Polyamine transporter of the SLC45 family, a 12-transmembrane protein with the major facilitator superfamily fold and an inserted regulatory plug domain that places two conserved basic residues into an electronegative central cavity forming the polyamine binding site. SLC45A4 recognises and translocates putrescine, cadaverine, spermidine and spermine, with highest affinity for the shorter diamines and lowest for spermine; it does not transport amino acids, and neither GABA nor sucrose binds the purified protein, despite the family having been named for sequence similarity to plant sucrose transporters. Transport is optimal at slightly alkaline pH. In the nervous system the protein is enriched across dorsal root ganglion sensory neuron subtypes including nociceptors, where it supports the excitability of C-polymodal nociceptors and contributes to the perception of noxious heat and tonic chemical pain; common variants at the human locus associate with chronic pain intensity. In transformed epithelial cells it supports the de novo synthesis of GABA from ornithine through the arginine-ornithine-putrescine pathway by supplying putrescine for oxidation by diamine oxidases, without itself transporting GABA. Its subcellular location is disputed: work on overexpressed protein in neurons and cell lines places it in the plasma membrane, while work on endogenous protein in lung-cancer lines places it in the peroxisomal membrane, and it is not yet known whether the difference reflects cell type, expression level, or a protein with two pools. Beyond the transporter domain, the C-terminus binds a wide range of PDZ-domain scaffolding proteins.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0008506 sucrose:proton symporter activity | IBA GO_REF:0000033 | REMOVE | Summary: Family-level assertion that SLC45A4 is a proton-coupled sucrose symporter. Two independent 2025 studies tested sucrose directly against the human protein and found no interaction and no sucrose catabolism. Reason: Donor tracing shows this is NOT a simple plant-to-animal leak, which is the usual reason such a term is challenged. The IBA WITH/FROM list mixes plant SUC transporters (Arabidopsis AT1G71880 etc., rice SUT1/SUT5) with three mouse SLC45 paralogs, and those mouse genes (Slc45a2 P58355, Slc45a3 Q8K0H7, Slc45a4 Q0P5V9) each carry their own IDA for GO:0008506. So the PAINT node placement is internally coherent. The failure lies in the seed evidence: all three mouse IDAs come from one heterologous yeast-expression study (PMID:25164149) reporting a Km of about 5 mM, an affinity with no physiological meaning for a disaccharide that mammals hydrolyse in the intestinal lumen and never expose to the cytosolic face of a membrane. The human protein has now been tested directly: purified SLC45A4 is not thermally stabilised by sucrose while polyamines destabilise it strongly (PMID:40836097), the cryo-EM structure resolves an acidic polyamine-recognition site rather than a sugar site, and isotope tracing found no sucrose catabolism attributable to SLC45A4 (PMID:41266324). UniProt has already added a CAUTION to Q5BKX6 on this point. The mouse IDAs on Slc45a2/3/4 should be revisited by the relevant curators for the same reason. Propagation Review Root cause: SOURCE WEAK OR INFERRED Failure modes: SOURCE EVIDENCE WEAK FUNCTIONAL DIVERGENCE CONTEXT OR TISSUE MISMATCH Sources checked: PANTHER:PTN000751434 · SLC45 / plant SUC sucrose-transporter node SOURCE WEAK OR INFERRED Node placement itself is defensible: the clade contains both plant SUC transporters and mammalian SLC45 paralogs carrying experimental sucrose annotations. The weakness is in the seed experiments, not in the tree. UniProtKB:Q0P5V9 · mouse Slc45a4 SOURCE WEAK OR INFERRED Sole WITH/FROM of the human ISS. Its own GO:0008506 is an IDA from PMID:25164149, a Saccharomyces cerevisiae heterologous expression study with Km ~5 mM for sucrose - a non-physiological affinity for a disaccharide mammals never present to any cytosolic membrane face. UniProtKB:P58355 · mouse Slc45a2 SOURCE WEAK OR INFERRED Same single yeast study (PMID:25164149); SLC45A2 is a melanosomal protein, so the yeast sucrose flux is unlikely to reflect its cellular activity either. UniProtKB:Q8K0H7 · mouse Slc45a3 SOURCE WEAK OR INFERRED Same single yeast study (PMID:25164149). UniProtKB:Q10R54 · Oryza sativa SUT1 SUPPORTS SOURCE BUT NOT TARGET Bona fide plant sucrose:H+ symporter; ~26% identity to human SLC45A4 and no shared physiological context. UniProtKB:Q69JW3 · Oryza sativa SUT5 SUPPORTS SOURCE BUT NOT TARGET As for SUT1: plant-specific sucrose phloem loading. AGI_LocusCode:AT1G71880 · Arabidopsis SUC1 SUPPORTS SOURCE BUT NOT TARGET The homology anchor for the original family name; the Nature 2025 study measured only ~26% identity to human SLC45A4 and found no sucrose binding to the purified human protein. Supporting Evidence: PMID:40836097 However, neither GABA nor sucrose elicited a change in the thermal stability of purified SLC45A4 PMID:40836097 Notably, we observed a marked decrease in thermal stability in the presence of biogenic amines, with the polyamines Spm and Spd eliciting the largest response PMID:40836097 the human SLC45A4 protein shares only about 26% sequence identity with the A. thaliana SUC1 transporter PMID:41266324 Our observations did not support the sucrose-transporting activity. PMID:25164149 Heterologous expression of the three members SLC45A2, SLC45A3 and SLC45A4 in Saccharomyces cerevisiae confirmed that they are indeed sucrose transporters. PMID:25164149 [(14)C]Sucrose-uptake measurements revealed intermediate transport affinities with Km values of approximately 5 mM. |
| GO:0016020 membrane | IBA GO_REF:0000033 | ACCEPT | Summary: SLC45A4 is a 12-transmembrane-helix MFS-fold integral membrane protein; that it is active in a membrane is not in dispute. Reason: Uninformative but correct, and notably the only compartment statement that survives the disagreement between the plasma-membrane (PMID:40836097, PMID:41075780) and peroxisome-membrane (PMID:41266324) reports. Retained as-is rather than sharpened, because sharpening it would mean picking a side of an unresolved question. Supporting Evidence: PMID:40836097 Cell-based assays show that SLC45A4 is a selective plasma membrane polyamine transporter, and the cryo-electron microscopy (cryo-EM) structure reveals a regulatory domain and basis for polyamine recognition. PMID:41266324 SLC45A4 functions as a putrescine transporter localized to the peroxisome membrane to facilitate GABA production. |
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping of the "Cell membrane" annotation, which UniProt itself sourced from PMID:40836097. Reason: Correct as a derived annotation: the UniProt Cell membrane statement it maps carries ECO:0000269|PubMed:40836097. Same action as the IDA for the same term, per same-term-same-action. The caveat recorded there (this is an overexpression result, and an independent study localises endogenous SLC45A4 to peroxisomes) applies equally here. Supporting Evidence: PMID:40836097 When electroporated into DRG neurons, eGFP-tagged SLC45A4 is trafficked to the plasma membrane |
| GO:0015203 polyamine transmembrane transporter activity | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA machine-learning assignment of polyamine transporter activity, which the 2025 experimental work independently confirms. Reason: An unusual case of an ARBA prediction being overtaken by, and agreeing with, direct experiment. This is the term on which all three 2025 studies converge and is the least contested statement about the protein. Supporting Evidence: PMID:40836097 Cell-based radioactive uptake assays further confirmed that SLC45A4 functions as a non-selective polyamine transporter with an optimum pH of 7.5–8.5 PMID:41075780 This approach identifies UNC93A and SLC45A4 as candidate plasma membrane transporters for acetylglucosamine and polyamines, respectively. |
| GO:0015847 putrescine transport | IEA GO_REF:0000108 | ACCEPT | Summary: Logical inference from the putrescine transmembrane transporter activity MF term. Reason: The inference is sound and the underlying MF now has both IDA support (competition and thermal-shift data in PMID:40836097) and an independent, if differently localised, claim from PMID:41266324 that putrescine is the physiological substrate. Supporting Evidence: PMID:40836097 The longest polyamine, Spm, has an half-maximum inhibitory concentration (IC50) of 240 µM, followed by Spd (123 µM), with the smaller polyamines Put and cadaverine (Cad) having the highest affinity (74 µM and 67 µM, respectively) PMID:41266324 SLC45A4 functions as a putrescine transporter localized to the peroxisome membrane to facilitate GABA production. |
| GO:0019233 sensory perception of pain | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA assignment of a pain role, matching the mouse knockout phenotype. Reason: Same action as the IMP for this term. The ARBA model is almost certainly reading the post-2025 UniProt FUNCTION text rather than reasoning independently, but the assertion it makes is the one the primary data support. Supporting Evidence: PMID:40836097 Mice lacking SLC45A4 show normal mechanosensitivity but reduced sensitivity to noxious heat- and algogen-induced tonic pain that is associated with reduced excitability of C-polymodal nociceptors. |
| GO:0022857 transmembrane transporter activity | IEA GO_REF:0000002 | MODIFY | Summary: Root-level transporter activity from the InterPro MFS signature IPR011701. Reason: Correct but far too general now that the substrate class is established by direct experiment in three independent 2025 papers. Replaced with polyamine transmembrane transporter activity, the most specific MF term that all three studies agree on (the compartment, not the substrate class, is what is contested). Proposed replacements: polyamine transmembrane transporter activity Supporting Evidence: PMID:40836097 Our results identify SLC45A4 as a plasma membrane polyamine transporter. PMID:41075780 Isotope-labeled tracing confirmed these predictions, demonstrating that SLC45A4 and UNC93A mediate the transport of polyamine and acetylglucosamine species across the plasma membrane. |
| GO:0055085 transmembrane transport | IEA GO_REF:0000002 | MODIFY | Summary: Root-level transport process from the InterPro MFS signature. Reason: Correct but too general, and a specific child is already independently annotated to this gene. Replaced with polyamine transmembrane transport. Proposed replacements: polyamine transmembrane transport Supporting Evidence: PMID:40836097 Cell-based radioactive uptake assays further confirmed that SLC45A4 functions as a non-selective polyamine transporter with an optimum pH of 7.5–8.5 |
| GO:1902047 polyamine transmembrane transport | IEA GO_REF:0000108 | ACCEPT | Summary: Logical inference from polyamine transmembrane transporter activity. Reason: Sound inference from a now experimentally established MF, and the correct process-level summary of what this protein does whichever membrane it sits in. Supporting Evidence: PMID:40836097 Cell-based radioactive uptake assays further confirmed that SLC45A4 functions as a non-selective polyamine transporter with an optimum pH of 7.5–8.5 |
| GO:1903710 spermine transmembrane transport | IEA GO_REF:0000108 | ACCEPT | Summary: Logical inference from spermine transmembrane transporter activity. Reason: Follows from the GO:0000297 IDA. Spermine support is by thermal destabilisation of the purified protein and by competition against 14C-spermidine uptake (IC50 240 uM) rather than by a direct spermine flux measurement, but recognition of spermine by the transport site is well established. Supporting Evidence: PMID:40836097 Notably, we observed a marked decrease in thermal stability in the presence of biogenic amines, with the polyamines Spm and Spd eliciting the largest response PMID:40836097 The longest polyamine, Spm, has an half-maximum inhibitory concentration (IC50) of 240 µM, followed by Spd (123 µM), with the smaller polyamines Put and cadaverine (Cad) having the highest affinity (74 µM and 67 µM, respectively) |
| GO:1903711 spermidine transmembrane transport | IEA GO_REF:0000108 | ACCEPT | Summary: Logical inference from spermidine transmembrane transporter activity. Reason: Follows from the GO:0015606 IDA, which rests on the strongest single dataset in the paper: a direct 14C-spermidine uptake time course in neuronal N2a cells expressing human or mouse SLC45A4 versus empty vector. Supporting Evidence: PMID:40836097 Cell-based radioactive uptake assays further confirmed that SLC45A4 functions as a non-selective polyamine transporter with an optimum pH of 7.5–8.5 |
| GO:0005515 protein binding | IPI PMID:36115835 Quantitative fragmentomics allow affinity mapping of interac... | MODIFY | Summary: Eighteen IPI partners from a single quantitative PDZ-domain holdup screen; all partners are PDZ-domain scaffolds (DLG1-4, GRIP1/2, MPDZ, PATJ, PDZK1, MAGI1, LNX1/2, WHRN, NHERF4, APBA2, IL16, FRMPD2). Reason: Per project guidance, bare protein binding carries no functional information and should not be treated as a molecular function. The underlying data are real and the pattern is informative - a uniformly PDZ-scaffold partner set points to a functional C-terminal PDZ-binding motif that would plausibly anchor SLC45A4 at a membrane and could bear on the plasma-membrane versus peroxisome question. If a curator wishes to retain the interaction evidence, GO:0030165 (PDZ domain binding) is the informative term; the individual partner identities belong in IntAct, where they already are. Replaced by GO:0030165 PDZ domain binding, which states what the interaction accomplishes. Proposed replacements: PDZ domain binding |
| GO:0000297 spermine transmembrane transporter activity | IDA PMID:41075780 Machine-learning-guided discovery of SLC25A45 as a mediator ... | ACCEPT | Summary: Machine-learning-guided deorphanization: HEK293T SLC45A4-knockout cells re-expressing SLC45A4 cDNA show altered intracellular and extracellular polyamine species, with isotope tracing using d6-spermidine and 13C4-putrescine. Reason: Same action as the other GO:0000297 IDA. Deferring to the curator, who read the full text. Two honest caveats for the record. First, this paper's headline subject is SLC25A45 (see reference_review) and SLC45A4 is the secondary subject, so the citation looks transposed but is not. Second, the cellular signature - intracellular acetylspermidine depleted and 13C4-acetylputrescine raised in the medium on SLC45A4 expression - reads more like export than import of acetylated polyamines. GO MF terms are direction-agnostic so the term choice is unaffected, but the authors themselves note that oocyte and proteoliposome assays are still needed to settle the mechanism. Supporting Evidence: PMID:41075780 Consistent with the machine learning predictions, acetylspermidine levels were depleted by nearly 4-fold, while gamma-aminobutyric acid (GABA) accumulated by ~5.5 fold upon SLC45A4 expression PMID:41075780 Furthermore, SLC45A4 expression resulted in elevated levels of [13C4]acetylputrescine in the media PMID:41075780 However, further structural studies as well as liposome-based in vitro reconstituted uptake for SLC25A45 and Xenopus larvae oocytes assays for SLC45A4 and UNC93A should be performed to definitively study the function of these transporters. |
| GO:0000297 spermine transmembrane transporter activity | IDA PMID:40836097 SLC45A4 is a pain gene encoding a neuronal polyamine transpo... | ACCEPT | Summary: Spermine is recognised by purified human SLC45A4 (largest thermal destabilisation of any metabolite tested) and competes 14C-spermidine uptake with an IC50 of 240 uM. Reason: Core molecular function. Evidence for spermine specifically is binding plus competition rather than a direct spermine flux; the paper measured radiolabelled flux for spermidine. That is a normal basis for a substrate call in a transporter paper and I defer to the curator, but a reviewer should know that spermine is the weakest-affinity member of the panel (IC50 240 uM versus 67-74 uM for putrescine and cadaverine). Supporting Evidence: PMID:40836097 Notably, we observed a marked decrease in thermal stability in the presence of biogenic amines, with the polyamines Spm and Spd eliciting the largest response PMID:40836097 The longest polyamine, Spm, has an half-maximum inhibitory concentration (IC50) of 240 µM, followed by Spd (123 µM), with the smaller polyamines Put and cadaverine (Cad) having the highest affinity (74 µM and 67 µM, respectively) |
| GO:0005886 plasma membrane | IDA PMID:40836097 SLC45A4 is a pain gene encoding a neuronal polyamine transpo... | ACCEPT | Summary: eGFP-tagged human SLC45A4 electroporated into mouse DRG neurons traffics to the plasma membrane, and cell-based uptake assays measure transport across the surface membrane of intact N2a cells. Reason: This is the contested half of the record and GOA carries only this side of it. Kept, for three reasons: the uptake assays only work if a functional pool reaches the surface; a third study independently reports plasma-membrane polyamine transport (PMID:41075780); and is_active_in is the right qualifier for what was measured. But the conflict is real and should not be hidden. PMID:41266324 localises ENDOGENOUS SLC45A4 in A549/H1299 cells to the peroxisome membrane by antibody immunofluorescence (co-localising with ABCD3, not with actin, E-cadherin, giantin, calnexin or Mitotracker) and confirms it biochemically by PEX26-HA peroxisome immunoprecipitation and by density-gradient peroxisome isolation. Methodologically that is the stronger localisation dataset, since it uses endogenous protein; the plasma-membrane evidence rests on overexpressed tagged constructs, which routinely saturate organellar targeting. The two are not formally incompatible - a polyamine carrier could have both a surface pool and a peroxisomal pool, and the cell types differ completely (sensory neurons versus transformed lung epithelium). I therefore keep this annotation and propose GO:0005778 alongside it rather than replacing it. Supporting Evidence: PMID:40836097 When electroporated into DRG neurons, eGFP-tagged SLC45A4 is trafficked to the plasma membrane PMID:40836097 Cell-based assays show that SLC45A4 is a selective plasma membrane polyamine transporter, and the cryo-electron microscopy (cryo-EM) structure reveals a regulatory domain and basis for polyamine recognition. PMID:41075780 Isotope-labeled tracing confirmed these predictions, demonstrating that SLC45A4 and UNC93A mediate the transport of polyamine and acetylglucosamine species across the plasma membrane. PMID:41266324 SLC45A4 (green signal) is clearly detected around the cell nucleus, without overlapping actin signal (Fig. 5a, top panel), suggesting organellar localization rather than plasma membrane localization. PMID:41266324 This is also confirmed by the absence of co-localization with another plasma membrane marker, E-cadherin, as shown in Supplementary Fig. 10a. |
| GO:0015203 polyamine transmembrane transporter activity | IDA PMID:40836097 SLC45A4 is a pain gene encoding a neuronal polyamine transpo... | ACCEPT | Summary: Purified human SLC45A4 is destabilised by biogenic amines but not by GABA or sucrose; 14C-spermidine uptake into cells expressing SLC45A4 is competed by putrescine, cadaverine, spermidine and spermine but not by L-lysine, L-ornithine or L-arginine; cryo-EM resolves a polyamine-recognition site. Reason: The core molecular function and the single claim on which all three 2025 studies agree. Note the paper uses "selective" in two senses that look contradictory in isolation: selective FOR polyamines over amino acids and sugars, but non-selective AMONG polyamines. This is also why the apparent substrate dispute in the two paper titles (polyamine versus putrescine) is largely illusory - putrescine is a polyamine and is in fact the highest-affinity ligand in this dataset. Supporting Evidence: PMID:40836097 Cell-based radioactive uptake assays further confirmed that SLC45A4 functions as a non-selective polyamine transporter with an optimum pH of 7.5–8.5 PMID:40836097 Notably, we observed a marked decrease in thermal stability in the presence of biogenic amines, with the polyamines Spm and Spd eliciting the largest response PMID:40836097 The longest polyamine, Spm, has an half-maximum inhibitory concentration (IC50) of 240 µM, followed by Spd (123 µM), with the smaller polyamines Put and cadaverine (Cad) having the highest affinity (74 µM and 67 µM, respectively) PMID:40836097 Our results identify SLC45A4 as a plasma membrane polyamine transporter. |
| GO:0015489 putrescine transmembrane transporter activity | IDA PMID:40836097 SLC45A4 is a pain gene encoding a neuronal polyamine transpo... | ACCEPT | Summary: Putrescine is among the highest-affinity competitors of 14C-spermidine uptake (IC50 74 uM) and destabilises the purified protein. Reason: Accepted, and worth highlighting: this is the annotation where the two 2025 papers actually converge. PMID:41266324 independently concludes that putrescine is the physiological substrate, reaching that conclusion from a completely different direction (SLC45A4 knockout causes intracellular putrescine build-up with reduced GABA production from ornithine). The disagreement between the papers is about the membrane, not about the chemistry. Supporting Evidence: PMID:40836097 The longest polyamine, Spm, has an half-maximum inhibitory concentration (IC50) of 240 µM, followed by Spd (123 µM), with the smaller polyamines Put and cadaverine (Cad) having the highest affinity (74 µM and 67 µM, respectively) PMID:41266324 SLC45A4 functions as a putrescine transporter localized to the peroxisome membrane to facilitate GABA production. PMID:41266324 we observed that the SLC45A4 KO significantly decreased the production of GABA from ornithine, but did not completely eliminate it |
| GO:0015606 spermidine transmembrane transporter activity | IDA PMID:40836097 SLC45A4 is a pain gene encoding a neuronal polyamine transpo... | ACCEPT | Summary: Direct 14C-spermidine uptake time course in neuronal N2a cells overexpressing human or mouse SLC45A4 versus empty-vector control, with a pH optimum of 7.5-8.5. Reason: The best-supported of the four substrate MF annotations, because spermidine is the only polyamine for which a direct radiolabelled flux was measured rather than inferred from competition or thermal shift. Independently corroborated by d6-spermidine tracing in PMID:41075780. Supporting Evidence: PMID:40836097 Cell-based radioactive uptake assays further confirmed that SLC45A4 functions as a non-selective polyamine transporter with an optimum pH of 7.5–8.5 PMID:41075780 Consistent with the machine learning predictions, acetylspermidine levels were depleted by nearly 4-fold, while gamma-aminobutyric acid (GABA) accumulated by ~5.5 fold upon SLC45A4 expression |
| GO:0019233 sensory perception of pain | IMP PMID:40836097 SLC45A4 is a pain gene encoding a neuronal polyamine transpo... | ACCEPT | Summary: Slc45a4 knockout mice show reduced sensitivity to noxious heat and to algogen-induced tonic pain with reduced C-polymodal nociceptor excitability; human SLC45A4 variants associate with chronic pain intensity in UK Biobank and FinnGen. Reason: Well supported and specific: the phenotype is modality-selective (noxious heat and tonic chemical pain affected, mechanosensitivity normal), which argues against a non-specific sickness or activity confound, and it is mechanistically tied to reduced nociceptor excitability - the expected consequence of losing a neuronal polyamine transporter, since polyamines gate inwardly rectifying K+ channels and AMPA receptors. Retained as a genuine biological-process role of the neuronal pool rather than demoted, because the human genetics and the mouse phenotype agree. Supporting Evidence: PMID:40836097 Mice lacking SLC45A4 show normal mechanosensitivity but reduced sensitivity to noxious heat- and algogen-induced tonic pain that is associated with reduced excitability of C-polymodal nociceptors. PMID:40836097 In the mouse nervous system, Slc45a4 expression is enriched in all sensory neuron subtypes within the dorsal root ganglion, including nociceptors. |
| GO:0008506 sucrose:proton symporter activity | ISS GO_REF:0000024 | REMOVE | Summary: Curator-judgment ortholog transfer from mouse Slc45a4 (WITH/FROM UniProtKB:Q0P5V9), made in 2016. Reason: Same action as the IBA for this term. This ISS is a clean one-to-one ortholog transfer, so it inherits exactly the strength of its single source: the mouse Slc45a4 IDA from PMID:25164149, a yeast heterologous expression study with a ~5 mM sucrose Km. Nine years later the human protein has been purified and tested directly and does not bind sucrose, while the same assay shows strong polyamine binding; cells expressing SLC45A4 show no sucrose catabolism. The transfer was reasonable in 2016 and is not reasonable now. Propagation Review Root cause: SOURCE WEAK OR INFERRED Failure modes: SOURCE EVIDENCE WEAK FUNCTIONAL DIVERGENCE CONTEXT OR TISSUE MISMATCH Sources checked: PANTHER:PTN000751434 · SLC45 / plant SUC sucrose-transporter node SOURCE WEAK OR INFERRED Node placement itself is defensible: the clade contains both plant SUC transporters and mammalian SLC45 paralogs carrying experimental sucrose annotations. The weakness is in the seed experiments, not in the tree. UniProtKB:Q0P5V9 · mouse Slc45a4 SOURCE WEAK OR INFERRED Sole WITH/FROM of the human ISS. Its own GO:0008506 is an IDA from PMID:25164149, a Saccharomyces cerevisiae heterologous expression study with Km ~5 mM for sucrose - a non-physiological affinity for a disaccharide mammals never present to any cytosolic membrane face. UniProtKB:P58355 · mouse Slc45a2 SOURCE WEAK OR INFERRED Same single yeast study (PMID:25164149); SLC45A2 is a melanosomal protein, so the yeast sucrose flux is unlikely to reflect its cellular activity either. UniProtKB:Q8K0H7 · mouse Slc45a3 SOURCE WEAK OR INFERRED Same single yeast study (PMID:25164149). UniProtKB:Q10R54 · Oryza sativa SUT1 SUPPORTS SOURCE BUT NOT TARGET Bona fide plant sucrose:H+ symporter; ~26% identity to human SLC45A4 and no shared physiological context. UniProtKB:Q69JW3 · Oryza sativa SUT5 SUPPORTS SOURCE BUT NOT TARGET As for SUT1: plant-specific sucrose phloem loading. AGI_LocusCode:AT1G71880 · Arabidopsis SUC1 SUPPORTS SOURCE BUT NOT TARGET The homology anchor for the original family name; the Nature 2025 study measured only ~26% identity to human SLC45A4 and found no sucrose binding to the purified human protein. Supporting Evidence: PMID:25164149 Heterologous expression of the three members SLC45A2, SLC45A3 and SLC45A4 in Saccharomyces cerevisiae confirmed that they are indeed sucrose transporters. PMID:25164149 [(14)C]Sucrose-uptake measurements revealed intermediate transport affinities with Km values of approximately 5 mM. PMID:40836097 However, neither GABA nor sucrose elicited a change in the thermal stability of purified SLC45A4 PMID:41266324 Our observations did not support the sucrose-transporting activity. |
| GO:0015770 sucrose transport | ISS GO_REF:0000024 | REMOVE | Summary: Process-level counterpart of the sucrose:proton symporter ISS, transferred from mouse Slc45a4. Reason: Removed for the same reasons as its molecular-function counterpart, and with an additional biological argument that applies specifically at the process level: there is no cellular sucrose transport process in mammals for this protein to participate in. Dietary sucrose is hydrolysed to glucose and fructose by brush-border sucrase-isomaltase before absorption, and sucrose is not a mammalian metabolite, so a mammalian sucrose transport process is not something an SLC45A4 molecule could be involved in even if the yeast-measured activity were real. Propagation Review Root cause: SOURCE WEAK OR INFERRED Failure modes: SOURCE EVIDENCE WEAK FUNCTIONAL DIVERGENCE CONTEXT OR TISSUE MISMATCH Sources checked: PANTHER:PTN000751434 · SLC45 / plant SUC sucrose-transporter node SOURCE WEAK OR INFERRED Node placement itself is defensible: the clade contains both plant SUC transporters and mammalian SLC45 paralogs carrying experimental sucrose annotations. The weakness is in the seed experiments, not in the tree. UniProtKB:Q0P5V9 · mouse Slc45a4 SOURCE WEAK OR INFERRED Sole WITH/FROM of the human ISS. Its own GO:0008506 is an IDA from PMID:25164149, a Saccharomyces cerevisiae heterologous expression study with Km ~5 mM for sucrose - a non-physiological affinity for a disaccharide mammals never present to any cytosolic membrane face. UniProtKB:P58355 · mouse Slc45a2 SOURCE WEAK OR INFERRED Same single yeast study (PMID:25164149); SLC45A2 is a melanosomal protein, so the yeast sucrose flux is unlikely to reflect its cellular activity either. UniProtKB:Q8K0H7 · mouse Slc45a3 SOURCE WEAK OR INFERRED Same single yeast study (PMID:25164149). UniProtKB:Q10R54 · Oryza sativa SUT1 SUPPORTS SOURCE BUT NOT TARGET Bona fide plant sucrose:H+ symporter; ~26% identity to human SLC45A4 and no shared physiological context. UniProtKB:Q69JW3 · Oryza sativa SUT5 SUPPORTS SOURCE BUT NOT TARGET As for SUT1: plant-specific sucrose phloem loading. AGI_LocusCode:AT1G71880 · Arabidopsis SUC1 SUPPORTS SOURCE BUT NOT TARGET The homology anchor for the original family name; the Nature 2025 study measured only ~26% identity to human SLC45A4 and found no sucrose binding to the purified human protein. Supporting Evidence: PMID:25164149 [(14)C]Sucrose-uptake measurements revealed intermediate transport affinities with Km values of approximately 5 mM. PMID:40836097 However, neither GABA nor sucrose elicited a change in the thermal stability of purified SLC45A4 PMID:41266324 Our observations did not support the sucrose-transporting activity. |
| GO:0005778 peroxisomal membrane | IDA PMID:41266324 SLC45A4 encodes a peroxisomal putrescine transporter that pr... | NEW | Summary: Reviewer-proposed annotation. Endogenous SLC45A4 in A549 cells co-localises with the peroxisomal marker ABCD3 and not with actin, E-cadherin, giantin, calnexin or Mitotracker, and is enriched with ABCD3 and catalase in peroxisomes isolated both by PEX26-HA immunoprecipitation and by density-gradient centrifugation. Reason: Proposed because GOA currently records only one side of an active localisation dispute. This is the side with endogenous protein and orthogonal biochemical fractionation, so its absence from GOA misrepresents the state of the evidence. Proposed ALONGSIDE, not instead of, GO:0005886: the two results come from incomparable systems (endogenous protein in transformed lung epithelium versus overexpressed eGFP fusions in sensory neurons and N2a cells) and a dual-pool transporter is entirely plausible. A curator who prefers to wait for an independent replication of the peroxisomal result before annotating it would be taking a defensible position; what is not defensible is leaving the record silent about it. Supporting Evidence: PMID:41266324 SLC45A4 functions as a putrescine transporter localized to the peroxisome membrane to facilitate GABA production. PMID:41266324 SLC45A4 (green signal) is clearly detected around the cell nucleus, without overlapping actin signal (Fig. 5a, top panel), suggesting organellar localization rather than plasma membrane localization. PMID:41266324 This is also confirmed by the absence of co-localization with another plasma membrane marker, E-cadherin, as shown in Supplementary Fig. 10a. PMID:41266324 Moreover, SLC45A4 signal strongly co-localized with ABCD3, which is a peroxisomal transporter of long-chain fatty acids and is frequently used as a peroxisomal marker PMID:41266324 In addition, we observed no overlap between SLC45A4 signals and Golgi marker, Giantin; endoplasmic reticulum marker, Calnexin or mitochondrial marker, Mitotracker PMID:41266324 the results showed a strong enrichment of SLC45 A4 protein in the HA-IP fraction, associated with an enrichment of peroxisome-specific proteins ABCD3 and catalase. |
| GO:0009449 GABA biosynthetic process | IMP PMID:41266324 SLC45A4 encodes a peroxisomal putrescine transporter that pr... | NEW | Summary: Reviewer-proposed annotation. SLC45A4 knockout in A549 and H1299 cells reduces GABA produced from 2H6-ornithine via the arginine-ornithine-putrescine pathway and causes intracellular putrescine to accumulate, while GABA production from exogenously supplied putrescine is unaffected. Reason: Proposed because this is the only biological process to which SLC45A4 has been tied by loss-of-function experiments outside the nervous system, and GOA has no process annotation covering it. The differential result is what makes the claim mechanistically specific rather than a generic metabolic knock-on: knockout blocks GABA made from ornithine-derived (intracellular) putrescine but not from extracellular putrescine, which is what a transporter delivering putrescine to an intracellular oxidative compartment would predict. Two limitations to note. SLC45A4 is not itself a GABA transporter, so the involvement is upstream substrate delivery, not GABA handling; and directionality of putrescine flux across the peroxisomal membrane was inferred from metabolite pools, not measured. A curator preferring acts_upstream_of_or_within to involved_in would be justified. Supporting Evidence: PMID:41266324 we observed that the SLC45A4 KO significantly decreased the production of GABA from ornithine, but did not completely eliminate it PMID:41266324 These results indicate that although it regulates cellular GABA levels, SLC45A4 does not function directly as a GABA transporter. PMID:41266324 SLC45A4 functions as a putrescine transporter localized to the peroxisome membrane to facilitate GABA production. |
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Download this section (compressed HTML)Q: Which membrane does endogenous SLC45A4 occupy, and does the answer depend on cell type? PMID:40836097 and PMID:41075780 place it at the plasma membrane using overexpressed tagged constructs in sensory neurons, N2a and HEK293T cells; PMID:41266324 places endogenous protein at the peroxisomal membrane in A549 and H1299 cells using a validated antibody plus two peroxisome isolation methods. The decisive experiment is endogenous-protein localisation in dorsal root ganglion neurons with a peroxisomal marker included, which neither group has done.
Suggested experts: Simon Newstead, David L Bennett, Xiaoyang Su, Joanne L Parker
Q: Can the two models be unified as one transporter with two pools? A surface pool importing polyamines into the neuronal cytosol and a peroxisomal pool delivering putrescine to luminal diamine oxidase are not mutually exclusive, and the cryo-EM plug domain is an obvious candidate regulator of trafficking. Is there a targeting signal, splice variant (Q5BKX6 has three isoforms) or PDZ interaction that partitions SLC45A4 between the two?
Suggested experts: Simon Newstead, Xiaoyang Su
Q: Which direction does SLC45A4 move polyamines, and what is the coupling ion? The Nature study measured spermidine uptake into cells with a pH optimum of 7.5-8.5, whereas the Cell Metab data show intracellular acetylspermidine depletion and acetylputrescine accumulation in the medium, which reads as export. Reconstitution into proteoliposomes with imposed gradients would settle both the direction and whether transport is proton-coupled as the SLC45 family name implies.
Suggested experts: Simon Newstead, Kivanc Birsoy, Joanne L Parker
Q: Should the mouse Slc45a2, Slc45a3 and Slc45a4 IDAs for GO:0008506 (sucrose:proton symporter activity), all from PMID:25164149, be re-curated? The human IBA and ISS both trace to them, and the human protein has now been shown not to bind sucrose. If the mouse annotations stand, the phylogenetic node will keep re-propagating a term that direct experiment on the human orthologue contradicts.
Q: Does the uniformly PDZ-scaffold interactome of SLC45A4 (DLG1-4, GRIP1/2, MPDZ, PATJ, PDZK1, MAGI1, LNX1/2, WHRN, NHERF4) reflect a functional C-terminal PDZ-binding motif in neurons, and does disrupting it change surface expression or the pain phenotype? This would also test whether PDZ anchoring is what distinguishes the plasma-membrane pool from any intracellular pool.
Suggested experts: David L Bennett
Experiment: Knock a small epitope tag into the endogenous SLC45A4 locus (e.g. HiBiT or 3xHA by CRISPR knock-in) in both a DRG-derived neuronal model and A549 cells, then image against plasma-membrane (Na+/K+-ATPase, E-cadherin) and peroxisomal (ABCD3, catalase, PEX14) markers in parallel in the two backgrounds, with identical antibodies and acquisition settings. Quantify by Manders/Pearson coefficients rather than by representative images. Run peroxisome immunoprecipitation (PEX26-HA) and surface biotinylation on the same lysates to give orthogonal, quantitative partitioning.
Hypothesis: Endogenous SLC45A4 occupies different membranes in sensory neurons and in transformed epithelial cells.
Type: endogenous epitope knock-in, confocal colocalisation, surface biotinylation, organelle immunoprecipitation
Experiment: Purify human SLC45A4 (the construct used for cryo-EM) and reconstitute into proteoliposomes. Measure 14C-putrescine and 14C-spermidine flux under imposed pH gradients of both signs, with and without internal unlabelled polyamine to test for counter-transport, and with valinomycin/K+ to clamp membrane potential and test electrogenicity. Test acetylspermidine and acetylputrescine as substrates directly, since those are the species that changed in the Cell Metab cellular data.
Hypothesis: SLC45A4 is a proton-coupled polyamine antiporter whose net direction depends on the imposed gradients, which would reconcile the uptake and efflux observations.
Type: proteoliposome reconstitution, radiolabelled flux, gradient and potential clamping
Experiment: Compare 2H6-ornithine to 2H6-GABA flux in SLC45A4 knockout versus wild-type cells on a PEX-deficient background (e.g. PEX19 or PEX5 knockout, which mislocalises peroxisomal matrix and membrane proteins). If the SLC45A4 requirement for ornithine-derived GABA disappears when peroxisome biogenesis is blocked, the peroxisomal pool is the functional one for this pathway. Include a plasma-membrane-restricted SLC45A4 variant (peroxisomal targeting disrupted) as a rescue arm.
Hypothesis: The peroxisomal pool of SLC45A4 supplies putrescine to luminal diamine oxidase for GABA synthesis, and is dispensable in cells lacking peroxisomes.
Type: stable isotope tracing with epistatic peroxisome-biogenesis knockouts, targeted-variant rescue
Experiment: Repeat the yeast complementation and 14C-sucrose uptake design of PMID:25164149 side by side for human SLC45A4, mouse Slc45a4 and a plant SUC positive control, in glucose-free medium, and in parallel test 14C-sucrose flux in the reconstituted proteoliposome system above. A clean negative in both formats would justify retiring the sucrose annotations across the mammalian SLC45 family rather than only on human SLC45A4.
Hypothesis: Sucrose is not a substrate of human SLC45A4 under any condition, including the glucose-free condition that PMID:41266324 could not exclude.
Type: yeast heterologous uptake replication, proteoliposome flux
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