LPIN3 is a magnesium-dependent phosphatidate phosphatase of the mammalian lipin family. Its conserved DXDXT catalytic motif hydrolyzes phosphatidic acid to diacylglycerol and orthophosphate, supplying diacylglycerol for triacylglycerol and phospholipid synthesis. Recombinant lipin-3 has phosphatidate-specific PAP1 activity under the tested conditions, and its catalytic activity and association with phosphatidate are comparatively insensitive to phosphorylation in vitro. LPIN3 is observed in a cytosolic pool, while pathway models place catalysis at the endoplasmic-reticulum membrane where the lipid substrate is accessible; stable endogenous human LPIN3 residence at that membrane has not been demonstrated. LPIN3 expression is prominent in intestine and other gastrointestinal regions. Human adipocyte perturbations indicate substantial functional redundancy among LPIN1, LPIN2, and LPIN3. Mouse genetics similarly shows cooperative Lpin1/Lpin3 control of adipose PAP activity and combined Lpin2/Lpin3 control of intestinal phospholipid homeostasis and chylomicron production, rather than an isolated human LPIN3-specific role. LPIN3 can associate with LPIN1 in experimental oligomerization systems, but a constitutive endogenous complex is not established. Human LPIN3 has two curated splice isoforms; isoform 2 contains a one-residue insertion, with no demonstrated isoform-specific function. Its conserved LXXIL motif and inferred nuclear localization are consistent with inherited transcriptional coactivation and fatty-acid metabolic roles, whose LPIN3-specific mechanisms remain incompletely characterized. Mouse lipin-3 differs from lipin-1 in acute insulin-dependent phosphorylation; this does not establish absence of every insulin-related role in human LPIN3.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: Retain the phylogenetically inferred nuclear pool of LPIN3. Reason: PTHR12181 places nuclear localization at PTN000256565. The annotation is reviewed for location, not its machine-supplied relationship wording. A conserved polybasic region and the separate UniProt-derived nuclear annotation are consistent with this localization; no LPIN3-specific exclusion was established. |
| GO:0008195 phosphatidate phosphatase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Accepted as the defining catalytic activity of LPIN3. Reason: Recombinant lipin-3 has directly demonstrated magnesium-dependent, phosphatidate-specific PAP1 activity, and LPIN3 retains the conserved DXDXT catalytic motif. The broad phylogenetic transfer is independently corroborated for this paralog. Propagation Review Root cause: NO FAILURE CORE Sources checked: FB:FBgn0263593 SUPPORTS TRANSFER Exact WITH/FROM source consistent with conserved lipin phosphatidate phosphatase activity. MGI:MGI:1891340 SUPPORTS TRANSFER Exact WITH/FROM source consistent with conserved lipin phosphatidate phosphatase activity. MGI:MGI:1891341 SUPPORTS TRANSFER Exact WITH/FROM source consistent with conserved lipin phosphatidate phosphatase activity. MGI:MGI:1891342 SUPPORTS TRANSFER Exact WITH/FROM source consistent with conserved lipin phosphatidate phosphatase activity. PANTHER:PTN000256564 SUPPORTS TRANSFER Exact ancestral-node source consistent with conserved lipin phosphatidate phosphatase activity. SGD:S000004775 SUPPORTS TRANSFER Exact WITH/FROM source consistent with conserved lipin phosphatidate phosphatase activity. UniProtKB:Q14693 SUPPORTS TRANSFER Exact human paralog source consistent with independently demonstrated LPIN3 activity. UniProtKB:Q92539 SUPPORTS TRANSFER Exact human paralog source consistent with independently demonstrated LPIN3 activity. |
| GO:0009062 fatty acid catabolic process | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Retain fatty-acid catabolism as a non-core phylogenetic process inference. Reason: The ancestral placement at PTN000256565 represents more than an inference from PAP chemistry. LPIN3 PAP activity supplies biosynthetic DAG, but this specialization does not exclude a separate inherited transcriptional role influencing fatty-acid catabolism. No target-specific loss of that role was identified. Retain the process as an inference pending the focused report; the PAP reaction itself is not evidence of fatty-acid breakdown. |
| GO:0019432 triglyceride biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: Accepted as the principal biosynthetic process supplied by LPIN3 phosphatidate phosphatase activity. Reason: LPIN3 directly generates diacylglycerol from phosphatidate, providing the immediate precursor for triacylglycerol synthesis; Reactome also places human LPIN3 in this pathway. This is a mechanistically appropriate process-level transfer. Propagation Review Root cause: NO FAILURE CORE Sources checked: FB:FBgn0263593 SUPPORTS TRANSFER Exact WITH/FROM source consistent with the conserved PA-to-DAG contribution to triglyceride synthesis. MGI:MGI:1891340 SUPPORTS TRANSFER Exact WITH/FROM source consistent with the conserved PA-to-DAG contribution to triglyceride synthesis. MGI:MGI:1891341 SUPPORTS TRANSFER Exact WITH/FROM source consistent with the conserved PA-to-DAG contribution to triglyceride synthesis. PANTHER:PTN000256565 SUPPORTS TRANSFER Exact ancestral-node source consistent with the conserved PA-to-DAG contribution to triglyceride synthesis. SGD:S000004775 SUPPORTS TRANSFER Exact WITH/FROM source consistent with the conserved PA-to-DAG contribution to triglyceride synthesis. UniProtKB:Q14693 SUPPORTS TRANSFER Exact human paralog source consistent with independently demonstrated LPIN3 catalysis. |
| GO:0003713 transcription coactivator activity | IBA GO_REF:0000033 | ACCEPT | Summary: Retain the inherited nuclear coactivation function while distinguishing it from direct LPIN3 experimental evidence. Reason: PTHR12181 places transcription coactivation and positive regulation of RNA polymerase II transcription at PTN000953144. Published assays directly establish coactivation in other lipins, and LPIN3 retains the LXXIL motif and inferred nuclear localization. These facts do not independently prove LPIN3 coactivation, but there is no demonstrated LPIN3-specific loss that would overturn the curated ancestral placement. The previous over-annotation judgment relied on missing direct experiments. Retain the IBA while the focused assessment examines functional divergence. |
| GO:0032869 cellular response to insulin stimulus | IBA GO_REF:0000033 | UNDECIDED | Summary: The broad insulin-response process remains unresolved after distinguishing phosphorylation and expression readouts. Reason: PTN000953144 is the current ancestral insulin-response IBD. PMID:28982975 directly finds no acute insulin/Torin-dependent phosphorylation of mouse lipin-3, rather than demonstrating absence of every insulin-related function of human LPIN3. Conversely, PMID:18245816 finds that insulin suppresses dex-induced mouse Lpin3 mRNA while having no effect alone. Expression responsiveness does not by itself establish a gene-product function in the process. These condition-specific results require assessment of which role the IBD inherits; neither a blanket rejection nor direct human experimental support is justified. Propagation Review Root cause: UNRESOLVED Sources checked: PANTHER:PTN000953144 · PTN000953144 UNRESOLVED Ortholog experiments separate absent acute phosphorylation from condition-dependent transcript regulation; the inherited process role remains unresolved. Supporting Evidence: PMID:28982975 insulin and Torin 1 treatment had no effect on the levels of phosphate incorporation into lipin 3. PMID:18245816 CPTcAMP or insulin alone had no significant effect relative to the nontreated control, but both decreased the dex-induced increase in lipin-3 mRNA |
| GO:0045944 positive regulation of transcription by RNA polymerase II | IBA GO_REF:0000033 | ACCEPT | Summary: Retain the inherited nuclear coactivation function while distinguishing it from direct LPIN3 experimental evidence. Reason: PTHR12181 places transcription coactivation and positive regulation of RNA polymerase II transcription at PTN000953144. Published assays directly establish coactivation in other lipins, and LPIN3 retains the LXXIL motif and inferred nuclear localization. These facts do not independently prove LPIN3 coactivation, but there is no demonstrated LPIN3-specific loss that would overturn the curated ancestral placement. The previous over-annotation judgment relied on missing direct experiments. Retain the IBA while the focused assessment examines functional divergence. |
| GO:0005634 nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: Accepted as a plausible LPIN3 localization supported by the curated UniProt subcellular-location mapping. Reason: LPIN3 contains a predicted nuclear-localization signal and UniProt assigns a nuclear location. The IEA correctly represents the mapped source, while direct endogenous localization and isoform dependence remain unresolved. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB-SubCell:SL-0191 SUPPORTS TRANSFER Exact WITH/FROM vocabulary source for the UniProt nuclear-location mapping. |
| GO:0008195 phosphatidate phosphatase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Accepted as a well-supported automated assignment of LPIN3 phosphatidate phosphatase activity. Reason: The combined mapping from the mouse ortholog, conserved reaction and EC identifiers agrees with directly demonstrated phosphatidate-specific PAP1 activity of recombinant lipin-3 and its intact DXDXT motif. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:Q99PI4 SUPPORTS TRANSFER Exact mouse Lpin3 ortholog source with conserved catalytic activity. ensembl:ENSMUSP00000043053 SUPPORTS TRANSFER Exact mouse ortholog protein source supporting the conserved assignment. RHEA:27429 SUPPORTS TRANSFER Exact reaction source matching phosphatidate hydrolysis to diacylglycerol and phosphate. EC:3.1.3.4 SUPPORTS TRANSFER Exact enzyme-class source matching phosphatidate phosphatase activity. |
| GO:0005829 cytosol | IDA GO_REF:0000052 | ACCEPT | Summary: Accepted as direct imaging-based evidence for a cytosolic LPIN3 pool. Reason: The Human Protein Atlas immunofluorescence curation directly supports cytosolic localization. This is compatible with soluble lipin proteins partitioning between cytosol and substrate-containing membranes. |
| GO:0008195 phosphatidate phosphatase activity | ISS GO_REF:0000024 | ACCEPT | Summary: Accepted orthology transfer of LPIN3 phosphatidate phosphatase activity. Reason: Mouse Lpin3 is the direct ortholog and the transferred catalytic activity is independently corroborated by recombinant lipin-3 biochemistry and the conserved DXDXT motif. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:Q99PI4 SUPPORTS TRANSFER Exact mouse Lpin3 ortholog source; the catalytic function is conserved and independently supported for LPIN3. |
| GO:0005789 endoplasmic reticulum membrane | TAS Reactome:R-HSA-1483203 | ACCEPT | Summary: Accepted as the membrane context in which LPIN3 accesses phosphatidate for catalysis. Reason: Reactome places LPIN3 with LPIN1 and LPIN2 at the endoplasmic-reticulum membrane for phosphatidate-to-diacylglycerol conversion. LPIN3 catalytic activity is established, but stable ER association has not been demonstrated directly for human LPIN3, so the annotation should not be read as a stable membrane residency claim. |
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Download this section (compressed HTML)Q: What fraction of phosphatidate phosphatase activity and glycerolipid flux is supplied by LPIN3 in human gastrointestinal epithelium, adipocytes, liver, and other tissues?
Q: Which lipid, phosphorylation, or protein signals recruit endogenous LPIN3 from the cytosolic pool to phosphatidate-containing membranes?
Q: Does human LPIN3 directly coactivate a nuclear receptor or other transcription factor, and if so, which endogenous target genes and physiological contexts are involved?
Q: Which mouse intestinal and adipose phenotypes persist when Lpin3 is perturbed alone, rather than jointly with Lpin2 or Lpin1?
Q: Do Q9BQK8-1 and Q9BQK8-2 differ in expression, PAP kinetics, localization, regulation, or interaction with other lipins?
Q: Does endogenous LPIN3 form stable homo- or hetero-oligomers, or are the reported associations transient and context-dependent?
Experiment: Generate paralog-resolved LPIN1, LPIN2, and LPIN3 single and combinatorial knockouts in human intestinal organoids and adipocytes, then quantify phosphatidate and diacylglycerol species, PAP activity, and isotope-traced triacylglycerol and phospholipid synthesis.
Hypothesis: LPIN3 makes a tissue-dependent contribution to PA-to-DAG flux that is masked by LPIN1 or LPIN2 compensation in standard cultured cells.
Experiment: Endogenously tag LPIN3 and perform live-cell imaging with ER and phosphatidate sensors, coupled to acute lipid, nutrient, and phosphorylation perturbations and catalytic-dead controls.
Hypothesis: LPIN3 is recruited transiently from the cytosol to phosphatidate-rich ER domains rather than residing stably at the ER membrane.
Experiment: Compare purified and endogenously expressed Q9BQK8-1 and Q9BQK8-2 using kinetic, membrane-binding, phosphoproteomic, localization, and rescue assays.
Hypothesis: The one-residue insertion in isoform 2 has little effect on intrinsic PAP catalysis but may alter regulatory interactions or membrane recruitment.
Experiment: Test LPIN3-dependent nuclear-receptor coactivation with catalytic-dead and LXXIL-mutant rescue constructs, reporter assays, endogenous target-gene measurements, and chromatin occupancy in human gastrointestinal cells.
Hypothesis: If LPIN3 has a nuclear coactivator function, it will depend on the LXXIL motif and be separable from phosphatidate phosphatase catalysis.
Experiment: Measure endogenous LPIN3 self-association and association with LPIN1/LPIN2 by calibrated co-immunoprecipitation, proximity labeling, and single-molecule or cross-linking methods across tissues and metabolic states.
Hypothesis: LPIN3 oligomerization is regulated and context-dependent rather than a fixed stable complex required for PAP activity.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The quantitative contribution of endogenous human LPIN3 to phosphatidate-to-DAG flux relative to LPIN1 and LPIN2 is unknown across tissues. Recombinant activity is direct, but human adipocyte experiments reveal substantial paralog compensation.
Gap: Native human LPIN3 membrane recruitment, residence time, and organelle specificity are unresolved. Cytosolic localization is observed and ER catalysis is modeled, but stable endogenous LPIN3 association with the ER has not been directly demonstrated.
Gap: Lipin-3 phosphoregulation has been characterized in vitro, but the signals that control endogenous LPIN3 abundance, phosphatidate access, and catalytic output in human cells remain poorly defined and should not be inferred from LPIN1 insulin or glucocorticoid responses.
Gap: LPIN3-specific contributions to human gastrointestinal lipid handling, adipogenesis, and lipoprotein production remain unresolved. Current physiology is derived mainly from mouse double-mutant or cooperative-paralog experiments.
Gap: A direct transcriptional coactivator function has not been shown for LPIN3. The conserved LXXIL motif and nuclear localization are suggestive, but available direct family experiments establish coactivation for LPIN1 and LPIN2.
Gap: No study has compared Q9BQK8-1 with the one-residue-insertion isoform Q9BQK8-2 for catalytic activity, localization, regulation, tissue expression, or partner binding.
Gap: The stoichiometry, tissue distribution, and functional effect of endogenous LPIN3 oligomerization are unknown. Experimental LPIN1-LPIN3 association does not establish a constitutive stable human complex.
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