SLC3A1 encodes rBAT, a heavily glycosylated type II membrane protein that is expressed chiefly at the apical brush border of kidney proximal-tubule and intestinal epithelial cells. rBAT forms a disulfide-linked heterodimer with the SLC7A9 light chain and promotes assembly, maturation, oligomerization, and delivery of the heteromeric amino-acid transporter to the cell surface. The SLC7 light chain supplies the transmembrane transport pathway, whereas SLC3A1 is the heavy-chain accessory and trafficking subunit. The resulting complex supports renal and intestinal reabsorption of cystine and dibasic amino acids; loss-of-function SLC3A1 variants cause cystinuria, with poorly soluble urinary cystine producing recurrent stones. The extracellular alpha-amylase-like fold is structurally important but lacks demonstrated glucosidase activity.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0031526 brush border membrane | IBA GO_REF:0000033 | ACCEPT | Summary: SLC3A1 functions at the apical brush border of proximal-tubule and intestinal epithelial cells as the rBAT heavy chain of heteromeric amino-acid transporters. Reason: The phylogenetic inference agrees with direct biochemical detection of the native rBAT-SLC7A9 heterodimer in human kidney brush-border membranes. Supporting Evidence: PMID:12167606 human kidney brush-border membranes showed that rBAT and b(0,+)AT |
| GO:0006865 amino acid transport | IBA GO_REF:0000033 | ACCEPT | Summary: rBAT is required for assembly and surface delivery of transport-competent SLC7A9-containing complexes and is therefore involved in amino-acid transport. Reason: This biological-process annotation does not claim that SLC3A1 forms the substrate pathway; it correctly records the heavy chain's essential contribution. Supporting Evidence: PMID:32817565 The light chain constitutes the transport subunit whereas the heavy chain mediates trafficking to the plasma membrane |
| GO:0003333 amino acid transmembrane transport | IEA GO_REF:0000108 | ACCEPT | Summary: SLC3A1 contributes to amino-acid transmembrane transport as the obligatory heavy-chain assembly and trafficking partner of the catalytic SLC7 light chain. Reason: The process term is appropriate for a required complex subunit when its non-catalytic role is kept distinct from an enables molecular-function assertion. Supporting Evidence: PMID:10588648 b(0,+)AT as the catalytic subunit that associates by a disulfide bond with rBAT file:human/SLC3A1/SLC3A1-deep-research-manual.md SLC3A1 is therefore a transporter-activating heavy chain and an essential complex component, not the amino-acid permeation pathway itself. |
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: SLC3A1 is a single-pass membrane protein that functions in cell-surface heteromeric transport complexes. Reason: Plasma-membrane localization is directly supported by structural and kidney biochemical studies and is the functional destination promoted by the heavy chain. Supporting Evidence: PMID:32817565 heavy chain mediates trafficking to the plasma membrane |
| GO:0005975 carbohydrate metabolic process | IEA GO_REF:0000002 | REMOVE | Summary: The annotation is an InterPro transfer from the alpha-amylase-like fold in the extracellular domain, not evidence of carbohydrate metabolism by SLC3A1. Reason: Purified human b0,+AT-rBAT was tested directly and had no alpha-glucosidase activity. The fold has been repurposed for complex architecture and stability. Supporting Evidence: PMID:32494597 The results showed that the complex has no Ξ±-glucosidase activity |
| GO:0016020 membrane | IEA GO_REF:0000117 | ACCEPT | Summary: SLC3A1 is an integral single-pass membrane glycoprotein. Reason: The term is broad but correct and consistent with the more specific accepted plasma-, apical-, and brush-border-membrane annotations. Supporting Evidence: PMID:32494597 Both proteins are type II membrane glycoproteins |
| GO:0016324 apical plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: rBAT is concentrated at the apical membrane of kidney and intestinal epithelial cells. Reason: The reviewed localization mapping is concordant with human brush-border biochemistry and the physiological direction of cystine reabsorption. Supporting Evidence: PMID:12167606 the main apical reabsorption system for cystine in the kidney |
| GO:1990822 basic amino acid transmembrane transport | IEA GO_REF:0000108 | ACCEPT | Summary: SLC3A1-containing heteromers participate in the exchange and epithelial reabsorption of dibasic amino acids, including arginine and lysine. Reason: This process annotation correctly reflects the required heavy-chain contribution without assigning catalytic transporter activity to SLC3A1 alone. Supporting Evidence: PMID:8054986 high-affinity transport of cystine and dibasic amino acids in kidney and intestine |
| GO:0005515 protein binding | IPI PMID:32494597 Cryo-EM structure of the human heteromeric amino acid transp... | MODIFY | Summary: The resolved interaction is the disulfide-linked rBAT-SLC7A9 heterodimer, not an uncharacterized generic protein-binding event. Reason: Protein heterodimerization activity captures the experimentally defined molecular interaction and is more informative than protein binding. Proposed replacements: protein heterodimerization activity Supporting Evidence: PMID:32494597 forming heterodimer with rBAT |
| GO:0005774 vacuolar membrane | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: The transfer originates from immunogold detection of rat NBAT on plasmalemmal and vacuolar membranes in forebrain neurons. Reason: The source-side localization is experimental and should not be overruled, but this transferred neuronal compartment is not the established epithelial brush-border site of SLC3A1's core transport role in humans. Supporting Evidence: PMID:9987991 immunogold labeling for NBAT was distributed along plasmalemmal and vacuolar membranes |
| GO:0006865 amino acid transport | IEA GO_REF:0000120 | ACCEPT | Summary: SLC3A1 is involved in amino-acid transport through assembly and surface delivery of transport-competent heteromers. Reason: Multiple independent experimental studies establish this complex-level biological role, while assigning catalysis to the SLC7 light chain. Supporting Evidence: PMID:32817565 heavy chain mediates trafficking to the plasma membrane and maturation of the functional complex |
| GO:0031526 brush border membrane | IEA GO_REF:0000107 | ACCEPT | Summary: Orthology transfer places human SLC3A1 at the epithelial brush border. Reason: The transferred location is independently confirmed in human kidney brush-border membranes, so there is no propagation-specific concern. Supporting Evidence: PMID:12167606 human kidney brush-border membranes showed that rBAT and b(0,+)AT |
| GO:0044877 protein-containing complex binding | IEA GO_REF:0000107 | MODIFY | Summary: SLC3A1 binds the specific SLC7A9 light chain to form a disulfide-linked heterodimeric transporter complex. Reason: Protein-containing complex binding is generic and implies binding to a pre-existing complex. Protein heterodimerization activity precisely describes the native heavy-light-chain assembly. Proposed replacements: protein heterodimerization activity Supporting Evidence: PMID:10588648 associates by a disulfide bond with rBAT |
| GO:0031526 brush border membrane | IDA PMID:32494597 Cryo-EM structure of the human heteromeric amino acid transp... | ACCEPT | Summary: rBAT is an apical epithelial brush-border component of the b0,+AT-rBAT complex. Reason: The structural study's biological context agrees with direct human kidney evidence and the established reabsorptive function of the complex. Supporting Evidence: PMID:12167606 human kidney brush-border membranes showed that rBAT and b(0,+)AT |
| GO:1902495 transmembrane transporter complex | IPI PMID:32494597 Cryo-EM structure of the human heteromeric amino acid transp... | MODIFY | Summary: SLC3A1 is a heavy-chain component of the b0,+AT-rBAT amino-acid transport complex. Reason: Complex membership is correct, but amino acid transport complex is a more informative child term for this structurally resolved heteromer. Proposed replacements: amino acid transport complex Supporting Evidence: PMID:32494597 b0,+AT-rBAT exists as a dimer of heterodimer |
| GO:0005886 plasma membrane | EXP PMID:12167606 rBAT-b(0,+)AT heterodimer is the main apical reabsorption sy... | ACCEPT | Summary: Native rBAT-SLC7A9 heterodimers occur in human kidney brush-border plasma membranes. Reason: The direct kidney biochemical evidence supports the functional cell-surface location. Supporting Evidence: PMID:12167606 human kidney brush-border membranes showed that rBAT and b(0,+)AT |
| GO:0005886 plasma membrane | EXP PMID:32817565 Structural basis for amino acid exchange by a human heterome... | ACCEPT | Summary: The rBAT heavy chain mediates maturation and plasma-membrane trafficking of the functional heteromer. Reason: The experimentally supported location is central to SLC3A1's accessory role. Supporting Evidence: PMID:32817565 heavy chain mediates trafficking to the plasma membrane and maturation of the functional complex |
| GO:0016324 apical plasma membrane | ISS GO_REF:0000024 | ACCEPT | Summary: Human SLC3A1 functions at the apical epithelial membrane, consistent with the orthology transfer. Reason: Direct human kidney brush-border evidence independently validates the transferred location. Supporting Evidence: PMID:12167606 the main apical reabsorption system for cystine in the kidney |
| GO:0046982 protein heterodimerization activity | IDA PMID:32494597 Cryo-EM structure of the human heteromeric amino acid transp... | ACCEPT | Summary: rBAT forms a disulfide-linked heterodimer with SLC7A9. Reason: Cryo-EM resolves the nonidentical heavy and light subunits and their interface, making heterodimerization the specific direct molecular activity of SLC3A1. Supporting Evidence: PMID:32494597 forming heterodimer with rBAT |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-5655702 | ACCEPT | Summary: Reactome places defective SLC3A1 within the plasma-membrane SLC7A9-SLC3A1 exchange complex. Reason: The pathway location is consistent with direct structural and kidney localization evidence. |
| GO:0005515 protein binding | IPI PMID:12167606 rBAT-b(0,+)AT heterodimer is the main apical reabsorption sy... | MODIFY | Summary: The paper demonstrates the native rBAT-SLC7A9 heterodimer rather than generic protein binding. Reason: Protein heterodimerization activity is the informative molecular-function term for the interaction. Proposed replacements: protein heterodimerization activity Supporting Evidence: PMID:12167606 rBAT and b(0,+)AT were solely expressed as heterodimers of identical size |
| GO:0031526 brush border membrane | IDA PMID:12167606 rBAT-b(0,+)AT heterodimer is the main apical reabsorption sy... | ACCEPT | Summary: Human kidney brush-border membranes contain the native rBAT-SLC7A9 heterodimer. Reason: This is direct biochemical evidence for the core epithelial localization. Supporting Evidence: PMID:12167606 human kidney brush-border membranes showed that rBAT and b(0,+)AT |
| GO:0070062 extracellular exosome | HDA PMID:19056867 Large-scale proteomics and phosphoproteomics of urinary exos... | UNDECIDED | Summary: SLC3A1 was reported in a large-scale human urinary-exosome proteomic dataset. Reason: Urinary-exosome detection is plausible for an abundant apical renal membrane protein but does not establish an exosomal function. The local abstract does not expose the SLC3A1-specific peptide result, so the experimental annotation cannot be manually verified and is not overruled. Supporting Evidence: PMID:19056867 analysis identified 1132 proteins unambiguously |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-379432 | ACCEPT | Summary: Reactome models SLC3A1 in the plasma-membrane SLC7A9-SLC3A1 amino-acid exchange complex. Reason: The pathway assignment agrees with direct structural and localization evidence. |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-5660890 | ACCEPT | Summary: Reactome places SLC3A1 in the plasma-membrane complex affected by defective SLC7A9. Reason: The modeled location is independently established and remains correct in the disease pathway. |
| GO:0005886 plasma membrane | TAS PMID:8054986 Cystinuria caused by mutations in rBAT, a gene involved in t... | ACCEPT | Summary: SLC3A1 is the membrane heavy chain whose loss disrupts epithelial cystine and dibasic-amino-acid transport. Reason: Although the abstract does not isolate membrane localization, plasma-membrane placement is firmly established by later direct studies and supports the curator's statement. Supporting Evidence: PMID:32817565 heavy chain mediates trafficking to the plasma membrane |
| GO:0006865 amino acid transport | TAS PMID:10799513 Differential influence of the 4F2 heavy chain and the protei... | ACCEPT | Summary: Co-expression of rBAT with SLC7A9 produces a functionally competent b0,+-like transport system. Reason: The biological-process statement correctly records SLC3A1 involvement through complex assembly and modulation without requiring direct permease activity. Supporting Evidence: PMID:10799513 constitute functionally competent b(0,+)-like amino acid transport systems |
| GO:0015171 amino acid transmembrane transporter activity | TAS PMID:10799513 Differential influence of the 4F2 heavy chain and the protei... | MODIFY | Summary: The rBAT-SLC7A9 complex transports amino acids, but SLC7A9 is the catalytic transmembrane transport subunit and SLC3A1 regulates assembly, trafficking, and affinity. Reason: Transporter activator activity captures the heavy chain's direct function; amino-acid transporter activity belongs to the complex/light chain and should be represented as a contributed molecular function for SLC3A1. Proposed replacements: transporter activator activity Supporting Evidence: PMID:10588648 b(0,+)AT as the catalytic subunit that associates by a disulfide bond with rBAT |
| GO:0015174 basic amino acid transmembrane transporter activity | TAS PMID:8054986 Cystinuria caused by mutations in rBAT, a gene involved in t... | MODIFY | Summary: SLC3A1 variants abolish complex-level dibasic-amino-acid transport, but rBAT does not form the transmembrane substrate pathway. Reason: The historical oocyte result reflects activation of an endogenous light-chain transporter. Transporter activator activity is the correct direct heavy-chain function. Proposed replacements: transporter activator activity Supporting Evidence: PMID:8054986 M467T nearly abolished the amino acid transport activity induced by rBAT in Xenopus oocytes |
| GO:0015184 L-cystine transmembrane transporter activity | TAS PMID:8054986 Cystinuria caused by mutations in rBAT, a gene involved in t... | MODIFY | Summary: SLC3A1 is essential for cystine transport by the heteromer, but SLC7A9 is the catalytic light chain and rBAT is the heavy-chain assembly/trafficking subunit. Reason: The direct enables annotation conflates a complex phenotype with SLC3A1 catalysis. Replace it with transporter activator activity and retain L-cystine transport as a process. Proposed replacements: transporter activator activity Supporting Evidence: PMID:10588648 b(0,+)AT as the catalytic subunit that associates by a disulfide bond with rBAT |
| GO:0015802 basic amino acid transport | TAS PMID:8054986 Cystinuria caused by mutations in rBAT, a gene involved in t... | ACCEPT | Summary: rBAT is required for epithelial transport of dibasic amino acids by the SLC7A9-SLC3A1 complex. Reason: This process annotation is supported by human disease variants and transport assays and does not assign catalytic permease activity to the heavy chain. Supporting Evidence: PMID:8054986 defective transepithelial transport of cystine and dibasic amino acids in the kidney and intestine |
| GO:0015811 L-cystine transport | TAS PMID:8054986 Cystinuria caused by mutations in rBAT, a gene involved in t... | ACCEPT | Summary: SLC3A1 is required for heteromeric cystine transport and epithelial reabsorption. Reason: Cystinuria genetics and kidney biochemistry establish the heavy chain's indispensable involvement, while later work assigns catalysis to SLC7A9. Supporting Evidence: PMID:12167606 responsible for virtually all apical cystine reabsorption |
| GO:0016020 membrane | TAS PMID:8054986 Cystinuria caused by mutations in rBAT, a gene involved in t... | ACCEPT | Summary: rBAT is a membrane glycoprotein required for the epithelial cystine-transport complex. Reason: The broad membrane annotation is correct and consistent with direct structural evidence. Supporting Evidence: PMID:32494597 Both proteins are type II membrane glycoproteins |
| GO:1990297 renal amino acid absorption | IDA PMID:12167606 rBAT-b(0,+)AT heterodimer is the main apical reabsorption sy... | NEW | Summary: The native rBAT-SLC7A9 heterodimer is responsible for apical renal cystine reabsorption. Reason: Renal amino acid absorption is the specific physiological process demonstrated by human kidney brush-border biochemistry and cystine-reabsorption data and is more informative than the existing generic amino-acid-transport terms. Supporting Evidence: PMID:12167606 responsible for virtually all apical cystine reabsorption |
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Download this section (compressed HTML)Q: What fraction of cystine reabsorption in each human proximal-tubule segment is mediated by SLC3A1-SLC7A9 versus SLC3A1-SLC7A13?
Q: Do any of the six noncanonical SLC3A1 splice isoforms form stable surface heteromers or exert tissue-specific dominant-negative effects?
Q: Does the neuronal vacuolar-membrane localization transferred from rat occur for endogenous human SLC3A1, and does it have a transport-related function?
Experiment: Introduce interface, glycosylation, and extracellular-domain variants into SLC3A1-null human proximal-tubule organoids; separately measure heterodimer assembly, ER exit, apical abundance, and isotope-labeled cystine/arginine exchange.
Hypothesis: Human SLC3A1 activates SLC7A9 transport primarily by promoting heterodimer maturation and apical delivery rather than by contributing to the substrate pathway.
Type: Separation-of-function rescue in human kidney organoids
Experiment: Use segment-resolved spatial proteomics and endogenous cross-linking in human kidney, followed by reconstitution of each purified heteromer and matched counterflow assays for cystine, basic, neutral, and acidic amino acids.
Hypothesis: SLC7A9 and SLC7A13 partner with SLC3A1 in different proximal-tubule segments and confer distinct exchange substrates.
Type: Partner-resolved spatial proteomics and transport reconstitution
Experiment: Express each verified isoform at matched levels with SLC7A9 in polarized human epithelial cells and compare disulfide-linked assembly, glycan maturation, apical trafficking, turnover, and transport activation.
Hypothesis: Most noncanonical SLC3A1 isoforms fail to assemble a stable apical amino-acid transport complex.
Type: Isoform-resolved trafficking and functional assay
What is not known β curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The relative physiological contributions of SLC7A9 and SLC7A13 as SLC3A1 partners along distinct human proximal-tubule segments are unresolved.
OPEN BIOLOGY BP_DARK
What is known: The SLC7A9-rBAT complex is directly demonstrated in human kidney, whereas the SLC7A13-rBAT late-proximal-tubule model is currently supported mainly by orthology.
Significance: Partner-specific resolution is needed to explain segmental cystine handling and residual transport in different cystinuria genotypes.
What would resolve it: Quantify endogenous SLC3A1-SLC7A9 and SLC3A1-SLC7A13 complexes across human proximal-tubule segments and measure partner-specific exchange in organoids.
Provenance (the field's own admissions):
Gap: The physiological function of the large extracellular glycosidase-like domain, beyond structural stabilization and oligomerization, remains incompletely defined.
OPEN BIOLOGY MF_DARK
What is known: The domain binds calcium and resembles glycosidases, but purified complex lacks alpha-glucosidase activity and no alternative catalytic substrate is established.
Significance: Resolving this domain's role may explain how extracellular variants alter complex folding, trafficking, and cystinuria severity.
What would resolve it: Compare structure, trafficking, partner affinity, and transport activation for targeted domain and calcium-site mutants without assuming enzymatic activity.
Provenance (the field's own admissions):
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