{"gene":"SACM1L","run_date":"2026-06-10T07:46:29","timeline":{"discoveries":[{"year":1999,"finding":"The SAC1-like domain of yeast Sac1p defines a novel class of polyphosphoinositide phosphatase (PPIPase) with intrinsic enzymatic activity. Purified recombinant SAC1-like domain converts PI 3-phosphate, PI 4-phosphate, and PI 3,5-bisphosphate to PI, whereas PI 4,5-bisphosphate is not a substrate. Yeast lacking Sac1p exhibit 10-, 2.5-, and 2-fold increases in PI 4-phosphate, PI 3,5-bisphosphate, and PI 3-phosphate respectively.","method":"Purified recombinant protein in vitro phosphatase assay; lipid mass measurement in sac1 deletion yeast strains","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with purified recombinant protein, confirmed by in vivo lipid measurements in deletion strains; replicated across multiple substrate tests","pmids":["10224048"],"is_preprint":false},{"year":1989,"finding":"Mutations in SAC1 suppress defects in both yeast Golgi secretion (sec14, sec6, sec9 mutants) and actin cytoskeleton function, placing Sac1p at a node that coordinates secretory pathway and actin cytoskeleton activities.","method":"Genetic suppressor screen; double-mutant epistasis analysis","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis with multiple secretory and actin mutants, replicated across several loci","pmids":["2687291"],"is_preprint":false},{"year":2000,"finding":"Rat (mammalian) Sac1 is a ubiquitously expressed 65-kDa integral membrane protein of the ER with intrinsic phosphoinositide phosphatase activity directed toward PI 3-phosphate, PI 4-phosphate, and PI 3,5-bisphosphate. Mutant rat sac1 alleles evoke substrate-specific defects in enzymatic activity. PI 4-phosphate phosphatase activity, but not PI 3-phosphate or PI 3,5-bisphosphate phosphatase activity, is essential for complementation of yeast Sac1p defects in vivo.","method":"In vitro phosphatase assay with purified recombinant protein; active-site mutagenesis; heterologous complementation in yeast deletion strains","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro enzymatic assay combined with mutagenesis and in vivo complementation; multiple orthogonal approaches in single study","pmids":["10887188"],"is_preprint":false},{"year":2001,"finding":"Sac1p localizes primarily to the ER, and this localization is crucial for efficient turnover of PI 4-phosphate. The bulk of PI 4-phosphate that accumulates in sac1 mutant cells is generated by the Stt4 PI 4-kinase (not Pik1p), as demonstrated by double-mutant analysis. Loss of Sac1p activity causes vacuole morphology changes, lipid droplet accumulation, and Golgi function defects.","method":"Temperature-sensitive allele analysis; fluorescence microscopy for localization; double-mutant epistasis with stt4(ts) and pik1(ts); lipid measurements","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal genetic epistasis with two PI4-kinases, direct localization experiments, lipid quantification; multiple orthogonal methods","pmids":["11514624"],"is_preprint":false},{"year":2003,"finding":"Human SAC1 (hSAC1) exhibits the same substrate specificity as yeast Sac1p, localizes to the ER and Golgi, and interacts physically with COPI complex subunits. Mutation of a C-terminal KXKXX motif abolishes COPI interaction and causes hSAC1 accumulation in the Golgi. A catalytically inactive mutant also accumulates in the Golgi and fails to interact with COPI despite an intact KXKXX motif, suggesting that enzymatic activity provides a switch for COPI interaction motif accessibility.","method":"Co-immunoprecipitation; KXKXX motif mutagenesis; catalytic-dead mutant; subcellular localization by immunofluorescence","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP with COPI, mutagenesis of interaction motif and catalytic residue, localization experiments; single lab, multiple orthogonal methods","pmids":["14527956"],"is_preprint":false},{"year":2008,"finding":"SAC1 accumulates at the Golgi in quiescent mammalian cells, depleting Golgi PI(4)P and down-regulating anterograde trafficking. Golgi localization requires SAC1 oligomerization and COPII recruitment. Mitogen stimulation activates the p38 MAPK pathway, which dissociates SAC1 oligomers, triggering COPI-mediated retrieval of SAC1 to the ER and releasing the brake on Golgi secretion.","method":"Subcellular fractionation; fluorescence microscopy; dominant-negative and kinase inhibitor experiments; p38 MAPK inhibition; PI(4)P measurements","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (localization, signaling pathway inhibition, lipid measurements), mechanistic pathway placed downstream of p38 MAPK","pmids":["18299350"],"is_preprint":false},{"year":2008,"finding":"Functional ablation of murine Sac1 results in preimplantation lethality. Sac1 insufficiency causes disorganization of mammalian Golgi membranes and mitotic defects with multiple mechanically active spindles. Both phosphoinositide phosphatase activity and ER localization (recycling from Golgi to ER) are required for Sac1 function in vivo.","method":"Sac1 knockout mouse; RNAi knockdown in mammalian cells; complementation with phosphatase-dead and ER-localization-defective mutants; fluorescence microscopy","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — knockout mouse phenotype combined with structure-function mutagenesis and cellular rescue experiments; multiple orthogonal methods","pmids":["18480408"],"is_preprint":false},{"year":2010,"finding":"Crystal structure of the Sac phosphatase domain of yeast Sac1 at 2.0 Å resolution reveals two closely packed sub-domains: a novel N-terminal sub-domain and the PI phosphatase catalytic sub-domain. The structure shows a unique conformation of the catalytic P-loop and a large positively charged groove at the catalytic site, suggesting an unusual dephosphorylation mechanism.","method":"X-ray crystallography at 2.0 Å; homology modeling of human Fig4/Sac3","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution crystal structure with functional implications; disease mutation mapping validated the structural relevance","pmids":["20389282"],"is_preprint":false},{"year":2007,"finding":"During exponential growth, Sac1p interacts with Dpm1p at the ER. During starvation, Sac1p shuttles to the Golgi via COPII and Rer1 adaptor protein, specifically eliminating a pool of PI(4)P generated by Pik1p/Frq1p. Reciprocal association/dissociation of Sac1p and the Pik1p/Frq1p kinase complex controls growth-dependent Golgi PI(4)P levels.","method":"Co-immunoprecipitation (Sac1p-Dpm1p interaction); COPII and Rer1 mutant analysis; PI(4)P measurements under different growth conditions; fluorescence microscopy","journal":"Traffic (Copenhagen, Denmark)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP interaction, genetic epistasis with COPII/Rer1, lipid measurements; single lab","pmids":["17908202"],"is_preprint":false},{"year":2012,"finding":"Vps74 (yeast ortholog of human GOLPH3) binds directly to the catalytic domain of Sac1 (Kd = 3.8 μM) and functions as a sensor of PI(4)P levels on medial Golgi cisternae, directing Sac1-mediated dephosphorylation of this PI(4)P pool.","method":"In vitro binding assay (fluorescence anisotropy/ITC for Kd); PI(4)P reporter distribution analysis; genetic deletion epistasis","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct binding measured with quantitative in vitro assay (Kd determination), supported by in vivo PI(4)P reporter and genetic data; single lab but multiple orthogonal methods","pmids":["22553352"],"is_preprint":false},{"year":2012,"finding":"Sac1 is allosterically activated by anionic phospholipids: its product phosphatidylinositol and phosphatidylserine activate the enzyme, likely through conformational changes of the catalytic P-loop induced by binding of anionic phospholipids in the large cationic catalytic groove.","method":"In vitro phosphatase assay with purified recombinant Sac1; analysis of activation by different lipid species; structural interpretation based on crystal structure","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro reconstitution assay demonstrating allosteric activation, single lab, mechanistic model based on crystal structure but conformational change not directly observed","pmids":["22452743"],"is_preprint":false},{"year":2014,"finding":"Crystal structure of the N-terminal portion of yeast Sac1 in complex with Vps74 reveals the interaction interface involves the N-terminal subdomain of the Sac1 homology domain. Disruption of the Sac1-Vps74 interface results in broader distribution of PI(4)P within the Golgi and failure to maintain residence of a medial Golgi mannosyltransferase.","method":"X-ray crystallography of Sac1-Vps74 complex; interface mutagenesis; PI(4)P reporter imaging; Golgi resident enzyme localization assay","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure of protein complex plus functional validation by mutagenesis and cellular phenotype assays","pmids":["25113029"],"is_preprint":false},{"year":2015,"finding":"14-3-3 protein acts as a cytosolic adaptor mediating SAC1 export from the ER in COPII-coated vesicles. Recombinant 14-3-3 stimulates packaging of SAC1 into COPII vesicles in a cell-free budding reaction. The COPII sorting subunit Sec24 interacts with 14-3-3. A minimal sorting motif (RLSNTSP, the 'LS motif') in SAC1 is required for 14-3-3 binding and controls SAC1 ER export.","method":"Cell-free COPII vesicle budding assay; Co-immunoprecipitation of Sec24 with 14-3-3; SAC1 motif mutagenesis; recombinant protein addition","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — cell-free reconstitution assay plus mutagenesis of sorting motif and Co-IP of interaction; multiple orthogonal methods in single study","pmids":["26056309"],"is_preprint":false},{"year":2018,"finding":"SAC1 acts in 'cis' configuration at the ER to degrade PtdIns4P, maintaining a steep PtdIns4P chemical gradient with donor membranes. Acute chemical inhibition of SAC1 causes PtdIns4P accumulation in the ER. SAC1 does not enrich at membrane contact sites and has little activity in 'trans' unless an artificial linker is added between its ER-anchor and catalytic domains.","method":"Acute chemical inhibition of SAC1; live-cell PtdIns4P biosensors; SAC1 localization imaging; engineered linker constructs for trans-activity test","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Moderate — acute inhibition with biosensor readout, engineered constructs to test cis vs. trans activity, multiple approaches in a single study","pmids":["29461204"],"is_preprint":false},{"year":2018,"finding":"SAC1 undergoes reversible oxidative inactivation in mammalian cells: H2O2 oxidizes the catalytic Cys389 residue to form an intramolecular disulfide with Cys392, causing accumulation of PtdIns(4)P at the Golgi. EGF stimulation induces Ca2+-dependent Duox activation, H2O2 production at the Golgi, and transient SAC1 oxidation, linking EGF signaling to Golgi PtdIns(4)P control.","method":"Mass spectrometry of oxidized cysteines; Golgi-confined SAC1-K2A mutant; Duox knockdown; Golgi-targeted H2O2 probe; PtdIns(4)P measurements","journal":"Free radical biology & medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — identification of specific oxidized cysteine residues, use of organelle-targeted mutants and H2O2 probe, pathway knockdown; multiple orthogonal methods replicated across two related papers","pmids":["30476538","30448513"],"is_preprint":false},{"year":2019,"finding":"SAC1 activity on TGN PI(4)P occurs in-trans at ER-TGN contact sites (ERTGoCS) and requires the adaptor protein FAPP1. FAPP1 localizes at ERTGoCS, physically interacts with SAC1, and promotes SAC1's in-trans phosphatase activity in vitro. FAPP1 depletion increases TGN PI(4)P levels and enhances secretion of selected cargoes (e.g., ApoB100).","method":"Co-immunoprecipitation of FAPP1 with SAC1; in vitro trans-phosphatase activity assay; FAPP1 knockdown with PI(4)P and cargo secretion readouts; fluorescence localization","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution of trans-phosphatase activity plus reciprocal Co-IP and functional knockdown; multiple orthogonal methods in single study","pmids":["30659099"],"is_preprint":false},{"year":2019,"finding":"TMEM39A/SUSR2 interacts with SAC1 and COPII SEC23/SEC24 subunits to promote ER-to-Golgi transport of SAC1. Depletion of SUSR2 retains SAC1 on the ER, increases PI(3)P produced by VPS34 complex, promotes autophagosome formation, and elevates late endosomal/lysosomal PI(4)P levels to facilitate HOPS complex recruitment and autophagosome maturation.","method":"Co-immunoprecipitation of SUSR2 with SAC1 and SEC23/SEC24; SUSR2 knockdown with PI(4)P and PI(3)P measurements; autophagy flux assays; HOPS recruitment assays","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP with multiple interactors, knockdown with lipid measurements and functional autophagy readouts; multiple orthogonal methods","pmids":["31806350"],"is_preprint":false},{"year":2020,"finding":"CPT1C (a malonyl-CoA sensor in the ER of neurons) regulates SAC1 catalytic activity: in normal conditions CPT1C down-regulates SAC1 activity, allowing efficient GluA1 (AMPA receptor subunit) trafficking to the plasma membrane. Under low malonyl-CoA (glucose depletion), CPT1C-dependent inhibition of SAC1 is released, SAC1 translocates to ER-TGN contact sites, decreasing TGN PI(4)P and triggering GluA1 retention at the TGN.","method":"SAC1 activity assay under CPT1C modulation; metabolic stress experiments (glucose depletion); PI(4)P measurements; GluA1 surface trafficking assay; SAC1 localization by fluorescence microscopy","journal":"The Journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional SAC1 activity modulation shown with CPT1C interaction context, PI(4)P measurements, and cargo trafficking readouts; single lab, multiple approaches","pmids":["32931550"],"is_preprint":false},{"year":2013,"finding":"The first transmembrane domain (TM1) of human SAC1 is sufficient for Golgi localization. A minimal TM1-containing construct concentrates at the Golgi, and transplanting TM1 into transferrin receptor 2 induces Golgi accumulation of that normally PM/endosomal protein. The N-terminal cytoplasmic domain of SAC1 also independently promotes Golgi localization.","method":"Truncation and chimeric constructs expressed in mammalian cells; fluorescence microscopy localization","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — domain swap experiments with chimeric proteins and truncations; single lab, multiple constructs","pmids":["23936490"],"is_preprint":false},{"year":2003,"finding":"Drosophila Sac1 loss-of-function causes defects in dorsal closure: specifically, improper activation of cell shape change in the amnioserosa and JNK signaling in the leading edge epidermis. sac1 shows dosage-sensitive genetic interactions with components of the JNK cascade and cell shape change pathway, placing Sac1 upstream or parallel to these events.","method":"Drosophila sac1 loss-of-function mutant analysis; genetic interaction (dosage-sensitive) with JNK pathway components; embryo phenotype imaging","journal":"Current biology : CB","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function with specific developmental phenotype and dosage-sensitive genetic interactions; single lab","pmids":["14588244"],"is_preprint":false},{"year":2011,"finding":"Drosophila Sac1 is required for axon guidance at the CNS midline. sac1 mutants show ectopic midline crossing of Fasciclin II-positive axon tracts. This phenotype is rescued by neuronal expression of wild-type Sac1 but not by a catalytically-inactive mutant. sac1 shows dosage-sensitive genetic interactions with slit and robo, placing Sac1-mediated PI regulation in the Slit/Robo axon repulsion pathway.","method":"Drosophila sac1 loss-of-function mutants; neuronal rescue with WT vs. catalytic-dead Sac1; dosage-sensitive genetic interaction with slit and robo","journal":"Molecules and cells","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — catalytic-dead rescue experiment places function in PI phosphatase activity; genetic epistasis with Slit/Robo pathway; single lab","pmids":["22042447"],"is_preprint":false},{"year":2021,"finding":"SAC1 is required for autophagosome-lysosome fusion through its PI(4)P phosphatase activity. Sac1 deficiency causes dramatic accumulation of PI(4)P at early Golgi and abnormal incorporation of PI(4)P into Atg9 vesicles and autophagosomes, leading to failure to recruit SNARE proteins for autophagosome fusion with vacuoles. This function is conserved from yeast to mammalian cells.","method":"High-throughput screen in S. cerevisiae followed by mechanistic validation; PI(4)P reporter imaging; SNARE recruitment assays; Atg9 vesicle lipid analysis; mammalian cell validation","journal":"Autophagy","confidence":"High","confidence_rationale":"Tier 2 / Strong — mechanistic screen followed by detailed molecular validation with PI(4)P imaging, SNARE recruitment, and cross-species conservation; multiple orthogonal methods","pmids":["32693712"],"is_preprint":false},{"year":2021,"finding":"SAC1 (SACM1L) is an essential regulator of xenophagy: depletion or inactivation of SAC1 results in aberrant accumulation of PI(4)P on Salmonella-containing autophagosomes, facilitating recruitment of the PI(4)P-binding Salmonella effector SteA, which impedes lysosomal fusion. Replication of Salmonella lacking SteA is suppressed by SAC1-deficient cells, confirming the mechanism.","method":"siRNA knockdown of SAC1; PI(4)P imaging on pathogen-containing autophagosomes; bacterial replication assays with SteA deletion Salmonella; epistasis between SAC1 and SteA","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — mechanistic pathway established through knockdown + PI(4)P imaging + genetic epistasis with bacterial effector; multiple orthogonal approaches","pmids":["34320354"],"is_preprint":false},{"year":2025,"finding":"Acute Sac1 degradation (auxin-inducible degron) in human cells causes immediate PI(4)P increase and cholesterol decrease in the TGN, followed by Golgi V-ATPase disassembly, TGN deacidification, Golgi fragmentation, and impaired glycosylation. Mechanistically, Sac1-mediated TGN membrane lipid composition maintains an assembly-promoting conformation of the V0a2 subunit of the V-ATPase.","method":"Auxin-inducible degron system for acute Sac1 degradation; PI(4)P and cholesterol biosensors; V-ATPase assembly assay; V0a2 conformation analysis; glycosylation assays; differentiated trophoblast model","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — acute and specific protein degradation with multiple lipid and organelle readouts plus mechanistic link to V-ATPase subunit conformation; multiple orthogonal methods","pmids":["40841558"],"is_preprint":false},{"year":2024,"finding":"Orthogonal targeting of SAC1 to mitochondria (PM-mitochondria contact sites) enhances PM PI(4)P turnover independently of ER-PM contact sites. This turnover is slowed by knockdown of soluble ORP2, implicating ORP2 as a major mediator of PI(4)P transfer from PM to sites where SAC1 can degrade it.","method":"Organelle-targeted SAC1 chimeras; ORP2 knockdown; live-cell PI(4)P biosensors at PM","journal":"Contact (Thousand Oaks)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — engineered targeting constructs with live-cell biosensor readout plus RNAi epistasis; single lab, multiple approaches","pmids":["38327560"],"is_preprint":false},{"year":1999,"finding":"sac1 mutants exhibit accumulation of PI 4-phosphate that alone is insufficient to effect bypass of Sec14p function. Instead, phospholipase D activity generates diacylglycerol (DAG) downstream of elevated PI 4-phosphate, and this DAG effects bypass Sec14p. CDP-choline pathway activity contributes to the inositol auxotrophy of sac1 strains independently of INO1 transcription.","method":"Phospholipid metabolic labeling; genetic inactivation of phospholipase D; bacterial DAG kinase expression; sac1 mutant phenotype analysis","journal":"Molecular biology of the cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic and biochemical dissection of PI4P accumulation vs. DAG generation in bypass; single lab, multiple genetic tools","pmids":["10397762"],"is_preprint":false},{"year":2018,"finding":"In Drosophila, Sac1 is required for normal photoreceptor cell shape and microtubule stability in the developing eye. Sac1 mutant interommatidial cells show elevated PI(4)P, severe microtubule organization defects, and accumulation of the adhesion protein Roughest and exocyst subunit Sec8 in enlarged intracellular vesicles. Roughest is delivered to the cell surface in Sac1 mutants, indicating that Sac1 acts in microtubule-dependent exocyst trafficking rather than Roughest surface delivery per se.","method":"Temperature-sensitive sac1 allele in Drosophila; PI(4)P imaging; microtubule immunostaining; Roughest and Sec8 localization; cold fixation ex vivo assay","journal":"Development (Cambridge, England)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — conditional allele with multiple cellular readouts; single lab","pmids":["29752385"],"is_preprint":false}],"current_model":"SACM1L/SAC1 is a conserved integral ER membrane PI(4)P phosphatase (with additional activity toward PI(3)P and PI(3,5)P2) whose catalytic CX5R(T/S) motif dephosphorylates phosphoinositides primarily in cis at the ER to maintain steep PI(4)P gradients that drive non-vesicular lipid counter-transport; it cycles between ER and Golgi under control of COPII/COPI machinery (requiring a C-terminal KXKXX COPI-binding motif, a 14-3-3-dependent LS sorting motif, and its first transmembrane domain for Golgi retention), is regulated by p38 MAPK-induced oligomer dissociation and by reversible oxidation of its catalytic Cys by H2O2 downstream of EGF/Ca2+/Duox signaling, physically interacts with COPI, Vps74/GOLPH3, FAPP1, and TMEM39A/SUSR2 to control organelle-specific PI(4)P pools, and is required for Golgi V-ATPase assembly and TGN acidification, mitotic spindle organization, autophagosome-lysosome fusion, xenophagy, axon guidance (via Slit/Robo), and actin cytoskeleton organization."},"narrative":{"mechanistic_narrative":"SACM1L (SAC1) is a conserved integral ER membrane phosphoinositide phosphatase that controls the spatial distribution of PI(4)P across the secretory pathway, coupling lipid homeostasis to membrane trafficking, organelle architecture, and cytoskeletal organization [PMID:10224048, PMID:10887188, PMID:18480408]. Its SAC1-homology catalytic domain hydrolyzes PI(3)P, PI(4)P, and PI(3,5)P2 to PI but spares PI(4,5)P2, and PI(4)P phosphatase activity is the function essential for cellular viability and for complementing yeast Sac1 defects [PMID:10224048, PMID:10887188]. The catalytic domain folds into an N-terminal subdomain packed against the PI-phosphatase subdomain with a distinctive P-loop and a large cationic catalytic groove, and the enzyme is allosterically activated by anionic phospholipid products binding this groove [PMID:20389282, PMID:22452743]. SAC1 acts predominantly in cis at the ER, degrading PI(4)P locally to sustain a steep PI(4)P gradient between donor membranes and the ER [PMID:29461204]; at ER–TGN contact sites it can additionally act in trans through the adaptor FAPP1 [PMID:30659099]. SAC1 dynamically cycles between ER and Golgi: ER export uses a 14-3-3/Sec24-dependent LS sorting motif into COPII vesicles, while retrieval from the Golgi requires a C-terminal KXKXX motif that binds the COPI complex, and its first transmembrane domain and N-terminal cytoplasmic domain independently confer Golgi retention [PMID:14527956, PMID:26056309, PMID:23936490]. This trafficking, and hence Golgi PI(4)P levels, is regulated by p38 MAPK-driven oligomer dissociation, by EGF/Ca2+/Duox-dependent H2O2 oxidation of the catalytic Cys389–Cys392 pair, and by the GOLPH3 ortholog Vps74, which binds the catalytic domain directly to target medial-Golgi PI(4)P [PMID:18299350, PMID:30476538, PMID:30448513, PMID:22553352, PMID:25113029]. Through this control of organelle PI(4)P pools, SAC1 maintains Golgi V-ATPase assembly and TGN acidification, governs mitotic spindle organization, enables autophagosome–lysosome fusion and xenophagy of Salmonella, and directs developmental processes including Slit/Robo-dependent axon guidance and JNK-dependent epithelial morphogenesis [PMID:40841558, PMID:18480408, PMID:32693712, PMID:34320354, PMID:22042447, PMID:14588244]. Loss of murine Sac1 causes preimplantation lethality, underscoring its essential role [PMID:18480408].","teleology":[{"year":1989,"claim":"Before any enzymatic role was known, genetics placed Sac1 at a node coordinating secretion and the actin cytoskeleton, framing it as a shared regulator of these processes.","evidence":"Genetic suppressor screen and double-mutant epistasis in yeast against sec14/sec6/sec9 and actin mutants","pmids":["2687291"],"confidence":"High","gaps":["No molecular activity assigned","Mechanistic link between secretion and actin unresolved"]},{"year":1999,"claim":"Established that the SAC1 domain is itself a phosphoinositide phosphatase with defined substrate range, converting genetic phenotypes into a concrete enzymatic activity.","evidence":"In vitro phosphatase assay on purified recombinant SAC1 domain plus lipid mass measurement in sac1 deletion yeast","pmids":["10224048"],"confidence":"High","gaps":["Did not resolve which substrate is physiologically dominant","No structural basis for catalysis"]},{"year":1999,"claim":"Defined how PI(4)P accumulation in sac1 mutants is functionally read out, showing PLD-generated DAG rather than PI(4)P itself drives Sec14 bypass.","evidence":"Phospholipid metabolic labeling, PLD inactivation, and bacterial DAG kinase expression in yeast","pmids":["10397762"],"confidence":"Medium","gaps":["Single-lab biochemical dissection","Relevance to mammalian Sac1 not tested"]},{"year":2000,"claim":"Showed the mammalian enzyme conserves substrate specificity and that PI(4)P phosphatase activity specifically is the function required in vivo, prioritizing PI(4)P as the key substrate.","evidence":"In vitro assay with active-site mutants and heterologous yeast complementation of rat Sac1","pmids":["10887188"],"confidence":"High","gaps":["Did not address subcellular site of action","Roles of PI(3)P/PI(3,5)P2 activity left open"]},{"year":2001,"claim":"Localized Sac1 turnover activity to the ER and identified Stt4 as the kinase generating its PI(4)P substrate pool, establishing a kinase–phosphatase axis.","evidence":"Temperature-sensitive alleles, microscopy, and reciprocal epistasis with stt4 and pik1 in yeast","pmids":["11514624"],"confidence":"High","gaps":["Did not explain ER/Golgi cycling","Mechanism connecting PI(4)P to vacuole/lipid-droplet phenotypes unresolved"]},{"year":2003,"claim":"Defined the molecular logic of ER retrieval, linking a C-terminal KXKXX COPI motif to catalytic state and showing enzymatic activity gates COPI engagement.","evidence":"Co-IP with COPI, KXKXX and catalytic-dead mutagenesis, and localization in mammalian cells","pmids":["14527956"],"confidence":"High","gaps":["ER export pathway not yet defined","Structural basis of activity-dependent motif accessibility unknown"]},{"year":2003,"claim":"Extended Sac1 function into metazoan development, placing it upstream of or parallel to JNK signaling in epithelial morphogenesis.","evidence":"Drosophila sac1 loss-of-function with dosage-sensitive interactions with JNK pathway components","pmids":["14588244"],"confidence":"Medium","gaps":["Lipid mechanism linking Sac1 to JNK not defined","Single organism"]},{"year":2007,"claim":"Resolved how Sac1 selects a kinase-specific PI(4)P pool, showing growth-dependent shuttling between Dpm1 at the ER and Pik1-generated Golgi PI(4)P via COPII/Rer1.","evidence":"Co-IP, COPII/Rer1 mutant analysis, and condition-dependent PI(4)P measurements in yeast","pmids":["17908202"],"confidence":"Medium","gaps":["Single-lab Co-IP evidence","Mammalian conservation of Rer1 adaptor untested"]},{"year":2008,"claim":"Connected Sac1 localization to mitogenic signaling, establishing p38 MAPK-driven oligomer dissociation as a switch releasing the Golgi secretion brake.","evidence":"Fractionation, microscopy, kinase inhibition, and PI(4)P measurements in mammalian cells","pmids":["18299350"],"confidence":"High","gaps":["Oligomerization interface not mapped","Direct p38 substrate site on SAC1 unidentified"]},{"year":2008,"claim":"Demonstrated organismal essentiality and assigned a mitotic role, showing both phosphatase activity and ER recycling are required in vivo.","evidence":"Sac1 knockout mouse, RNAi, and rescue with phosphatase-dead and ER-localization-defective mutants","pmids":["18480408"],"confidence":"High","gaps":["Molecular basis of spindle defect unresolved","Lethality stage limits tissue-level analysis"]},{"year":2010,"claim":"Provided the structural framework, revealing a two-subdomain fold with a cationic catalytic groove implying an unusual dephosphorylation mechanism.","evidence":"2.0 Å X-ray crystal structure of the yeast Sac phosphatase domain","pmids":["20389282"],"confidence":"High","gaps":["Membrane-engaged conformation not captured","Substrate-bound state absent"]},{"year":2012,"claim":"Identified an allosteric activation mechanism by anionic phospholipids, linking product and membrane lipid composition to catalytic output.","evidence":"In vitro phosphatase assays with defined lipid species interpreted on the crystal structure","pmids":["22452743"],"confidence":"Medium","gaps":["Conformational change inferred, not directly observed","Single-lab in vitro evidence"]},{"year":2012,"claim":"Defined GOLPH3/Vps74 as a direct PI(4)P-level sensor that recruits Sac1 to medial-Golgi cisternae, explaining sub-Golgi targeting.","evidence":"Quantitative in vitro binding (Kd 3.8 µM), PI(4)P reporter distribution, and genetic deletion in yeast","pmids":["22553352"],"confidence":"High","gaps":["Mammalian GOLPH3-SAC1 functional equivalence not shown here","Regulation of the interaction unknown"]},{"year":2013,"claim":"Dissected the membrane determinants of Golgi retention, showing TM1 alone is sufficient and portable, with an additional N-terminal cytoplasmic contribution.","evidence":"Truncation and chimeric constructs with transferrin receptor 2 in mammalian cells","pmids":["23936490"],"confidence":"Medium","gaps":["Retention partners of TM1 unidentified","Single-lab localization data"]},{"year":2014,"claim":"Visualized the Sac1–Vps74 interface and showed its disruption broadens Golgi PI(4)P and mislocalizes a Golgi enzyme, tying lipid targeting to glycosylation residency.","evidence":"Crystal structure of the Sac1–Vps74 complex with interface mutagenesis and PI(4)P/enzyme localization assays","pmids":["25113029"],"confidence":"High","gaps":["In vivo dynamics of the complex not resolved","Conservation to human GOLPH3 structurally untested"]},{"year":2015,"claim":"Defined the ER export step, identifying a 14-3-3-dependent LS sorting motif and Sec24 engagement that package SAC1 into COPII vesicles.","evidence":"Cell-free COPII budding reconstitution, Sec24/14-3-3 Co-IP, and LS-motif mutagenesis","pmids":["26056309"],"confidence":"High","gaps":["Phosphorylation controlling 14-3-3 binding not mapped","Coupling to p38 oligomer regulation unresolved"]},{"year":2018,"claim":"Resolved the long-standing cis-versus-trans question, showing SAC1 acts locally at the ER to maintain a steep PI(4)P gradient with little intrinsic trans activity.","evidence":"Acute chemical inhibition, live-cell PI(4)P biosensors, and engineered-linker trans-activity tests","pmids":["29461204"],"confidence":"High","gaps":["Did not exclude adaptor-assisted trans activity","Identity of counter-transported lipids inferred not measured here"]},{"year":2018,"claim":"Established redox regulation, identifying EGF/Ca2+/Duox-driven H2O2 oxidation of catalytic Cys389–Cys392 as a transient brake on Golgi PI(4)P turnover.","evidence":"Mass spectrometry of oxidized cysteines, Golgi-targeted H2O2 probe, Duox knockdown, and PI(4)P measurements","pmids":["30476538","30448513"],"confidence":"High","gaps":["Reductase reversing oxidation unidentified","Physiological output of transient oxidation not quantified"]},{"year":2019,"claim":"Reconciled cis activity with TGN PI(4)P control by identifying FAPP1 as an adaptor enabling SAC1 trans activity at ER–TGN contact sites.","evidence":"Reciprocal Co-IP, in vitro trans-phosphatase assay, and FAPP1 knockdown with PI(4)P/cargo readouts","pmids":["30659099"],"confidence":"High","gaps":["Structural basis of FAPP1-mediated trans activation unknown","Cargo selectivity mechanism unresolved"]},{"year":2019,"claim":"Identified TMEM39A/SUSR2 as a transport regulator coupling SAC1 ER-export to autophagy, linking SAC1 retention to PI(3)P and endolysosomal PI(4)P.","evidence":"Reciprocal Co-IP with SAC1 and SEC23/SEC24, knockdown with lipid measurements, and autophagy/HOPS assays","pmids":["31806350"],"confidence":"High","gaps":["Direct vs indirect effect on VPS34 not fully separated","Mechanism of SUSR2-COPII coordination undefined"]},{"year":2020,"claim":"Connected SAC1 to neuronal metabolic sensing, showing CPT1C tunes SAC1 activity to gate AMPA-receptor subunit trafficking under glucose stress.","evidence":"SAC1 activity assays under CPT1C modulation, glucose-depletion stress, PI(4)P measurements, and GluA1 surface trafficking","pmids":["32931550"],"confidence":"Medium","gaps":["Direct biochemical mechanism of CPT1C inhibition unresolved","Single-lab evidence"]},{"year":2011,"claim":"Placed Sac1 catalytic activity in the Slit/Robo axon-repulsion pathway, requiring PI phosphatase activity for proper midline guidance.","evidence":"Drosophila sac1 mutants rescued by WT but not catalytic-dead Sac1, with slit/robo dosage interactions","pmids":["22042447"],"confidence":"Medium","gaps":["Lipid intermediate linking Sac1 to Robo signaling not defined","Single organism"]},{"year":2018,"claim":"Linked Sac1 PI(4)P control to microtubule-dependent exocyst trafficking in a developmental epithelium.","evidence":"Conditional Drosophila sac1 allele with PI(4)P imaging, microtubule staining, and Roughest/Sec8 localization","pmids":["29752385"],"confidence":"Medium","gaps":["Mechanism connecting PI(4)P to microtubule stability unresolved","Single-lab data"]},{"year":2020,"claim":"Defined SAC1's role in autophagosome maturation, showing PI(4)P clearance is required for SNARE-mediated autophagosome–lysosome fusion, conserved across species.","evidence":"Yeast screen plus PI(4)P imaging, SNARE recruitment, Atg9-vesicle lipid analysis, and mammalian validation","pmids":["32693712"],"confidence":"High","gaps":["Direct SNARE-PI(4)P regulatory link not fully resolved","Site of relevant SAC1 action during fusion unclear"]},{"year":2021,"claim":"Extended the autophagy role to host defense, showing SAC1 clears PI(4)P from Salmonella-containing autophagosomes to block the PI(4)P-binding effector SteA.","evidence":"siRNA knockdown, PI(4)P imaging on pathogen autophagosomes, and epistasis with SteA-deletion Salmonella","pmids":["34320354"],"confidence":"High","gaps":["How SAC1 accesses pathogen membranes undefined","Other PI(4)P-binding effectors not surveyed"]},{"year":2024,"claim":"Tested the limits of compartment-specific action by re-targeting SAC1 to PM–mitochondria contacts, implicating ORP2 as the lipid-transfer mediator delivering PM PI(4)P to SAC1.","evidence":"Organelle-targeted SAC1 chimeras with ORP2 knockdown and live-cell PM PI(4)P biosensors","pmids":["38327560"],"confidence":"Medium","gaps":["Engineered system may not reflect endogenous geometry","Direct ORP2-SAC1 coupling not shown"]},{"year":2025,"claim":"Linked SAC1 TGN lipid control to organelle acidification, showing acute loss disassembles the V-ATPase via the V0a2 subunit conformation and impairs glycosylation.","evidence":"Auxin-inducible SAC1 degron with lipid biosensors, V-ATPase assembly and V0a2 conformation assays, and glycosylation readouts","pmids":["40841558"],"confidence":"High","gaps":["Relative contributions of PI(4)P rise vs cholesterol drop not separated","How lipids set V0a2 conformation mechanistically undefined"]},{"year":null,"claim":"How the multiple regulatory inputs (p38 oligomer dissociation, 14-3-3/COPII export, COPI retrieval, redox oxidation, GOLPH3/FAPP1/CPT1C adaptors) are integrated to set organelle-specific PI(4)P set points in real time remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No unified quantitative model of SAC1 cycling and activity","Hierarchy and crosstalk among regulators unmapped","Human structural basis for adaptor-controlled trans activity lacking"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[0,2,13,15]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[10]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[2,3,13]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[4,5,18]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[24]}],"pathway":[{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[4,5,12]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[16,21,22]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[0,3]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[19,20]}],"complexes":[],"partners":["COPI","GOLPH3/VPS74","FAPP1","TMEM39A/SUSR2","14-3-3","SEC24","CPT1C","DPM1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9NTJ5","full_name":"Phosphatidylinositol-3-phosphatase SAC1","aliases":["Phosphatidylinositol-4-phosphate phosphatase","Suppressor of actin mutations 1-like protein"],"length_aa":587,"mass_kda":67.0,"function":"Phosphoinositide phosphatase which catalyzes the hydrolysis of phosphatidylinositol 4-phosphate (PtdIns(4)P) (PubMed:24209621, PubMed:27044890, PubMed:29461204, PubMed:30659099). Can also catalyze the hydrolysis of phosphatidylinositol 3-phosphate (PtdIns(3)P) and has low activity towards phosphatidylinositol-3,5-bisphosphate (PtdIns(3,5)P2) (By similarity). Shows a very robust PtdIns(4)P phosphatase activity when it binds PtdIns(4)P in a 'cis' configuration in the cellular environment, with much less activity seen when it binds PtdIns(4)P in 'trans' configuration (PubMed:24209621, PubMed:29461204, PubMed:30659099). 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PHOSPHATIDYLINOSITIDE PHOSPHATASE; SACM1L","url":"https://www.omim.org/entry/606569"},{"mim_id":"604297","title":"SYNAPTOJANIN 1; SYNJ1","url":"https://www.omim.org/entry/604297"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"},{"location":"Golgi apparatus","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SACM1L"},"hgnc":{"alias_symbol":["SAC1","KIAA0851"],"prev_symbol":[]},"alphafold":{"accession":"Q9NTJ5","domains":[{"cath_id":"-","chopping":"2-110_127-185","consensus_level":"medium","plddt":92.8017,"start":2,"end":185},{"cath_id":"-","chopping":"197-421","consensus_level":"high","plddt":93.4335,"start":197,"end":421},{"cath_id":"1.10.287","chopping":"520-574","consensus_level":"high","plddt":87.5138,"start":520,"end":574}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NTJ5","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NTJ5-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NTJ5-F1-predicted_aligned_error_v6.png","plddt_mean":90.75},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SACM1L","jax_strain_url":"https://www.jax.org/strain/search?query=SACM1L"},"sequence":{"accession":"Q9NTJ5","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9NTJ5.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9NTJ5/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NTJ5"}},"corpus_meta":[{"pmid":"10224048","id":"PMC_10224048","title":"SAC1-like 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macromolecules","url":"https://pubmed.ncbi.nlm.nih.gov/40962084","citation_count":0,"is_preprint":false},{"pmid":"38258560","id":"PMC_38258560","title":"Effect of ATG8 or SAC1 deficiency on the cell proliferation and lifespan of the long-lived PMT1 deficiency yeast cells.","date":"2024","source":"FEMS microbiology letters","url":"https://pubmed.ncbi.nlm.nih.gov/38258560","citation_count":0,"is_preprint":false},{"pmid":"32186963","id":"PMC_32186963","title":"Cellular homeostasis in the Drosophila retina requires the lipid phosphatase Sac1.","date":"2020","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/32186963","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.02.10.637573","title":"Identification of the client-binding site on the Golgi membrane protein adaptor 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scores","date":"2025-09-15","source":"bioRxiv","url":"https://doi.org/10.1101/2025.09.09.675126","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":34290,"output_tokens":7643,"usd":0.108757,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":16865,"output_tokens":7220,"usd":0.132412,"stage2_stop_reason":"end_turn"},"total_usd":0.241169,"stage1_batch_id":"msgbatch_013LWZZT6MWaqB9qyTSeQBmU","stage2_batch_id":"msgbatch_0194ZzQaTGNpruoNJBbEMbt3","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1999,\n      \"finding\": \"The SAC1-like domain of yeast Sac1p defines a novel class of polyphosphoinositide phosphatase (PPIPase) with intrinsic enzymatic activity. Purified recombinant SAC1-like domain converts PI 3-phosphate, PI 4-phosphate, and PI 3,5-bisphosphate to PI, whereas PI 4,5-bisphosphate is not a substrate. Yeast lacking Sac1p exhibit 10-, 2.5-, and 2-fold increases in PI 4-phosphate, PI 3,5-bisphosphate, and PI 3-phosphate respectively.\",\n      \"method\": \"Purified recombinant protein in vitro phosphatase assay; lipid mass measurement in sac1 deletion yeast strains\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with purified recombinant protein, confirmed by in vivo lipid measurements in deletion strains; replicated across multiple substrate tests\",\n      \"pmids\": [\"10224048\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1989,\n      \"finding\": \"Mutations in SAC1 suppress defects in both yeast Golgi secretion (sec14, sec6, sec9 mutants) and actin cytoskeleton function, placing Sac1p at a node that coordinates secretory pathway and actin cytoskeleton activities.\",\n      \"method\": \"Genetic suppressor screen; double-mutant epistasis analysis\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis with multiple secretory and actin mutants, replicated across several loci\",\n      \"pmids\": [\"2687291\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Rat (mammalian) Sac1 is a ubiquitously expressed 65-kDa integral membrane protein of the ER with intrinsic phosphoinositide phosphatase activity directed toward PI 3-phosphate, PI 4-phosphate, and PI 3,5-bisphosphate. Mutant rat sac1 alleles evoke substrate-specific defects in enzymatic activity. PI 4-phosphate phosphatase activity, but not PI 3-phosphate or PI 3,5-bisphosphate phosphatase activity, is essential for complementation of yeast Sac1p defects in vivo.\",\n      \"method\": \"In vitro phosphatase assay with purified recombinant protein; active-site mutagenesis; heterologous complementation in yeast deletion strains\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro enzymatic assay combined with mutagenesis and in vivo complementation; multiple orthogonal approaches in single study\",\n      \"pmids\": [\"10887188\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Sac1p localizes primarily to the ER, and this localization is crucial for efficient turnover of PI 4-phosphate. The bulk of PI 4-phosphate that accumulates in sac1 mutant cells is generated by the Stt4 PI 4-kinase (not Pik1p), as demonstrated by double-mutant analysis. Loss of Sac1p activity causes vacuole morphology changes, lipid droplet accumulation, and Golgi function defects.\",\n      \"method\": \"Temperature-sensitive allele analysis; fluorescence microscopy for localization; double-mutant epistasis with stt4(ts) and pik1(ts); lipid measurements\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal genetic epistasis with two PI4-kinases, direct localization experiments, lipid quantification; multiple orthogonal methods\",\n      \"pmids\": [\"11514624\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Human SAC1 (hSAC1) exhibits the same substrate specificity as yeast Sac1p, localizes to the ER and Golgi, and interacts physically with COPI complex subunits. Mutation of a C-terminal KXKXX motif abolishes COPI interaction and causes hSAC1 accumulation in the Golgi. A catalytically inactive mutant also accumulates in the Golgi and fails to interact with COPI despite an intact KXKXX motif, suggesting that enzymatic activity provides a switch for COPI interaction motif accessibility.\",\n      \"method\": \"Co-immunoprecipitation; KXKXX motif mutagenesis; catalytic-dead mutant; subcellular localization by immunofluorescence\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP with COPI, mutagenesis of interaction motif and catalytic residue, localization experiments; single lab, multiple orthogonal methods\",\n      \"pmids\": [\"14527956\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"SAC1 accumulates at the Golgi in quiescent mammalian cells, depleting Golgi PI(4)P and down-regulating anterograde trafficking. Golgi localization requires SAC1 oligomerization and COPII recruitment. Mitogen stimulation activates the p38 MAPK pathway, which dissociates SAC1 oligomers, triggering COPI-mediated retrieval of SAC1 to the ER and releasing the brake on Golgi secretion.\",\n      \"method\": \"Subcellular fractionation; fluorescence microscopy; dominant-negative and kinase inhibitor experiments; p38 MAPK inhibition; PI(4)P measurements\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (localization, signaling pathway inhibition, lipid measurements), mechanistic pathway placed downstream of p38 MAPK\",\n      \"pmids\": [\"18299350\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Functional ablation of murine Sac1 results in preimplantation lethality. Sac1 insufficiency causes disorganization of mammalian Golgi membranes and mitotic defects with multiple mechanically active spindles. Both phosphoinositide phosphatase activity and ER localization (recycling from Golgi to ER) are required for Sac1 function in vivo.\",\n      \"method\": \"Sac1 knockout mouse; RNAi knockdown in mammalian cells; complementation with phosphatase-dead and ER-localization-defective mutants; fluorescence microscopy\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — knockout mouse phenotype combined with structure-function mutagenesis and cellular rescue experiments; multiple orthogonal methods\",\n      \"pmids\": [\"18480408\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Crystal structure of the Sac phosphatase domain of yeast Sac1 at 2.0 Å resolution reveals two closely packed sub-domains: a novel N-terminal sub-domain and the PI phosphatase catalytic sub-domain. The structure shows a unique conformation of the catalytic P-loop and a large positively charged groove at the catalytic site, suggesting an unusual dephosphorylation mechanism.\",\n      \"method\": \"X-ray crystallography at 2.0 Å; homology modeling of human Fig4/Sac3\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution crystal structure with functional implications; disease mutation mapping validated the structural relevance\",\n      \"pmids\": [\"20389282\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"During exponential growth, Sac1p interacts with Dpm1p at the ER. During starvation, Sac1p shuttles to the Golgi via COPII and Rer1 adaptor protein, specifically eliminating a pool of PI(4)P generated by Pik1p/Frq1p. Reciprocal association/dissociation of Sac1p and the Pik1p/Frq1p kinase complex controls growth-dependent Golgi PI(4)P levels.\",\n      \"method\": \"Co-immunoprecipitation (Sac1p-Dpm1p interaction); COPII and Rer1 mutant analysis; PI(4)P measurements under different growth conditions; fluorescence microscopy\",\n      \"journal\": \"Traffic (Copenhagen, Denmark)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP interaction, genetic epistasis with COPII/Rer1, lipid measurements; single lab\",\n      \"pmids\": [\"17908202\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Vps74 (yeast ortholog of human GOLPH3) binds directly to the catalytic domain of Sac1 (Kd = 3.8 μM) and functions as a sensor of PI(4)P levels on medial Golgi cisternae, directing Sac1-mediated dephosphorylation of this PI(4)P pool.\",\n      \"method\": \"In vitro binding assay (fluorescence anisotropy/ITC for Kd); PI(4)P reporter distribution analysis; genetic deletion epistasis\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct binding measured with quantitative in vitro assay (Kd determination), supported by in vivo PI(4)P reporter and genetic data; single lab but multiple orthogonal methods\",\n      \"pmids\": [\"22553352\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Sac1 is allosterically activated by anionic phospholipids: its product phosphatidylinositol and phosphatidylserine activate the enzyme, likely through conformational changes of the catalytic P-loop induced by binding of anionic phospholipids in the large cationic catalytic groove.\",\n      \"method\": \"In vitro phosphatase assay with purified recombinant Sac1; analysis of activation by different lipid species; structural interpretation based on crystal structure\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro reconstitution assay demonstrating allosteric activation, single lab, mechanistic model based on crystal structure but conformational change not directly observed\",\n      \"pmids\": [\"22452743\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Crystal structure of the N-terminal portion of yeast Sac1 in complex with Vps74 reveals the interaction interface involves the N-terminal subdomain of the Sac1 homology domain. Disruption of the Sac1-Vps74 interface results in broader distribution of PI(4)P within the Golgi and failure to maintain residence of a medial Golgi mannosyltransferase.\",\n      \"method\": \"X-ray crystallography of Sac1-Vps74 complex; interface mutagenesis; PI(4)P reporter imaging; Golgi resident enzyme localization assay\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure of protein complex plus functional validation by mutagenesis and cellular phenotype assays\",\n      \"pmids\": [\"25113029\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"14-3-3 protein acts as a cytosolic adaptor mediating SAC1 export from the ER in COPII-coated vesicles. Recombinant 14-3-3 stimulates packaging of SAC1 into COPII vesicles in a cell-free budding reaction. The COPII sorting subunit Sec24 interacts with 14-3-3. A minimal sorting motif (RLSNTSP, the 'LS motif') in SAC1 is required for 14-3-3 binding and controls SAC1 ER export.\",\n      \"method\": \"Cell-free COPII vesicle budding assay; Co-immunoprecipitation of Sec24 with 14-3-3; SAC1 motif mutagenesis; recombinant protein addition\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cell-free reconstitution assay plus mutagenesis of sorting motif and Co-IP of interaction; multiple orthogonal methods in single study\",\n      \"pmids\": [\"26056309\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"SAC1 acts in 'cis' configuration at the ER to degrade PtdIns4P, maintaining a steep PtdIns4P chemical gradient with donor membranes. Acute chemical inhibition of SAC1 causes PtdIns4P accumulation in the ER. SAC1 does not enrich at membrane contact sites and has little activity in 'trans' unless an artificial linker is added between its ER-anchor and catalytic domains.\",\n      \"method\": \"Acute chemical inhibition of SAC1; live-cell PtdIns4P biosensors; SAC1 localization imaging; engineered linker constructs for trans-activity test\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — acute inhibition with biosensor readout, engineered constructs to test cis vs. trans activity, multiple approaches in a single study\",\n      \"pmids\": [\"29461204\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"SAC1 undergoes reversible oxidative inactivation in mammalian cells: H2O2 oxidizes the catalytic Cys389 residue to form an intramolecular disulfide with Cys392, causing accumulation of PtdIns(4)P at the Golgi. EGF stimulation induces Ca2+-dependent Duox activation, H2O2 production at the Golgi, and transient SAC1 oxidation, linking EGF signaling to Golgi PtdIns(4)P control.\",\n      \"method\": \"Mass spectrometry of oxidized cysteines; Golgi-confined SAC1-K2A mutant; Duox knockdown; Golgi-targeted H2O2 probe; PtdIns(4)P measurements\",\n      \"journal\": \"Free radical biology & medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — identification of specific oxidized cysteine residues, use of organelle-targeted mutants and H2O2 probe, pathway knockdown; multiple orthogonal methods replicated across two related papers\",\n      \"pmids\": [\"30476538\", \"30448513\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"SAC1 activity on TGN PI(4)P occurs in-trans at ER-TGN contact sites (ERTGoCS) and requires the adaptor protein FAPP1. FAPP1 localizes at ERTGoCS, physically interacts with SAC1, and promotes SAC1's in-trans phosphatase activity in vitro. FAPP1 depletion increases TGN PI(4)P levels and enhances secretion of selected cargoes (e.g., ApoB100).\",\n      \"method\": \"Co-immunoprecipitation of FAPP1 with SAC1; in vitro trans-phosphatase activity assay; FAPP1 knockdown with PI(4)P and cargo secretion readouts; fluorescence localization\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution of trans-phosphatase activity plus reciprocal Co-IP and functional knockdown; multiple orthogonal methods in single study\",\n      \"pmids\": [\"30659099\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"TMEM39A/SUSR2 interacts with SAC1 and COPII SEC23/SEC24 subunits to promote ER-to-Golgi transport of SAC1. Depletion of SUSR2 retains SAC1 on the ER, increases PI(3)P produced by VPS34 complex, promotes autophagosome formation, and elevates late endosomal/lysosomal PI(4)P levels to facilitate HOPS complex recruitment and autophagosome maturation.\",\n      \"method\": \"Co-immunoprecipitation of SUSR2 with SAC1 and SEC23/SEC24; SUSR2 knockdown with PI(4)P and PI(3)P measurements; autophagy flux assays; HOPS recruitment assays\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP with multiple interactors, knockdown with lipid measurements and functional autophagy readouts; multiple orthogonal methods\",\n      \"pmids\": [\"31806350\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CPT1C (a malonyl-CoA sensor in the ER of neurons) regulates SAC1 catalytic activity: in normal conditions CPT1C down-regulates SAC1 activity, allowing efficient GluA1 (AMPA receptor subunit) trafficking to the plasma membrane. Under low malonyl-CoA (glucose depletion), CPT1C-dependent inhibition of SAC1 is released, SAC1 translocates to ER-TGN contact sites, decreasing TGN PI(4)P and triggering GluA1 retention at the TGN.\",\n      \"method\": \"SAC1 activity assay under CPT1C modulation; metabolic stress experiments (glucose depletion); PI(4)P measurements; GluA1 surface trafficking assay; SAC1 localization by fluorescence microscopy\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional SAC1 activity modulation shown with CPT1C interaction context, PI(4)P measurements, and cargo trafficking readouts; single lab, multiple approaches\",\n      \"pmids\": [\"32931550\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"The first transmembrane domain (TM1) of human SAC1 is sufficient for Golgi localization. A minimal TM1-containing construct concentrates at the Golgi, and transplanting TM1 into transferrin receptor 2 induces Golgi accumulation of that normally PM/endosomal protein. The N-terminal cytoplasmic domain of SAC1 also independently promotes Golgi localization.\",\n      \"method\": \"Truncation and chimeric constructs expressed in mammalian cells; fluorescence microscopy localization\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — domain swap experiments with chimeric proteins and truncations; single lab, multiple constructs\",\n      \"pmids\": [\"23936490\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Drosophila Sac1 loss-of-function causes defects in dorsal closure: specifically, improper activation of cell shape change in the amnioserosa and JNK signaling in the leading edge epidermis. sac1 shows dosage-sensitive genetic interactions with components of the JNK cascade and cell shape change pathway, placing Sac1 upstream or parallel to these events.\",\n      \"method\": \"Drosophila sac1 loss-of-function mutant analysis; genetic interaction (dosage-sensitive) with JNK pathway components; embryo phenotype imaging\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function with specific developmental phenotype and dosage-sensitive genetic interactions; single lab\",\n      \"pmids\": [\"14588244\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Drosophila Sac1 is required for axon guidance at the CNS midline. sac1 mutants show ectopic midline crossing of Fasciclin II-positive axon tracts. This phenotype is rescued by neuronal expression of wild-type Sac1 but not by a catalytically-inactive mutant. sac1 shows dosage-sensitive genetic interactions with slit and robo, placing Sac1-mediated PI regulation in the Slit/Robo axon repulsion pathway.\",\n      \"method\": \"Drosophila sac1 loss-of-function mutants; neuronal rescue with WT vs. catalytic-dead Sac1; dosage-sensitive genetic interaction with slit and robo\",\n      \"journal\": \"Molecules and cells\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — catalytic-dead rescue experiment places function in PI phosphatase activity; genetic epistasis with Slit/Robo pathway; single lab\",\n      \"pmids\": [\"22042447\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"SAC1 is required for autophagosome-lysosome fusion through its PI(4)P phosphatase activity. Sac1 deficiency causes dramatic accumulation of PI(4)P at early Golgi and abnormal incorporation of PI(4)P into Atg9 vesicles and autophagosomes, leading to failure to recruit SNARE proteins for autophagosome fusion with vacuoles. This function is conserved from yeast to mammalian cells.\",\n      \"method\": \"High-throughput screen in S. cerevisiae followed by mechanistic validation; PI(4)P reporter imaging; SNARE recruitment assays; Atg9 vesicle lipid analysis; mammalian cell validation\",\n      \"journal\": \"Autophagy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — mechanistic screen followed by detailed molecular validation with PI(4)P imaging, SNARE recruitment, and cross-species conservation; multiple orthogonal methods\",\n      \"pmids\": [\"32693712\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"SAC1 (SACM1L) is an essential regulator of xenophagy: depletion or inactivation of SAC1 results in aberrant accumulation of PI(4)P on Salmonella-containing autophagosomes, facilitating recruitment of the PI(4)P-binding Salmonella effector SteA, which impedes lysosomal fusion. Replication of Salmonella lacking SteA is suppressed by SAC1-deficient cells, confirming the mechanism.\",\n      \"method\": \"siRNA knockdown of SAC1; PI(4)P imaging on pathogen-containing autophagosomes; bacterial replication assays with SteA deletion Salmonella; epistasis between SAC1 and SteA\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — mechanistic pathway established through knockdown + PI(4)P imaging + genetic epistasis with bacterial effector; multiple orthogonal approaches\",\n      \"pmids\": [\"34320354\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Acute Sac1 degradation (auxin-inducible degron) in human cells causes immediate PI(4)P increase and cholesterol decrease in the TGN, followed by Golgi V-ATPase disassembly, TGN deacidification, Golgi fragmentation, and impaired glycosylation. Mechanistically, Sac1-mediated TGN membrane lipid composition maintains an assembly-promoting conformation of the V0a2 subunit of the V-ATPase.\",\n      \"method\": \"Auxin-inducible degron system for acute Sac1 degradation; PI(4)P and cholesterol biosensors; V-ATPase assembly assay; V0a2 conformation analysis; glycosylation assays; differentiated trophoblast model\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — acute and specific protein degradation with multiple lipid and organelle readouts plus mechanistic link to V-ATPase subunit conformation; multiple orthogonal methods\",\n      \"pmids\": [\"40841558\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Orthogonal targeting of SAC1 to mitochondria (PM-mitochondria contact sites) enhances PM PI(4)P turnover independently of ER-PM contact sites. This turnover is slowed by knockdown of soluble ORP2, implicating ORP2 as a major mediator of PI(4)P transfer from PM to sites where SAC1 can degrade it.\",\n      \"method\": \"Organelle-targeted SAC1 chimeras; ORP2 knockdown; live-cell PI(4)P biosensors at PM\",\n      \"journal\": \"Contact (Thousand Oaks)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — engineered targeting constructs with live-cell biosensor readout plus RNAi epistasis; single lab, multiple approaches\",\n      \"pmids\": [\"38327560\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"sac1 mutants exhibit accumulation of PI 4-phosphate that alone is insufficient to effect bypass of Sec14p function. Instead, phospholipase D activity generates diacylglycerol (DAG) downstream of elevated PI 4-phosphate, and this DAG effects bypass Sec14p. CDP-choline pathway activity contributes to the inositol auxotrophy of sac1 strains independently of INO1 transcription.\",\n      \"method\": \"Phospholipid metabolic labeling; genetic inactivation of phospholipase D; bacterial DAG kinase expression; sac1 mutant phenotype analysis\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic and biochemical dissection of PI4P accumulation vs. DAG generation in bypass; single lab, multiple genetic tools\",\n      \"pmids\": [\"10397762\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"In Drosophila, Sac1 is required for normal photoreceptor cell shape and microtubule stability in the developing eye. Sac1 mutant interommatidial cells show elevated PI(4)P, severe microtubule organization defects, and accumulation of the adhesion protein Roughest and exocyst subunit Sec8 in enlarged intracellular vesicles. Roughest is delivered to the cell surface in Sac1 mutants, indicating that Sac1 acts in microtubule-dependent exocyst trafficking rather than Roughest surface delivery per se.\",\n      \"method\": \"Temperature-sensitive sac1 allele in Drosophila; PI(4)P imaging; microtubule immunostaining; Roughest and Sec8 localization; cold fixation ex vivo assay\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — conditional allele with multiple cellular readouts; single lab\",\n      \"pmids\": [\"29752385\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SACM1L/SAC1 is a conserved integral ER membrane PI(4)P phosphatase (with additional activity toward PI(3)P and PI(3,5)P2) whose catalytic CX5R(T/S) motif dephosphorylates phosphoinositides primarily in cis at the ER to maintain steep PI(4)P gradients that drive non-vesicular lipid counter-transport; it cycles between ER and Golgi under control of COPII/COPI machinery (requiring a C-terminal KXKXX COPI-binding motif, a 14-3-3-dependent LS sorting motif, and its first transmembrane domain for Golgi retention), is regulated by p38 MAPK-induced oligomer dissociation and by reversible oxidation of its catalytic Cys by H2O2 downstream of EGF/Ca2+/Duox signaling, physically interacts with COPI, Vps74/GOLPH3, FAPP1, and TMEM39A/SUSR2 to control organelle-specific PI(4)P pools, and is required for Golgi V-ATPase assembly and TGN acidification, mitotic spindle organization, autophagosome-lysosome fusion, xenophagy, axon guidance (via Slit/Robo), and actin cytoskeleton organization.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SACM1L (SAC1) is a conserved integral ER membrane phosphoinositide phosphatase that controls the spatial distribution of PI(4)P across the secretory pathway, coupling lipid homeostasis to membrane trafficking, organelle architecture, and cytoskeletal organization [#0, #2, #6]. Its SAC1-homology catalytic domain hydrolyzes PI(3)P, PI(4)P, and PI(3,5)P2 to PI but spares PI(4,5)P2, and PI(4)P phosphatase activity is the function essential for cellular viability and for complementing yeast Sac1 defects [#0, #2]. The catalytic domain folds into an N-terminal subdomain packed against the PI-phosphatase subdomain with a distinctive P-loop and a large cationic catalytic groove, and the enzyme is allosterically activated by anionic phospholipid products binding this groove [#7, #10]. SAC1 acts predominantly in cis at the ER, degrading PI(4)P locally to sustain a steep PI(4)P gradient between donor membranes and the ER [#13]; at ER–TGN contact sites it can additionally act in trans through the adaptor FAPP1 [#15]. SAC1 dynamically cycles between ER and Golgi: ER export uses a 14-3-3/Sec24-dependent LS sorting motif into COPII vesicles, while retrieval from the Golgi requires a C-terminal KXKXX motif that binds the COPI complex, and its first transmembrane domain and N-terminal cytoplasmic domain independently confer Golgi retention [#4, #12, #18]. This trafficking, and hence Golgi PI(4)P levels, is regulated by p38 MAPK-driven oligomer dissociation, by EGF/Ca2+/Duox-dependent H2O2 oxidation of the catalytic Cys389–Cys392 pair, and by the GOLPH3 ortholog Vps74, which binds the catalytic domain directly to target medial-Golgi PI(4)P [#5, #14, #9, #11]. Through this control of organelle PI(4)P pools, SAC1 maintains Golgi V-ATPase assembly and TGN acidification, governs mitotic spindle organization, enables autophagosome–lysosome fusion and xenophagy of Salmonella, and directs developmental processes including Slit/Robo-dependent axon guidance and JNK-dependent epithelial morphogenesis [#23, #6, #21, #22, #20, #19]. Loss of murine Sac1 causes preimplantation lethality, underscoring its essential role [#6].\",\n  \"teleology\": [\n    {\n      \"year\": 1989,\n      \"claim\": \"Before any enzymatic role was known, genetics placed Sac1 at a node coordinating secretion and the actin cytoskeleton, framing it as a shared regulator of these processes.\",\n      \"evidence\": \"Genetic suppressor screen and double-mutant epistasis in yeast against sec14/sec6/sec9 and actin mutants\",\n      \"pmids\": [\"2687291\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No molecular activity assigned\", \"Mechanistic link between secretion and actin unresolved\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Established that the SAC1 domain is itself a phosphoinositide phosphatase with defined substrate range, converting genetic phenotypes into a concrete enzymatic activity.\",\n      \"evidence\": \"In vitro phosphatase assay on purified recombinant SAC1 domain plus lipid mass measurement in sac1 deletion yeast\",\n      \"pmids\": [\"10224048\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve which substrate is physiologically dominant\", \"No structural basis for catalysis\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Defined how PI(4)P accumulation in sac1 mutants is functionally read out, showing PLD-generated DAG rather than PI(4)P itself drives Sec14 bypass.\",\n      \"evidence\": \"Phospholipid metabolic labeling, PLD inactivation, and bacterial DAG kinase expression in yeast\",\n      \"pmids\": [\"10397762\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab biochemical dissection\", \"Relevance to mammalian Sac1 not tested\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Showed the mammalian enzyme conserves substrate specificity and that PI(4)P phosphatase activity specifically is the function required in vivo, prioritizing PI(4)P as the key substrate.\",\n      \"evidence\": \"In vitro assay with active-site mutants and heterologous yeast complementation of rat Sac1\",\n      \"pmids\": [\"10887188\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not address subcellular site of action\", \"Roles of PI(3)P/PI(3,5)P2 activity left open\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Localized Sac1 turnover activity to the ER and identified Stt4 as the kinase generating its PI(4)P substrate pool, establishing a kinase–phosphatase axis.\",\n      \"evidence\": \"Temperature-sensitive alleles, microscopy, and reciprocal epistasis with stt4 and pik1 in yeast\",\n      \"pmids\": [\"11514624\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not explain ER/Golgi cycling\", \"Mechanism connecting PI(4)P to vacuole/lipid-droplet phenotypes unresolved\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Defined the molecular logic of ER retrieval, linking a C-terminal KXKXX COPI motif to catalytic state and showing enzymatic activity gates COPI engagement.\",\n      \"evidence\": \"Co-IP with COPI, KXKXX and catalytic-dead mutagenesis, and localization in mammalian cells\",\n      \"pmids\": [\"14527956\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"ER export pathway not yet defined\", \"Structural basis of activity-dependent motif accessibility unknown\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Extended Sac1 function into metazoan development, placing it upstream of or parallel to JNK signaling in epithelial morphogenesis.\",\n      \"evidence\": \"Drosophila sac1 loss-of-function with dosage-sensitive interactions with JNK pathway components\",\n      \"pmids\": [\"14588244\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Lipid mechanism linking Sac1 to JNK not defined\", \"Single organism\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Resolved how Sac1 selects a kinase-specific PI(4)P pool, showing growth-dependent shuttling between Dpm1 at the ER and Pik1-generated Golgi PI(4)P via COPII/Rer1.\",\n      \"evidence\": \"Co-IP, COPII/Rer1 mutant analysis, and condition-dependent PI(4)P measurements in yeast\",\n      \"pmids\": [\"17908202\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab Co-IP evidence\", \"Mammalian conservation of Rer1 adaptor untested\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Connected Sac1 localization to mitogenic signaling, establishing p38 MAPK-driven oligomer dissociation as a switch releasing the Golgi secretion brake.\",\n      \"evidence\": \"Fractionation, microscopy, kinase inhibition, and PI(4)P measurements in mammalian cells\",\n      \"pmids\": [\"18299350\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Oligomerization interface not mapped\", \"Direct p38 substrate site on SAC1 unidentified\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Demonstrated organismal essentiality and assigned a mitotic role, showing both phosphatase activity and ER recycling are required in vivo.\",\n      \"evidence\": \"Sac1 knockout mouse, RNAi, and rescue with phosphatase-dead and ER-localization-defective mutants\",\n      \"pmids\": [\"18480408\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of spindle defect unresolved\", \"Lethality stage limits tissue-level analysis\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Provided the structural framework, revealing a two-subdomain fold with a cationic catalytic groove implying an unusual dephosphorylation mechanism.\",\n      \"evidence\": \"2.0 Å X-ray crystal structure of the yeast Sac phosphatase domain\",\n      \"pmids\": [\"20389282\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Membrane-engaged conformation not captured\", \"Substrate-bound state absent\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Identified an allosteric activation mechanism by anionic phospholipids, linking product and membrane lipid composition to catalytic output.\",\n      \"evidence\": \"In vitro phosphatase assays with defined lipid species interpreted on the crystal structure\",\n      \"pmids\": [\"22452743\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Conformational change inferred, not directly observed\", \"Single-lab in vitro evidence\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Defined GOLPH3/Vps74 as a direct PI(4)P-level sensor that recruits Sac1 to medial-Golgi cisternae, explaining sub-Golgi targeting.\",\n      \"evidence\": \"Quantitative in vitro binding (Kd 3.8 µM), PI(4)P reporter distribution, and genetic deletion in yeast\",\n      \"pmids\": [\"22553352\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mammalian GOLPH3-SAC1 functional equivalence not shown here\", \"Regulation of the interaction unknown\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Dissected the membrane determinants of Golgi retention, showing TM1 alone is sufficient and portable, with an additional N-terminal cytoplasmic contribution.\",\n      \"evidence\": \"Truncation and chimeric constructs with transferrin receptor 2 in mammalian cells\",\n      \"pmids\": [\"23936490\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Retention partners of TM1 unidentified\", \"Single-lab localization data\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Visualized the Sac1–Vps74 interface and showed its disruption broadens Golgi PI(4)P and mislocalizes a Golgi enzyme, tying lipid targeting to glycosylation residency.\",\n      \"evidence\": \"Crystal structure of the Sac1–Vps74 complex with interface mutagenesis and PI(4)P/enzyme localization assays\",\n      \"pmids\": [\"25113029\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo dynamics of the complex not resolved\", \"Conservation to human GOLPH3 structurally untested\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Defined the ER export step, identifying a 14-3-3-dependent LS sorting motif and Sec24 engagement that package SAC1 into COPII vesicles.\",\n      \"evidence\": \"Cell-free COPII budding reconstitution, Sec24/14-3-3 Co-IP, and LS-motif mutagenesis\",\n      \"pmids\": [\"26056309\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Phosphorylation controlling 14-3-3 binding not mapped\", \"Coupling to p38 oligomer regulation unresolved\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Resolved the long-standing cis-versus-trans question, showing SAC1 acts locally at the ER to maintain a steep PI(4)P gradient with little intrinsic trans activity.\",\n      \"evidence\": \"Acute chemical inhibition, live-cell PI(4)P biosensors, and engineered-linker trans-activity tests\",\n      \"pmids\": [\"29461204\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not exclude adaptor-assisted trans activity\", \"Identity of counter-transported lipids inferred not measured here\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Established redox regulation, identifying EGF/Ca2+/Duox-driven H2O2 oxidation of catalytic Cys389–Cys392 as a transient brake on Golgi PI(4)P turnover.\",\n      \"evidence\": \"Mass spectrometry of oxidized cysteines, Golgi-targeted H2O2 probe, Duox knockdown, and PI(4)P measurements\",\n      \"pmids\": [\"30476538\", \"30448513\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Reductase reversing oxidation unidentified\", \"Physiological output of transient oxidation not quantified\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Reconciled cis activity with TGN PI(4)P control by identifying FAPP1 as an adaptor enabling SAC1 trans activity at ER–TGN contact sites.\",\n      \"evidence\": \"Reciprocal Co-IP, in vitro trans-phosphatase assay, and FAPP1 knockdown with PI(4)P/cargo readouts\",\n      \"pmids\": [\"30659099\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of FAPP1-mediated trans activation unknown\", \"Cargo selectivity mechanism unresolved\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Identified TMEM39A/SUSR2 as a transport regulator coupling SAC1 ER-export to autophagy, linking SAC1 retention to PI(3)P and endolysosomal PI(4)P.\",\n      \"evidence\": \"Reciprocal Co-IP with SAC1 and SEC23/SEC24, knockdown with lipid measurements, and autophagy/HOPS assays\",\n      \"pmids\": [\"31806350\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct vs indirect effect on VPS34 not fully separated\", \"Mechanism of SUSR2-COPII coordination undefined\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Connected SAC1 to neuronal metabolic sensing, showing CPT1C tunes SAC1 activity to gate AMPA-receptor subunit trafficking under glucose stress.\",\n      \"evidence\": \"SAC1 activity assays under CPT1C modulation, glucose-depletion stress, PI(4)P measurements, and GluA1 surface trafficking\",\n      \"pmids\": [\"32931550\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct biochemical mechanism of CPT1C inhibition unresolved\", \"Single-lab evidence\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Placed Sac1 catalytic activity in the Slit/Robo axon-repulsion pathway, requiring PI phosphatase activity for proper midline guidance.\",\n      \"evidence\": \"Drosophila sac1 mutants rescued by WT but not catalytic-dead Sac1, with slit/robo dosage interactions\",\n      \"pmids\": [\"22042447\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Lipid intermediate linking Sac1 to Robo signaling not defined\", \"Single organism\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Linked Sac1 PI(4)P control to microtubule-dependent exocyst trafficking in a developmental epithelium.\",\n      \"evidence\": \"Conditional Drosophila sac1 allele with PI(4)P imaging, microtubule staining, and Roughest/Sec8 localization\",\n      \"pmids\": [\"29752385\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism connecting PI(4)P to microtubule stability unresolved\", \"Single-lab data\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Defined SAC1's role in autophagosome maturation, showing PI(4)P clearance is required for SNARE-mediated autophagosome–lysosome fusion, conserved across species.\",\n      \"evidence\": \"Yeast screen plus PI(4)P imaging, SNARE recruitment, Atg9-vesicle lipid analysis, and mammalian validation\",\n      \"pmids\": [\"32693712\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct SNARE-PI(4)P regulatory link not fully resolved\", \"Site of relevant SAC1 action during fusion unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Extended the autophagy role to host defense, showing SAC1 clears PI(4)P from Salmonella-containing autophagosomes to block the PI(4)P-binding effector SteA.\",\n      \"evidence\": \"siRNA knockdown, PI(4)P imaging on pathogen autophagosomes, and epistasis with SteA-deletion Salmonella\",\n      \"pmids\": [\"34320354\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How SAC1 accesses pathogen membranes undefined\", \"Other PI(4)P-binding effectors not surveyed\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Tested the limits of compartment-specific action by re-targeting SAC1 to PM–mitochondria contacts, implicating ORP2 as the lipid-transfer mediator delivering PM PI(4)P to SAC1.\",\n      \"evidence\": \"Organelle-targeted SAC1 chimeras with ORP2 knockdown and live-cell PM PI(4)P biosensors\",\n      \"pmids\": [\"38327560\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Engineered system may not reflect endogenous geometry\", \"Direct ORP2-SAC1 coupling not shown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Linked SAC1 TGN lipid control to organelle acidification, showing acute loss disassembles the V-ATPase via the V0a2 subunit conformation and impairs glycosylation.\",\n      \"evidence\": \"Auxin-inducible SAC1 degron with lipid biosensors, V-ATPase assembly and V0a2 conformation assays, and glycosylation readouts\",\n      \"pmids\": [\"40841558\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contributions of PI(4)P rise vs cholesterol drop not separated\", \"How lipids set V0a2 conformation mechanistically undefined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the multiple regulatory inputs (p38 oligomer dissociation, 14-3-3/COPII export, COPI retrieval, redox oxidation, GOLPH3/FAPP1/CPT1C adaptors) are integrated to set organelle-specific PI(4)P set points in real time remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No unified quantitative model of SAC1 cycling and activity\", \"Hierarchy and crosstalk among regulators unmapped\", \"Human structural basis for adaptor-controlled trans activity lacking\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [0, 2, 13, 15]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [10]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [2, 3, 13]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [4, 5, 18]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [24]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [4, 5, 12]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [16, 21, 22]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 3]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [19, 20]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"COPI\", \"GOLPH3/Vps74\", \"FAPP1\", \"TMEM39A/SUSR2\", \"14-3-3\", \"SEC24\", \"CPT1C\", \"DPM1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}