SLC48A1 (HRG1) is a small (146 aa) multi-pass membrane protein of the HRG family that transports heme across endolysosomal membranes. Discovered as the heme-responsive permease of the heme-auxotrophic nematode C. elegans, the human protein binds and transports heme, with a conserved histidine in the second transmembrane segment required for activity. It resides in late endosome and lysosome membranes and is strongly expressed in reticuloendothelial macrophages, where it is induced by heme and recruited to the phagolysosomal membrane during erythrophagocytosis to move heme released from digested hemoglobin into the cytosol for degradation by heme oxygenase and recycling of its iron. Loss of the mouse ortholog traps heme in enlarged macrophage lysosomes as crystalline hemozoin and impairs heme-iron recycling, and the zebrafish orthologs are needed for kidney macrophage heme-iron recycling. SLC48A1 is also highly expressed in myelinating oligodendrocytes, where heme import supports myelin integrity. A role in V-ATPase assembly and endosomal acidification has been reported in cultured cells.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | REMOVE | Summary: Generic protein binding from a high-throughput yeast two-hybrid interactome screen (HI-II-14, PMID:25416956; partner MEOX2). The partner is not known to be functionally linked to heme transport. Reason: Bare GO:0005515 protein binding is uninformative under the project protein-binding policy. Removal does not assert that the binary interaction is false; it was detected in a systematic Y2H screen and no follow-up links it to an HRG1 activity. Most partners in these screens are themselves membrane proteins (a frequent feature of Y2H hits for small multi-pass proteins), and none suggests a more specific molecular function for HRG1. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Generic protein binding from a high-throughput yeast two-hybrid interactome screen (HuRI, PMID:32296183). The partner is not known to be functionally linked to heme transport. Reason: Bare GO:0005515 protein binding is uninformative under the project protein-binding policy. Removal does not assert that the binary interaction is false; it was detected in a systematic Y2H screen and no follow-up links it to an HRG1 activity. Most partners in these screens are themselves membrane proteins (a frequent feature of Y2H hits for small multi-pass proteins), and none suggests a more specific molecular function for HRG1. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Generic protein binding from a high-throughput yeast two-hybrid interactome screen (HuRI, PMID:32296183). The partner is not known to be functionally linked to heme transport. Reason: Bare GO:0005515 protein binding is uninformative under the project protein-binding policy. Removal does not assert that the binary interaction is false; it was detected in a systematic Y2H screen and no follow-up links it to an HRG1 activity. Most partners in these screens are themselves membrane proteins (a frequent feature of Y2H hits for small multi-pass proteins), and none suggests a more specific molecular function for HRG1. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Generic protein binding from a high-throughput yeast two-hybrid interactome screen (HuRI, PMID:32296183). The partner is not known to be functionally linked to heme transport. Reason: Bare GO:0005515 protein binding is uninformative under the project protein-binding policy. Removal does not assert that the binary interaction is false; it was detected in a systematic Y2H screen and no follow-up links it to an HRG1 activity. Most partners in these screens are themselves membrane proteins (a frequent feature of Y2H hits for small multi-pass proteins), and none suggests a more specific molecular function for HRG1. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Generic protein binding from a high-throughput yeast two-hybrid interactome screen (HuRI, PMID:32296183). The partner is not known to be functionally linked to heme transport. Reason: Bare GO:0005515 protein binding is uninformative under the project protein-binding policy. Removal does not assert that the binary interaction is false; it was detected in a systematic Y2H screen and no follow-up links it to an HRG1 activity. Most partners in these screens are themselves membrane proteins (a frequent feature of Y2H hits for small multi-pass proteins), and none suggests a more specific molecular function for HRG1. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Generic protein binding from a high-throughput yeast two-hybrid interactome screen (HuRI, PMID:32296183). The partner is not known to be functionally linked to heme transport. Reason: Bare GO:0005515 protein binding is uninformative under the project protein-binding policy. Removal does not assert that the binary interaction is false; it was detected in a systematic Y2H screen and no follow-up links it to an HRG1 activity. Most partners in these screens are themselves membrane proteins (a frequent feature of Y2H hits for small multi-pass proteins), and none suggests a more specific molecular function for HRG1. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Generic protein binding from a high-throughput yeast two-hybrid interactome screen (HuRI, PMID:32296183). The partner is not known to be functionally linked to heme transport. Reason: Bare GO:0005515 protein binding is uninformative under the project protein-binding policy. Removal does not assert that the binary interaction is false; it was detected in a systematic Y2H screen and no follow-up links it to an HRG1 activity. Most partners in these screens are themselves membrane proteins (a frequent feature of Y2H hits for small multi-pass proteins), and none suggests a more specific molecular function for HRG1. |
| GO:0005764 lysosome | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA machine-learning mapping placing HRG1 in the lysosome. Reason: Root organelle term (lysosome) from an electronic rule. Checked against the experimental record rather than the rule: human and worm HRG1 co-localize with LAMP1 in HEK293 cells, endogenous mouse Hrg1 is endolysosomal and co-localizes with Lamp1 on phagolysosomes during erythrophagocytosis, and Slc48a1-null macrophages accumulate heme in LAMP1-positive lysosomes. The organelle claim holds. Per the organelle policy the root term is kept; the membrane sub-compartment is already captured by the lysosomal membrane rows. One study (PMID:19875448) found overexpressed HA-HRG-1 in endosomes but not LAMP1-positive lysosomes in HeLa cells, but the weight of evidence, including endogenous protein in macrophages, supports lysosomal residence. Supporting Evidence: PMID:23395172 worm and human HRG1 proteins co-localize with LAMP1 and the small GTPases Rab7 and Rab11 in HEK293 cells PMID:23395172 during EP, Hrg1 and Lamp1 protein strongly colocalized on the phagolysosomal membrane PMID:31571584 showed that all of the hemozoin in isolated-F4/80+ macrophages from KO bone marrow is enclosed within LAMP1-positive compartments |
| GO:0005765 lysosomal membrane | EXP PMID:23395172 HRG1 is essential for heme transport from the phagolysosome ... | ACCEPT | Summary: Lysosomal membrane from White et al. 2013: Hrg1 co-localizes with Lamp1 on phagolysosomal membranes during erythrophagocytosis in macrophages. Reason: HRG1 is a four-TM heme permease of the endolysosomal system. Human HRG1 co-localizes with LAMP1, Rab7 and Rab11 when expressed in HEK293 cells, endogenous Hrg1 is endolysosomal in macrophages and is recruited to Lamp1-positive phagolysosomal membranes during erythrophagocytosis, and heme accumulates in LAMP1-positive lysosomes of Slc48a1-null macrophages. The lysosomal (phagolysosomal) membrane is where the core heme export activity acts. O'Callaghan et al. (PMID:19875448) reported overexpressed HRG-1 in endosomes but not lysosomes in HeLa cells; this is noted as a discrepancy but does not outweigh the endogenous-protein and knockout data. Supporting Evidence: PMID:23395172 during EP, Hrg1 and Lamp1 protein strongly colocalized on the phagolysosomal membrane PMID:23395172 Hrg1 was recruited to the phagolysosomal membranes specifically during phagocytosis of RBCs but not latex beads PMID:23395172 worm and human HRG1 proteins co-localize with LAMP1 and the small GTPases Rab7 and Rab11 in HEK293 cells |
| GO:0005765 lysosomal membrane | IBA GO_REF:0000033 | ACCEPT | Summary: IBA (PTN002418776) lysosomal membrane, seeded by mouse Slc48a1, human SLC48A1 and C. elegans hrg-1 experimental annotations. Reason: The PAINT node covers the HRG1 clade, whose characterized members (worm HRG-1, human and mouse HRG1) reside in endolysosomal membranes; human SLC48A1 appearing among the sources reflects its own experimental grounding. No evidence of loss in the human lineage. HRG1 is a four-TM heme permease of the endolysosomal system. Human HRG1 co-localizes with LAMP1, Rab7 and Rab11 when expressed in HEK293 cells, endogenous Hrg1 is endolysosomal in macrophages and is recruited to Lamp1-positive phagolysosomal membranes during erythrophagocytosis, and heme accumulates in LAMP1-positive lysosomes of Slc48a1-null macrophages. The lysosomal (phagolysosomal) membrane is where the core heme export activity acts. O'Callaghan et al. (PMID:19875448) reported overexpressed HRG-1 in endosomes but not lysosomes in HeLa cells; this is noted as a discrepancy but does not outweigh the endogenous-protein and knockout data. Supporting Evidence: PMID:23395172 worm and human HRG1 proteins co-localize with LAMP1 and the small GTPases Rab7 and Rab11 in HEK293 cells PMID:23395172 during EP, Hrg1 and Lamp1 protein strongly colocalized on the phagolysosomal membrane PMID:31571584 showed that all of the hemozoin in isolated-F4/80+ macrophages from KO bone marrow is enclosed within LAMP1-positive compartments |
| GO:0005765 lysosomal membrane | IDA PMID:18418376 Haem homeostasis is regulated by the conserved and concerted... | ACCEPT | Summary: IDA lysosomal membrane from Rajagopal et al. 2008, where human HRG1 was expressed in HEK293 cells and co-localized with worm HRG-1 and LAMP1. Reason: The cached record is abstract only, but the abstract states that human and worm proteins co-localize, and White et al. 2013 summarize the result as co-localization with LAMP1, Rab7 and Rab11 in HEK293 cells. Consistent with all later work. HRG1 is a four-TM heme permease of the endolysosomal system. Human HRG1 co-localizes with LAMP1, Rab7 and Rab11 when expressed in HEK293 cells, endogenous Hrg1 is endolysosomal in macrophages and is recruited to Lamp1-positive phagolysosomal membranes during erythrophagocytosis, and heme accumulates in LAMP1-positive lysosomes of Slc48a1-null macrophages. The lysosomal (phagolysosomal) membrane is where the core heme export activity acts. O'Callaghan et al. (PMID:19875448) reported overexpressed HRG-1 in endosomes but not lysosomes in HeLa cells; this is noted as a discrepancy but does not outweigh the endogenous-protein and knockout data. Supporting Evidence: PMID:18418376 Human and worm proteins localize together, and bind and transport haem, thus establishing an evolutionarily conserved function for HRG-1 PMID:23395172 worm and human HRG1 proteins co-localize with LAMP1 and the small GTPases Rab7 and Rab11 in HEK293 cells |
| GO:0005765 lysosomal membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular location mapping (SL-0157 lysosome membrane), based on PMID:18418376 and PMID:23395172. Reason: HRG1 is a four-TM heme permease of the endolysosomal system. Human HRG1 co-localizes with LAMP1, Rab7 and Rab11 when expressed in HEK293 cells, endogenous Hrg1 is endolysosomal in macrophages and is recruited to Lamp1-positive phagolysosomal membranes during erythrophagocytosis, and heme accumulates in LAMP1-positive lysosomes of Slc48a1-null macrophages. The lysosomal (phagolysosomal) membrane is where the core heme export activity acts. O'Callaghan et al. (PMID:19875448) reported overexpressed HRG-1 in endosomes but not lysosomes in HeLa cells; this is noted as a discrepancy but does not outweigh the endogenous-protein and knockout data. Supporting Evidence: file:human/SLC48A1/SLC48A1-uniprot.txt Lysosome membrane PMID:23395172 during EP, Hrg1 and Lamp1 protein strongly colocalized on the phagolysosomal membrane |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: IBA plasma membrane (PTN002418776), seeded by human SLC48A1 and C. elegans hrg-4. Reason: The PAINT node includes the worm plasma-membrane paralog HRG-4 as a donor; the human gene also carries its own IDA. A minor plasma-membrane pool is supported: O'Callaghan et al. detected HRG-1 at the surface of live HeLa cells by flow cytometry, increased by serum starvation, and Fogarty et al. found surface HRG-1 in invasive cancer cell lines. In the C. elegans family the plasma-membrane heme importer is the paralog HRG-4, whereas HRG-1 itself is endolysosomal, and the physiological function of mammalian HRG1 (heme export from the phagolysosome) is at the lysosomal membrane. The location is plausible (transit through the plasma membrane is expected for a di-leucine-sorted endolysosomal protein, and heme uptake assays in cultured cells are consistent with some surface activity) but peripheral to the core function. Supporting Evidence: PMID:19875448 Using the polyclonal anti-HRG-1 antibody, HRG-1 was detected by flow cytometry at the plasma membrane of live HeLa cells PMID:24141772 HRG-1 and the V-ATPase are co-expressed at the plasma membrane |
| GO:0005886 plasma membrane | IDA PMID:18418376 Haem homeostasis is regulated by the conserved and concerted... | KEEP AS NON CORE | Summary: IDA plasma membrane from Rajagopal et al. 2008 (abstract-only in cache). Reason: The cached abstract does not describe the plasma-membrane observation; we defer to the curator, who had the full text. Independent studies support a surface pool. A minor plasma-membrane pool is supported: O'Callaghan et al. detected HRG-1 at the surface of live HeLa cells by flow cytometry, increased by serum starvation, and Fogarty et al. found surface HRG-1 in invasive cancer cell lines. In the C. elegans family the plasma-membrane heme importer is the paralog HRG-4, whereas HRG-1 itself is endolysosomal, and the physiological function of mammalian HRG1 (heme export from the phagolysosome) is at the lysosomal membrane. The location is plausible (transit through the plasma membrane is expected for a di-leucine-sorted endolysosomal protein, and heme uptake assays in cultured cells are consistent with some surface activity) but peripheral to the core function. Supporting Evidence: PMID:19875448 Using the polyclonal anti-HRG-1 antibody, HRG-1 was detected by flow cytometry at the plasma membrane of live HeLa cells PMID:24141772 HRG-1 and the V-ATPase are co-expressed at the plasma membrane |
| GO:0010008 endosome membrane | EXP PMID:23395172 HRG1 is essential for heme transport from the phagolysosome ... | ACCEPT | Summary: Endosome membrane from White et al. 2013: endogenous Hrg1 localized primarily to the endolysosome in macrophages. Reason: HRG1 resides in endosomal and lysosomal membranes; UniProt describes it as regulating heme availability "through the endosomal or lysosomal compartment". Human HRG1 co-localizes with Rab7 and Rab11 endosomal markers, and O'Callaghan et al. found HRG-1 throughout endosomal compartments. Endosome membrane is a genuine and functionally relevant location (late endosomes mature into the lysosomes and phagolysosomes where heme is exported). Supporting Evidence: PMID:23395172 Consistent with the previously observed location of ectopic human HRG1::GFP in HEK293 cells, endogenous Hrg1 was localized primarily to the endolysosome in BMDM PMID:23395172 worm and human HRG1 proteins co-localize with LAMP1 and the small GTPases Rab7 and Rab11 in HEK293 cells |
| GO:0010008 endosome membrane | IDA PMID:18418376 Haem homeostasis is regulated by the conserved and concerted... | ACCEPT | Summary: IDA endosome membrane from Rajagopal et al. 2008 (abstract only cached). Reason: Curator had the full text; supported by the Rab7/Rab11 co-localization summarized in PMID:23395172 and by PMID:19875448. HRG1 resides in endosomal and lysosomal membranes; UniProt describes it as regulating heme availability "through the endosomal or lysosomal compartment". Human HRG1 co-localizes with Rab7 and Rab11 endosomal markers, and O'Callaghan et al. found HRG-1 throughout endosomal compartments. Endosome membrane is a genuine and functionally relevant location (late endosomes mature into the lysosomes and phagolysosomes where heme is exported). Supporting Evidence: PMID:23395172 worm and human HRG1 proteins co-localize with LAMP1 and the small GTPases Rab7 and Rab11 in HEK293 cells PMID:19875448 We found that HRG-1 is present in endosomes but not in lysosomes |
| GO:0010008 endosome membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular location mapping (SL-0100 endosome membrane). Reason: HRG1 resides in endosomal and lysosomal membranes; UniProt describes it as regulating heme availability "through the endosomal or lysosomal compartment". Human HRG1 co-localizes with Rab7 and Rab11 endosomal markers, and O'Callaghan et al. found HRG-1 throughout endosomal compartments. Endosome membrane is a genuine and functionally relevant location (late endosomes mature into the lysosomes and phagolysosomes where heme is exported). Supporting Evidence: file:human/SLC48A1/SLC48A1-uniprot.txt Endosome membrane PMID:23395172 worm and human HRG1 proteins co-localize with LAMP1 and the small GTPases Rab7 and Rab11 in HEK293 cells |
| GO:0015232 heme transmembrane transporter activity | IBA GO_REF:0000033 | ACCEPT | Summary: IBA heme transmembrane transporter activity for the HRG1 clade (PTN002418776), seeded by mouse, human, worm and zebrafish members. Reason: Function placed at the HRG clade node with experimental support from nematode, fish and mammalian members; the human gene is among the donors because it has its own experimental evidence. Heme transmembrane transporter activity is the core molecular function of HRG1. Human and worm HRG-1 bind and transport heme; human HRG1 rescues heme-dependent growth of heme-synthesis-deficient (hem1-deletion) yeast at sub-micromolar heme, a TMD2 histidine is required, and the population variant P36L abolishes transport. In vivo, loss of the mouse ortholog traps heme in macrophage lysosomes as hemozoin, and zebrafish Hrg1a/Hrg1b are functional heme transporters. UniProt records the reaction heme b(in) = heme b(out) (RHEA:75443). Supporting Evidence: PMID:18418376 Human and worm proteins localize together, and bind and transport haem, thus establishing an evolutionarily conserved function for HRG-1 PMID:22174408 mutation of the synonymous histidine in TMD2 of hHRG-1 eliminates heme transport activity PMID:30248094 we show that zebrafish Hrg1a and its paralog Hrg1b are functional heme transporters |
| GO:0015232 heme transmembrane transporter activity | IDA PMID:18418376 Haem homeostasis is regulated by the conserved and concerted... | ACCEPT | Summary: IDA heme transmembrane transporter activity from Rajagopal et al. 2008. Reason: Heme transmembrane transporter activity is the core molecular function of HRG1. Human and worm HRG-1 bind and transport heme; human HRG1 rescues heme-dependent growth of heme-synthesis-deficient (hem1-deletion) yeast at sub-micromolar heme, a TMD2 histidine is required, and the population variant P36L abolishes transport. In vivo, loss of the mouse ortholog traps heme in macrophage lysosomes as hemozoin, and zebrafish Hrg1a/Hrg1b are functional heme transporters. UniProt records the reaction heme b(in) = heme b(out) (RHEA:75443). Supporting Evidence: PMID:18418376 Human and worm proteins localize together, and bind and transport haem, thus establishing an evolutionarily conserved function for HRG-1 PMID:23395172 The P36L variant was unable to rescue the growth of hem1Ξ, even in the presence of 10 Β΅M heme, whereas wildtype HRG1 restored growth of yeast with as little as 0.25 Β΅M heme PMID:22174408 mutation of the synonymous histidine in TMD2 of hHRG-1 eliminates heme transport activity |
| GO:0015232 heme transmembrane transporter activity | IEA GO_REF:0000120 | ACCEPT | Summary: Combined IEA (InterPro IPR026218 HRG family, ARBA). Reason: The HRG family signature (IPR026218) is diagnostic of the HRG1-related heme permeases. Heme transmembrane transporter activity is the core molecular function of HRG1. Human and worm HRG-1 bind and transport heme; human HRG1 rescues heme-dependent growth of heme-synthesis-deficient (hem1-deletion) yeast at sub-micromolar heme, a TMD2 histidine is required, and the population variant P36L abolishes transport. In vivo, loss of the mouse ortholog traps heme in macrophage lysosomes as hemozoin, and zebrafish Hrg1a/Hrg1b are functional heme transporters. UniProt records the reaction heme b(in) = heme b(out) (RHEA:75443). Supporting Evidence: file:human/SLC48A1/SLC48A1-uniprot.txt DR InterPro; IPR026218; HRG. file:human/SLC48A1/SLC48A1-uniprot.txt Heme transporter that regulates intracellular heme availability through the endosomal or lysosomal compartment PMID:23395172 The P36L variant was unable to rescue the growth of hem1Ξ, even in the presence of 10 Β΅M heme, whereas wildtype HRG1 restored growth of yeast with as little as 0.25 Β΅M heme |
| GO:0015886 heme transport | IBA GO_REF:0000033 | MODIFY | Summary: IBA heme transport for the HRG1 clade (PTN002418776). Reason: HRG1 moves heme across a membrane, so the process is correct, but GO:0015886 heme transport is broader than the evidence supports. The specific process is heme transmembrane transport (GO:0035351), and in macrophages the physiological direction is from the (phago)lysosomal lumen to the cytosol, i.e. heme export from vacuole to cytoplasm (GO:0140357, a descendant of GO:0035351 via vacuolar transmembrane transport). Both are proposed: GO:0035351 covers the general transport demonstrated in yeast and cultured cells; GO:0140357 captures the in vivo role shown by knockdown in macrophages and by heme/hemozoin accumulation in lysosomes of Slc48a1-null mice. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: PANTHER:PTN002418776 SUPPORTS TRANSFER The HRG1 clade node correctly carries heme transport; the function is conserved across nematode, fish and mammalian members. Only the term is broader than the transmembrane, lysosome-to-cytosol transport the members perform. Proposed replacements: heme transmembrane transport heme export from vacuole to cytoplasm Supporting Evidence: PMID:18418376 Human and worm proteins localize together, and bind and transport haem, thus establishing an evolutionarily conserved function for HRG-1 PMID:23395172 Hrg1 knockdown resulted in a significant attenuation of Hmox1 and Fpn1 mRNA induction during EP PMID:31571584 These animals are heme tolerant because they sequester heme within enlarged lysosomes in the RES macrophages and form crystalline hemozoin |
| GO:0015886 heme transport | IDA PMID:18418376 Haem homeostasis is regulated by the conserved and concerted... | MODIFY | Summary: IDA heme transport from Rajagopal et al. 2008 (human HRG1 binds and transports heme). Reason: The assays (heme analog uptake and transport by heterologously expressed human HRG1) demonstrate transmembrane heme transport; they do not establish the compartment. Refine to GO:0035351 heme transmembrane transport. HRG1 moves heme across a membrane, so the process is correct, but GO:0015886 heme transport is broader than the evidence supports. The specific process is heme transmembrane transport (GO:0035351), and in macrophages the physiological direction is from the (phago)lysosomal lumen to the cytosol, i.e. heme export from vacuole to cytoplasm (GO:0140357, a descendant of GO:0035351 via vacuolar transmembrane transport). Both are proposed: GO:0035351 covers the general transport demonstrated in yeast and cultured cells; GO:0140357 captures the in vivo role shown by knockdown in macrophages and by heme/hemozoin accumulation in lysosomes of Slc48a1-null mice. Proposed replacements: heme transmembrane transport Supporting Evidence: PMID:18418376 Human and worm proteins localize together, and bind and transport haem, thus establishing an evolutionarily conserved function for HRG-1 PMID:22174408 mutation of the synonymous histidine in TMD2 of hHRG-1 eliminates heme transport activity |
| GO:0015886 heme transport | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO mapping from IPR026218 (HRG family). Reason: HRG1 moves heme across a membrane, so the process is correct, but GO:0015886 heme transport is broader than the evidence supports. The specific process is heme transmembrane transport (GO:0035351), and in macrophages the physiological direction is from the (phago)lysosomal lumen to the cytosol, i.e. heme export from vacuole to cytoplasm (GO:0140357, a descendant of GO:0035351 via vacuolar transmembrane transport). Both are proposed: GO:0035351 covers the general transport demonstrated in yeast and cultured cells; GO:0140357 captures the in vivo role shown by knockdown in macrophages and by heme/hemozoin accumulation in lysosomes of Slc48a1-null mice. Proposed replacements: heme transmembrane transport heme export from vacuole to cytoplasm Supporting Evidence: file:human/SLC48A1/SLC48A1-uniprot.txt DR InterPro; IPR026218; HRG. file:human/SLC48A1/SLC48A1-uniprot.txt transports heme from the phagolysosome to the cytoplasm during erythrophagocytosis (EP) PMID:34713217 SLC48A1-deficient mice are unable to transport heme from RES phagolysosomes into the cytoplasm. |
| GO:0015886 heme transport | IMP PMID:23395172 HRG1 is essential for heme transport from the phagolysosome ... | MODIFY | Summary: IMP heme transport from White et al. 2013: Hrg1 knockdown in macrophages attenuates heme-dependent Hmox1/Fpn1 induction during erythrophagocytosis, and the human P36L variant is transport-defective. Reason: The experiments show HRG1 moving heme out of the phagolysosome into the cytosol during erythrophagocytosis; the specific term is GO:0140357 heme export from vacuole to cytoplasm (phagolysosome is a lytic vacuole). Supported in vivo by heme accumulation in lysosomes of Slc48a1-null mice. HRG1 moves heme across a membrane, so the process is correct, but GO:0015886 heme transport is broader than the evidence supports. The specific process is heme transmembrane transport (GO:0035351), and in macrophages the physiological direction is from the (phago)lysosomal lumen to the cytosol, i.e. heme export from vacuole to cytoplasm (GO:0140357, a descendant of GO:0035351 via vacuolar transmembrane transport). Both are proposed: GO:0035351 covers the general transport demonstrated in yeast and cultured cells; GO:0140357 captures the in vivo role shown by knockdown in macrophages and by heme/hemozoin accumulation in lysosomes of Slc48a1-null mice. Proposed replacements: heme export from vacuole to cytoplasm Supporting Evidence: PMID:23395172 Hrg1 knockdown resulted in a significant attenuation of Hmox1 and Fpn1 mRNA induction during EP PMID:23395172 Hrg1 must transport heme from the lumen into the cytosol based on its membrane orientation and topology PMID:23395172 The P36L variant was unable to rescue the growth of hem1Ξ, even in the presence of 10 Β΅M heme, whereas wildtype HRG1 restored growth of yeast with as little as 0.25 Β΅M heme PMID:31571584 These animals are heme tolerant because they sequester heme within enlarged lysosomes in the RES macrophages and form crystalline hemozoin PMID:34713217 SLC48A1-deficient mice are unable to transport heme from RES phagolysosomes into the cytoplasm. |
| GO:0020037 heme binding | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: IBA heme binding for the HRG1 clade. Reason: HRG1 binds its substrate heme (hemin-agarose binding reported by Rajagopal et al. 2008 and reused by O'Callaghan et al. 2010), and conserved histidine/tyrosine residues needed for transport are thought to coordinate heme. This is substrate binding intrinsic to the transport mechanism rather than a separate function (HRG1 is not a hemoprotein carrying heme as a prosthetic group), so it is kept as non-core alongside the accepted transporter activity. Supporting Evidence: PMID:18418376 Human and worm proteins localize together, and bind and transport haem, thus establishing an evolutionarily conserved function for HRG-1 PMID:22174408 mutation of the synonymous histidine in TMD2 of hHRG-1 eliminates heme transport activity |
| GO:0020037 heme binding | IDA PMID:18418376 Haem homeostasis is regulated by the conserved and concerted... | KEEP AS NON CORE | Summary: IDA heme binding from Rajagopal et al. 2008. Reason: HRG1 binds its substrate heme (hemin-agarose binding reported by Rajagopal et al. 2008 and reused by O'Callaghan et al. 2010), and conserved histidine/tyrosine residues needed for transport are thought to coordinate heme. This is substrate binding intrinsic to the transport mechanism rather than a separate function (HRG1 is not a hemoprotein carrying heme as a prosthetic group), so it is kept as non-core alongside the accepted transporter activity. Supporting Evidence: PMID:18418376 Human and worm proteins localize together, and bind and transport haem, thus establishing an evolutionarily conserved function for HRG-1 |
| GO:0030670 phagocytic vesicle membrane | IEA GO_REF:0000044 | MODIFY | Summary: UniProt subcellular location mapping (SL-0205 phagosome membrane), transferred from mouse Q9D8M3. Reason: Correct but general. HRG1 is recruited specifically to the phagolysosomal membrane (co-localizing with Lamp1) during erythrophagocytosis, not to early phagosomes; the specific term GO:0061474 phagolysosome membrane (is_a phagocytic vesicle membrane and lysosomal membrane) is where the core heme export activity acts. Proposed replacements: phagolysosome membrane Supporting Evidence: file:human/SLC48A1/SLC48A1-uniprot.txt In macrophages, specifically localizes to the phagolysosomal membranes during erythrophagocytosis PMID:23395172 Hrg1 was recruited to the phagolysosomal membranes specifically during phagocytosis of RBCs but not latex beads PMID:23395172 during EP, Hrg1 and Lamp1 protein strongly colocalized on the phagolysosomal membrane |
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Download this section (compressed HTML)Q: What is the transport mechanism of HRG1 (uniporter, coupled to protons, or channel-like), and is transport driven by the lysosomal pH gradient? No structure or reconstituted-liposome transport assay has been reported.
Q: Does HRG1 genuinely associate with and regulate the V-ATPase (c subunit), as reported in HeLa and cancer cells, and is that association needed for heme export? The localization and topology reported in those studies conflict with macrophage data.
Q: Do loss-of-function SLC48A1 variants (e.g. P36L) cause iron disorders or hemozoin accumulation in humans?
Q: In oligodendrocytes, does HRG1 import heme across the plasma membrane or export it from endolysosomes after endocytic uptake?
Experiment: Reconstitute purified human HRG1 (wild type, P36L and TMD2 histidine mutants) into proteoliposomes and measure heme or fluorescent heme-analog translocation under imposed pH gradients and membrane potentials.
Hypothesis: HRG1 is sufficient for heme translocation and its activity depends on the pH gradient across the lysosomal membrane.
Type: in vitro transport assay
Experiment: Generate SLC48A1-knockout human iPSC-derived macrophages, feed senescent red cells and quantify lysosomal heme/hemozoin, HMOX1 induction and ferroportin-dependent iron export, with rescue by wild-type versus P36L HRG1.
Hypothesis: Human SLC48A1 loss-of-function causes macrophage heme sequestration.
Type: genetic knockout with rescue
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