ETNPPL (formerly AGXT2L1) encodes ethanolamine-phosphate phospho-lyase, a pyridoxal 5'-phosphate (PLP)-dependent enzyme of the class-III aminotransferase fold that irreversibly degrades O-phosphoethanolamine to acetaldehyde, ammonia and inorganic phosphate. Although it evolved from transaminases, the purified human enzyme has no significant aminotransferase activity and is highly selective for phosphoethanolamine, with an active site tuned to bind dianionic groups; its crystal structure (PDB 6TOR) shows the class-III fold with bound pyridoxamine phosphate. Because phosphoethanolamine is the precursor of phosphatidylethanolamine in the CDP-ethanolamine (Kennedy) pathway, ETNPPL removes substrate from phospholipid synthesis; loss of the mouse enzyme increases conversion of ethanolamine to phosphoethanolamine and phosphatidylethanolamine in hepatocytes, and overexpression in hepatoma cells lowers both. ETNPPL is expressed mainly in brain, where it is an astrocyte-selective, fasting- and glucocorticoid-induced gene, and in liver. Antibody staining in human and mouse brain places the protein in the nucleus and cytoplasm; a mitochondrial location has been inferred but not shown experimentally. Altered ETNPPL expression has been reported in neuropsychiatric disorders, gliomas and liver disease.
Definition: The chemical reactions and pathways resulting in the breakdown of O-phosphoethanolamine (ethanolamine phosphate).
Justification: ETNPPL catalyses the only known dedicated step of phosphoethanolamine breakdown in animals; GO currently offers only the broad parent organophosphate catabolic process or ceramide phosphoethanolamine catabolic process, which is a different substrate.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005515 protein binding | IPI PMID:28514442 Architecture of the human interactome defines protein commun... | REMOVE | Summary: High-throughput interactome (BioPlex/HuRI-type) detection of an interaction between ETNPPL and ZC3HC1 (Q86WB0). Reason: Bare protein binding carries no functional information. The ZC3HC1 interaction comes from a proteome-scale screen and has no known bearing on the phospho-lyase function of ETNPPL; no more informative molecular function term is supported by this paper. Removal does not imply the interaction is false. |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | REMOVE | Summary: Second proteome-scale interactome detection of the same ETNPPL-ZC3HC1 interaction. Reason: As for the other ZC3HC1 row, generic protein binding from a high-throughput screen is uninformative and no specific molecular function can be derived from it. Removal does not imply the interaction is false. |
| GO:0005739 mitochondrion | IEA GO_REF:0000044 | UNDECIDED | Summary: Mapped from the UniProt subcellular location "Mitochondrion", which UniProt itself marks as a curator inference (ECO:0000305) rather than an experimental observation. Reason: No experimental study has localized ETNPPL to mitochondria. The direct localization data available point elsewhere: immunofluorescence in human cortex (with a peptide-competition control) found ETNPPL in the cytoplasm or the nucleus, and immunohistochemistry in mouse brain with knockout-validated monoclonal antibodies found it predominantly nuclear. Tissue-level staining cannot exclude a mitochondrial pool, and the mitochondrial assignment may reflect analogy with AGXT2, a mitochondrial class-III transaminase in the same PANTHER family. Note that PAINT propagated mitochondrion from the AGXT2 node (PTN002485563) but not to the ETNPPL node, which received only the phospho-lyase activity. Neither accepted nor removed until fractionation or co-localization data exist. Supporting Evidence: file:human/ETNPPL/ETNPPL-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion {ECO:0000305}. PMID:32218467 ETNPPL is detected in the cytoplasm (yellow arrow) or the nucleus (red arrow). PMID:36794261 In the mouse brain, we observed that ETNPPL was predominantly localized in the cell nuclei |
| GO:0005759 mitochondrial matrix | IEA GO_REF:0000117 | UNDECIDED | Summary: ARBA machine-learning rule assigning mitochondrial matrix. Reason: The ARBA prediction is not independent evidence, and there is no experimental support for a matrix location; antibody studies in human and mouse brain detected ETNPPL in the nucleus and cytoplasm. Kept undecided, consistent with the parent mitochondrion row, pending direct localization data. Supporting Evidence: PMID:32218467 ETNPPL is detected in the cytoplasm (yellow arrow) or the nucleus (red arrow). PMID:36794261 In the mouse brain, we observed that ETNPPL was predominantly localized in the cell nuclei |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-5696415 | UNDECIDED | Summary: Reactome places the phosphoethanolamine phospho-lyase reaction in mitochondria, citing Veiga-da-Cunha et al. 2012. Reason: The Reactome statement is a traceable author statement whose cited source (PMID:22241472) characterised the purified recombinant enzyme and does not, in the abstract or the cached text, report a localization experiment. Experimental antibody staining in human and mouse brain detects the protein in the nucleus and cytoplasm. The matrix location is therefore unconfirmed rather than refuted. Supporting Evidence: Reactome:R-HSA-5696415 In mitochondria, ethanolamine-phosphate phospho-lyase and 5-phosphohydroxy-L-lysine phospho-lyase (ETNPPL and PHYKPL respectively) are two closely related pyridoxal-phosphate-dependent, homotetrameric ammoniophospholyases PMID:36794261 In the mouse brain, we observed that ETNPPL was predominantly localized in the cell nuclei |
| GO:0030170 pyridoxal phosphate binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro class-III aminotransferase signature mapping to PLP binding. Reason: ETNPPL is a PLP-dependent enzyme. UniProt records PLP as cofactor from experimental work on the purified human protein, and the crystal structure of the human enzyme contains the related cofactor pyridoxamine 5'-phosphate in the active site. PLP binding is integral to the catalytic function. Supporting Evidence: file:human/ETNPPL/ETNPPL-uniprot.txt Name=pyridoxal 5'-phosphate; Xref=ChEBI:CHEBI:597326; PMID:25327712 Human O-phosphoethanolamine (PEA) phospho-lyase is a pyridoxal 5'-phosphate (PLP) dependent enzyme that catalyzes the degradation of PEA to acetaldehyde, phosphate and ammonia. PMID:32254049 the crystal structure of hEtnppl in complex with pyridoxamine 5'-phosphate was determined at 2.05 Å resolution |
| GO:0050459 ethanolamine-phosphate phospho-lyase activity | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT IBD at node PTN002904331 (Euteleostomi ETNPPL clade, PANTHER PTHR45688:SF1), seeded by the experimental annotation of human ETNPPL itself. Reason: ETNPPL lies inside the clade at which the curator placed the activity, and the target's own IDA is the descendant evidence for the node, which is expected rather than circular. The IBA adds the claim that the activity is shared by vertebrate SF1 orthologues. The node is limited to the vertebrate ETNPPL clade; it is not a basis for transferring the phosphoethanolamine-specific activity to proteins outside SF1, such as PHYKPL (SF6) or the fly CG8745 (SF13). Supporting Evidence: PMID:22241472 AGXT2L1 catalyzed a similar reaction on phosphoethanolamine, converting it to ammonia, inorganic phosphate, and acetaldehyde. |
| GO:0050459 ethanolamine-phosphate phospho-lyase activity | IDA PMID:22241472 Molecular identification of hydroxylysine kinase and of ammo... | ACCEPT | Summary: Purified recombinant human AGXT2L1/ETNPPL converts phosphoethanolamine to ammonia, inorganic phosphate and acetaldehyde in a PLP-dependent reaction (Km 680 uM, Vmax 1.46 umol/min/mg at pH 7.4). Reason: This is the defining activity of ETNPPL, shown directly with the human enzyme and independently confirmed by the Peracchi laboratory, which found the enzyme at least 500-fold more active as a lyase than as an aminotransferase and very selective for O-phosphoethanolamine. In vivo, loss of mouse Etnppl increases flux of ethanolamine into phosphoethanolamine and phosphatidylethanolamine, consistent with this activity operating in cells. Supporting Evidence: PMID:22241472 AGXT2L1 catalyzed a similar reaction on phosphoethanolamine, converting it to ammonia, inorganic phosphate, and acetaldehyde. PMID:23761375 is at least 500-fold more active as a lyase than as an aminotransferase file:human/ETNPPL/ETNPPL-uniprot.txt Reaction=phosphoethanolamine + H2O = acetaldehyde + NH4(+) + phosphate; |
| GO:0050459 ethanolamine-phosphate phospho-lyase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic mapping from the UniProt catalytic activity (RHEA:17889, EC 4.2.3.2), which is itself based on experimental evidence. Reason: Consistent with the IDA and IBA rows for the same term; the underlying UniProt catalytic-activity statement is experimentally supported (ECO:0000269, PubMed:22241472). Supporting Evidence: PMID:22241472 AGXT2L1 catalyzed a similar reaction on phosphoethanolamine, converting it to ammonia, inorganic phosphate, and acetaldehyde. |
| GO:0046434 organophosphate catabolic process | IDA PMID:22241472 Molecular identification of hydroxylysine kinase and of ammo... | NEW | Summary: ETNPPL itself catalyses the breakdown of the organophosphate O-phosphoethanolamine; no biological process annotation currently exists. Reason: Participation test: ETNPPL is the enzyme that performs the catabolic step (irreversible elimination of phosphate and ammonia from phosphoethanolamine), not a substrate or a required factor. The cellular relevance of this catabolism is supported by mouse Etnppl knockout hepatocytes, which convert more ethanolamine to phosphoethanolamine and phosphatidylethanolamine, and by ETNPPL overexpression in Huh7 cells, which reduces cellular phosphoethanolamine. GO has no phosphoethanolamine-specific catabolic term (OLS search returned only ceramide phosphoethanolamine catabolic process), so the nearest accurate parent, organophosphate catabolic process, is used and a specific term is proposed. The lipid and lipoprotein phenotypes of knockout mice are downstream consequences and are not annotated. Supporting Evidence: PMID:22241472 AGXT2L1 catalyzed a similar reaction on phosphoethanolamine, converting it to ammonia, inorganic phosphate, and acetaldehyde. PMID:39312446 Primary hepatocytes isolated from Etnppl-/- mice showed increased conversion of [3H]ethanolamine to [3H]p-ETN and [3H]PE compared to Etnppl+/+ mice. PMID:40154596 We show that ETNPPL reduces cellular pEtn synthesized from ethanolamine |
| GO:0005634 nucleus | IDA PMID:32218467 Transformation Foci in IDH1-mutated Gliomas Show STAT3 Phosp... | NEW | Summary: Immunofluorescence of human cortex and glioma cells, with two antibodies and a peptide-competition control, detects ETNPPL in the nucleus or the cytoplasm; mouse brain staining with knockout-validated antibodies shows predominantly nuclear ETNPPL. Reason: Two independent studies with specificity controls (peptide blocking and a second antibody in human tissue; knockout validation in mouse) report nuclear ETNPPL in astrocytes and glioma cells. The functional significance of the nuclear pool is unknown, so this location is non-core. Supporting Evidence: PMID:32218467 ETNPPL is detected in the cytoplasm (yellow arrow) or the nucleus (red arrow). PMID:36794261 In the mouse brain, we observed that ETNPPL was predominantly localized in the cell nuclei |
Loading supporting content…
Download this section (compressed HTML)Q: Where in the cell does ETNPPL act? Antibody staining finds it mainly in the nucleus and cytoplasm, while databases infer a mitochondrial matrix location. Is there a mitochondrial pool, and does the nuclear pool have catalytic activity?
Q: Is the phosphoethanolamine phospho-lyase activity conserved in the invertebrate PTHR45688:SF13 relatives such as Drosophila CG8745, which branch outside the vertebrate ETNPPL (SF1) and PHYKPL (SF6) subfamilies, or do those proteins act on a different phosphorylated amine?
Q: What is the physiological purpose of irreversibly diverting phosphoethanolamine from phospholipid synthesis in fasting astrocytes?
Experiment: Fractionate astrocytes or hepatocytes expressing endogenous ETNPPL into nuclear, cytosolic and mitochondrial (with protease protection) fractions and co-stain with organelle markers, using knockout cells as negative controls.
Hypothesis: ETNPPL is predominantly nucleocytoplasmic rather than mitochondrial.
Type: Subcellular fractionation and imaging
Experiment: Express Drosophila CG8745 and other SF13 members and assay against phosphoethanolamine, 5-phosphohydroxy-L-lysine and related phosphorylated amines, with human ETNPPL and PHYKPL as positive controls.
Hypothesis: The substrate preference of SF13 members differs from or overlaps with vertebrate ETNPPL, indicating whether activity can be transferred across subfamilies.
Type: Enzyme assay of homologues
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)