Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0005739 mitochondrion	IBA GO_REF:0000033	KEEP AS NON CORE	Summary: Phylogenetic (IBA) assignment placing ORNT1 activity in the mitochondrion. This is correct but general; the specific and experimentally supported location is the mitochondrial inner membrane. Reason: ORNT1 is a mitochondrial carrier that acts in the mitochondrion; the IBA call across the SLC25 ornithine-carrier clade is sound. It is a correct parent of the more specific mitochondrial inner membrane annotation, so it is retained as accurate though non-core. The precise location is captured by GO:0005743.
GO:0000064 L-ornithine transmembrane transporter activity	IBA GO_REF:0000033	ACCEPT	Summary: Phylogenetic (IBA) assignment of L-ornithine transmembrane transporter activity, the core molecular function of ORNT1 and its fungal orthologs (N. crassa ARG13, S. cerevisiae ARG11/ORT1). Reason: This is the core molecular function, directly supported by reconstitution assays and by the disease mechanism, and is consistent across the ornithine-carrier phylogenetic clade. Supporting Evidence: PMID:10369256 Our results show that ORNT1 encodes the mitochondrial ornithine transporter involved in UC function and is defective in HHH syndrome.
GO:1990575 mitochondrial L-ornithine transmembrane transport	IBA GO_REF:0000033	ACCEPT	Summary: Phylogenetic (IBA) assignment of the biological process of transporting L-ornithine across the mitochondrial membrane, the process this carrier mediates. Reason: Correctly captures the core biological process (movement of L-ornithine across the inner mitochondrial membrane) that ORNT1 performs as part of urea-cycle function. Supporting Evidence: PMID:10369256 ORNT1 expression restores ornithine metabolism in fibroblasts from patients with hyperammonaemia-hyperornithinaemia-homocitrullinuria (HHH) syndrome.
GO:0000064 L-ornithine transmembrane transporter activity	IEA GO_REF:0000117	ACCEPT	Summary: ARBA machine-learning electronic annotation of the core L-ornithine transmembrane transporter activity. Reason: Duplicates the well-supported core molecular function (also assigned by IBA, EXP, and TAS). The electronic mapping is at the correct level of specificity.
GO:0005743 mitochondrial inner membrane	IEA GO_REF:0000120	ACCEPT	Summary: Electronic (multi-method IEA) assignment of mitochondrial inner membrane location, from UniProt subcellular-location mapping (SL-0168). Reason: Correct core location for this multi-pass SLC25 inner-membrane carrier; concordant with the UniProt subcellular location and the ISS-from-ortholog call.
GO:0015297 antiporter activity	IEA GO_REF:0000117	ACCEPT	Summary: ARBA electronic annotation of generic antiporter activity, reflecting the exchange (antiport) transport mechanism of ORNT1. Reason: ORNT1 is genuinely an antiporter (ornithine/citrulline exchange), so the term is correct although general. It is retained as a true parent of the more informative specific antiport function; the physiologically meaningful antiport is the arginine:ornithine / ornithine:citrulline exchange captured by GO:0043858 and by the ornithine-transporter terms.
GO:0031966 mitochondrial membrane	IEA GO_REF:0000044	ACCEPT	Summary: Electronic annotation (UniProt subcellular-location mapping, SL-0171) of mitochondrial membrane location. Reason: Correct but less specific than mitochondrial inner membrane. Retained as a true parent; the precise, experimentally consistent location is GO:0005743.
GO:0000050 urea cycle	TAS Reactome:R-HSA-70635	ACCEPT	Summary: Reactome-traceable assignment placing ORNT1 in the urea cycle, which it connects by shuttling ornithine and citrulline across the inner mitochondrial membrane. Reason: Core biological process. ORNT1 physically links the cytosolic and matrix halves of the urea cycle; its loss causes the urea-cycle disorder HHH syndrome. Supporting Evidence: PMID:10369256 ORNT1 encodes the mitochondrial ornithine transporter involved in UC function and is defective in HHH syndrome.
GO:0005743 mitochondrial inner membrane	TAS Reactome:R-HSA-9956519	ACCEPT	Summary: Reactome-traceable assignment of mitochondrial inner membrane location (from the Reactome reaction describing ornithine/citrulline translocation). Reason: Correct core location. Although the source Reactome event concerns disease variants that fail to translocate, the localization to the inner membrane is a correct statement about the wild-type protein.
GO:0000064 L-ornithine transmembrane transporter activity	TAS Reactome:R-HSA-9956519	ACCEPT	Summary: Reactome-traceable assignment of the core L-ornithine transmembrane transporter activity. Reason: Duplicates the well-supported core molecular function. Correct level of specificity.
GO:0005743 mitochondrial inner membrane	ISS GO_REF:0000024	ACCEPT	Summary: Sequence-similarity (ISS) transfer of mitochondrial inner membrane location from the S. cerevisiae ortholog ORT1/Q12375. Reason: Correct core location; concordant with UniProt and electronic annotations and with the multi-pass SLC25 topology.
GO:0031966 mitochondrial membrane	EXP PMID:10369256 Hyperornithinaemia-hyperammonaemia-homocitrullinuria syndrom...	ACCEPT	Summary: Experimental localization of ORNT1 to the mitochondrial membrane in the original disease-gene study. Reason: Experimentally supported mitochondrial membrane localization. The term is correct though more general than the inner-membrane call; retained as a true and experimentally grounded parent. Supporting Evidence: PMID:10369256 mutations in a gene encoding a mitochondrial ornithine transporter
GO:0043858 arginine:ornithine antiporter activity	EXP PMID:12807890 The mitochondrial ornithine transporter. Bacterial expressio...	ACCEPT	Summary: Experimentally demonstrated arginine/ornithine antiport by reconstituted, purified ORNT1 (ORC1). This is a specific antiport activity of the carrier and corresponds to the UniProt/Rhea reaction L-arginine(out) + L-ornithine(in) = L-arginine(in) + L-ornithine(out) (RHEA:34991). Reason: Directly measured for reconstituted ORNT1 and matches the GO term direction (arginine(out) + ornithine(in) = arginine(in) + ornithine(out)). It represents a real, specific antiport function of the carrier. The physiologically dominant exchange is ornithine/citrulline, but arginine:ornithine antiport is a genuine biochemical activity and captures the carrier antiport mechanism more precisely than generic antiporter activity. Supporting Evidence: PMID:12807890 Both transport L-isomers of ornithine, lysine, arginine, and citrulline by exchange and by unidirectional mechanisms, and they are inactivated by the same inhibitors.
GO:0005739 mitochondrion	HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an...	KEEP AS NON CORE	Summary: High-throughput mitochondrial-proteome study detecting ORNT1 in the mitochondrion. Reason: Consistent with the well-established mitochondrial localization, though less specific than the experimentally supported inner-membrane location and derived from a high-throughput proteomic dataset. Retained as a correct but corroborative, non-core localization annotation.
GO:0015189 L-lysine transmembrane transporter activity	IDA PMID:12807890 The mitochondrial ornithine transporter. Bacterial expressio...	KEEP AS NON CORE	Summary: Direct assay of reconstituted ORNT1 showed transport of L-lysine, reflecting the carrier broad basic-amino-acid specificity (UniProt Rhea reaction L-ornithine(out) + L-lysine(in) = L-ornithine(in) + L-lysine(out), RHEA:70799). Reason: Experimentally demonstrated in the reconstituted system (do not remove an IDA activity). However, lysine is a lower-affinity substrate than ornithine (Km 0.8 vs 0.22 mM) and lysine transport is not the physiological role of the carrier; the urea-cycle-relevant function is ornithine/citrulline exchange. Marked non-core to reflect the broad in vitro specificity rather than the primary biological function. Supporting Evidence: PMID:12807890 Both transport L-isomers of ornithine, lysine, arginine, and citrulline by exchange and by unidirectional mechanisms, and they are inactivated by the same inhibitors.
GO:0015297 antiporter activity	IDA PMID:12807890 The mitochondrial ornithine transporter. Bacterial expressio...	ACCEPT	Summary: Direct demonstration that ORNT1 transports basic amino acids by an exchange (antiport) mechanism in reconstituted proteoliposomes. Reason: Correct, experimentally grounded description of the antiport mechanism, although general. Retained as a true parent of the specific antiport activities; the more informative specific term is GO:0043858 (arginine:ornithine antiporter). Supporting Evidence: PMID:12807890 Both transport L-isomers of ornithine, lysine, arginine, and citrulline by exchange and by unidirectional mechanisms, and they are inactivated by the same inhibitors.
GO:0061459 L-arginine transmembrane transporter activity	IDA PMID:12807890 The mitochondrial ornithine transporter. Bacterial expressio...	KEEP AS NON CORE	Summary: Direct assay of reconstituted ORNT1 showed transport of L-arginine, reflecting the carrier broad basic-amino-acid specificity (UniProt Rhea reaction RHEA:34991). Reason: Experimentally demonstrated (do not remove an IDA activity). Arginine is a lower-affinity substrate than ornithine (Km 1.58 vs 0.22 mM) and arginine transport is not the carrier principal physiological role, which is ornithine/citrulline exchange in the urea cycle. Marked non-core to reflect the broad in vitro specificity. Supporting Evidence: PMID:12807890 Both transport L-isomers of ornithine, lysine, arginine, and citrulline by exchange and by unidirectional mechanisms, and they are inactivated by the same inhibitors.
GO:1903401 L-lysine transmembrane transport	IDA PMID:12807890 The mitochondrial ornithine transporter. Bacterial expressio...	KEEP AS NON CORE	Summary: Biological-process counterpart of the demonstrated L-lysine transport activity by reconstituted ORNT1. Reason: Experimentally supported but non-core; a manifestation of the carrier broad basic-amino-acid specificity rather than its urea-cycle role. Supporting Evidence: PMID:12807890 Both transport L-isomers of ornithine, lysine, arginine, and citrulline by exchange and by unidirectional mechanisms, and they are inactivated by the same inhibitors.
GO:1903826 L-arginine transmembrane transport	IDA PMID:12807890 The mitochondrial ornithine transporter. Bacterial expressio...	KEEP AS NON CORE	Summary: Biological-process counterpart of the demonstrated L-arginine transport activity by reconstituted ORNT1. Reason: Experimentally supported but non-core; reflects broad basic-amino-acid specificity, not the primary urea-cycle function. Supporting Evidence: PMID:12807890 Both transport L-isomers of ornithine, lysine, arginine, and citrulline by exchange and by unidirectional mechanisms, and they are inactivated by the same inhibitors.
GO:0005739 mitochondrion	IDA PMID:10369256 Hyperornithinaemia-hyperammonaemia-homocitrullinuria syndrom...	KEEP AS NON CORE	Summary: Direct experimental localization of ORNT1 to the mitochondrion in the original disease-gene identification study. Reason: Experimentally supported mitochondrial localization, consistent with the more specific inner-membrane location. Retained as a correct parent, non-core relative to the specific mitochondrial inner membrane location. Supporting Evidence: PMID:10369256 mutations in a gene encoding a mitochondrial ornithine transporter
GO:0000064 L-ornithine transmembrane transporter activity	EXP PMID:12807890 The mitochondrial ornithine transporter. Bacterial expressio...	ACCEPT	Summary: Experimental demonstration (reconstituted, purified ORNT1) of L-ornithine transport, establishing the core molecular function. Reason: Directly measured core molecular function; ornithine is the highest-affinity substrate (Km 0.22 mM). This is the defining activity of the carrier. Supporting Evidence: PMID:12807890 Both transport L-isomers of ornithine, lysine, arginine, and citrulline by exchange and by unidirectional mechanisms, and they are inactivated by the same inhibitors.
GO:0000064 L-ornithine transmembrane transporter activity	EXP PMID:12948741 Cloning and characterization of human ORNT2: a second mitoch...	ACCEPT	Summary: Experimental support for L-ornithine transporter activity in the study characterizing ORNT2 and confirming the ornithine-transport function shared with ORNT1 (functional rescue of ornithine metabolism in HHH patient fibroblasts). Reason: Reinforces the core molecular function of the ORNT ornithine carriers. The paper demonstrates the ornithine-transport function that ORNT1 provides in the urea cycle and that ORNT2 can partially replace. Supporting Evidence: PMID:12948741 When ORNT2 is overexpressed transiently in cultured fibroblasts from HHH patients, it rescues the deficient ornithine metabolism in these cells.
GO:0005743 mitochondrial inner membrane	TAS Reactome:R-HSA-70634	ACCEPT	Summary: Reactome-traceable assignment of mitochondrial inner membrane location, from the reaction describing cytosolic/mitochondrial ornithine-citrulline exchange. Reason: Correct core location, concordant with all other localization annotations.
GO:0000050 urea cycle	TAS PMID:10369256 Hyperornithinaemia-hyperammonaemia-homocitrullinuria syndrom...	ACCEPT	Summary: Author-stated (TAS) involvement of ORNT1 in the urea cycle, from the disease-gene identification study. Reason: Core biological process. ORNT1 connects the cytosolic and matrix reactions of the urea cycle; its deficiency causes the urea-cycle disorder HHH syndrome. Supporting Evidence: PMID:10369256 ORNT1 encodes the mitochondrial ornithine transporter involved in UC function and is defective in HHH syndrome.
GO:0000064 L-ornithine transmembrane transporter activity	TAS PMID:10369256 Hyperornithinaemia-hyperammonaemia-homocitrullinuria syndrom...	ACCEPT	Summary: Author-stated (TAS) L-ornithine transporter activity for ORNT1 in the disease-gene study. Reason: Duplicates the well-supported core molecular function. Supporting Evidence: PMID:10369256 mutations in a gene encoding a mitochondrial ornithine transporter
GO:0005743 mitochondrial inner membrane	TAS PMID:10369256 Hyperornithinaemia-hyperammonaemia-homocitrullinuria syndrom...	ACCEPT	Summary: Author-stated (TAS) mitochondrial inner membrane location for ORNT1. Reason: Correct core location, consistent with experimental and electronic annotations. Supporting Evidence: PMID:10369256 transport ornithine across the mitochondrial inner membrane
GO:0016020 membrane	TAS PMID:10369256 Hyperornithinaemia-hyperammonaemia-homocitrullinuria syndrom...	MARK AS OVER ANNOTATED	Summary: Author-stated (TAS) generic membrane location, superseded by the specific mitochondrial inner membrane annotations. Reason: Not incorrect (ORNT1 is a multi-pass membrane protein), but GO:0016020 is uninformative given the well-established mitochondrial inner membrane localization. It is a legacy general term that adds no biological information beyond the specific inner-membrane annotations.
GO:1990575 mitochondrial L-ornithine transmembrane transport	TAS PMID:10369256 Hyperornithinaemia-hyperammonaemia-homocitrullinuria syndrom...	ACCEPT	Summary: Author-stated (TAS) biological process of L-ornithine transport across the mitochondrial membrane. Reason: Core biological process, concordant with the IBA assignment and the urea-cycle role. Supporting Evidence: PMID:10369256 transport ornithine across the mitochondrial inner membrane

Core Functions

Ornithine/citrulline antiport across the inner mitochondrial membrane that couples the cytosolic and matrix halves of the urea cycle, importing cytosolic L-ornithine into the matrix in exchange for exporting matrix L-citrulline.

Molecular Function:

L-ornithine transmembrane transporter activity

Directly Involved In:

urea cycle

Cellular Locations:

mitochondrial inner membrane

Supporting Evidence:

PMID:12807890
Both transport L-isomers of ornithine, lysine, arginine, and citrulline by exchange and by unidirectional mechanisms, and they are inactivated by the same inhibitors.
PMID:10369256
ORNT1 encodes the mitochondrial ornithine transporter involved in UC function and is defective in HHH syndrome.

Transmembrane movement of L-ornithine across the mitochondrial inner membrane, the transport process that ORNT1 mediates.

Molecular Function:

L-ornithine transmembrane transporter activity

Directly Involved In:

mitochondrial L-ornithine transmembrane transport

Cellular Locations:

mitochondrial inner membrane

Supporting Evidence:

PMID:10369256
ORNT1 expression restores ornithine metabolism in fibroblasts from patients with hyperammonaemia-hyperornithinaemia-homocitrullinuria (HHH) syndrome.

References

GO_REF:0000024

Manual transfer of experimentally-verified manual GO annotation data to orthologs by curator judgment of sequence similarity

GO_REF:0000033

Annotation inferences using phylogenetic trees

GO_REF:0000044

Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt

GO_REF:0000117

Electronic Gene Ontology annotations created by ARBA machine learning models

GO_REF:0000120

Combined Automated Annotation using Multiple IEA Methods

PMID:10369256

Hyperornithinaemia-hyperammonaemia-homocitrullinuria syndrome is caused by mutations in a gene encoding a mitochondrial ornithine transporter.

Identified ORNT1 (SLC25A15) by orthology to fungal mitochondrial ornithine carriers, showed it restores ornithine metabolism in HHH patient fibroblasts, and established that its mutations cause HHH syndrome.

"ORNT1 encodes the mitochondrial ornithine transporter involved in UC function and is defective in HHH syndrome."

PMID:12807890

The mitochondrial ornithine transporter. Bacterial expression, reconstitution, functional characterization, and tissue distribution of two human isoforms.

Reconstituted, purified ORNT1 (ORC1) transports L-ornithine, L-lysine, L-arginine and L-citrulline by exchange and unidirectional mechanisms; ORNT2 (ORC2) has broader specificity; both are most highly expressed in liver.

"Both transport L-isomers of ornithine, lysine, arginine, and citrulline by exchange and by unidirectional mechanisms, and they are inactivated by the same inhibitors."

PMID:12948741

Cloning and characterization of human ORNT2: a second mitochondrial ornithine transporter that can rescue a defective ORNT1 in patients with the hyperornithinemia-hyperammonemia-homocitrullinuria syndrome, a urea cycle disorder.

ORNT2 (SLC25A2) is a paralog ~88% identical to ORNT1 that targets to mitochondria and rescues deficient ornithine metabolism in HHH patient fibroblasts, explaining partial functional redundancy.

"When ORNT2 is overexpressed transiently in cultured fibroblasts from HHH patients, it rescues the deficient ornithine metabolism in these cells."

PMID:34800366

Quantitative high-confidence human mitochondrial proteome and its dynamics in cellular context.

High-throughput mitochondrial-proteome study detecting SLC25A15 in the mitochondrion.

PMID:22262851

Substrate specificity of the two mitochondrial ornithine carriers can be swapped by single mutation in substrate binding site.

Mutagenesis of substrate-binding residues (R179, E180, W224) in ORNT1 shifts relative ornithine versus arginine/lysine transport, defining the substrate-specificity determinants of the carrier.

Reactome:R-HSA-70634

ornithine (cytosolic) + citrulline (mitochondrial) => ornithine (mitochondrial) + citrulline (cytosolic)

Reactome:R-HSA-70635

Urea cycle

Reactome:R-HSA-9956519

SLC25A15 variants don't translocate ornithine and citrulline

Deep Research

Falcon

(SLC25A15-deep-research-falcon.md)

Comprehensive Research Report: SLC25A15 (Mitochondrial Ornithine Transporter 1 / ORC1 / ORNT1) Falcon Edison Scientific Literature 40 citations 2 artifacts 2026-07-05T13:56:54.667433

Edison artifact artifact-00 (text/markdown)

Edison artifact artifact-01 (text/markdown)

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Comprehensive Research Report: SLC25A15 (Mitochondrial Ornithine Transporter 1 / ORC1 / ORNT1)

1. Gene and Protein Identity

SLC25A15 (Solute Carrier Family 25 Member 15) encodes Mitochondrial Ornithine Transporter 1, also known as Ornithine Carrier 1 (ORC1) or ORNT1 (UniProt: Q9Y619). It is a member of the mitochondrial carrier family (SLC25/TC 2.A.29), a large family of transport proteins embedded in the inner mitochondrial membrane that facilitate the translocation of diverse small molecules between the mitochondrial intermembrane space and the matrix (palmieri2013themitochondrialtransporter pages 3-6). The gene is located on human chromosome 13q14 and encodes a 301 amino acid protein (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11).

The following table summarizes the key properties of SLC25A15/ORC1:

Property	Summary
Gene name	SLC25A15 (solute carrier family 25 member 15) (OpenTargets Search: -SLC25A15, gutierrezaguilar2013physiologicalandpathological pages 3-4)
Aliases	ORC1, ORNT1 (gutierrezaguilar2013physiologicalandpathological pages 3-4, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11)
Protein name	Mitochondrial ornithine transporter 1 / ornithine carrier 1 (ORC1) (gao2024cancertherapeuticpotential pages 6-7, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11)
UniProt ID	Q9Y619 (provided target identity)
Chromosomal location	13q14 / chromosome 13 locus reported for human SLC25A15 (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11, tessa2009identificationofnovel pages 3-4)
Protein length	301 amino acids (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11)
Subcellular localization	Inner mitochondrial membrane; carrier with N- and C-termini exposed to the cytosolic/intermembrane-space side (tessa2009identificationofnovel pages 5-6, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11)
Primary substrates	L-ornithine, L-citrulline, L-arginine, L-lysine; transport is stereoselective for the L-forms in ORC1 (monne2012substratespecificityof pages 1-2, monne2012substratespecificityof pages 3-4)
Transport mechanism	Electroneutral antiport, classically exchanging cytosolic ornithine for matrix citrulline + H+ in a strict 1:1 mode (gao2024cancertherapeuticpotential pages 6-7, monne2012substratespecificityof pages 3-4, palmieri2020diseasescausedby pages 13-15)
Tissue expression	Highest/major expression reported in liver, pancreas, lung, kidney; broader tissue expression also noted (gutierrezaguilar2013physiologicalandpathological pages 3-4, ahmed2024theroleof pages 16-18)
Key substrate-binding / mechanistic residues	E77, R179, E180 directly contribute to substrate binding; W224 and R275 help couple binding to conformational change/translocation (monne2012substratespecificityof pages 1-2, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11, monne2012substratespecificityof pages 7-8)
Structural class	Member of the mitochondrial carrier family (SLC25) with 6 transmembrane α-helices and tripartite architecture (tessa2009identificationofnovel pages 5-6, palmieri2013themitochondrialtransporter pages 3-6)
Associated disease	Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome / ornithine translocase deficiency (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11, palmieri2020diseasescausedby pages 13-15, OpenTargets Search: -SLC25A15)
Paralogues / related compensatory carriers	ORC2 / SLC25A2: 87% identical paralogue with lower affinity and broader substrate range; ORNT3 / SLC25A29: related basic amino acid carrier that can rescue ornithine metabolism in HHH fibroblasts (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 8-9, gutierrezaguilar2013physiologicalandpathological pages 3-4, camacho2009thehumanand pages 6-7)

Table: This table summarizes the core identity, localization, transport properties, structural determinants, disease relevance, and related paralogues of human SLC25A15/ORC1. It is useful as a compact reference for functional annotation and pathway interpretation.

2. Primary Transport Function and Substrate Specificity

SLC25A15/ORC1 functions as an electroneutral antiporter at the inner mitochondrial membrane, catalyzing the 1:1 exchange of cytoplasmic L-ornithine for mitochondrial matrix L-citrulline plus a proton (H+) (palmieri2020diseasescausedby pages 13-15, monne2012substratespecificityof pages 3-4). This transport is the physiologically essential step that bridges the mitochondrial and cytoplasmic segments of the urea cycle (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11, cunningham202020000picometersunder pages 4-5). The proton co-transport neutralizes the positive charge of ornithine during the exchange process, rendering the transport electrically neutral (gao2024cancertherapeuticpotential pages 6-7).

The substrate specificity of ORC1 is restricted to the L-forms of ornithine, citrulline, lysine, and arginine (monne2012substratespecificityof pages 1-2). Among these, ORC1 shows highest affinity for L-ornithine, followed by L-arginine and L-lysine (mentel2021learningfromyeast pages 20-21). The stereospecificity for L-isomers is a distinguishing feature of ORC1 compared to ORC2, which can also transport D-isomers and histidine (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 8-9, mentel2021learningfromyeast pages 21-22). ORC1 operates as a strict 1:1 antiporter, meaning it exchanges substrates in a one-to-one stoichiometric ratio across the membrane (monne2012substratespecificityof pages 3-4).

Transport activity has been characterized through reconstitution of bacterially expressed ORC1 protein into proteoliposomes, enabling detailed kinetic and substrate specificity measurements (tessa2009identificationofnovel pages 1-2, tessa2009identificationofnovel pages 3-4).

3. Protein Structure and Transport Mechanism

ORC1 adopts the characteristic tripartite architecture of the SLC25 mitochondrial carrier family, consisting of three tandemly repeated homologous domains, each approximately 100 amino acids in length and containing two hydrophobic transmembrane stretches (palmieri2013themitochondrialtransporter pages 3-6). The protein forms a barrel-like structure composed of six transmembrane α-helices (H1–H6) arranged counter-clockwise that surround a central cavity open toward the cytosolic (intermembrane space) side (tessa2009identificationofnovel pages 5-6, palmieri2013themitochondrialtransporter pages 3-6). Three additional short α-helices (h12, h34, h56) lie parallel to the membrane plane on the matrix side (tessa2009identificationofnovel pages 5-6). Both the N- and C-termini are exposed on the cytosolic side (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11).

3.1 Substrate Binding Site

The substrate binding site is located within the central cavity and consists of three major contact points (monne2012substratespecificityof pages 1-2, monne2012substratespecificityof pages 2-3):

Contact Point I: Glu-77 (E77) likely interacts with the terminal amino group of the substrate side chain (monne2012substratespecificityof pages 1-2, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11).
Contact Point II: Arg-179 (R179) and Glu-180 (E180) bind the Cα carboxylate and amino group of the substrates, respectively. R179 is a critical determinant of substrate specificity — the difference between R179 (in ORC1) and Q179 (in ORC2) is largely responsible for the differing substrate profiles of the two isoforms, and swapping this single residue can interchange their substrate specificities (monne2012substratespecificityof pages 1-2, monne2012substratespecificityof pages 3-4, monne2012substratespecificityof pages 8-9).
Contact Point III: Arg-275 (R275), a highly conserved residue, is connected to R179 through Trp-224 (W224) via cation-π interactions, and is important for triggering substrate-induced conformational changes required for translocation (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11, monne2012substratespecificityof pages 7-8).

Additional residues Asn-74 (N74) and Asn-78 (N78) contribute to the substrate binding pocket (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11, monne2012substratespecificityof pages 7-8).

3.2 Gating and Transport Cycle

Transport follows a "single binding center gated pore" model (also described as an alternating-access mechanism) involving opening and closing of gates on the cytoplasmic and matrix sides to alternately expose the common substrate binding site (monne2012substratespecificityof pages 1-2). Three characteristic signature sequence motifs (PFDTMK, PTELVK, and PVDCIK) contain proline residues that kink the odd-numbered transmembrane helices (tessa2009identificationofnovel pages 5-6). A salt-bridge network formed by charged residues (D31-K131, K34-D231, K234-E128) closes the cavity on the matrix side and constitutes the matrix gate (tessa2009identificationofnovel pages 5-6, palmieri2013themitochondrialtransporter pages 3-6). Substrate binding to R179, as the rate-limiting step, triggers conformational changes that open the matrix gate, allowing substrate translocation (monne2012substratespecificityof pages 8-9).

The structural model of ORC1 was built based on the crystal structure of the bovine ADP/ATP carrier (tessa2009identificationofnovel pages 5-6, monne2012substratespecificityof pages 2-3).

4. Role in the Urea Cycle and Ornithine Metabolism

The urea cycle is a metabolic pathway that detoxifies nitrogen by converting ammonia into urea, operating across both the mitochondrial matrix and the cytoplasm of hepatocytes. SLC25A15/ORC1 provides the essential transport link between these two compartments by importing cytoplasmic ornithine into the mitochondrial matrix while simultaneously exporting citrulline to the cytoplasm (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11, palmieri2020diseasescausedby pages 13-15, cunningham202020000picometersunder pages 4-5).

In the mitochondrial matrix, imported ornithine serves as a substrate for ornithine transcarbamylase (OTC), which combines ornithine with carbamyl phosphate (produced by carbamoyl phosphate synthetase I) to generate citrulline (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11). The citrulline produced is then exported to the cytoplasm via ORC1, where argininosuccinate synthase reacts citrulline with ATP and aspartate to generate argininosuccinate, continuing the cytoplasmic portion of the urea cycle (cunningham202020000picometersunder pages 4-5).

Beyond the urea cycle, ORC1 also supports arginine biosynthesis and polyamine synthesis, as export of mitochondrial ornithine to the cytosol provides the substrate for ornithine decarboxylase, a key enzyme in polyamine production (mentel2021learningfromyeast pages 20-21, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 8-9).

5. Subcellular Localization and Tissue Expression

ORC1 is an integral membrane protein of the inner mitochondrial membrane (tessa2009identificationofnovel pages 5-6, palmieri2013themitochondrialtransporter pages 3-6). It is expressed broadly across tissues, with highest expression levels reported in liver, pancreas, lung, and kidney (gutierrezaguilar2013physiologicalandpathological pages 3-4, ahmed2024theroleof pages 16-18). Expression in the liver is particularly important given that the urea cycle is principally operative in periportal hepatocytes (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 8-9). Expression has also been noted in testes, small intestine, and other organs (cai2026argininetransportersin pages 10-12, gutierrezaguilar2013physiologicalandpathological pages 3-4).

6. Paralogues and Functional Redundancy

Three human mitochondrial carriers can transport ornithine:

ORC2/SLC25A2 (ORNT2): Shares 87% sequence identity with ORC1 but has lower affinity for ornithine and citrulline and broader substrate specificity, including the ability to transport histidine, homocitrulline, and D-isomers of amino acids (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 8-9, arco2005newmitochondrialcarriers pages 10-11). ORC2 expression is consistently lower than ORC1 in all tissues examined (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 8-9, gutierrezaguilar2013physiologicalandpathological pages 3-4). ORC2 originated through retrotransposition and orthologues are found only in mammalian genomes (arco2005newmitochondrialcarriers pages 10-11).
SLC25A29 (ORNT3): A related basic amino acid carrier that can rescue ornithine metabolism in fibroblasts from HHH syndrome patients, demonstrating compensatory capacity (camacho2009thehumanand pages 6-7, mentel2021learningfromyeast pages 21-22). ORNT3 shows higher expression in the central nervous system than in the liver, suggesting tissue-specific compensatory roles (camacho2009thehumanand pages 6-7).

The redundancy provided by ORC2 and SLC25A29 may explain the variable severity and relatively milder phenotypes of HHH syndrome compared to deficiencies in urea cycle enzymes themselves (mentel2021learningfromyeast pages 20-21, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 8-9, arco2005newmitochondrialcarriers pages 10-11).

7. Disease Associations

7.1 HHH Syndrome (Hyperornithinemia-Hyperammonemia-Homocitrullinuria)

Loss-of-function mutations in SLC25A15 cause HHH syndrome (OMIM #238970), a rare autosomal recessive disorder of the urea cycle (palmieri2020diseasescausedby pages 13-15, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11). Over 100 patients have been identified worldwide, with major prevalence clusters in Canada, Italy, and Japan (palmieri2020diseasescausedby pages 13-15). The molecular pathogenesis proceeds through a defined cascade:

Impaired ornithine transport into mitochondria reduces the availability of ornithine for OTC in the matrix (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11).
Cytoplasmic ornithine accumulates (hyperornithinemia) while mitochondrial ornithine is depleted (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11).
Urea cycle dysfunction leads to ammonia accumulation (hyperammonemia) (palmieri2020diseasescausedby pages 13-15, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11).
Accumulated carbamyl phosphate reacts with lysine to form homocitrulline (homocitrullinuria), a hallmark metabolite (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11, tunalı2014anovelmutation pages 1-2).
High cytoplasmic ornithine inhibits arginine-glycine amidotransferase (AGAT), causing secondary creatine deficiency, which compounds neurological vulnerability (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11).
Excessive ornithine and homocitrulline cause protein and lipid oxidation, impairing brain oxidative phosphorylation and Krebs cycle function (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11).

Clinical manifestations include acute episodes of vomiting, confusion, and coma; chronic features include developmental delay, intellectual disability, spastic paraplegia, cerebellar ataxia, seizures, and pyramidal dysfunction (palmieri2020diseasescausedby pages 13-15, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11). More than 35 disease-causing mutations have been identified (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11).

The following table summarizes the spectrum of known pathogenic mutations:

Mutation	Type	Residual transport activity	Structural location	Population prevalence / recurrence	Functional/pathogenic note
F188del	In-frame deletion	Defective; markedly reduced activity	Likely TM4 / pore region	~30% of reported HHH patients; enriched in French-Canadian cases (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 16-16)	One of the two most common HHH alleles; affects a residue lining the internal translocation pore, consistent with impaired substrate translocation (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11)
R179* (also reported as R179X)	Nonsense	Predicted null	Contact-point II / pore-facing region, near matrix gate	~15% of reported HHH patients; recurrent in Japanese and Middle Eastern/Palestinian families (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11, tessa2009identificationofnovel pages 3-4, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 16-16, dweikat2022clinicalheterogeneityof pages 9-9)	Truncates ORC1; residue 179 is a key determinant of substrate recognition in wild-type ORC1, so loss is expected to abolish transport (monne2012substratespecificityof pages 1-2, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11)
M37R	Missense	~0% (virtually incapable of transport)	TM1 / matrix salt-bridge network region (H1) (tessa2009identificationofnovel pages 6-8, tessa2009identificationofnovel pages 5-6)	Novel/reported in HHH families; no major founder prevalence stated (tessa2009identificationofnovel pages 6-8)	Disrupts salt-bridge networks critical for carrier gating and strongly impairs transport of ornithine, arginine, lysine, and citrulline (tessa2009identificationofnovel pages 6-8, tessa2009identificationofnovel pages 5-6)
L71Q	Missense	Reduced; within reported mutant range of ~4–19% of wild type	TM2 / H2 helix package (tessa2009identificationofnovel pages 6-8, tessa2009identificationofnovel pages 5-6)	Reported in HHH families; no major founder prevalence stated (tessa2009identificationofnovel pages 6-8)	Hydrophilic substitution perturbs H2 helix packing and carrier structural integrity (tessa2009identificationofnovel pages 6-8)
A15V	Missense	Nearly abolished / dramatically inhibited	N-terminal region / early TM1 vicinity (tunalı2014anovelmutation pages 1-2)	Identified in a Turkish patient (tunalı2014anovelmutation pages 1-2, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 16-16)	Functional assay showed near-complete loss of ornithine transport, supporting pathogenicity (tunalı2014anovelmutation pages 1-2)
G27E	Missense	Defective; exact % not stated	TM1 / pore-proximal region	Recurrent in Japanese patients; also reported in Palestinian families (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 16-16, dweikat2022clinicalheterogeneityof pages 9-9)	Associated with decreased ornithine transport activity in liver mitochondria/ORC1 deficiency (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 16-16, dweikat2022clinicalheterogeneityof pages 9-9)
G216S	Missense	Reduced; within reported mutant range of ~4–19% of wild type	Likely TM5 / pore-facing region (tessa2009identificationofnovel pages 6-8, tessa2009identificationofnovel pages 5-6)	Reported in HHH families (tessa2009identificationofnovel pages 6-8, tessa2009identificationofnovel pages 3-4)	Alters carrier structure/function and contributes to markedly reduced transport (tessa2009identificationofnovel pages 6-8, tessa2009identificationofnovel pages 5-6)
T272I	Missense	Reduced; within reported mutant range of ~4–19% of wild type	TM6 region (tessa2009identificationofnovel pages 6-8)	Reported in HHH families (tessa2009identificationofnovel pages 6-8, tessa2009identificationofnovel pages 3-4)	Causes steric interference with crucial intramolecular interactions required for catalytic function (tessa2009identificationofnovel pages 6-8)
L283F	Missense	Reduced; within reported mutant range of ~4–19% of wild type	TM6 region / late C-terminal helix (tessa2009identificationofnovel pages 6-8)	Reported in HHH families (tessa2009identificationofnovel pages 6-8)	Bulky substitution likely perturbs helix packing and transport pathway geometry (tessa2009identificationofnovel pages 6-8)
E180K	Missense	Pathogenic; exact residual % not stated	Contact-point II substrate-binding site near residue R179 (monne2012substratespecificityof pages 1-2, cunningham202020000picometersunder pages 4-5, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11)	Reported missense HHH allele (cunningham202020000picometersunder pages 4-5)	E180 is a key substrate-binding residue in wild-type ORC1; substitution is expected to disrupt ligand recognition and translocation (monne2012substratespecificityof pages 1-2, martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11)
F188D	Missense	Pathogenic; exact residual % not stated	Around residue 188 in pore/TM4 region (cunningham202020000picometersunder pages 4-5)	Reported missense HHH allele (cunningham202020000picometersunder pages 4-5)	Missense change near the recurrent F188 hotspot; associated with HHH syndrome and impaired urea-cycle transport (cunningham202020000picometersunder pages 4-5)
P126R	Missense	Defective; exact % not stated	Likely TM3 / central cavity region (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 16-16)	Reported in affected families (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 16-16)	Likely disrupts conserved carrier helix geometry important for alternating-access transport (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 16-16, tessa2009identificationofnovel pages 5-6)
R275X	Nonsense	Predicted null	TM6 / contact-point III vicinity; R275 is mechanistically important in wild type (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11, monne2012substratespecificityof pages 7-8)	Reported in affected families (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 16-16)	Premature stop removes C-terminal region; wild-type Arg275 contributes to substrate-triggered conformational change (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11, monne2012substratespecificityof pages 7-8)
T32R	Missense	Defective; exact % not stated	TM1 / matrix-gate neighborhood (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 16-16, tessa2009identificationofnovel pages 5-6)	Reported in affected families (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 16-16)	Likely perturbs local charge environment near matrix-side gating residues (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 16-16, tessa2009identificationofnovel pages 5-6)
p.K245X	Nonsense	Predicted null	C-terminal half / likely TM5-TM6 region (tessa2009identificationofnovel pages 6-8, tessa2009identificationofnovel pages 5-6)	Identified among 13 mutations in HHH families (tessa2009identificationofnovel pages 5-6, tessa2009identificationofnovel pages 3-4)	Premature truncation expected to abolish functional carrier assembly/transport (tessa2009identificationofnovel pages 6-8, tessa2009identificationofnovel pages 5-6)
p.S175fsX192	Frameshift	Predicted null	Mid-protein / around TM4 entry (tessa2009identificationofnovel pages 5-6)	Reported in HHH families (tessa2009identificationofnovel pages 5-6)	Frameshift with premature termination; expected severe loss of ORC1 function (tessa2009identificationofnovel pages 5-6)
c.552-555delTTTC (p.Phe185SerfsTer8)	Frameshift deletion	Predicted null	Around residue 185 / pore hotspot region (dweikat2022clinicalheterogeneityof pages 9-9)	Novel homozygous variant in 9 Palestinian patients (dweikat2022clinicalheterogeneityof pages 9-9)	Expected nonsense-mediated decay or severely truncated protein; expands the HHH molecular spectrum (dweikat2022clinicalheterogeneityof pages 9-9)
c.446delG (p.Ser149ThrfsTer45)	Frameshift deletion	Predicted null	Mid-protein / central carrier region (dweikat2022clinicalheterogeneityof pages 9-9)	Recurrent homozygous variant in 4 Palestinian patients (dweikat2022clinicalheterogeneityof pages 9-9)	Frameshift expected to abrogate transporter function; associated with marked clinical heterogeneity despite shared genotype (dweikat2022clinicalheterogeneityof pages 9-9)
Overall missense class	Missense	Typically ~4–19% of wild-type transport, except severe alleles such as M37R (~0%) and A15V (near-abolished) (tessa2009identificationofnovel pages 5-6, tessa2009identificationofnovel pages 1-2, tunalı2014anovelmutation pages 1-2)	Frequently in transmembrane helices H1–H6 and residues protruding into the internal pore (tessa2009identificationofnovel pages 6-8, tessa2009identificationofnovel pages 5-6)	No clear genotype-phenotype correlation overall (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11, tessa2009identificationofnovel pages 1-2)	Many pathogenic residues cluster in the translocation pore or gating networks, interfering with substrate binding, helix movement, or conformational switching (martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 9-11, tessa2009identificationofnovel pages 6-8, monne2012substratespecificityof pages 7-8)

Table: This table summarizes representative disease-causing SLC25A15 variants reported in hyperornithinemia-hyperammonemia-homocitrullinuria syndrome, including mutation class, functional impact, structural context, and recurrence. It is useful for connecting genotype to transporter mechanism and clinical interpretation.

7.2 Cancer Associations

Emerging evidence implicates SLC25A15 in cancer biology in a context-dependent manner:

Hepatocellular carcinoma (HCC): ORC1 is downregulated in HCC and functions as a tumor suppressor. Its downregulation leads to ammonia accumulation, which activates mTORC1, promotes glutamine uptake, and redirects glutamine metabolism toward reductive carboxylation and de novo lipogenesis, thereby driving HCC progression. ORC1 downregulation also confers resistance to anti-PD-L1 therapy (ahmed2024theroleof pages 16-18).
Diffuse large B-cell lymphoma (DLBCL): In contrast, SLC25A15 is highly expressed in DLBCL where it serves as a survival factor. It supports purine nucleotide homeostasis and genomic stability; its downregulation leads to impaired purine nucleotide synthesis, dGTP depletion, and DNA damage, identifying it as a potential metabolic vulnerability and therapeutic target (su2026pufabiflavone pages 12-14, su2026pufabiflavone pages 7-12, su2026pufabiflavone pages 1-2).

OpenTargets disease-target association analysis confirms strong associations of SLC25A15 with ornithine translocase deficiency (score 0.85) and HHH syndrome (score 0.84), as well as weaker associations with inflammatory bowel disease and autoimmune CNS disorders (OpenTargets Search: -SLC25A15).

8. Summary

SLC25A15/ORC1 is the principal human mitochondrial ornithine transporter, embedded in the inner mitochondrial membrane, where it catalyzes the electroneutral 1:1 antiport exchange of cytoplasmic L-ornithine for mitochondrial L-citrulline plus a proton. Its primary biological role is to bridge the mitochondrial and cytoplasmic reactions of the urea cycle, ensuring both ornithine availability for OTC in the matrix and citrulline export for argininosuccinate synthase in the cytosol. The protein adopts the canonical six-transmembrane-helix architecture of the SLC25 family, with substrate binding determined by key residues at three contact points (E77, R179/E180, R275). Loss-of-function mutations cause HHH syndrome, a rare urea cycle disorder, while emerging evidence points to context-dependent roles in cancer metabolism. Paralogous carriers ORC2 and SLC25A29 provide partial functional redundancy that may modulate disease severity.

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(tunalı2014anovelmutation pages 1-2): Nagehan Ersoy Tunalı, Carlo M.T. Marobbio, N. Ozan Tiryakioğlu, Giuseppe Punzi, Seha K. Saygılı, Hasan Önal, and Ferdinando Palmieri. A novel mutation in the slc25a15 gene in a turkish patient with hhh syndrome: functional analysis of the mutant protein. Molecular Genetics and Metabolism, 112(1):25-29, May 2014. URL: https://doi.org/10.1016/j.ymgme.2014.03.002, doi:10.1016/j.ymgme.2014.03.002. This article has 34 citations and is from a peer-reviewed journal.
(martinelli2015thehyperornithinemia–hyperammonemiahomocitrullinuriasyndrome pages 16-16): Diego Martinelli, Daria Diodato, Emanuela Ponzi, Magnus Monné, Sara Boenzi, Enrico Bertini, Giuseppe Fiermonte, and Carlo Dionisi-Vici. The hyperornithinemia–hyperammonemia-homocitrullinuria syndrome. Orphanet Journal of Rare Diseases, Mar 2015. URL: https://doi.org/10.1186/s13023-015-0242-9, doi:10.1186/s13023-015-0242-9. This article has 121 citations and is from a peer-reviewed journal.
(dweikat2022clinicalheterogeneityof pages 9-9): Imad Dweikat and Reham Khalaf-Nazzal. Clinical heterogeneity of hyperornithinemia-hyperammonemia-homocitrullinuria syndrome in thirteen palestinian patients and report of a novel variant in the slc25a15 gene. Frontiers in Genetics, Nov 2022. URL: https://doi.org/10.3389/fgene.2022.1004598, doi:10.3389/fgene.2022.1004598. This article has 7 citations and is from a peer-reviewed journal.
(tessa2009identificationofnovel pages 6-8): Alessandra Tessa, Giuseppe Fiermonte, Carlo Dionisi-Vici, Eleonora Paradies, Matthias R. Baumgartner, Yin-Hsiu Chien, Carmela Loguercio, Helene Ogier de Baulny, Marie-Cecile Nassogne, Manuel Schiff, Federica Deodato, Giancarlo Parenti, S. Lane Rutledge, M. Antonia Vilaseca, Mariarosa A.B. Melone, Gioacchino Scarano, Luiz Aldamiz-Echevarría, Guy Besley, John Walter, Eugenia Martinez-Hernandez, Jose M. Hernandez, Ciro L. Pierri, Ferdinando Palmieri, and Filippo M. Santorelli. Identification of novel mutations in the slc25a15 gene in hyperornithinemia‐hyperammonemia‐homocitrullinuria (hhh) syndrome: a clinical, molecular, and functional study. Human Mutation, 30:741-748, May 2009. URL: https://doi.org/10.1002/humu.20930, doi:10.1002/humu.20930. This article has 88 citations and is from a domain leading peer-reviewed journal.
(su2026pufabiflavone pages 12-14): Chang Su, Guige Lu, Lijia Ou, Liang Liang, Caiqin Wang, Yizi He, Ruolan Zeng, Yajun Li, Hui Zhou, and Ling Xiao. Puf, a biflavone monomer, triggers dna damage through slc25a15 downregulation and purine metabolic suppression in dlbcl. Journal of Translational Medicine, Feb 2026. URL: https://doi.org/10.1186/s12967-026-07797-9, doi:10.1186/s12967-026-07797-9. This article has 0 citations and is from a peer-reviewed journal.
(su2026pufabiflavone pages 7-12): Chang Su, Guige Lu, Lijia Ou, Liang Liang, Caiqin Wang, Yizi He, Ruolan Zeng, Yajun Li, Hui Zhou, and Ling Xiao. Puf, a biflavone monomer, triggers dna damage through slc25a15 downregulation and purine metabolic suppression in dlbcl. Journal of Translational Medicine, Feb 2026. URL: https://doi.org/10.1186/s12967-026-07797-9, doi:10.1186/s12967-026-07797-9. This article has 0 citations and is from a peer-reviewed journal.
(su2026pufabiflavone pages 1-2): Chang Su, Guige Lu, Lijia Ou, Liang Liang, Caiqin Wang, Yizi He, Ruolan Zeng, Yajun Li, Hui Zhou, and Ling Xiao. Puf, a biflavone monomer, triggers dna damage through slc25a15 downregulation and purine metabolic suppression in dlbcl. Journal of Translational Medicine, Feb 2026. URL: https://doi.org/10.1186/s12967-026-07797-9, doi:10.1186/s12967-026-07797-9. This article has 0 citations and is from a peer-reviewed journal.

Artifacts

Citations

palmieri2013themitochondrialtransporter pages 3-6
gao2024cancertherapeuticpotential pages 6-7
monne2012substratespecificityof pages 1-2
mentel2021learningfromyeast pages 20-21
monne2012substratespecificityof pages 3-4
tessa2009identificationofnovel pages 5-6
monne2012substratespecificityof pages 8-9
arco2005newmitochondrialcarriers pages 10-11
camacho2009thehumanand pages 6-7
palmieri2020diseasescausedby pages 13-15
tessa2009identificationofnovel pages 6-8
tunalı2014anovelmutation pages 1-2
dweikat2022clinicalheterogeneityof pages 9-9
ahmed2024theroleof pages 16-18
gutierrezaguilar2013physiologicalandpathological pages 3-4
tessa2009identificationofnovel pages 3-4
monne2012substratespecificityof pages 7-8
mentel2021learningfromyeast pages 21-22
tessa2009identificationofnovel pages 1-2
monne2012substratespecificityof pages 2-3
cai2026argininetransportersin pages 10-12
su2026pufabiflavone pages 12-14
su2026pufabiflavone pages 7-12
su2026pufabiflavone pages 1-2
https://doi.org/10.1016/j.mam.2012.05.005,
https://doi.org/10.1186/s13023-015-0242-9,
https://doi.org/10.1042/bj20121753,
https://doi.org/10.1177/10732748241287905,
https://doi.org/10.1002/humu.20930,
https://doi.org/10.1074/jbc.m111.324855,
https://doi.org/10.3390/biom10040655,
https://doi.org/10.3390/ijms26010092,
https://doi.org/10.1203/pdr.0b013e3181a283c1,
https://doi.org/10.15252/embr.202050071,
https://doi.org/10.3390/microorganisms9102044,
https://doi.org/10.3390/biom16010132,
https://doi.org/10.1007/s00018-005-5197-x,
https://doi.org/10.1016/j.ymgme.2014.03.002,
https://doi.org/10.3389/fgene.2022.1004598,
https://doi.org/10.1186/s12967-026-07797-9,

📚 Additional Documentation

Notes

(SLC25A15-notes.md)

SLC25A15 / ORNT1 (Q9Y619) review notes

Identity and family

Mitochondrial ornithine transporter 1 (ORNT1); Solute carrier family 25 member 15.
301 aa, mitochondrial carrier (SLC25 / TC 2.A.29) family: three Solcar repeats, six TM helices, MCP domain (UniProt Q9Y619).
TCDB 2.A.29.19.2. Paralog SLC25A2/ORNT2 (~88% identical, retrotransposon-derived) has overlapping, broader specificity and partially compensates for ORNT1 loss.

Core molecular function

Mitochondrial inner-membrane ornithine/citrulline antiporter: imports cytosolic L-ornithine into the matrix in exchange for exporting matrix L-citrulline (with an H+ making transport electroneutral).
UniProt FUNCTION: "Mitochondrial ornithine-citrulline antiporter ... Catalyzes the exchange between cytosolic ornithine and mitochondrial citrulline plus an H(+), the proton compensates the positive charge of ornithine thus leading to an electroneutral transport. Plays a crucial role in the urea cycle, by connecting the cytosolic and the intramitochondrial reactions of the urea cycle" (UniProt Q9Y619, [PubMed:12807890], [PubMed:22262851]).
Rhea catalytic activity: RHEA:70787 L-citrulline(in) + L-ornithine(out) + H+(in) = L-citrulline(out) + L-ornithine(in) + H+(out).

Broad basic-amino-acid specificity (in vitro)

PMID:12807890 ORC1=ORNT1; ORC2=ORNT2.
PMID:12807890
Km values (UniProt, PubMed:12807890): ornithine 0.22 mM, lysine 0.8 mM, arginine 1.58 mM, citrulline 2.52 mM — ornithine highest affinity, consistent with ornithine being the primary physiological substrate.
Additional Rhea reactions in UniProt: L-arginine/L-ornithine antiport (RHEA:34991, EXP PubMed:12807890), L-ornithine/L-lysine antiport (RHEA:70799, EXP PubMed:12807890), plus by-similarity ornithine/H+ and lysine/H+ uniport.

Substrate-specificity determinants

[PMID:22262851 title "Substrate specificity of the two mitochondrial ornithine carriers can be swapped by single mutation in substrate binding site."] Mutagenesis (R179, E180, W224) alters relative ornithine vs arginine/lysine transport (UniProt MUTAGEN features).

Localization

Mitochondrial inner membrane; multi-pass membrane protein (UniProt SUBCELLULAR LOCATION; ISS from yeast ortholog Q12375). Original disease paper localized to mitochondrial membrane (EXP PubMed:10369256). HTP mito proteome (PMID:34800366).

Urea cycle role & disease

PMID:10369256
Neurospora crassa ARG13 / S. cerevisiae ARG11 are the fungal orthologs; identified ORNT1 by orthology; expression high in liver and dietary-protein regulated (PMID:10369256).
HHH syndrome (hyperornithinemia-hyperammonemia-homocitrullinuria; MIM:238970; MONDO:0009393): autosomal recessive urea-cycle disorder from biallelic SLC25A15 LoF. Confirmed by dismech KB (Hyperornithinemia_Hyperammonemia_Homocitrullinuria_Syndrome.yaml). Many pathogenic HHH variants (G27R, E180K, F188del, R275Q, etc.) abolish/reduce transport.
PMID:12948741 ORNT2 rescues defective ornithine metabolism in HHH patient fibroblasts.

GO term verification (OLS, current labels)

GO:0000064 L-ornithine transmembrane transporter activity — MF, correct core.
GO:1990575 mitochondrial L-ornithine transmembrane transport — BP; "L-ornithine is transported across a mitochondrial membrane" — correct core BP.
GO:0000050 urea cycle — BP, correct.
GO:0005743 mitochondrial inner membrane — CC, correct core location.
GO:0043858 arginine:ornithine antiporter activity — MF; direction matches RHEA:34991; EXP-supported.
GO:0015189 L-lysine / GO:0061459 L-arginine transmembrane transporter activity — IDA-supported broad specificity; non-core (physiological role is ornithine/citrulline).
GO:0015297 antiporter activity — parent MF; correct but general.
No dedicated "ornithine:citrulline antiporter" MF term exists in GO (searched OLS).

Curation stance

Core: GO:0000064 (MF), ornithine/citrulline antiport captured by GO:0043858 + GO:0015297, GO:1990575 (BP ornithine transport), GO:0000050 (urea cycle BP), GO:0005743 (CC).
Non-core but experimentally real: lysine/arginine transporter MF + BP (broad basic-AA specificity in reconstituted system).
Keep general CC (mitochondrion, mitochondrial membrane, membrane) as ACCEPT/non-core supporting the specific inner-membrane call.
Reactome R-HSA-9956519 "SLC25A15 variants don't translocate ornithine and citrulline" is a variant/disease reaction but is used here to support the WT normal function annotations (GO:0005743, GO:0000064) — accept, these are correct for the gene.

📄 View Raw YAML

id: Q9Y619
gene_symbol: SLC25A15
product_type: PROTEIN
status: INITIALIZED
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  SLC25A15 (ORNT1, mitochondrial ornithine transporter 1) is a member of the SLC25
  mitochondrial carrier family that resides in the inner mitochondrial membrane as a
  multi-pass membrane protein built from three tandem Solcar (mitochondrial carrier)
  repeats. It catalyzes the electroneutral ornithine/citrulline antiport that couples
  the cytosolic and matrix halves of the urea cycle; cytosolic L-ornithine is imported
  into the matrix in exchange for the export of matrix L-citrulline, together with a
  proton that neutralizes the ornithine charge. In vitro the carrier also exchanges the
  basic amino acids L-lysine and L-arginine, and can perform ornithine/H+ and lysine/H+
  uniport, reflecting a broad basic-amino-acid specificity, but ornithine is its
  highest-affinity and principal physiological substrate. The protein is most highly
  expressed in liver, pancreas, testis, lung and small intestine, consistent with its
  urea-cycle role. Loss of function causes the autosomal recessive urea-cycle disorder
  hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome. A closely related
  paralog, SLC25A2/ORNT2, has overlapping and broader substrate specificity and
  partially compensates for defective ORNT1.
existing_annotations:
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: >-
      Phylogenetic (IBA) assignment placing ORNT1 activity in the mitochondrion. This is
      correct but general; the specific and experimentally supported location is the
      mitochondrial inner membrane.
    action: KEEP_AS_NON_CORE
    reason: >-
      ORNT1 is a mitochondrial carrier that acts in the mitochondrion; the IBA call
      across the SLC25 ornithine-carrier clade is sound. It is a correct parent of the
      more specific mitochondrial inner membrane annotation, so it is retained as
      accurate though non-core. The precise location is captured by GO:0005743.
- term:
    id: GO:0000064
    label: L-ornithine transmembrane transporter activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: >-
      Phylogenetic (IBA) assignment of L-ornithine transmembrane transporter activity,
      the core molecular function of ORNT1 and its fungal orthologs (N. crassa ARG13,
      S. cerevisiae ARG11/ORT1).
    action: ACCEPT
    reason: >-
      This is the core molecular function, directly supported by reconstitution assays
      and by the disease mechanism, and is consistent across the ornithine-carrier
      phylogenetic clade.
    supported_by:
    - reference_id: PMID:10369256
      supporting_text: >-
        Our results show that ORNT1 encodes the mitochondrial ornithine transporter
        involved in UC function and is defective in HHH syndrome.
- term:
    id: GO:1990575
    label: mitochondrial L-ornithine transmembrane transport
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: >-
      Phylogenetic (IBA) assignment of the biological process of transporting L-ornithine
      across the mitochondrial membrane, the process this carrier mediates.
    action: ACCEPT
    reason: >-
      Correctly captures the core biological process (movement of L-ornithine across the
      inner mitochondrial membrane) that ORNT1 performs as part of urea-cycle function.
    supported_by:
    - reference_id: PMID:10369256
      supporting_text: >-
        ORNT1 expression restores ornithine metabolism in fibroblasts from patients with
        hyperammonaemia-hyperornithinaemia-homocitrullinuria (HHH) syndrome.
- term:
    id: GO:0000064
    label: L-ornithine transmembrane transporter activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  qualifier: enables
  review:
    summary: >-
      ARBA machine-learning electronic annotation of the core L-ornithine transmembrane
      transporter activity.
    action: ACCEPT
    reason: >-
      Duplicates the well-supported core molecular function (also assigned by IBA, EXP,
      and TAS). The electronic mapping is at the correct level of specificity.
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: located_in
  review:
    summary: >-
      Electronic (multi-method IEA) assignment of mitochondrial inner membrane location,
      from UniProt subcellular-location mapping (SL-0168).
    action: ACCEPT
    reason: >-
      Correct core location for this multi-pass SLC25 inner-membrane carrier; concordant
      with the UniProt subcellular location and the ISS-from-ortholog call.
- term:
    id: GO:0015297
    label: antiporter activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  qualifier: enables
  review:
    summary: >-
      ARBA electronic annotation of generic antiporter activity, reflecting the exchange
      (antiport) transport mechanism of ORNT1.
    action: ACCEPT
    reason: >-
      ORNT1 is genuinely an antiporter (ornithine/citrulline exchange), so the term is
      correct although general. It is retained as a true parent of the more informative
      specific antiport function; the physiologically meaningful antiport is the
      arginine:ornithine / ornithine:citrulline exchange captured by GO:0043858 and by
      the ornithine-transporter terms.
- term:
    id: GO:0031966
    label: mitochondrial membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: >-
      Electronic annotation (UniProt subcellular-location mapping, SL-0171) of
      mitochondrial membrane location.
    action: ACCEPT
    reason: >-
      Correct but less specific than mitochondrial inner membrane. Retained as a true
      parent; the precise, experimentally consistent location is GO:0005743.
- term:
    id: GO:0000050
    label: urea cycle
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-70635
  qualifier: involved_in
  review:
    summary: >-
      Reactome-traceable assignment placing ORNT1 in the urea cycle, which it connects by
      shuttling ornithine and citrulline across the inner mitochondrial membrane.
    action: ACCEPT
    reason: >-
      Core biological process. ORNT1 physically links the cytosolic and matrix halves of
      the urea cycle; its loss causes the urea-cycle disorder HHH syndrome.
    supported_by:
    - reference_id: PMID:10369256
      supporting_text: >-
        ORNT1 encodes the mitochondrial ornithine transporter involved in UC function and
        is defective in HHH syndrome.
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9956519
  qualifier: located_in
  review:
    summary: >-
      Reactome-traceable assignment of mitochondrial inner membrane location (from the
      Reactome reaction describing ornithine/citrulline translocation).
    action: ACCEPT
    reason: >-
      Correct core location. Although the source Reactome event concerns disease variants
      that fail to translocate, the localization to the inner membrane is a correct
      statement about the wild-type protein.
- term:
    id: GO:0000064
    label: L-ornithine transmembrane transporter activity
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9956519
  qualifier: enables
  review:
    summary: >-
      Reactome-traceable assignment of the core L-ornithine transmembrane transporter
      activity.
    action: ACCEPT
    reason: >-
      Duplicates the well-supported core molecular function. Correct level of specificity.
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: located_in
  review:
    summary: >-
      Sequence-similarity (ISS) transfer of mitochondrial inner membrane location from
      the S. cerevisiae ortholog ORT1/Q12375.
    action: ACCEPT
    reason: >-
      Correct core location; concordant with UniProt and electronic annotations and with
      the multi-pass SLC25 topology.
- term:
    id: GO:0031966
    label: mitochondrial membrane
  evidence_type: EXP
  original_reference_id: PMID:10369256
  qualifier: located_in
  review:
    summary: >-
      Experimental localization of ORNT1 to the mitochondrial membrane in the original
      disease-gene study.
    action: ACCEPT
    reason: >-
      Experimentally supported mitochondrial membrane localization. The term is correct
      though more general than the inner-membrane call; retained as a true and
      experimentally grounded parent.
    supported_by:
    - reference_id: PMID:10369256
      supporting_text: mutations in a gene encoding a mitochondrial ornithine transporter
- term:
    id: GO:0043858
    label: arginine:ornithine antiporter activity
  evidence_type: EXP
  original_reference_id: PMID:12807890
  qualifier: enables
  review:
    summary: >-
      Experimentally demonstrated arginine/ornithine antiport by reconstituted, purified
      ORNT1 (ORC1). This is a specific antiport activity of the carrier and corresponds to
      the UniProt/Rhea reaction L-arginine(out) + L-ornithine(in) = L-arginine(in) +
      L-ornithine(out) (RHEA:34991).
    action: ACCEPT
    reason: >-
      Directly measured for reconstituted ORNT1 and matches the GO term direction
      (arginine(out) + ornithine(in) = arginine(in) + ornithine(out)). It represents a
      real, specific antiport function of the carrier. The physiologically dominant
      exchange is ornithine/citrulline, but arginine:ornithine antiport is a genuine
      biochemical activity and captures the carrier antiport mechanism more precisely than
      generic antiporter activity.
    supported_by:
    - reference_id: PMID:12807890
      supporting_text: >-
        Both transport L-isomers of ornithine, lysine, arginine, and citrulline by
        exchange and by unidirectional mechanisms, and they are inactivated by the same
        inhibitors.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: HTP
  original_reference_id: PMID:34800366
  qualifier: located_in
  review:
    summary: >-
      High-throughput mitochondrial-proteome study detecting ORNT1 in the mitochondrion.
    action: KEEP_AS_NON_CORE
    reason: >-
      Consistent with the well-established mitochondrial localization, though less specific
      than the experimentally supported inner-membrane location and derived from a
      high-throughput proteomic dataset. Retained as a correct but corroborative,
      non-core localization annotation.
- term:
    id: GO:0015189
    label: L-lysine transmembrane transporter activity
  evidence_type: IDA
  original_reference_id: PMID:12807890
  qualifier: enables
  review:
    summary: >-
      Direct assay of reconstituted ORNT1 showed transport of L-lysine, reflecting the
      carrier broad basic-amino-acid specificity (UniProt Rhea reaction L-ornithine(out) +
      L-lysine(in) = L-ornithine(in) + L-lysine(out), RHEA:70799).
    action: KEEP_AS_NON_CORE
    reason: >-
      Experimentally demonstrated in the reconstituted system (do not remove an IDA
      activity). However, lysine is a lower-affinity substrate than ornithine (Km 0.8 vs
      0.22 mM) and lysine transport is not the physiological role of the carrier; the
      urea-cycle-relevant function is ornithine/citrulline exchange. Marked non-core to
      reflect the broad in vitro specificity rather than the primary biological function.
    supported_by:
    - reference_id: PMID:12807890
      supporting_text: >-
        Both transport L-isomers of ornithine, lysine, arginine, and citrulline by
        exchange and by unidirectional mechanisms, and they are inactivated by the same
        inhibitors.
- term:
    id: GO:0015297
    label: antiporter activity
  evidence_type: IDA
  original_reference_id: PMID:12807890
  qualifier: enables
  review:
    summary: >-
      Direct demonstration that ORNT1 transports basic amino acids by an exchange
      (antiport) mechanism in reconstituted proteoliposomes.
    action: ACCEPT
    reason: >-
      Correct, experimentally grounded description of the antiport mechanism, although
      general. Retained as a true parent of the specific antiport activities; the more
      informative specific term is GO:0043858 (arginine:ornithine antiporter).
    supported_by:
    - reference_id: PMID:12807890
      supporting_text: >-
        Both transport L-isomers of ornithine, lysine, arginine, and citrulline by
        exchange and by unidirectional mechanisms, and they are inactivated by the same
        inhibitors.
- term:
    id: GO:0061459
    label: L-arginine transmembrane transporter activity
  evidence_type: IDA
  original_reference_id: PMID:12807890
  qualifier: enables
  review:
    summary: >-
      Direct assay of reconstituted ORNT1 showed transport of L-arginine, reflecting the
      carrier broad basic-amino-acid specificity (UniProt Rhea reaction RHEA:34991).
    action: KEEP_AS_NON_CORE
    reason: >-
      Experimentally demonstrated (do not remove an IDA activity). Arginine is a
      lower-affinity substrate than ornithine (Km 1.58 vs 0.22 mM) and arginine transport
      is not the carrier principal physiological role, which is ornithine/citrulline
      exchange in the urea cycle. Marked non-core to reflect the broad in vitro
      specificity.
    supported_by:
    - reference_id: PMID:12807890
      supporting_text: >-
        Both transport L-isomers of ornithine, lysine, arginine, and citrulline by
        exchange and by unidirectional mechanisms, and they are inactivated by the same
        inhibitors.
- term:
    id: GO:1903401
    label: L-lysine transmembrane transport
  evidence_type: IDA
  original_reference_id: PMID:12807890
  qualifier: involved_in
  review:
    summary: >-
      Biological-process counterpart of the demonstrated L-lysine transport activity by
      reconstituted ORNT1.
    action: KEEP_AS_NON_CORE
    reason: >-
      Experimentally supported but non-core; a manifestation of the carrier broad
      basic-amino-acid specificity rather than its urea-cycle role.
    supported_by:
    - reference_id: PMID:12807890
      supporting_text: >-
        Both transport L-isomers of ornithine, lysine, arginine, and citrulline by
        exchange and by unidirectional mechanisms, and they are inactivated by the same
        inhibitors.
- term:
    id: GO:1903826
    label: L-arginine transmembrane transport
  evidence_type: IDA
  original_reference_id: PMID:12807890
  qualifier: involved_in
  review:
    summary: >-
      Biological-process counterpart of the demonstrated L-arginine transport activity by
      reconstituted ORNT1.
    action: KEEP_AS_NON_CORE
    reason: >-
      Experimentally supported but non-core; reflects broad basic-amino-acid specificity,
      not the primary urea-cycle function.
    supported_by:
    - reference_id: PMID:12807890
      supporting_text: >-
        Both transport L-isomers of ornithine, lysine, arginine, and citrulline by
        exchange and by unidirectional mechanisms, and they are inactivated by the same
        inhibitors.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IDA
  original_reference_id: PMID:10369256
  qualifier: located_in
  review:
    summary: >-
      Direct experimental localization of ORNT1 to the mitochondrion in the original
      disease-gene identification study.
    action: KEEP_AS_NON_CORE
    reason: >-
      Experimentally supported mitochondrial localization, consistent with the more
      specific inner-membrane location. Retained as a correct parent, non-core relative
      to the specific mitochondrial inner membrane location.
    supported_by:
    - reference_id: PMID:10369256
      supporting_text: mutations in a gene encoding a mitochondrial ornithine transporter
- term:
    id: GO:0000064
    label: L-ornithine transmembrane transporter activity
  evidence_type: EXP
  original_reference_id: PMID:12807890
  qualifier: enables
  review:
    summary: >-
      Experimental demonstration (reconstituted, purified ORNT1) of L-ornithine transport,
      establishing the core molecular function.
    action: ACCEPT
    reason: >-
      Directly measured core molecular function; ornithine is the highest-affinity
      substrate (Km 0.22 mM). This is the defining activity of the carrier.
    supported_by:
    - reference_id: PMID:12807890
      supporting_text: >-
        Both transport L-isomers of ornithine, lysine, arginine, and citrulline by
        exchange and by unidirectional mechanisms, and they are inactivated by the same
        inhibitors.
- term:
    id: GO:0000064
    label: L-ornithine transmembrane transporter activity
  evidence_type: EXP
  original_reference_id: PMID:12948741
  qualifier: enables
  review:
    summary: >-
      Experimental support for L-ornithine transporter activity in the study
      characterizing ORNT2 and confirming the ornithine-transport function shared with
      ORNT1 (functional rescue of ornithine metabolism in HHH patient fibroblasts).
    action: ACCEPT
    reason: >-
      Reinforces the core molecular function of the ORNT ornithine carriers. The paper
      demonstrates the ornithine-transport function that ORNT1 provides in the urea cycle
      and that ORNT2 can partially replace.
    supported_by:
    - reference_id: PMID:12948741
      supporting_text: >-
        When ORNT2 is overexpressed transiently in cultured fibroblasts from HHH patients,
        it rescues the deficient ornithine metabolism in these cells.
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-70634
  qualifier: located_in
  review:
    summary: >-
      Reactome-traceable assignment of mitochondrial inner membrane location, from the
      reaction describing cytosolic/mitochondrial ornithine-citrulline exchange.
    action: ACCEPT
    reason: >-
      Correct core location, concordant with all other localization annotations.
- term:
    id: GO:0000050
    label: urea cycle
  evidence_type: TAS
  original_reference_id: PMID:10369256
  qualifier: involved_in
  review:
    summary: >-
      Author-stated (TAS) involvement of ORNT1 in the urea cycle, from the disease-gene
      identification study.
    action: ACCEPT
    reason: >-
      Core biological process. ORNT1 connects the cytosolic and matrix reactions of the
      urea cycle; its deficiency causes the urea-cycle disorder HHH syndrome.
    supported_by:
    - reference_id: PMID:10369256
      supporting_text: >-
        ORNT1 encodes the mitochondrial ornithine transporter involved in UC function and
        is defective in HHH syndrome.
- term:
    id: GO:0000064
    label: L-ornithine transmembrane transporter activity
  evidence_type: TAS
  original_reference_id: PMID:10369256
  qualifier: enables
  review:
    summary: >-
      Author-stated (TAS) L-ornithine transporter activity for ORNT1 in the disease-gene
      study.
    action: ACCEPT
    reason: Duplicates the well-supported core molecular function.
    supported_by:
    - reference_id: PMID:10369256
      supporting_text: mutations in a gene encoding a mitochondrial ornithine transporter
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: TAS
  original_reference_id: PMID:10369256
  qualifier: located_in
  review:
    summary: Author-stated (TAS) mitochondrial inner membrane location for ORNT1.
    action: ACCEPT
    reason: >-
      Correct core location, consistent with experimental and electronic annotations.
    supported_by:
    - reference_id: PMID:10369256
      supporting_text: transport ornithine across the mitochondrial inner membrane
- term:
    id: GO:0016020
    label: membrane
  evidence_type: TAS
  original_reference_id: PMID:10369256
  qualifier: located_in
  review:
    summary: >-
      Author-stated (TAS) generic membrane location, superseded by the specific
      mitochondrial inner membrane annotations.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Not incorrect (ORNT1 is a multi-pass membrane protein), but GO:0016020 is
      uninformative given the well-established mitochondrial inner membrane localization.
      It is a legacy general term that adds no biological information beyond the specific
      inner-membrane annotations.
- term:
    id: GO:1990575
    label: mitochondrial L-ornithine transmembrane transport
  evidence_type: TAS
  original_reference_id: PMID:10369256
  qualifier: involved_in
  review:
    summary: >-
      Author-stated (TAS) biological process of L-ornithine transport across the
      mitochondrial membrane.
    action: ACCEPT
    reason: >-
      Core biological process, concordant with the IBA assignment and the urea-cycle role.
    supported_by:
    - reference_id: PMID:10369256
      supporting_text: transport ornithine across the mitochondrial inner membrane
core_functions:
- description: >-
    Ornithine/citrulline antiport across the inner mitochondrial membrane that couples the
    cytosolic and matrix halves of the urea cycle, importing cytosolic L-ornithine into
    the matrix in exchange for exporting matrix L-citrulline.
  molecular_function:
    id: GO:0000064
    label: L-ornithine transmembrane transporter activity
  directly_involved_in:
  - id: GO:0000050
    label: urea cycle
  locations:
  - id: GO:0005743
    label: mitochondrial inner membrane
  supported_by:
  - reference_id: PMID:12807890
    supporting_text: >-
      Both transport L-isomers of ornithine, lysine, arginine, and citrulline by exchange
      and by unidirectional mechanisms, and they are inactivated by the same inhibitors.
  - reference_id: PMID:10369256
    supporting_text: >-
      ORNT1 encodes the mitochondrial ornithine transporter involved in UC function and is
      defective in HHH syndrome.
- description: >-
    Transmembrane movement of L-ornithine across the mitochondrial inner membrane, the
    transport process that ORNT1 mediates.
  molecular_function:
    id: GO:0000064
    label: L-ornithine transmembrane transporter activity
  directly_involved_in:
  - id: GO:1990575
    label: mitochondrial L-ornithine transmembrane transport
  locations:
  - id: GO:0005743
    label: mitochondrial inner membrane
  supported_by:
  - reference_id: PMID:10369256
    supporting_text: >-
      ORNT1 expression restores ornithine metabolism in fibroblasts from patients with
      hyperammonaemia-hyperornithinaemia-homocitrullinuria (HHH) syndrome.
references:
- id: GO_REF:0000024
  title: Manual transfer of experimentally-verified manual GO annotation data to orthologs
    by curator judgment of sequence similarity
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000044
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location
    vocabulary mapping, accompanied by conservative changes to GO terms applied by
    UniProt
  findings: []
- id: GO_REF:0000117
  title: Electronic Gene Ontology annotations created by ARBA machine learning models
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:10369256
  title: Hyperornithinaemia-hyperammonaemia-homocitrullinuria syndrome is caused by
    mutations in a gene encoding a mitochondrial ornithine transporter.
  findings:
  - statement: >-
      Identified ORNT1 (SLC25A15) by orthology to fungal mitochondrial ornithine carriers,
      showed it restores ornithine metabolism in HHH patient fibroblasts, and established
      that its mutations cause HHH syndrome.
    supporting_text: >-
      ORNT1 encodes the mitochondrial ornithine transporter involved in UC function and is
      defective in HHH syndrome.
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      PubMed-verified original disease-gene study establishing ORNT1 as the mitochondrial
      ornithine transporter and its role in the urea cycle and HHH syndrome. Cached record
      is abstract-only.
- id: PMID:12807890
  title: The mitochondrial ornithine transporter. Bacterial expression, reconstitution,
    functional characterization, and tissue distribution of two human isoforms.
  findings:
  - statement: >-
      Reconstituted, purified ORNT1 (ORC1) transports L-ornithine, L-lysine, L-arginine
      and L-citrulline by exchange and unidirectional mechanisms; ORNT2 (ORC2) has broader
      specificity; both are most highly expressed in liver.
    supporting_text: >-
      Both transport L-isomers of ornithine, lysine, arginine, and citrulline by exchange
      and by unidirectional mechanisms, and they are inactivated by the same inhibitors.
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      PubMed-verified biochemical characterization of purified, reconstituted human
      ORNT1/ORNT2, establishing substrate specificity (ornithine highest affinity) and
      antiport mechanism. Cached record is abstract-only.
- id: PMID:12948741
  title: 'Cloning and characterization of human ORNT2: a second mitochondrial ornithine
    transporter that can rescue a defective ORNT1 in patients with the hyperornithinemia-hyperammonemia-homocitrullinuria
    syndrome, a urea cycle disorder.'
  findings:
  - statement: >-
      ORNT2 (SLC25A2) is a paralog ~88% identical to ORNT1 that targets to mitochondria
      and rescues deficient ornithine metabolism in HHH patient fibroblasts, explaining
      partial functional redundancy.
    supporting_text: >-
      When ORNT2 is overexpressed transiently in cultured fibroblasts from HHH patients,
      it rescues the deficient ornithine metabolism in these cells.
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      PubMed-verified; primarily characterizes the paralog ORNT2 but confirms the shared
      ornithine-transport function relevant to ORNT1. Abstract-only.
- id: PMID:34800366
  title: Quantitative high-confidence human mitochondrial proteome and its dynamics
    in cellular context.
  findings:
  - statement: >-
      High-throughput mitochondrial-proteome study detecting SLC25A15 in the mitochondrion.
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: >-
      High-throughput proteomic detection supporting mitochondrial localization;
      corroborative rather than functionally decisive.
- id: PMID:22262851
  title: Substrate specificity of the two mitochondrial ornithine carriers can be
    swapped by single mutation in substrate binding site.
  findings:
  - statement: >-
      Mutagenesis of substrate-binding residues (R179, E180, W224) in ORNT1 shifts
      relative ornithine versus arginine/lysine transport, defining the
      substrate-specificity determinants of the carrier.
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      UniProt-cited mechanistic study of ORNT1 substrate binding; supports the
      ornithine-carrier function and its broad basic-amino-acid specificity. Not cached;
      cited from the UniProt record (FUNCTION and MUTAGEN features).
- id: Reactome:R-HSA-70634
  title: ornithine (cytosolic) + citrulline (mitochondrial) => ornithine (mitochondrial)
    + citrulline (cytosolic)
  findings: []
- id: Reactome:R-HSA-70635
  title: Urea cycle
  findings: []
- id: Reactome:R-HSA-9956519
  title: SLC25A15 variants don't translocate ornithine and citrulline
  findings: []

SLC25A15

Gene Description

Existing Annotations Review

Core Functions

Ornithine/citrulline antiport across the inner mitochondrial membrane that couples the cytosolic and matrix halves of the urea cycle, importing cytosolic L-ornithine into the matrix in exchange for exporting matrix L-citrulline.

Transmembrane movement of L-ornithine across the mitochondrial inner membrane, the transport process that ORNT1 mediates.

References

Deep Research

Falcon

Comprehensive Research Report: SLC25A15 (Mitochondrial Ornithine Transporter 1 / ORC1 / ORNT1)

1. Gene and Protein Identity

2. Primary Transport Function and Substrate Specificity

3. Protein Structure and Transport Mechanism

3.1 Substrate Binding Site

3.2 Gating and Transport Cycle

4. Role in the Urea Cycle and Ornithine Metabolism

5. Subcellular Localization and Tissue Expression

6. Paralogues and Functional Redundancy

7. Disease Associations

7.1 HHH Syndrome (Hyperornithinemia-Hyperammonemia-Homocitrullinuria)

7.2 Cancer Associations

8. Summary

Artifacts

Citations

📚 Additional Documentation

Notes

SLC25A15 / ORNT1 (Q9Y619) review notes

Identity and family

Core molecular function

Broad basic-amino-acid specificity (in vitro)

Substrate-specificity determinants

Localization

Urea cycle role & disease

GO term verification (OLS, current labels)

Curation stance

📄 View Raw YAML