| Annotation question | Best-supported conclusion | Direct evidence / quantitative result | Evidence strength | Key citation (author/year/DOI URL) |
|---|---|---|---|---|
| Identity: is this the correct EUG1? | Yes. The literature matches **Saccharomyces cerevisiae EUG1 / YDR518W**, a **PDI-family ER protein** with **two thioredoxin-like active sites of the CXXS type**, consistent with UniProt P32474. | Primary literature on yeast PDI homologues explicitly places **EUG1** among the nonessential S. cerevisiae PDI-family genes and notes its **two CXXS motifs**; a later study tabulates **YDR518W EUG1** with motifs at **positions 62 and 405**. (pqac-00000010, pqac-00000013) | **Strong** | Nørgaard et al. 2001, https://doi.org/10.1083/jcb.152.3.553; Hacioglu et al. 2010, https://doi.org/10.1016/j.mad.2010.09.006 |
| Localization: where does Eug1p function? | Eug1p is best supported as a **soluble lumenal endoplasmic reticulum (ER) protein**. | Reviews of yeast PDI homologues state that **all are ER lumenal proteins except Eps1**, and PDI-family members are identified by **signal sequence plus ER retention signal (K/H)DEL**. Foundational work cited in these papers originally identified EUG1 as an ER protein. (pqac-00000010, pqac-00000011) | **Moderate to strong** | Nørgaard et al. 2001, https://doi.org/10.1083/jcb.152.3.553 |
| Catalytic chemistry / active-site constraint | Eug1p is a **thiol-disulfide isomerase–like/shuffling** PDI-family protein with **limited oxidase capacity** because its active sites are **CXXS, not CXXC**. | Nørgaard et al. state Eug1p has **one cysteine residue in each active site**, rendering it **incapable of forming the internal disulfide bond required for oxidation**, but suited for shuffling reactions; overexpression of **CXXC-converted EUG1** rescues strains that native EUG1 cannot. Laboissière et al. independently showed the same principle in PDI: **CGHS (“shufflease”)** supports growth, whereas **SGHC** does not. (pqac-00000008, pqac-00000006, pqac-00000016, pqac-00000017) | **Strong** | Nørgaard et al. 2001, https://doi.org/10.1083/jcb.152.3.553; Laboissière et al. 1995, https://doi.org/10.1074/jbc.270.47.28006 |
| What reaction is catalyzed? | The most defensible annotation is **rearrangement/isomerization of disulfide bonds in ER client proteins**, rather than efficient net oxidation of dithiols. | Direct Eug1p-specific biochemical kinetics were not recovered here, but in vivo genetics show native EUG1 behaves as a **CXXS-dependent shuffler/reductase-like PDI homolog** whose function improves when active sites are converted to **CXXC**. Family-level assays in related PDI constructs support the mechanistic interpretation. (pqac-00000008, pqac-00000016) | **Moderate** | Nørgaard et al. 2001, https://doi.org/10.1083/jcb.152.3.553; Laboissière et al. 1995, https://doi.org/10.1074/jbc.270.47.28006 |
| Physiological substrate evidence: does Eug1p act on CPY? | **CPY/proCPY is supported as an informative in vivo substrate/reporters system**, but not as a uniquely specific physiological substrate of Eug1p. | In **Δpdi1 [EUG1]** strains, **proCPY maturation is almost arrested** with accumulation of the **ER p1 form**; the **steady-state proCPY pool is partially reduced**, indicating compromised oxidation when Eug1p substitutes for Pdi1p. This shows Eug1p can engage the CPY folding pathway but does so inefficiently on its own. (pqac-00000009, pqac-00000006) | **Strong for CPY pathway involvement; weak for specificity** | Nørgaard et al. 2001, https://doi.org/10.1083/jcb.152.3.553 |
| Substrate specificity: what proteins does Eug1p prefer? | **No well-defined Eug1p-specific substrate repertoire is established** from the retrieved literature. | Available evidence is from complementation and CPY reporter assays rather than substrate-trapping or direct substrate panels. Reviews discuss PDI-family substrate selectivity broadly, but not a specific Eug1p client spectrum. (pqac-00000010, pqac-00000003) | **Limited / unresolved** | Nørgaard et al. 2001, https://doi.org/10.1083/jcb.152.3.553; Xiao et al. 2004, https://doi.org/10.1074/jbc.M409210200 |
| Genetic essentiality and redundancy | **EUG1 is nonessential** and functionally redundant only in part; it cannot replace all essential Pdi1p functions by itself. | Deletion of **EUG1** caused **no obvious growth defect** under standard conditions, but **overexpression of EUG1** rescued **pdi1Δ** only when **MPD1 and MPD2** were present. By contrast, **MPD1** alone could rescue all tested deletion combinations. (pqac-00000008, pqac-00000007, pqac-00000010) | **Strong** | Nørgaard et al. 2001, https://doi.org/10.1083/jcb.152.3.553 |
| Quantitative expression context | EUG1 is normally expressed at a **much lower level than PDI1**, which likely contributes to its limited standalone capacity. | Promoter-lacZ data showed **EUG1 reporter activity ~1.173 Miller units** versus **PDI1 ~26.28 Miller units**; thus EUG1 promoter output was about **4–5% of PDI1** in that assay. (pqac-00000007) | **Strong** | Nørgaard et al. 2001, https://doi.org/10.1083/jcb.152.3.553 |
| Relationship to Ero1 oxidative folding pathway | Eug1p function is genetically linked to the canonical **Ero1-dependent ER oxidative folding pathway**, not an independent oxidation route. | In a **pdi1-deleted ero1-1** background, **none of the homologues** rescued viability when expressed from the **PDI1 promoter**; authors concluded the simplest explanation is that **Ero1p transfers oxidizing equivalents to the homologues** as well. For Eug1p specifically, its inability to oxidize on its own is consistent with this dependency. (pqac-00000007, pqac-00000006, pqac-00000003) | **Moderate to strong** | Nørgaard et al. 2001, https://doi.org/10.1083/jcb.152.3.553; Xiao et al. 2004, https://doi.org/10.1074/jbc.M409210200 |
| UPR / ER-stress pathway placement | EUG1 is a **transcriptional target of the yeast unfolded protein response (UPR)** downstream of **Ire1/Hac1** and participates in ER proteostasis capacity. | Classic UPR literature lists **EUG1** among genes induced by Hac1p, and Schröder et al. report that activation of **ER chaperone genes EUG1 and SCJ1 is completely abolished** in the tested IRE1/HAC1-independent context, implying their normal ER-stress induction depends on the canonical pathway. A 2023 review summarizes UPR-driven induction of ER folding/modification genes for applied engineering. (pqac-00000019, pqac-00000014) | **Moderate** | Cox et al. 1996, https://doi.org/10.1016/S0092-8674(00)81360-4; Schröder et al. 2003, https://doi.org/10.1046/j.1365-2958.2003.03585.x; Ishiwata-Kimata & Kimata 2023, https://doi.org/10.3390/jof9100989 |
| Aging phenotype | EUG1 contributes modestly to replicative lifespan maintenance, but this is likely secondary to its ER proteostasis role rather than its primary annotation. | Deletion of **EUG1** decreased replicative lifespan by **13%** in one study; authors note Eug1 may act as a **chaperone** in addition to thiol-disulfide isomerase activity. (pqac-00000013) | **Moderate** | Hacioglu et al. 2010, https://doi.org/10.1016/j.mad.2010.09.006 |
| Current applications / real-world implementation | There is **no strong evidence for EUG1-specific industrial or biomedical deployment**. Its main current relevance is as part of the **yeast ER folding/UPR network** that informs strain engineering for protein secretion. | Recent review literature (2023) emphasizes **UPR engineering** and ER expansion to improve secretion and lipid/terpenoid production in yeasts, but does **not identify EUG1 alone** as a validated engineering lever. Thus any application claim should remain network-level, not gene-specific. (pqac-00000015, pqac-00000014) | **Moderate for network relevance; weak for EUG1-specific application** | Ishiwata-Kimata & Kimata 2023, https://doi.org/10.3390/jof9100989 |
| Knowledge gaps | Key unresolved points are **direct native substrates**, **quantitative Eug1p enzymology in vivo**, **structural basis of client selection**, and **whether recent (2023–2024) EUG1-specific advances exist**. | Searches recovered foundational studies and recent UPR reviews, but **no 2023–2024 Eug1-specific mechanistic paper**. Existing data are dominated by genetics and reporter substrates such as CPY rather than direct client mapping. (pqac-00000010, pqac-00000014, pqac-00000015) | **Strong as an assessment of the evidence landscape** | Nørgaard et al. 2001, https://doi.org/10.1083/jcb.152.3.553; Ishiwata-Kimata & Kimata 2023, https://doi.org/10.3390/jof9100989 |


*Table: This table summarizes the strongest available evidence for the identity, function, pathway context, and limitations of functional annotation for yeast EUG1/YDR518W/P32474. It is weighted toward direct experimental evidence and explicitly distinguishes established conclusions from unresolved questions.*