8402897 statement: SREBP-1 is a bHLH-ZIP protein that binds the sterol regulatory element and activates transcription. supporting_text: We report the cDNA cloning of human SREBP-1, a protein that binds SRE-1, activates transcription gocam_associations: - model: gomodel:6205c24300000880 activity: gomodel:6205c24300000880/6205c24300000989 title: SREBF1 DNA-binding transcription factor activity (GO:0000981), part of GO:0045542 - id: srebf2_tf label: SREBF2 transcription activation participant: selector_type: GENE_PRODUCT gene_product: preferred_term: SREBF2 term: id: UniProtKB:Q12772 label: Sterol regulatory element-binding protein 2 function: preferred_term: DNA-binding transcription activator activity, RNA polymerase II-specific term: id: GO:0001228 label: DNA-binding transcription activator activity, RNA polymerase II-specific locations: - preferred_term: nucleus term: id: GO:0005634 label: nucleus role_description: Mainly cholesterol synthesis and uptake genes. evidence: - source_id: PMID:7903453 statement: SREBP-2 binds the same element as SREBP-1a and activates SRE-1 reporters. supporting_text: In vivo it mimicked SREBP-1a in activating transcription of reporter genes containing SRE-1. gocam_associations: - model: gomodel:6205c24300000880 activity: gomodel:6205c24300000880/6205c24300000999 title: SREBF2 DNA-binding transcription factor activity (GO:0000981), part of GO:0045542 connections: - source: srebp_sterol_retention target: srebp_er_golgi_transport connection_type: NEGATIVELY_REGULATES description: >- Sterol-bound SCAP and INSIG hold SCAP-SREBP in the ER and block COPII loading. - source: srebp_er_golgi_transport target: srebp_site1_cleavage connection_type: PROVIDES_INPUT_FOR description: Delivers SREBP to the Golgi, where MBTPS1 resides. - source: srebp_site1_cleavage target: srebp_site2_cleavage connection_type: PROVIDES_INPUT_FOR description: Site-1 cleavage produces the intermediate that MBTPS2 can cut. - source: srebp_site2_cleavage target: srebp_transcription connection_type: PROVIDES_INPUT_FOR description: Releases the nuclear N-terminal SREBP fragment. - source: srebp_transcription target: srebp_sterol_retention connection_type: POSITIVELY_REGULATES description: >- SREBP induces INSIG1 transcription, closing the negative feedback loop (PMID:16399501, PMID:12242332). INSIG2 is not on this loop.