GPAT2 (glycerol-3-phosphate acyltransferase 2, mitochondrial) is an 800-residue member of the GPAT/DHAPAT lipid acyltransferase family. By orthology to the mouse enzyme it transfers a fatty acyl chain from acyl-CoA, with a preference for arachidonoyl-CoA, to the sn-1 position of sn-glycerol 3-phosphate to form lysophosphatidic acid, the first step of de novo phosphatidic acid and triacylglycerol synthesis. The protein sits in the mitochondrial outer membrane with two membrane-spanning segments and its catalytic HXXXXD motif facing the cytosol. Expression is largely restricted to the testis, where in mouse it supports the synthesis of arachidonate-rich triacylglycerol in spermatocytes. Mouse GPAT2 also binds the PIWI protein MILI (PIWIL2) and is needed for primary piRNA production in germline stem cells, a role that does not require its acyltransferase activity.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003841 1-acylglycerol-3-phosphate O-acyltransferase activity | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Ensembl Compara transfer from mouse Gpat2 (Q14DK4) of 1-acylglycerol-3-phosphate (LPA) acyltransferase activity. The mouse evidence is a 2-fold increase in AGPAT activity in membranes of CHO-K1 cells overexpressing GPAT2, with arachidonoyl-CoA. Reason: The horse protein is in the same PANTHER subfamily (PTHR12563:SF15) as mouse Gpat2, so the transfer stays within the GPAT2 ortholog group. The donor evidence is weak for an intrinsic activity: it is an increase in AGPAT activity in total membranes of overexpressing cells, and the authors state that they cannot tell a second activity of GPAT2 from an indirect effect. PAINT curators propagated GPAT activity but not AGPAT activity at node PTN000970957, which I treat as a deliberate decision. Kept as a possible secondary in vitro activity, not core. Supporting Evidence: PMID:22905194 These findings suggest that the increase in PA may be due to an AGPAT activity for this heterologously-expressed protein, but we cannot exclude the possibility that the protein has dual enzymatic activities. PMID:22905194 GPAT2 overexpression significantly increased both GPAT and AGPAT activities only when arachidonoyl-CoA was used as a substrate |
| GO:0004366 glycerol-3-phosphate O-acyltransferase activity | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT propagation of glycerol-3-phosphate O-acyltransferase activity from the eumetazoan GPAT1/GPAT2 node PTN000970957, which the horse protein descends from. Reason: GPAT activity is the ancestral function of the GPAT1/GPAT2 clade and has been shown directly for mouse Gpat2 in two independent cloning studies. The horse protein is a full-length GPAT2 ortholog (PTHR12563:SF15, 88% identical to human GPAT2) and keeps the HXXXXD catalytic motif at the same position. This is the core activity. Supporting Evidence: PMID:17689486 When Cos-7 cells were transiently transfected with GPAT2, NEM-sensitive GPAT activity increased 30%. PMID:17013544 xGPAT1 was distributed in the membrane fraction and showed GPAT activity file:HORSE/GPAT2/GPAT2-bioinformatics/RESULTS.md H205-K206-S207-L208-L209-D210, identical in sequence and position. |
| GO:0004366 glycerol-3-phosphate O-acyltransferase activity | IEA GO_REF:0000107 | ACCEPT | Summary: Ensembl Compara transfer of GPAT activity from mouse Gpat2 (Q14DK4), which has direct experimental evidence for this activity. Reason: Same claim as the IBA row; the donor is an ortholog in the same PANTHER subfamily with direct activity data, and the catalytic motif is conserved in horse. Core activity. Supporting Evidence: PMID:17689486 When Cos-7 cells were transiently transfected with GPAT2, NEM-sensitive GPAT activity increased 30%. file:HORSE/GPAT2/GPAT2-bioinformatics/RESULTS.md assignment PTHR12563:SF15 (GLYCEROL-3-PHOSPHATE ACYLTRANSFERASE 2, MITOCHONDRIAL). |
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | MODIFY | Summary: PAINT propagation of mitochondrial localization from node PTN000970957. Reason: Correct but general. Mouse Gpat2 is in the mitochondrial outer membrane, with two transmembrane segments and cytosol-facing termini, so the specific supported location is GO:0005741 mitochondrial outer membrane. Propagation Review Root cause: TERM SCOPING PROBLEM Sources checked: PANTHER:PTN000970957 SUPPORTS TRANSFER Eumetazoan GPAT1/GPAT2 node; horse GPAT2 (PTHR12563:SF15) descends from it. The mitochondrial placement is right for GPAT2; only the specificity is refined to the outer membrane. Proposed replacements: mitochondrial outer membrane Supporting Evidence: PMID:17689486 Confocal microscopy confirmed a mitochondrial location. PMID:22285183 These results suggest that the GPAT2 protein has two transmembrane segments and that the N-terminal and C-terminal regions of this protein face the cytoplasm. file:HORSE/GPAT2/GPAT2-bioinformatics/mouse-Q14DK4-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion outer membrane |
| GO:0005739 mitochondrion | IEA GO_REF:0000107 | MODIFY | Summary: Ensembl Compara transfer of mitochondrial localization from mouse Gpat2. Reason: Same claim as the IBA row; refine to GO:0005741 mitochondrial outer membrane, the location shown for the mouse ortholog. Proposed replacements: mitochondrial outer membrane Supporting Evidence: PMID:23611983 glycerol-3-phosphate acyltransferase 2 (GPAT2), a mitochondrial outer membrane protein for lysophosphatidic acid, bound to MILI |
| GO:0006629 lipid metabolic process | IEA GO_REF:0000002 | TOO GENERAL | Summary: InterPro2GO mapping from the GPAT/DHAPAT family signature. Reason: True but uninformative; entailed by GO:0006654 phosphatidic acid biosynthetic process and GO:0019432 triglyceride biosynthetic process, both accepted in this review. Supporting Evidence: file:HORSE/GPAT2/GPAT2-bioinformatics/RESULTS.md assignment PTHR12563:SF15 (GLYCEROL-3-PHOSPHATE ACYLTRANSFERASE 2, MITOCHONDRIAL). |
| GO:0006650 glycerophospholipid metabolic process | IBA GO_REF:0000033 | TOO GENERAL | Summary: PAINT propagation of glycerophospholipid metabolic process from node PTN000970957. Reason: True for a GPAT, whose product lysophosphatidic acid is the precursor of phosphatidic acid, but GO:0006654 phosphatidic acid biosynthetic process (accepted below) is an is_a descendant and already entails it. In overexpressing cells mouse GPAT2 channelled label mainly into TAG rather than phospholipid, so the broader phospholipid framing adds nothing. Supporting Evidence: PMID:17689486 In contrast, incorporation into phospholipid species was lower than in control cells. |
| GO:0006654 phosphatidic acid biosynthetic process | IEA GO_REF:0000107 | ACCEPT | Summary: Ensembl Compara transfer from mouse Gpat2 of phosphatidic acid biosynthesis. Reason: Glycerol-3-phosphate acylation to lysophosphatidic acid is the first step of de novo phosphatidic acid synthesis, and the GPAT activity is accepted as core. Overexpressed GPAT2 increased PA production with arachidonoyl-CoA. The process is the direct consequence of the enzyme's own catalytic step. Supporting Evidence: PMID:22905194 but we found enhanced PA production when arachidonoyl-CoA was used as substrate |
| GO:0016020 membrane | IEA GO_REF:0000044 | MODIFY | Summary: UniProt subcellular-location mapping to membrane (ARBA-based for this entry). Reason: Correct but general; the supported location is the mitochondrial outer membrane (GO:0005741), as for the mouse ortholog. Proposed replacements: mitochondrial outer membrane Supporting Evidence: PMID:22285183 These results suggest that the GPAT2 protein has two transmembrane segments and that the N-terminal and C-terminal regions of this protein face the cytoplasm. |
| GO:0016746 acyltransferase activity | IEA GO_REF:0000002 | TOO GENERAL | Summary: InterPro2GO mapping from the phospholipid/glycerol acyltransferase domain. Reason: True but entailed by GO:0004366 glycerol-3-phosphate O-acyltransferase activity, accepted in this review. Supporting Evidence: file:HORSE/GPAT2/GPAT2-bioinformatics/RESULTS.md H205-K206-S207-L208-L209-D210, identical in sequence and position. |
| GO:0016747 acyltransferase activity, transferring groups other than amino-acyl groups | IEA GO_REF:0000002 | TOO GENERAL | Summary: InterPro2GO mapping from the GPAT/DHAPAT family signature. Reason: True (the PTHR12563 family review scopes this term family-wide) but entailed by GO:0004366, accepted in this review. Supporting Evidence: file:HORSE/GPAT2/GPAT2-bioinformatics/RESULTS.md assignment PTHR12563:SF15 (GLYCEROL-3-PHOSPHATE ACYLTRANSFERASE 2, MITOCHONDRIAL). |
| GO:0019432 triglyceride biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT propagation of triglyceride biosynthesis from node PTN000970957. Reason: GPAT catalyses the first acylation of the glycerol-3-phosphate route to TAG, and for GPAT2 the TAG link is direct: overexpression raised oleate and arachidonate incorporation into TAG and produced arachidonate-rich TAG, matching the testis TAG peak in spermatocytes. The enzyme performs a step of the process, so this is participation. Supporting Evidence: PMID:22905194 corroborating GPAT2's role in synthesizing TAG that is rich in arachidonic acid |
| GO:0034587 piRNA processing | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Ensembl Compara transfer from mouse Gpat2, whose knockdown in germline stem cells impairs primary piRNA production; Gpat2 co-precipitates with MILI/PIWIL2. Reason: The donor evidence is more than a downstream phenotype: GPAT2 binds MILI on the mitochondrial outer membrane, its knockdown impairs primary piRNA biogenesis, and catalytic-dead HXXXXD mutants rescue, which points to a non-enzymatic (proposed scaffold) contribution within the pathway. It rests on a single study in cultured mouse germline stem cells, the mechanism is unknown, and PAINT did not propagate the term at PTN000970957 (the node also contains GPAM, which has no piRNA role). The horse protein is a GPAT2 ortholog in the same subfamily and is testis-expressed, so the transfer is plausible. Kept as a germline-specific, non-core role. Supporting Evidence: PMID:23611983 GPAT2 is not only one of the MILI bound proteins but also a protein essential for primary piRNA biogenesis. PMID:23611983 These data suggest that the acyltransferase activity of GPAT2 is dispensable for the biogenesis of piRNA. |
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Download this section (compressed HTML)Q: Does purified GPAT2 have intrinsic 1-acylglycerol-3-phosphate acyltransferase activity, or does the AGPAT increase in overexpressing membranes come from endogenous AGPATs?
Q: How does GPAT2 contribute to primary piRNA biogenesis without its acyltransferase activity, and is this role conserved outside rodents?
Experiment: Assay purified or reconstituted wild-type and HXXXXD-mutant GPAT2 with lysophosphatidic acid and arachidonoyl-CoA to test for intrinsic AGPAT activity.
Experiment: Knock out GPAT2 in a non-rodent mammalian germline model and measure primary piRNA populations and MILI/PIWIL2 localization to mitochondria.
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