{"gene":"PREB","run_date":"2026-06-10T06:43:35","timeline":{"discoveries":[{"year":1993,"finding":"Yeast Sec12 (PREB ortholog) encodes a guanine-nucleotide exchange factor (GEF) for the Sar1 GTPase: a purified cytoplasmic fragment of Sec12 promotes guanine-nucleotide dissociation from Sar1, while the temperature-sensitive mutant Sec12-1 retains only ~15% of wild-type GEF activity. GTP hydrolysis by Sar1 is not enhanced by Sec12 alone but is stimulated >50-fold by a mixture of Sec12 and the GAP Sec23, supporting a model in which Sec12 recruits Sar1-GTP to the ER membrane to initiate vesicle budding.","method":"In vitro GEF assay with purified recombinant proteins; mutant Sec12-1 activity comparison; GTPase assay with Sec12 + Sec23","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with purified proteins, mutagenesis (Sec12-1 mutant), replicated across multiple assays in a landmark study","pmids":["8377826"],"is_preprint":false},{"year":1992,"finding":"Fission yeast Sec12 homologue (Stl1p/Spo14) and a plant Arabidopsis homologue (Stl2p) are functional homologues of budding yeast Sec12: their cytoplasmic domains resemble Sec12p and expression of either gene complements a sec12 null mutation, demonstrating conservation of the ER-to-Golgi vesicle budding mechanism across eukaryotes.","method":"Complementation of sec12 null mutation by heterologous cDNA expression; sequence analysis","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — genetic complementation of null mutant, replicated with two independent orthologous sequences","pmids":["1396601"],"is_preprint":false},{"year":1999,"finding":"PREB (designated as such in mammals) was cloned from rat pituitary cDNA as a novel WD-motif protein that binds specifically to the prolactin (PRL) promoter element 1P in a sequence-specific manner (distinct from Pit-1 binding site). PREB localizes to the nucleus in GH3 pituitary cells and transactivates a PRL promoter construct in PREB-negative C6 cells; a GAL4-PREB fusion strongly transactivates a reporter and mediates further stimulation by protein kinase A (PKA), establishing PREB as a PKA-responsive transcriptional activator at the PRL promoter.","method":"Expression cloning from pituitary cDNA library; gel shift/DNA-binding assay; transient transfection reporter assay; Western blot and immunocytochemistry for nuclear localization; GAL4 fusion transactivation assay with and without PKA","journal":"Molecular endocrinology","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — multiple orthogonal methods (binding assay, reporter assay, localization, PKA stimulation) in a single focused study on this protein","pmids":["10194769"],"is_preprint":false},{"year":2012,"finding":"In Pichia pastoris, Sec12 is concentrated at transitional ER (tER) sites through a direct physical interaction with the C-terminal domain of Sec16. Overexpression of Sec12 alone delocalizes it to the general ER, but co-overexpression of Sec16 retains Sec12 at tER sites. Biochemical pull-down confirms that the C-terminal fragment of Sec16 binds the cytosolic domain of Sec12. Human Sec12 (PREB) is similarly concentrated at tER sites, likely via association with Sec16A.","method":"Fluorescence localization with overexpression titration; heterologous expression of P. pastoris Sec12 in S. cerevisiae with and without P. pastoris Sec16; biochemical binding assay (pull-down) between Sec16 C-terminal fragment and Sec12 cytosolic domain; human Sec12 localization by imaging","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal genetic and biochemical evidence, multiple experimental systems including human Sec12 ortholog, multiple orthogonal methods","pmids":["22347445"],"is_preprint":false},{"year":2012,"finding":"Crystal structure of the catalytically active cytoplasmic domain of S. cerevisiae Sec12 at 1.36 Å resolution reveals a β-propeller fold. A conserved 'K loop' extending from the N-terminal blade is catalytically essential for GEF activity toward Sar1; structure-guided mutagenesis of K-loop residues abolishes activity in vitro and in vivo. A bound potassium ion stabilizes the K loop and is required for optimal guanine nucleotide exchange activity.","method":"X-ray crystallography (1.36 Å); structure-guided site-directed mutagenesis; in vitro GEF assay; in vivo yeast functional assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure combined with mutagenesis and both in vitro and in vivo functional validation in a single study","pmids":["23109340"],"is_preprint":false},{"year":2004,"finding":"In Pichia pastoris, Sec12 localizes to tER sites and exchanges rapidly between tER sites and the general ER (dynamic localization). The tER concentration of Sec12 is saturable and requires the cytosolic domain of Sec12 to interact with a tER partner component. Oligomerization of the Sec12 lumenal domain is required for efficient cytosolic-domain-mediated tER localization. Redistribution of Sec12 to the general ER does not perturb downstream tER components, suggesting Sec12 associates with a pre-existing tER scaffold.","method":"Fluorescence live imaging and photobleaching (FRAP-type); domain truncation and mutagenesis; overexpression saturation experiments","journal":"Developmental cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct live imaging with functional domain analysis; single lab but multiple orthogonal approaches","pmids":["15130490"],"is_preprint":false},{"year":2014,"finding":"Human Sec12 (PREB) is concentrated at ER exit sites (ERES) through direct interaction with cTAGE5, a component of the TANGO1/cTAGE5 collagen cargo receptor complex at ERES. This concentration of Sec12 at ERES is specifically required for secretion of collagen VII but not other secretory proteins. Depletion of Sec12 from ERES (e.g., by cTAGE5 knockdown) selectively impairs collagen export, identifying a cargo-specific requirement for locally elevated Sar1-GTP generated by ERES-concentrated Sec12.","method":"Co-immunoprecipitation (direct interaction between Sec12 and cTAGE5); siRNA knockdown of cTAGE5/Sec12; collagen secretion assay; immunofluorescence localization of Sec12 at ERES","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP establishing direct interaction, KD with specific cargo secretion readout, localization experiments; multiple orthogonal methods in single focused study","pmids":["25202031"],"is_preprint":false},{"year":1999,"finding":"In yeast, the kinase-dead (inactive) form of the casein kinase I Hrr25p suppresses the sec12-4 temperature-sensitive mutation. The suppressor allele hrr25-2 (T176I in kinase domain) has markedly reduced kinase activity, and overexpression of another kinase-dead Hrr25 mutant (K38A) also suppresses sec12-4. This indicates that Hrr25p kinase activity negatively regulates Sec12-dependent vesicle budding from the ER.","method":"Genetic suppressor cloning; kinase assay of immunoprecipitated Hrr25p mutant; growth and secretion assays in sec12-4 background","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis with biochemical kinase activity confirmation; single lab with multiple alleles tested","pmids":["9920934"],"is_preprint":false},{"year":2003,"finding":"The S. pombe Sec12 homologue Spo14/Stl1 is essential for ER-to-Golgi protein transport and for forespore membrane assembly during meiosis. In spo14 mutant cells, ER-like membranes accumulate and the ER/Golgi marker Rer1 is retained in the ER. Overproduction of the S. pombe Sar1 homologue Psr1 suppresses both the sporulation defect and cold-sensitive growth of spo14 mutants, placing Spo14/Sec12 upstream of Sar1 in the ER exit pathway.","method":"Genetic complementation; electron microscopy of membrane morphology; Rer1 localization assay; Sar1 overproduction suppression","journal":"Molecular biology of the cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis (Sar1 overproduction suppression) with organelle morphology readout; single lab with multiple orthogonal approaches","pmids":["12631727"],"is_preprint":false},{"year":2018,"finding":"TANGO1, cTAGE5, and SEC12 (PREB) are co-packaged with procollagen I (PC1) into large COPII carriers during ER export. SEC12 is particularly enriched around ER membranes and large COPII carriers containing PC1, whereas it is excluded from small (<100 nm) COPII vesicles. A split-GFP reconstitution system targeting SEC12 to PC1 via the luminal domain of TANGO1 enriches SEC12 around PC1 and generates enlarged PC1 carriers, demonstrating that SEC12 co-packaging is causally required for generation of large COPII carriers for bulky cargo.","method":"Vesicle budding assay with cargo fractionation; split-GFP reconstitution system; fluorescence imaging of SEC12 distribution around COPII carriers; immunoblotting of carrier fractions","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstitution with split-GFP system plus biochemical fractionation; multiple orthogonal approaches in a single rigorous study","pmids":["30545919"],"is_preprint":false},{"year":2021,"finding":"Under stress conditions, SEC12 (PREB) mediates formation of a novel ERGIC-ERES membrane contact by directly interacting with TMED9. This contact (as close as 2–5 nm) allows ERES-located SEC12 to transactivate COPII assembly on the ERGIC membrane, promoting formation of ERGIC-derived COPII vesicles that serve as autophagosome membrane precursors. A portion of SEC12 also relocates to the ERGIC. This ERGIC-ERES contact is physically and functionally distinct from the TFG-mediated ERGIC-ERES adjunction involved in secretory transport.","method":"Co-immunoprecipitation (TMED9–SEC12 interaction); electron microscopy measuring inter-membrane distance; siRNA knockdown with autophagosome biogenesis readout; COPII assembly assay on ERGIC; fluorescence imaging of SEC12 relocalization","journal":"Cell research","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, EM structural measurement, functional KD with autophagy readout, multiple orthogonal methods; published in peer-reviewed journal","pmids":["34561617"],"is_preprint":false}],"current_model":"PREB/SEC12 is a type II ER transmembrane protein with a β-propeller cytoplasmic domain that functions as the guanine nucleotide exchange factor (GEF) for the small GTPase Sar1, initiating COPII coat assembly for ER-to-Golgi vesicle budding; it is concentrated at ER exit sites (ERES) through interactions with Sec16 and cTAGE5, where locally elevated Sar1-GTP activity is required for export of large cargo such as collagens, and under stress it additionally forms ERGIC-ERES contacts via TMED9 to supply membrane for autophagosome biogenesis; in the pituitary, the mammalian PREB protein also localizes to the nucleus and acts as a PKA-responsive transcriptional activator of the prolactin promoter."},"narrative":{"mechanistic_narrative":"PREB/SEC12 is the guanine nucleotide exchange factor (GEF) for the small GTPase Sar1 and serves as the initiating factor for COPII-dependent ER-to-Golgi vesicle budding, a role conserved from yeast and plants to mammals [PMID:8377826, PMID:1396601]. Its catalytically active cytoplasmic domain folds into a β-propeller whose conserved, potassium-stabilized 'K loop' is essential for nucleotide exchange on Sar1, while GTP hydrolysis is driven in concert with the GAP Sec23 to recruit Sar1-GTP and seed coat assembly [PMID:8377826, PMID:23109340]. SEC12 is dynamically concentrated at ER exit sites (transitional ER) through direct interaction with the C-terminal domain of Sec16, providing locally elevated Sar1-GTP generation at sites of cargo export [PMID:22347445, PMID:15130490]. At these sites SEC12 directly binds the collagen cargo-receptor component cTAGE5 and is co-packaged with TANGO1 into large COPII carriers, a localized enrichment specifically required for export of bulky cargo such as collagen, while being excluded from small COPII vesicles [PMID:25202031, PMID:30545919]. Under stress, SEC12 additionally engages TMED9 to form ERGIC–ERES membrane contacts that transactivate COPII assembly on the ERGIC to supply autophagosome precursor membrane [PMID:34561617]. Separately, in pituitary cells the mammalian protein localizes to the nucleus and acts as a sequence-specific, PKA-responsive transcriptional activator of the prolactin promoter [PMID:10194769].","teleology":[{"year":1992,"claim":"Established that the ER-to-Golgi budding factor encoded by Sec12 is functionally conserved across eukaryotes, indicating a universal vesicle-budding mechanism rather than a yeast-specific one.","evidence":"Heterologous complementation of a budding-yeast sec12 null by fission yeast and Arabidopsis homologues, with sequence analysis","pmids":["1396601"],"confidence":"High","gaps":["Did not define the biochemical activity of the conserved domain","No direct evidence linking the conserved cytoplasmic domain to a specific GTPase"]},{"year":1993,"claim":"Defined the molecular activity of Sec12 as a GEF for Sar1 and placed it upstream of coat assembly, answering how vesicle budding is initiated at the ER.","evidence":"In vitro GEF and GTPase assays with purified proteins, Sec12-1 mutant comparison, and Sec12+Sec23 reconstitution in yeast","pmids":["8377826"],"confidence":"High","gaps":["Structural basis of nucleotide exchange unresolved","Mechanism of spatial restriction to ER exit sites not addressed"]},{"year":1999,"claim":"Identified a distinct nuclear, transcriptional role for the mammalian protein, showing PREB acts as a PKA-responsive activator at the prolactin promoter — a function separate from its membrane GEF activity.","evidence":"Expression cloning from pituitary cDNA, DNA-binding gel shift, reporter transactivation, immunocytochemistry, and GAL4 fusion with PKA stimulation","pmids":["10194769"],"confidence":"High","gaps":["Relationship between the nuclear transcriptional role and the ER membrane GEF role unresolved","No structural basis for promoter element 1P recognition"]},{"year":1999,"claim":"Revealed negative regulation of Sec12-dependent budding by a casein kinase I, showing the pathway is subject to kinase control.","evidence":"Genetic suppressor cloning of sec12-4 by kinase-dead Hrr25 alleles with kinase-activity confirmation and secretion assays in yeast","pmids":["9920934"],"confidence":"Medium","gaps":["No direct substrate (Sec12 phosphorylation) demonstrated","Mechanism by which kinase activity inhibits budding undefined","Genetic epistasis only; not biochemically reconstituted"]},{"year":2003,"claim":"Placed Sec12 genetically upstream of Sar1 in the ER exit pathway and extended its role to a developmental membrane-assembly process (meiotic forespore formation).","evidence":"Genetic complementation, EM membrane morphology, Rer1 localization, and Sar1 overproduction suppression in S. pombe","pmids":["12631727"],"confidence":"Medium","gaps":["Direct GEF activity not assayed in this system","Single-organism genetic epistasis"]},{"year":2004,"claim":"Showed Sec12 is dynamically and saturably concentrated at transitional ER via its cytosolic domain binding a pre-existing scaffold, addressing how the GEF is spatially organized.","evidence":"Live fluorescence imaging/photobleaching, domain truncation, and overexpression saturation in P. pastoris","pmids":["15130490"],"confidence":"Medium","gaps":["The specific tER partner was not molecularly identified in this study","Single-lab imaging system"]},{"year":2012,"claim":"Identified Sec16 as the direct partner that concentrates Sec12 at tER sites, resolving the molecular basis of its spatial targeting in yeast and human cells.","evidence":"Overexpression-titration imaging, heterologous expression with/without Sec16, and pull-down between Sec16 C-terminus and Sec12 cytosolic domain","pmids":["22347445"],"confidence":"High","gaps":["Quantitative contribution of Sec16 vs other partners to human Sec12 localization not dissected","Effect of the interaction on GEF activity not measured"]},{"year":2012,"claim":"Provided the structural mechanism of nucleotide exchange, defining the β-propeller fold and a potassium-stabilized K loop as catalytically essential.","evidence":"1.36 Å crystal structure of the S. cerevisiae cytoplasmic domain with structure-guided mutagenesis and in vitro/in vivo GEF assays","pmids":["23109340"],"confidence":"High","gaps":["No structure of the Sec12–Sar1 complex","Structure of the mammalian/full-length protein not determined"]},{"year":2014,"claim":"Established a cargo-specific function by showing ERES-concentrated SEC12, via direct cTAGE5 binding, is selectively required for collagen export.","evidence":"Reciprocal Co-IP of SEC12 and cTAGE5, siRNA knockdown with cargo-specific collagen secretion readout, and ERES localization imaging in human cells","pmids":["25202031"],"confidence":"High","gaps":["Whether locally elevated Sar1-GTP alone explains cargo selectivity not directly measured","Generality across collagen subtypes and other bulky cargos not fully defined"]},{"year":2018,"claim":"Demonstrated that SEC12 co-packaging is causally required to generate the large COPII carriers needed for bulky procollagen cargo, linking its local enrichment to carrier size control.","evidence":"Vesicle budding/cargo fractionation, split-GFP reconstitution targeting SEC12 to procollagen via TANGO1, and imaging of SEC12 distribution around carriers","pmids":["30545919"],"confidence":"High","gaps":["Mechanism converting local Sar1-GTP into membrane curvature/size remains incompletely defined","Quantitative stoichiometry on large carriers not established"]},{"year":2021,"claim":"Uncovered a stress-induced role in which SEC12, via direct TMED9 binding, builds ERGIC-ERES contacts that transactivate COPII on the ERGIC to supply autophagosome membrane, extending its function beyond secretory transport.","evidence":"Reciprocal Co-IP, EM inter-membrane distance measurement, siRNA knockdown with autophagy readout, and COPII assembly assay on ERGIC in human cells","pmids":["34561617"],"confidence":"High","gaps":["Signal that triggers SEC12 relocalization to ERGIC unclear","How TMED9-mediated contact is regulated relative to secretory ERES function undefined"]},{"year":null,"claim":"It remains unresolved how the mammalian protein's nuclear transcriptional activity and its ER membrane GEF activity are coordinated, and whether one pool regulates the other.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No study connects the prolactin-promoter transcriptional role to the COPII/ERES GEF role","No mechanism for partitioning PREB between nucleus and ER membrane"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,4]},{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[2]},{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[2]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[3,5,6]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[2]}],"pathway":[{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[0,6,9]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[10]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[2]}],"complexes":["COPII coat","TANGO1/cTAGE5 collagen export complex"],"partners":["SAR1","SEC16","CTAGE5","TANGO1","TMED9","SEC23"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9HCU5","full_name":"Guanine nucleotide-exchange factor SEC12","aliases":["Prolactin regulatory element-binding protein"],"length_aa":417,"mass_kda":45.5,"function":"Guanine nucleotide exchange factor (GEF) that regulates the assembly of the coat protein complex II/COPII in endoplasmic reticulum (ER) to Golgi vesicle-mediated transport. Selectively activates SAR1A and SAR1B by promoting the exchange of guanosine diphosphate (GDP) for guanosine triphosphate (GTP) in these small GTPases (PubMed:32358066). In their activated GTP-bound state, SAR1A and SAR1B insert into the membrane of the endoplasmic reticulum where they recruit the remainder of the coat protein complex II/COPII which is responsible for both the sorting of proteins and the deformation and budding of membranes into vesicles destined to the Golgi (PubMed:32358066) Was first identified based on its probable role in the regulation of pituitary gene transcription. Binds to the prolactin gene (PRL) promoter and seems to activate transcription","subcellular_location":"Endoplasmic reticulum membrane; Nucleus","url":"https://www.uniprot.org/uniprotkb/Q9HCU5/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/PREB","classification":"Common Essential","n_dependent_lines":1169,"n_total_lines":1208,"dependency_fraction":0.9677152317880795},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"CDC7","stoichiometry":10.0},{"gene":"SEC61B","stoichiometry":4.0},{"gene":"CANX","stoichiometry":0.2},{"gene":"TMED10","stoichiometry":0.2},{"gene":"CCDC47","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/PREB","total_profiled":1310},"omim":[{"mim_id":"612195","title":"ABHYDROLASE DOMAIN-CONTAINING PROTEIN 1; ABHD1","url":"https://www.omim.org/entry/612195"},{"mim_id":"606395","title":"PROLACTIN REGULATORY ELEMENT-BINDING PROTEIN; PREB","url":"https://www.omim.org/entry/606395"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Endoplasmic reticulum","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/PREB"},"hgnc":{"alias_symbol":["SEC12"],"prev_symbol":[]},"alphafold":{"accession":"Q9HCU5","domains":[{"cath_id":"2.130.10.10","chopping":"9-100_140-387","consensus_level":"medium","plddt":94.2233,"start":9,"end":387},{"cath_id":"1.20.5","chopping":"391-417","consensus_level":"medium","plddt":88.673,"start":391,"end":417}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9HCU5","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9HCU5-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9HCU5-F1-predicted_aligned_error_v6.png","plddt_mean":88.12},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PREB","jax_strain_url":"https://www.jax.org/strain/search?query=PREB"},"sequence":{"accession":"Q9HCU5","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9HCU5.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9HCU5/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9HCU5"}},"corpus_meta":[{"pmid":"3262202","id":"PMC_3262202","title":"Bcl-2 gene promotes haemopoietic cell survival and cooperates with c-myc to immortalize pre-B cells.","date":"1988","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/3262202","citation_count":2890,"is_preprint":false},{"pmid":"6438633","id":"PMC_6438633","title":"Enhancer-dependent expression of human kappa immunoglobulin genes introduced into mouse pre-B lymphocytes by electroporation.","date":"1984","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/6438633","citation_count":888,"is_preprint":false},{"pmid":"8377826","id":"PMC_8377826","title":"SEC12 encodes a guanine-nucleotide-exchange factor essential for transport vesicle budding from the ER.","date":"1993","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/8377826","citation_count":397,"is_preprint":false},{"pmid":"6984001","id":"PMC_6984001","title":"Surface antigen expression and immunoglobulin gene rearrangement during mouse pre-B cell development.","date":"1982","source":"Immunological reviews","url":"https://pubmed.ncbi.nlm.nih.gov/6984001","citation_count":381,"is_preprint":false},{"pmid":"24378843","id":"PMC_24378843","title":"Orchestrating B cell lymphopoiesis through interplay of IL-7 receptor and pre-B cell receptor signalling.","date":"2013","source":"Nature reviews. 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GTP hydrolysis by Sar1 is not enhanced by Sec12 alone but is stimulated >50-fold by a mixture of Sec12 and the GAP Sec23, supporting a model in which Sec12 recruits Sar1-GTP to the ER membrane to initiate vesicle budding.\",\n      \"method\": \"In vitro GEF assay with purified recombinant proteins; mutant Sec12-1 activity comparison; GTPase assay with Sec12 + Sec23\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with purified proteins, mutagenesis (Sec12-1 mutant), replicated across multiple assays in a landmark study\",\n      \"pmids\": [\"8377826\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"Fission yeast Sec12 homologue (Stl1p/Spo14) and a plant Arabidopsis homologue (Stl2p) are functional homologues of budding yeast Sec12: their cytoplasmic domains resemble Sec12p and expression of either gene complements a sec12 null mutation, demonstrating conservation of the ER-to-Golgi vesicle budding mechanism across eukaryotes.\",\n      \"method\": \"Complementation of sec12 null mutation by heterologous cDNA expression; sequence analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — genetic complementation of null mutant, replicated with two independent orthologous sequences\",\n      \"pmids\": [\"1396601\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"PREB (designated as such in mammals) was cloned from rat pituitary cDNA as a novel WD-motif protein that binds specifically to the prolactin (PRL) promoter element 1P in a sequence-specific manner (distinct from Pit-1 binding site). PREB localizes to the nucleus in GH3 pituitary cells and transactivates a PRL promoter construct in PREB-negative C6 cells; a GAL4-PREB fusion strongly transactivates a reporter and mediates further stimulation by protein kinase A (PKA), establishing PREB as a PKA-responsive transcriptional activator at the PRL promoter.\",\n      \"method\": \"Expression cloning from pituitary cDNA library; gel shift/DNA-binding assay; transient transfection reporter assay; Western blot and immunocytochemistry for nuclear localization; GAL4 fusion transactivation assay with and without PKA\",\n      \"journal\": \"Molecular endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — multiple orthogonal methods (binding assay, reporter assay, localization, PKA stimulation) in a single focused study on this protein\",\n      \"pmids\": [\"10194769\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"In Pichia pastoris, Sec12 is concentrated at transitional ER (tER) sites through a direct physical interaction with the C-terminal domain of Sec16. Overexpression of Sec12 alone delocalizes it to the general ER, but co-overexpression of Sec16 retains Sec12 at tER sites. Biochemical pull-down confirms that the C-terminal fragment of Sec16 binds the cytosolic domain of Sec12. Human Sec12 (PREB) is similarly concentrated at tER sites, likely via association with Sec16A.\",\n      \"method\": \"Fluorescence localization with overexpression titration; heterologous expression of P. pastoris Sec12 in S. cerevisiae with and without P. pastoris Sec16; biochemical binding assay (pull-down) between Sec16 C-terminal fragment and Sec12 cytosolic domain; human Sec12 localization by imaging\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal genetic and biochemical evidence, multiple experimental systems including human Sec12 ortholog, multiple orthogonal methods\",\n      \"pmids\": [\"22347445\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Crystal structure of the catalytically active cytoplasmic domain of S. cerevisiae Sec12 at 1.36 Å resolution reveals a β-propeller fold. A conserved 'K loop' extending from the N-terminal blade is catalytically essential for GEF activity toward Sar1; structure-guided mutagenesis of K-loop residues abolishes activity in vitro and in vivo. A bound potassium ion stabilizes the K loop and is required for optimal guanine nucleotide exchange activity.\",\n      \"method\": \"X-ray crystallography (1.36 Å); structure-guided site-directed mutagenesis; in vitro GEF assay; in vivo yeast functional assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure combined with mutagenesis and both in vitro and in vivo functional validation in a single study\",\n      \"pmids\": [\"23109340\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"In Pichia pastoris, Sec12 localizes to tER sites and exchanges rapidly between tER sites and the general ER (dynamic localization). The tER concentration of Sec12 is saturable and requires the cytosolic domain of Sec12 to interact with a tER partner component. Oligomerization of the Sec12 lumenal domain is required for efficient cytosolic-domain-mediated tER localization. Redistribution of Sec12 to the general ER does not perturb downstream tER components, suggesting Sec12 associates with a pre-existing tER scaffold.\",\n      \"method\": \"Fluorescence live imaging and photobleaching (FRAP-type); domain truncation and mutagenesis; overexpression saturation experiments\",\n      \"journal\": \"Developmental cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct live imaging with functional domain analysis; single lab but multiple orthogonal approaches\",\n      \"pmids\": [\"15130490\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Human Sec12 (PREB) is concentrated at ER exit sites (ERES) through direct interaction with cTAGE5, a component of the TANGO1/cTAGE5 collagen cargo receptor complex at ERES. This concentration of Sec12 at ERES is specifically required for secretion of collagen VII but not other secretory proteins. Depletion of Sec12 from ERES (e.g., by cTAGE5 knockdown) selectively impairs collagen export, identifying a cargo-specific requirement for locally elevated Sar1-GTP generated by ERES-concentrated Sec12.\",\n      \"method\": \"Co-immunoprecipitation (direct interaction between Sec12 and cTAGE5); siRNA knockdown of cTAGE5/Sec12; collagen secretion assay; immunofluorescence localization of Sec12 at ERES\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP establishing direct interaction, KD with specific cargo secretion readout, localization experiments; multiple orthogonal methods in single focused study\",\n      \"pmids\": [\"25202031\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"In yeast, the kinase-dead (inactive) form of the casein kinase I Hrr25p suppresses the sec12-4 temperature-sensitive mutation. The suppressor allele hrr25-2 (T176I in kinase domain) has markedly reduced kinase activity, and overexpression of another kinase-dead Hrr25 mutant (K38A) also suppresses sec12-4. This indicates that Hrr25p kinase activity negatively regulates Sec12-dependent vesicle budding from the ER.\",\n      \"method\": \"Genetic suppressor cloning; kinase assay of immunoprecipitated Hrr25p mutant; growth and secretion assays in sec12-4 background\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis with biochemical kinase activity confirmation; single lab with multiple alleles tested\",\n      \"pmids\": [\"9920934\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"The S. pombe Sec12 homologue Spo14/Stl1 is essential for ER-to-Golgi protein transport and for forespore membrane assembly during meiosis. In spo14 mutant cells, ER-like membranes accumulate and the ER/Golgi marker Rer1 is retained in the ER. Overproduction of the S. pombe Sar1 homologue Psr1 suppresses both the sporulation defect and cold-sensitive growth of spo14 mutants, placing Spo14/Sec12 upstream of Sar1 in the ER exit pathway.\",\n      \"method\": \"Genetic complementation; electron microscopy of membrane morphology; Rer1 localization assay; Sar1 overproduction suppression\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis (Sar1 overproduction suppression) with organelle morphology readout; single lab with multiple orthogonal approaches\",\n      \"pmids\": [\"12631727\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"TANGO1, cTAGE5, and SEC12 (PREB) are co-packaged with procollagen I (PC1) into large COPII carriers during ER export. SEC12 is particularly enriched around ER membranes and large COPII carriers containing PC1, whereas it is excluded from small (<100 nm) COPII vesicles. A split-GFP reconstitution system targeting SEC12 to PC1 via the luminal domain of TANGO1 enriches SEC12 around PC1 and generates enlarged PC1 carriers, demonstrating that SEC12 co-packaging is causally required for generation of large COPII carriers for bulky cargo.\",\n      \"method\": \"Vesicle budding assay with cargo fractionation; split-GFP reconstitution system; fluorescence imaging of SEC12 distribution around COPII carriers; immunoblotting of carrier fractions\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstitution with split-GFP system plus biochemical fractionation; multiple orthogonal approaches in a single rigorous study\",\n      \"pmids\": [\"30545919\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Under stress conditions, SEC12 (PREB) mediates formation of a novel ERGIC-ERES membrane contact by directly interacting with TMED9. This contact (as close as 2–5 nm) allows ERES-located SEC12 to transactivate COPII assembly on the ERGIC membrane, promoting formation of ERGIC-derived COPII vesicles that serve as autophagosome membrane precursors. A portion of SEC12 also relocates to the ERGIC. This ERGIC-ERES contact is physically and functionally distinct from the TFG-mediated ERGIC-ERES adjunction involved in secretory transport.\",\n      \"method\": \"Co-immunoprecipitation (TMED9–SEC12 interaction); electron microscopy measuring inter-membrane distance; siRNA knockdown with autophagosome biogenesis readout; COPII assembly assay on ERGIC; fluorescence imaging of SEC12 relocalization\",\n      \"journal\": \"Cell research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, EM structural measurement, functional KD with autophagy readout, multiple orthogonal methods; published in peer-reviewed journal\",\n      \"pmids\": [\"34561617\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PREB/SEC12 is a type II ER transmembrane protein with a β-propeller cytoplasmic domain that functions as the guanine nucleotide exchange factor (GEF) for the small GTPase Sar1, initiating COPII coat assembly for ER-to-Golgi vesicle budding; it is concentrated at ER exit sites (ERES) through interactions with Sec16 and cTAGE5, where locally elevated Sar1-GTP activity is required for export of large cargo such as collagens, and under stress it additionally forms ERGIC-ERES contacts via TMED9 to supply membrane for autophagosome biogenesis; in the pituitary, the mammalian PREB protein also localizes to the nucleus and acts as a PKA-responsive transcriptional activator of the prolactin promoter.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"PREB/SEC12 is the guanine nucleotide exchange factor (GEF) for the small GTPase Sar1 and serves as the initiating factor for COPII-dependent ER-to-Golgi vesicle budding, a role conserved from yeast and plants to mammals [#0, #1]. Its catalytically active cytoplasmic domain folds into a β-propeller whose conserved, potassium-stabilized 'K loop' is essential for nucleotide exchange on Sar1, while GTP hydrolysis is driven in concert with the GAP Sec23 to recruit Sar1-GTP and seed coat assembly [#0, #4]. SEC12 is dynamically concentrated at ER exit sites (transitional ER) through direct interaction with the C-terminal domain of Sec16, providing locally elevated Sar1-GTP generation at sites of cargo export [#3, #5]. At these sites SEC12 directly binds the collagen cargo-receptor component cTAGE5 and is co-packaged with TANGO1 into large COPII carriers, a localized enrichment specifically required for export of bulky cargo such as collagen, while being excluded from small COPII vesicles [#6, #9]. Under stress, SEC12 additionally engages TMED9 to form ERGIC–ERES membrane contacts that transactivate COPII assembly on the ERGIC to supply autophagosome precursor membrane [#10]. Separately, in pituitary cells the mammalian protein localizes to the nucleus and acts as a sequence-specific, PKA-responsive transcriptional activator of the prolactin promoter [#2].\",\n  \"teleology\": [\n    {\n      \"year\": 1992,\n      \"claim\": \"Established that the ER-to-Golgi budding factor encoded by Sec12 is functionally conserved across eukaryotes, indicating a universal vesicle-budding mechanism rather than a yeast-specific one.\",\n      \"evidence\": \"Heterologous complementation of a budding-yeast sec12 null by fission yeast and Arabidopsis homologues, with sequence analysis\",\n      \"pmids\": [\"1396601\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not define the biochemical activity of the conserved domain\", \"No direct evidence linking the conserved cytoplasmic domain to a specific GTPase\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Defined the molecular activity of Sec12 as a GEF for Sar1 and placed it upstream of coat assembly, answering how vesicle budding is initiated at the ER.\",\n      \"evidence\": \"In vitro GEF and GTPase assays with purified proteins, Sec12-1 mutant comparison, and Sec12+Sec23 reconstitution in yeast\",\n      \"pmids\": [\"8377826\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of nucleotide exchange unresolved\", \"Mechanism of spatial restriction to ER exit sites not addressed\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Identified a distinct nuclear, transcriptional role for the mammalian protein, showing PREB acts as a PKA-responsive activator at the prolactin promoter — a function separate from its membrane GEF activity.\",\n      \"evidence\": \"Expression cloning from pituitary cDNA, DNA-binding gel shift, reporter transactivation, immunocytochemistry, and GAL4 fusion with PKA stimulation\",\n      \"pmids\": [\"10194769\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relationship between the nuclear transcriptional role and the ER membrane GEF role unresolved\", \"No structural basis for promoter element 1P recognition\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Revealed negative regulation of Sec12-dependent budding by a casein kinase I, showing the pathway is subject to kinase control.\",\n      \"evidence\": \"Genetic suppressor cloning of sec12-4 by kinase-dead Hrr25 alleles with kinase-activity confirmation and secretion assays in yeast\",\n      \"pmids\": [\"9920934\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No direct substrate (Sec12 phosphorylation) demonstrated\", \"Mechanism by which kinase activity inhibits budding undefined\", \"Genetic epistasis only; not biochemically reconstituted\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Placed Sec12 genetically upstream of Sar1 in the ER exit pathway and extended its role to a developmental membrane-assembly process (meiotic forespore formation).\",\n      \"evidence\": \"Genetic complementation, EM membrane morphology, Rer1 localization, and Sar1 overproduction suppression in S. pombe\",\n      \"pmids\": [\"12631727\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct GEF activity not assayed in this system\", \"Single-organism genetic epistasis\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Showed Sec12 is dynamically and saturably concentrated at transitional ER via its cytosolic domain binding a pre-existing scaffold, addressing how the GEF is spatially organized.\",\n      \"evidence\": \"Live fluorescence imaging/photobleaching, domain truncation, and overexpression saturation in P. pastoris\",\n      \"pmids\": [\"15130490\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"The specific tER partner was not molecularly identified in this study\", \"Single-lab imaging system\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Identified Sec16 as the direct partner that concentrates Sec12 at tER sites, resolving the molecular basis of its spatial targeting in yeast and human cells.\",\n      \"evidence\": \"Overexpression-titration imaging, heterologous expression with/without Sec16, and pull-down between Sec16 C-terminus and Sec12 cytosolic domain\",\n      \"pmids\": [\"22347445\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Quantitative contribution of Sec16 vs other partners to human Sec12 localization not dissected\", \"Effect of the interaction on GEF activity not measured\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Provided the structural mechanism of nucleotide exchange, defining the β-propeller fold and a potassium-stabilized K loop as catalytically essential.\",\n      \"evidence\": \"1.36 Å crystal structure of the S. cerevisiae cytoplasmic domain with structure-guided mutagenesis and in vitro/in vivo GEF assays\",\n      \"pmids\": [\"23109340\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structure of the Sec12–Sar1 complex\", \"Structure of the mammalian/full-length protein not determined\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Established a cargo-specific function by showing ERES-concentrated SEC12, via direct cTAGE5 binding, is selectively required for collagen export.\",\n      \"evidence\": \"Reciprocal Co-IP of SEC12 and cTAGE5, siRNA knockdown with cargo-specific collagen secretion readout, and ERES localization imaging in human cells\",\n      \"pmids\": [\"25202031\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether locally elevated Sar1-GTP alone explains cargo selectivity not directly measured\", \"Generality across collagen subtypes and other bulky cargos not fully defined\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Demonstrated that SEC12 co-packaging is causally required to generate the large COPII carriers needed for bulky procollagen cargo, linking its local enrichment to carrier size control.\",\n      \"evidence\": \"Vesicle budding/cargo fractionation, split-GFP reconstitution targeting SEC12 to procollagen via TANGO1, and imaging of SEC12 distribution around carriers\",\n      \"pmids\": [\"30545919\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism converting local Sar1-GTP into membrane curvature/size remains incompletely defined\", \"Quantitative stoichiometry on large carriers not established\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Uncovered a stress-induced role in which SEC12, via direct TMED9 binding, builds ERGIC-ERES contacts that transactivate COPII on the ERGIC to supply autophagosome membrane, extending its function beyond secretory transport.\",\n      \"evidence\": \"Reciprocal Co-IP, EM inter-membrane distance measurement, siRNA knockdown with autophagy readout, and COPII assembly assay on ERGIC in human cells\",\n      \"pmids\": [\"34561617\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signal that triggers SEC12 relocalization to ERGIC unclear\", \"How TMED9-mediated contact is regulated relative to secretory ERES function undefined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unresolved how the mammalian protein's nuclear transcriptional activity and its ER membrane GEF activity are coordinated, and whether one pool regulates the other.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No study connects the prolactin-promoter transcriptional role to the COPII/ERES GEF role\", \"No mechanism for partitioning PREB between nucleus and ER membrane\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 4]},\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [2]},\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [3, 5, 6]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [0, 6, 9]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [10]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"complexes\": [\n      \"COPII coat\",\n      \"TANGO1/cTAGE5 collagen export complex\"\n    ],\n    \"partners\": [\n      \"SAR1\",\n      \"SEC16\",\n      \"cTAGE5\",\n      \"TANGO1\",\n      \"TMED9\",\n      \"SEC23\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}