Shu1 is a GPI-anchored cell surface heme receptor that enables high-affinity heme acquisition as an iron source during iron starvation in S. pombe. It binds hemin with micromolar affinity (KD ~2.2 ΞΌM) through a cysteine-rich region containing a partial CFEM-like motif (Cys72-Cys101). Under iron-limited conditions, Shu1 localizes to the plasma membrane where it captures extracellular heme, then undergoes ligand-induced endocytosis to deliver heme to the vacuole. This heme import pathway works in concert with the vacuolar ABC transporter Abc3, which exports heme or iron from the vacuole to the cytosol, completing the two-step heme assimilation process. Shu1 expression is tightly regulated by iron availability through the Fep1 repressor, ensuring it is produced only when cells need to scavenge iron from heme sources.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016192 vesicle-mediated transport | IEA GO_REF:0000108 | KEEP AS NON CORE | Summary: This IEA annotation appears to be based on inference from GO:0140488 (heme receptor activity). While Shu1 does undergo heme-induced internalization from plasma membrane to vacuole, this is more accurately described as receptor-mediated endocytosis rather than general vesicle-mediated transport. The annotation captures a real aspect of Shu1 function but is somewhat generic. |
| GO:0005773 vacuole | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: This generic IEA annotation is superseded by the more specific GO:0000324 (fungal-type vacuole) IDA annotation (PMID:28193844), which is ACCEPT'd in this review. Shu1 relocates to the vacuolar membrane upon heme binding, but the fungal-type vacuole term with experimental evidence better captures this localization. Keeping the generic vacuole term as non-core. Supporting Evidence: file:SCHPO/Shu1/Shu1-deep-research-falcon.md rapidly internalized to the vacuolar membrane** after binding hemin/ZnMP |
| GO:0005774 vacuolar membrane | IEA GO_REF:0000044 | ACCEPT | Summary: This IEA annotation is well-supported by experimental evidence. PMID:28193844 demonstrates that Shu1-HA4 relocates to the vacuolar membrane under high hemin concentrations, representing the internalized form of the protein after heme binding. |
| GO:0005886 plasma membrane | IEA GO_REF:0000120 | ACCEPT | Summary: This IEA annotation is strongly supported by experimental evidence. Multiple studies (PMID:25733668, PMID:28193844) demonstrate Shu1 localizes to the plasma membrane, particularly under low hemin concentrations. This represents the primary functional location for heme reception. |
| GO:0016020 membrane | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: This IEA annotation is correct but overly general. Shu1 is indeed a membrane protein (both plasma membrane and vacuolar membrane), but the more specific localization terms provide better functional information. |
| GO:0098552 side of membrane | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: This IEA annotation is too generic and not particularly informative. While Shu1 is GPI-anchored to the external side of the plasma membrane, the more specific GO:0009897 (external side of plasma membrane) term with experimental support provides better functional information. |
| GO:0140488 heme receptor activity | EXP PMID:25733668 Shu1 is a cell-surface protein involved in iron acquisition ... | ACCEPT | Summary: This experimental annotation perfectly captures Shu1 core molecular function. PMID:25733668 demonstrates Shu1 functions as a heme receptor with direct binding activity (KD ~2.2 ΞΌM) and is required for heme uptake. This is the primary molecular function of Shu1. Supporting Evidence: PMID:25733668 When a hem1 Ξ shu1 Ξ mutant strain was incubated in the absence of ALA and in the presence of hemin, cells were unable to grow unless an untagged shu1 + or HA 4 -tagged shu1 + allele was re-integrated and expressed in this mutant strain file:SCHPO/Shu1/Shu1-deep-research-falcon.md Direct biochemical evidence shows Shu1 binds hemin with **micromolar affinity (~2.2 Β΅M)** |
| GO:0140488 heme receptor activity | IDA PMID:28193844 Heme Assimilation in Schizosaccharomyces pombe Requires Cell... | ACCEPT | Summary: This IDA annotation provides additional experimental support for Shu1 heme receptor activity. PMID:28193844 demonstrates heme binding and receptor function through direct biochemical assays, complementing the evidence from PMID:25733668. Supporting Evidence: PMID:28193844 the heme analog zinc mesoporphyrin IX (ZnMP) first accumulates into vacuoles and then subsequently, within the cytoplasm in a rapid and Shu1-dependent manner file:SCHPO/Shu1/Shu1-deep-research-falcon.md a **cell-surface GPI-anchored heme receptor** that binds hemin or ZnMP |
| GO:0000324 fungal-type vacuole | IDA PMID:28193844 Heme Assimilation in Schizosaccharomyces pombe Requires Cell... | ACCEPT | Summary: This IDA annotation is well-supported and more specific than the generic vacuole term. PMID:28193844 shows Shu1 activity in fungal-type vacuoles after heme-induced internalization. This represents the secondary localization of Shu1 during the heme transport process. Supporting Evidence: PMID:28193844 When cells were treated with low concentrations of hemin, Shu1 localized at the cell surface, whereas under conditions of high concentrations of hemin, Shu1 was detected on vacuolar membrane file:SCHPO/Shu1/Shu1-deep-research-falcon.md rapidly internalized to the vacuolar membrane** after binding hemin/ZnMP |
| GO:0005886 plasma membrane | IDA PMID:28193844 Heme Assimilation in Schizosaccharomyces pombe Requires Cell... | ACCEPT | Summary: This IDA annotation provides strong experimental support for plasma membrane localization. PMID:28193844 shows Shu1-HA4 localizes to the cell surface (plasma membrane) under low hemin concentrations, which is the primary functional location for heme reception. Supporting Evidence: PMID:28193844 When cells were treated with low concentrations of hemin, Shu1 localized at the cell surface |
| GO:0015886 heme transport | IMP PMID:28193844 Heme Assimilation in Schizosaccharomyces pombe Requires Cell... | ACCEPT | Summary: This IMP annotation accurately captures Shu1 core biological process function. PMID:28193844 demonstrates through mutant phenotype analysis that Shu1 is required for heme transport, showing cells lacking Shu1 cannot efficiently transport heme. This is a core function. Supporting Evidence: PMID:28193844 the heme analog zinc mesoporphyrin IX (ZnMP) first accumulates into vacuoles and then subsequently, within the cytoplasm in a rapid and Shu1-dependent manner file:SCHPO/Shu1/Shu1-deep-research-falcon.md two distinct exogenous heme uptake pathways |
| GO:0020037 heme binding | IDA PMID:28193844 Heme Assimilation in Schizosaccharomyces pombe Requires Cell... | ACCEPT | Summary: This IDA annotation is supported by reference to previous work. PMID:28193844 cites PMID:25733668 which showed Shu1 binds hemin through hemin-agarose pulldown assays. This represents the fundamental molecular interaction. Supporting Evidence: PMID:28193844 Absorbance spectroscopy and hemin-agarose pulldown experiments have demonstrated that Shu1 binds to hemin file:SCHPO/Shu1/Shu1-deep-research-falcon.md Mutation of **C72/C87/C92/C101 β Ala** strongly reduces hemin-agarose retention and eliminates high-affinity binding behavior, indicating these residues (or a subset) are crucial for heme coordination. |
| GO:0020037 heme binding | IDA PMID:29549126 The major facilitator transporter Str3 is required for low-a... | ACCEPT | Summary: This IDA annotation provides additional experimental confirmation of heme binding activity. PMID:29549126 confirms Shu1 heme binding as part of studies comparing high-affinity (Shu1) vs low-affinity (Str3) heme transport systems. Supporting Evidence: PMID:29549126 In the fission yeast Schizosaccharomyces pombe, acquisition of exogenous heme is largely mediated by the cell membraneβassociated Shu1 |
| GO:0140420 heme import into cell | IMP PMID:29549126 The major facilitator transporter Str3 is required for low-a... | ACCEPT | Summary: This IMP annotation accurately describes Shu1 core biological process. PMID:29549126 demonstrates through mutant phenotype analysis that Shu1 is essential for cellular heme import, working as a high-affinity system. This precisely captures the physiological function. Supporting Evidence: PMID:29549126 Using a strain that cannot synthesize heme de novo ( hem1 Ξ) and lacks Shu1, we found that the heme-dependent growth deficit of this strain is rescued by hemin supplementation in the presence of Str3 file:SCHPO/Shu1/Shu1-deep-research-falcon.md cargo is delivered to the **vacuole**, followed by redistribution to the cytoplasm via the vacuolar **ABC transporter Abc3** |
| GO:0010106 cellular response to iron ion starvation | IMP PMID:25733668 Shu1 is a cell-surface protein involved in iron acquisition ... | KEEP AS NON CORE | Summary: This IMP annotation is well-supported by experimental evidence. PMID:25733668 shows Shu1 expression is induced under iron starvation conditions and repressed by iron repletion via Fep1. However, this represents the regulatory context rather than core function - Shu1 responds to iron starvation by enabling heme acquisition. Supporting Evidence: PMID:25733668 When iron levels are low, the transcription of shu1(+) is induced, although its expression is repressed when iron levels rise. The iron-dependent down-regulation of shu1(+) requires the GATA-type transcriptional repressor Fep1 file:SCHPO/Shu1/Shu1-deep-research-falcon.md shu1+ transcription is induced by low iron and repressed by iron repletion in a mechanism involving the GATA-type repressor **Fep1** |
| GO:0005886 plasma membrane | IDA PMID:25733668 Shu1 is a cell-surface protein involved in iron acquisition ... | ACCEPT | Summary: This IDA annotation is strongly supported by the original experimental evidence. PMID:25733668 shows HA4-tagged Shu1 localizes to the plasma membrane in functional shu1+-HA4 alleles, establishing the primary cellular location for heme receptor activity. Supporting Evidence: PMID:25733668 HA 4 -tagged Shu1 is localized at the plasma membrane when iron levels are low |
| GO:0098711 iron ion import across plasma membrane | IMP NOT PMID:25733668 Shu1 is a cell-surface protein involved in iron acquisition ... | ACCEPT | Summary: This annotation has a NOT qualifier in the original GOA data, indicating Shu1 is NOT involved in direct iron ion import. This is correct - Shu1 imports heme (which contains iron) rather than free iron ions. The NOT annotation properly distinguishes heme transport from direct iron transport. Supporting Evidence: PMID:25733668 their ability to acquire exogenous hemin or the fluorescent heme analog zinc mesoporphyrin IX is dependent on the expression of Shu1 file:SCHPO/Shu1/Shu1-deep-research-falcon.md Shu1-mediated heme use is **independent of Fio1/Fip1** oxidase-permease-mediated elemental iron uptake |
| GO:0140420 heme import into cell | IMP PMID:25733668 Shu1 is a cell-surface protein involved in iron acquisition ... | ACCEPT | Summary: This IMP annotation captures Shu1 core biological function perfectly. PMID:25733668 demonstrates through mutant analysis that Shu1 is required for acquisition of exogenous hemin and enables S. pombe to take up extracellular heme for cell growth. This is the primary physiological role. Supporting Evidence: PMID:25733668 that encodes a protein that enables S. pombe to take up extracellular heme for cell growth ... their ability to acquire exogenous hemin or the fluorescent heme analog zinc mesoporphyrin IX is dependent on the expression of Shu1 file:SCHPO/Shu1/Shu1-deep-research-falcon.md an **iron-regulated, cell-surface heme-binding protein** required for **assimilation of exogenous hemin/heme and heme analogs** |
| GO:0009897 external side of plasma membrane | TAS PMID:12845604 Genome-wide identification of fungal GPI proteins. | ACCEPT | Summary: This TAS annotation is based on computational prediction of GPI-anchoring from PMID:12845604 genome-wide analysis. This is well-supported by subsequent experimental evidence (PMID:28193844) showing PI-PLC cleavability confirming GPI-anchoring. The external side localization is functionally important for heme reception. Supporting Evidence: PMID:12845604 Glycosylphosphatidylinositol-modified (GPI) proteins PMID:28193844 Shaving experiments showed that Shu1 is released from membrane preparations when spheroplast lysates are incubated with phosphoinositide-specific phospholipase C (PI-PLC) |
| GO:0006879 intracellular iron ion homeostasis | IEA | NEW | Summary: intracellular iron ion homeostasis identified from core_functions analysis Reason: This biological process term captures Shu1's role in maintaining iron homeostasis by functioning as a high-affinity heme receptor for iron acquisition. Supporting Evidence: file:SCHPO/Shu1/Shu1-deep-research-falcon.md an **iron-regulated, cell-surface heme-binding protein** required for **assimilation of exogenous hemin/heme and heme analogs** |
| GO:0006898 receptor-mediated endocytosis | IEA | NEW | Summary: receptor-mediated endocytosis identified from core_functions analysis Reason: This biological process term reflects Shu1's mechanism of heme uptake through ligand-induced endocytic trafficking to deliver iron to intracellular compartments. Supporting Evidence: file:SCHPO/Shu1/Shu1-deep-research-falcon.md rapidly internalized to the vacuolar membrane** after binding hemin/ZnMP |
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Download this section (compressed HTML)Q: What is the molecular machinery driving heme-induced endocytic internalization of Shu1 from the plasma membrane to the vacuole, and how is this triggered by hemin binding (e.g. the proposed roles for ubiquitination, ESCRT factors)?
Q: How is heme handed off from vacuolar Shu1 to the vacuolar ABC transporter Abc3 for export to the cytosol, and are these factors part of a physical complex?
Q: How do the high-affinity (Shu1) and low-affinity (Str3) heme uptake routes functionally partition across different extracellular hemin concentrations and physiological conditions?
Q: Does the partial CFEM-like cysteine arrangement (C72/C87/C92/C101) coordinate the heme iron directly, and what is the structural basis of hemin recognition?
Experiment: Live-cell imaging of fluorescently tagged Shu1 combined with ZnMP uptake assays to track plasma-membrane-to-vacuole trafficking kinetics as a function of extracellular hemin concentration
Experiment: Structure determination (e.g. AlphaFold-guided mutagenesis or crystallography) of the cysteine-rich heme-binding region to define how the C72/C87/C92/C101 quartet coordinates hemin
Experiment: Genetic epistasis and co-fractionation experiments between Shu1, Abc3, and endocytic/ESCRT machinery to map the heme delivery pathway from cell surface to cytosol
Experiment: Comparative uptake assays in shu1Delta, str3Delta, and double mutants across a range of hemin concentrations to quantify the relative contributions of the high-affinity and low-affinity heme acquisition routes
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Download this section (compressed HTML)π View Pathway Visualization Interactive pathway diagram with detailed annotations