SLC25A20 is the mitochondrial carnitine/acylcarnitine carrier (also called carnitine-acylcarnitine translocase, CACT/CAC), an integral multi-pass protein of the mitochondrial inner membrane belonging to the mitochondrial carrier (SLC25) family. It catalyzes the electroneutral exchange (antiport) of long-chain O-acyl-L-carnitines for free L-carnitine across the inner membrane, and at lower rates the uniport of free carnitine. This activity is the central step of the mitochondrial carnitine shuttle: carnitine palmitoyltransferase 1 (CPT1) on the outer membrane converts acyl-CoA to acylcarnitine, SLC25A20 imports the acylcarnitine into the matrix while exporting free carnitine, and carnitine palmitoyltransferase 2 (CPT2) on the matrix face of the inner membrane regenerates acyl-CoA for fatty acid beta-oxidation. SLC25A20 is thus essential for delivering long-chain fatty acids into the matrix for energy production, especially during fasting and metabolic stress. Loss-of-function variants cause carnitine-acylcarnitine translocase deficiency, a severe autosomal-recessive long-chain fatty acid oxidation disorder presenting mostly in the neonatal period with hypoketotic hypoglycemia, hyperammonemia, cardiomyopathy, cardiac arrhythmia, and high mortality.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005740 mitochondrial envelope | IBA GO_REF:0000033 | ACCEPT | Summary: SLC25A20 is an integral protein of the mitochondrial inner membrane; the mitochondrial envelope is a correct but less precise cellular component. Reason: SLC25A20 is a multi-pass mitochondrial inner-membrane carrier, and the inner membrane is part of the mitochondrial envelope. This phylogenetic (IBA) call is consistent with the more specific inner-membrane annotations and with the UniProt subcellular location; retained as a correct broader component. The more informative location is captured by GO:0005743. Supporting Evidence: file:human/SLC25A20/SLC25A20-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion inner membrane; Multi-pass membrane |
| GO:0006839 mitochondrial transport | IBA GO_REF:0000033 | ACCEPT | Summary: SLC25A20 mediates transport across the mitochondrial inner membrane (acylcarnitine/carnitine exchange); mitochondrial transport is a correct broad parent process. Reason: As a mitochondrial carrier that moves acylcarnitines and carnitine across the inner membrane, SLC25A20 participates in mitochondrial transport. This is an accurate but general term; the specific carnitine/acylcarnitine transport processes are captured by GO:1902603 and GO:1902616 and the carnitine shuttle by GO:0006853. Supporting Evidence: file:human/SLC25A20/SLC25A20-uniprot.txt Mediates the electroneutral exchange of acylcarnitines (O- acyl-(R)-carnitine or L-acylcarnitine) of different acyl chain lengths |
| GO:0015227 O-acyl-L-carnitine transmembrane transporter activity | IBA GO_REF:0000033 | ACCEPT | Summary: SLC25A20 transports O-acyl-L-carnitines across the inner mitochondrial membrane; this is a correct molecular function, the transporter-parent of the more precise antiporter term. Reason: The phylogenetic call correctly assigns acylcarnitine transporter activity, supported by direct functional characterization of the human carrier. It is a valid parent of GO:0005476 (carnitine:O-acyl-L-carnitine antiporter activity), which is the mechanistically precise molecular function. Supporting Evidence: file:human/SLC25A20/SLC25A20-uniprot.txt it translocates the fatty acids in the form of acylcarnitines into the mitochondrial matrix |
| GO:1902603 carnitine transmembrane transport | IBA GO_REF:0000033 | ACCEPT | Summary: SLC25A20 transports free carnitine across the inner membrane (as the counter-substrate of the antiport and by lower-rate uniport). Reason: Carnitine transmembrane transport is directly supported: SLC25A20 exchanges acylcarnitine for free (R)-carnitine and also catalyzes unidirectional carnitine uniport. Correct process annotation. Supporting Evidence: file:human/SLC25A20/SLC25A20-uniprot.txt Catalyzes the unidirectional transport (uniport) of carnitine at lower rates than the antiport (exchange) |
| GO:0005743 mitochondrial inner membrane | IEA GO_REF:0000120 | ACCEPT | Summary: SLC25A20 is a multi-pass integral protein of the mitochondrial inner membrane; this is its precise, correct location. Reason: The electronic subcellular-location inference matches the UniProt curated location and the biology of the carnitine shuttle (inner-membrane carrier). Core cellular component. Supporting Evidence: file:human/SLC25A20/SLC25A20-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion inner membrane; Multi-pass membrane |
| GO:1902616 O-acyl-L-carnitine transmembrane transport | IEA GO_REF:0000120 | ACCEPT | Summary: SLC25A20 mediates transmembrane transport of O-acyl-L-carnitines into the matrix; correct process. Reason: This IEA process term is the biological-process counterpart of the acylcarnitine transporter/antiporter molecular function and is directly supported by the carrier's characterized activity. Accept. Supporting Evidence: file:human/SLC25A20/SLC25A20-uniprot.txt it translocates the fatty acids in the form of acylcarnitines into the mitochondrial matrix |
| GO:0005515 protein binding | IPI PMID:32814053 Interactome Mapping Provides a Network of Neurodegenerative ... | MARK AS OVER ANNOTATED | Summary: Bare "protein binding" from a large-scale yeast two-hybrid interactome screen (IntAct records an interaction with LAMP2, P13473-2). Uninformative as a molecular function for this carrier. Reason: Per curation guidelines, generic "protein binding" does not convey the protein's actual function and is not a core molecular function of a solute carrier. The interaction comes from a high-throughput neurodegenerative-disease Y2H interactome (IntAct with/from UniProtKB:P13473-2, LAMP2), with no evidence it reflects a functional partnership relevant to carnitine transport. Retained (experimental IPI, not removed) but flagged as over-annotated / non-core. Supporting Evidence: file:human/SLC25A20/SLC25A20-uniprot.txt O43772; P13473-2: LAMP2; NbExp=3; IntAct=EBI-727085, EBI-21591415; |
| GO:0006853 carnitine shuttle | IEA GO_REF:0000107 | ACCEPT | Summary: SLC25A20 is the inner-membrane translocase step of the mitochondrial carnitine shuttle; core biological process. Reason: The carnitine shuttle couples CPT1 (outer membrane), CACT/SLC25A20 (inner membrane exchange), and CPT2 (matrix) to deliver long-chain fatty acids for beta-oxidation. This orthology-based IEA is correct and central to the gene's function. Supporting Evidence: Reactome:R-HSA-200425 This transport system consists of the malonyl-CoA sensitive carnitine palmitoyltransferase I (CPT-I) localized in the mitochondrial outer membrane, the carnitine:acylcarnitine translocase, an integral inner membrane protein, and carnitine palmitoyltransferase II |
| GO:0006853 carnitine shuttle | TAS Reactome:R-HSA-200425 | ACCEPT | Summary: Reactome traceable assertion that SLC25A20 acts in the carnitine shuttle; duplicate of the IEA call and equally correct. Reason: Reactome curates SLC25A20 as the inner-membrane translocase of the carnitine shuttle. Consistent with the UniProt function and the disease mechanism; core process. Supporting Evidence: Reactome:R-HSA-200425 The mitochondrial carnitine system catalyzes the transport of long-chain fatty acids into the mitochondrial matrix where they undergo beta oxidation. |
| GO:0005739 mitochondrion | IDA GO_REF:0000052 | ACCEPT | Summary: Immunofluorescence (HPA) localizes SLC25A20 to the mitochondrion; correct but less precise than the inner-membrane annotation. Reason: Direct immunofluorescence localization to mitochondria is consistent with the curated inner-membrane location. Accept as a correct, if general, cellular component (GO:0005743 is the more informative term). Supporting Evidence: file:human/SLC25A20/SLC25A20-uniprot.txt GO:0005739; C:mitochondrion; IDA:HPA. |
| GO:0005476 carnitine:O-acyl-L-carnitine antiporter activity | EXP PMID:9399886 Cloning of the human carnitine-acylcarnitine carrier cDNA an... | ACCEPT | Summary: Experimentally supported core molecular function: SLC25A20 catalyzes the antiport (exchange) of O-acyl-L-carnitine for free carnitine across the inner mitochondrial membrane. Reason: This is the defining molecular function of the gene product. The cloned human carnitine-acylcarnitine carrier was shown to translocate long-chain fatty acids (as acylcarnitines) across the inner mitochondrial membrane, and reconstituted-carrier work (UniProt-cited PubMed:12892634, 18307102, 20347717) establishes the electroneutral antiport mechanism. Core MF. Supporting Evidence: PMID:9399886 The carnitine-acylcarnitine carrier (CAC) catalyzes the translocation of long-chain fatty acids across the inner mitochondrial membrane. file:human/SLC25A20/SLC25A20-uniprot.txt Mediates the electroneutral exchange of acylcarnitines (O- acyl-(R)-carnitine or L-acylcarnitine) of different acyl chain lengths |
| GO:0005476 carnitine:O-acyl-L-carnitine antiporter activity | TAS Reactome:R-HSA-9971094 | ACCEPT | Summary: Reactome traceable assertion of the carnitine:acylcarnitine antiporter activity; duplicate of the EXP call and equally correct. Reason: Reactome curates SLC25A20 as the carrier that exchanges acylcarnitine for carnitine across the inner membrane (and can reverse to export excess acylcarnitine down the substrate gradient). Same core molecular function. Supporting Evidence: Reactome:R-HSA-9971094 Mitochondrial carnitine/acylcarnitine carrier protein (SLC25A20, CACT) exports excess acylcarnitines out of mitochondria. |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | ACCEPT | Summary: SLC25A20 was identified in the high-confidence human mitochondrial proteome (MitoCoP) by quantitative mass spectrometry; consistent with mitochondrial localization. Reason: High-throughput proteomic identification in a curated high-confidence mitochondrial proteome corroborates mitochondrial localization. Correct but less precise than the inner-membrane annotation; retained. Supporting Evidence: PMID:34800366 defined a mitochondrial high-confidence proteome of >1,100 proteins (MitoCoP) |
| GO:0015227 O-acyl-L-carnitine transmembrane transporter activity | ISS GO_REF:0000024 | ACCEPT | Summary: Sequence-similarity transfer (from rodent ortholog UniProtKB:P97521) of acylcarnitine transporter activity; correct. Reason: The ISS transfer of O-acyl-L-carnitine transporter activity from the well-characterized ortholog is consistent with the experimentally established human activity. Valid parent of the antiporter MF; accept. Supporting Evidence: file:human/SLC25A20/SLC25A20-uniprot.txt it translocates the fatty acids in the form of acylcarnitines into the mitochondrial matrix |
| GO:1902603 carnitine transmembrane transport | ISS GO_REF:0000024 | ACCEPT | Summary: Sequence-similarity transfer of carnitine transmembrane transport; correct (carnitine is the counter-substrate and a uniport substrate). Reason: Consistent with the experimentally documented carnitine antiport/uniport by the human carrier. Duplicate of the IBA carnitine-transport call; accept. Supporting Evidence: file:human/SLC25A20/SLC25A20-uniprot.txt Catalyzes the unidirectional transport (uniport) of carnitine at lower rates than the antiport (exchange) |
| GO:0005739 mitochondrion | HDA PMID:20833797 Phosphoproteome analysis of functional mitochondria isolated... | ACCEPT | Summary: SLC25A20 was detected among inner-membrane solute carriers in the phosphoproteome of isolated human muscle mitochondria; supports mitochondrial localization. Reason: High-throughput proteomic detection in purified human muscle mitochondria (which included inner-membrane solute carriers) corroborates mitochondrial localization. Correct but less precise than the inner-membrane term. Supporting Evidence: PMID:20833797 outer or inner membrane solute carriers and channel proteins |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-200424 | ACCEPT | Summary: Reactome traceable assertion that the SLC25A20 carrier is embedded in the inner mitochondrial membrane; core location. Reason: Reactome curates the carnitine-acylcarnitine transporter as an inner-membrane protein mediating the acylcarnitine/carnitine exchange. Consistent with UniProt; core cellular component. Supporting Evidence: Reactome:R-HSA-200424 The carnitine-acylcarnitine transporter (SLC25A20 / CACT), embedded in the inner mitochondrial membrane |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9971094 | ACCEPT | Summary: Reactome traceable assertion of inner-membrane localization; duplicate of the R-HSA-200424 call and equally correct. Reason: SLC25A20 exports/exchanges acylcarnitine across the inner membrane as an integral IMM carrier; core cellular component. Supporting Evidence: file:human/SLC25A20/SLC25A20-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion inner membrane; Multi-pass membrane |
| GO:0006635 fatty acid beta-oxidation | IC PMID:9399886 Cloning of the human carnitine-acylcarnitine carrier cDNA an... | NEW | Summary: SLC25A20 is required for mitochondrial long-chain fatty acid beta-oxidation: by importing acylcarnitines into the matrix it delivers the substrate that CPT2 converts back to acyl-CoA for the beta-oxidation spiral. Reason: The carnitine shuttle exists to enable fatty acid beta-oxidation, and UniProt explicitly states SLC25A20 translocates fatty acids (as acylcarnitines) into the matrix "to undergo fatty acid beta-oxidation". Loss of SLC25A20 blocks long-chain FAO, causing CACT deficiency. This process is not captured elsewhere in the GOA set (which has the carnitine shuttle and transport processes but not the beta-oxidation process), so it is proposed as a NEW involved_in annotation. Inferred from the curator-established role (IC) with the cloning paper as anchor reference. Supporting Evidence: file:human/SLC25A20/SLC25A20-uniprot.txt it translocates the fatty acids in the form of acylcarnitines into the mitochondrial matrix, where the carnitine palmitoyltransferase 2 (CPT-2) activates them to undergo fatty acid beta-oxidation |
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Download this section (compressed HTML)Q: What is the physiological acyl-chain-length preference of human SLC25A20 in situ, and does it transport medium-chain acylcarnitines efficiently enough to explain the acylcarnitine profile seen in CACT deficiency?
Q: Does SLC25A20 have any physiologically relevant protein partners at the inner membrane (e.g., functional coupling to CPT2 or the beta-oxidation machinery), beyond the high-throughput LAMP2 interaction?
Experiment: Reconstitute recombinant human SLC25A20 into proteoliposomes and measure antiport kinetics (Km, Vmax) for a panel of acyl-carnitines of varying chain length versus free carnitine to define substrate specificity quantitatively.
Experiment: Use proximity labeling (BioID/APEX) of SLC25A20 in intact mitochondria to test for functional coupling with CPT2 and other inner-membrane fatty-acid oxidation components.
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