| Category | Key findings | Evidence type | Primary source(s) with publication date and URL |
|---|---|---|---|
| identity/structure | **shu1 = SPAC1F8.02c = UniProt Q92340** in *Schizosaccharomyces pombe*; encodes a **226 aa precursor/cell-surface heme-binding protein** with **N-terminal signal peptide**, **predicted GPI-anchor attachment near Ser199**, short C-terminal hydrophobic segment/pro-peptide, and a **partial CFEM-like cysteine-rich region** including **C72/C87/C92/C101** important for heme interaction (pqac-00000004, pqac-00000012, pqac-00000014) | Sequence analysis, genetics, biochemistry | Mourer T, Jacques J-F, Brault A, Bisaillon M, Labbé S. **2015-04**. *Shu1 Is a Cell-surface Protein Involved in Iron Acquisition from Heme in Schizosaccharomyces pombe*. J Biol Chem. https://doi.org/10.1074/jbc.m115.642058 ; Mourer T. **2019**. *Étude du mécanisme d'acquisition de l'hème par le récepteur Shu1 chez Schizosaccharomyces pombe* (thesis/monograph record) (pqac-00000004, pqac-00000012, pqac-00000014) |
| substrate specificity | Shu1 is required for utilization of **exogenous hemin/heme** and uptake of the fluorescent heme analog **ZnMP** when endogenous heme synthesis is blocked (*hem1Δ* background). Available evidence supports **heme/porphyrin uptake specificity**, not general ferric iron uptake; Shu1 does **not** substitute for the high-affinity elemental iron uptake system and cannot mediate growth on exogenous **FeCl3** alone (pqac-00000001, pqac-00000007, pqac-00000012) | Genetics, uptake assay | Mourer et al., **2015-04**, https://doi.org/10.1074/jbc.m115.642058 ; Mourer, **2019** (pqac-00000001, pqac-00000007, pqac-00000012) |
| binding affinity | Purified Shu1(21–200) binds hemin directly in vitro. **Spectral titration** of **10 µM hemin** with **0–7 µM Shu1** shifted the Soret peak from **~388–391 nm to ~407 nm** and gave an apparent **K_D ≈ 2.2 × 10^-6 M (2.2 µM)**. The **C72A/C87A/C92A/C101A** mutant greatly reduced hemin-agarose retention and failed to yield a measurable high-affinity interaction (pqac-00000004, pqac-00000005, pqac-00000006, pqac-00000023) | Biochemistry | Mourer et al., **2015-04**, https://doi.org/10.1074/jbc.m115.642058 ; Mourer, **2019** (pqac-00000004, pqac-00000005, pqac-00000006, pqac-00000023) |
| localization | HA-tagged Shu1 localizes to the **cell periphery/plasma membrane** under **low-iron** conditions; localization remains membrane-associated after **cell wall digestion** (spheroplasts), supporting plasma membrane rather than wall-only localization. Later work indicates Shu1 is **hemin-responsive**: at low hemin it is at the plasma membrane, while at higher extracellular hemin it is **internalized to the vacuole/vacuolar membrane** (pqac-00000002, pqac-00000003, pqac-00000013, pqac-00000015) | Microscopy, membrane fractionation | Mourer et al., **2015-04**, https://doi.org/10.1074/jbc.m115.642058 ; Mourer, **2019** (pqac-00000002, pqac-00000003, pqac-00000013, pqac-00000015) |
| regulation | **Iron starvation induces shu1+**, whereas iron repletion represses it through the GATA-type repressor **Fep1**. In the 2015 study, induction was detectable after **~90 min** of iron starvation and reached maximal levels by **~4 h**. Typical assay conditions included **Dip 250 µM** or **FeCl3 100 µM** for **3 h**. A 2024 study used the **shu1+ promoter** as a Fep1 ChIP positive control and reported **~7.3-fold** Fep1 enrichment at the promoter under iron-replete conditions versus **~1.2-fold** under low iron (pqac-00000004, pqac-00000014, pqac-00000019, pqac-00000025, pqac-00000009) | Regulatory, ChIP, expression analysis | Mourer et al., **2015-04**, https://doi.org/10.1074/jbc.m115.642058 ; Ping FLY et al. **2024-03**. *The flavohemoglobin Yhb1 is a new interacting partner of the heme transporter Str3* (preprint). https://doi.org/10.1101/2024.03.03.583214 (pqac-00000004, pqac-00000014, pqac-00000009) |
| pathway/model | Best-supported model: Shu1 is a **cell-surface GPI-anchored heme receptor/transporter** that captures extracellular heme/ZnMP for iron acquisition. Shu1-mediated heme use is **independent of Fio1/Fip1** oxidase-permease-mediated elemental iron uptake. Later mechanistic work proposes that Shu1-bound heme is trafficked via **endocytosis to the vacuole**, followed by export to the cytoplasm through the vacuolar **ABC transporter Abc3**; this is distinct from the parallel **Str3** pathway (pqac-00000000, pqac-00000003, pqac-00000008, pqac-00000014) | Genetics, cell biology, pathway inference | Mourer et al., **2015-04**, https://doi.org/10.1074/jbc.m115.642058 ; Ping et al., **2024-03**, https://doi.org/10.1101/2024.03.03.583214 ; Mourer, **2019** (pqac-00000000, pqac-00000003, pqac-00000008, pqac-00000014) |
| key assays & phenotypes | In *hem1Δ* cells (heme biosynthesis blocked), exogenous **ALA 200 µM** rescues viability and enables heme-uptake assays. In hemin-supported growth conditions (**hemin 0.075 µM**), **hem1Δ shu1Δ** cells show severe growth defects; one study reports **~6–8-fold less growth after 68 h** than **shu1+** controls/complemented strains. **ZnMP uptake assays** used **2 µM ZnMP** after **3 h** iron manipulation and **90 min** uptake; loss of shu1 **dramatically lowered ZnMP fluorescence**. Hemin-agarose pull-down and spectral titration support direct binding, while the cysteine-quartet mutant loses function without obvious mislocalization (pqac-00000001, pqac-00000007, pqac-00000019, pqac-00000024, pqac-00000015) | Genetics, fluorescence uptake assay, biochemistry, microscopy | Mourer et al., **2015-04**, https://doi.org/10.1074/jbc.m115.642058 ; Mourer, **2019** (pqac-00000001, pqac-00000007, pqac-00000019, pqac-00000024, pqac-00000015) |
| recent updates 2024 | A 2024 *S. pombe* preprint places Shu1 in a broader **dual heme-uptake network** with **Str3**. It describes Shu1 as the **high-affinity/cell-surface GPI-anchored route** for hemin/ZnMP uptake and notes mechanistic requirements for Shu1 trafficking/internalization involving **Ubi4-dependent ubiquitination, Ubc13, Nbr1, and ESCRT factors (Hse1, Sst6)**, whereas **Str3** is a separate 12-TM transporter route that remains at the plasma membrane and requires higher hemin concentrations than Shu1. This refines pathway context but does not overturn the core 2015 annotation (pqac-00000008, pqac-00000009, pqac-00000011) | Recent mechanistic synthesis, genetics, trafficking model | Ping FLY et al. **2024-03**. *The flavohemoglobin Yhb1 is a new interacting partner of the heme transporter Str3* (preprint). https://doi.org/10.1101/2024.03.03.583214 (pqac-00000008, pqac-00000009, pqac-00000011) |


*Table: This table summarizes experimentally supported functional annotation for fission yeast Shu1/Q92340, including structure, substrate, affinity, localization, regulation, and pathway context. It is designed as a compact evidence map linking each claim to primary sources and available quantitative data.*