| Topic | Key points | Representative evidence (with citation IDs) | Key sources (author-year, journal, URL, publication month/year) |
|---|---|---|---|
| Identity | KAR2 in *Saccharomyces cerevisiae* encodes Kar2p/BiP, the ER-luminal Hsp70-family chaperone homologous to mammalian BiP/GRP78; evidence aligns with UniProt P16474 and the expected ATPase and peptide-binding domain architecture. | Review and comparative sources identify ScKar2 as the ER BiP homolog and Hsp70-family chaperone; primary work describes canonical Hsp70 domain organization. (pqac-00000005, pqac-00000010) | Ishiwata-Kimata & Kimata 2023, *Journal of Fungi*, https://doi.org/10.3390/jof9100989, Oct 2023; Kabani et al. 2000, *Molecular and Cellular Biology*, https://doi.org/10.1128/MCB.20.18.6923-6934.2000, Sep 2000 |
| Localization/retention | Kar2p is ER luminal and retained/retrieved through a C-terminal HDEL-type motif; in tagged constructs the retrieval motif is FEHDEL. HDEL-dependent retrieval is important for growth and ER residency, with Erd2 as the receptor system. Imaging studies support ER localization. | FEHDEL motif documented for BiP/Kar2p; anti-Kar2 immunofluorescence and BiP-FP-HDEL imaging show ER localization; ERD2 determines specificity of the luminal ER protein retention system. (pqac-00000009, pqac-00000013, pqac-00000014, pqac-00000008) | Young et al. 2013, *Traffic*, https://doi.org/10.1111/tra.12041, Apr 2013; Hernández-Elvira et al. 2018, *Cells*, https://doi.org/10.3390/cells7080106, Aug 2018 |
| Primary biochemical activity | Kar2p is an ATP-dependent Hsp70 chaperone that binds unfolded polypeptides, couples nucleotide state to peptide binding/release, and supports ER protein folding/homeostasis. Domain architecture reported as ~44-kDa ATPase domain, ~18-kDa peptide-binding pocket, and ~10-kDa C-terminal domain. | Kar2 described as molecular ratchet/chaperone for unfolded ER clients; Sec63-mediated ATPase activation and Hsp70 domain architecture defined experimentally. (pqac-00000002, pqac-00000010) | Ishiwata-Kimata & Kimata 2023, *Journal of Fungi*, https://doi.org/10.3390/jof9100989, Oct 2023; Kabani et al. 2000, *Molecular and Cellular Biology*, https://doi.org/10.1128/MCB.20.18.6923-6934.2000, Sep 2000 |
| Translocation role | Kar2p acts during ER import/translocation, especially posttranslational translocation, where the Sec63 luminal J-domain recruits and activates Kar2p; Kar2 binding helps pull/ratchet nascent chains into the ER. Loss of BiP function blocks secretory protein translocation. | Sec63 J-domain binds Kar2p and activates its ATPase; sec63-1 mutants accumulate untranslocated preproteins; kar2 and sec63 mutant microsomes are defective in post- and cotranslational translocation; review reiterates molecular-ratchet role. (pqac-00000010, pqac-00000008, pqac-00000005) | Kabani et al. 2000, *Molecular and Cellular Biology*, https://doi.org/10.1128/MCB.20.18.6923-6934.2000, Sep 2000; Hernández-Elvira et al. 2018, *Cells*, https://doi.org/10.3390/cells7080106, Aug 2018; Ishiwata-Kimata & Kimata 2023, *Journal of Fungi*, https://doi.org/10.3390/jof9100989, Oct 2023 |
| UPR regulation | KAR2 is a classic UPR target gene. Under non-stress conditions Kar2 binds the Ire1 luminal domain and suppresses Ire1 self-association; during ER stress Kar2 dissociates, enabling Ire1 oligomerization and HAC1 mRNA splicing. Kar2 dissociation alone is not sufficient for full activation, indicating multilayered Ire1 control. | Review and mechanistic summaries cite Kar2-mediated repression of Ire1, stress-triggered dissociation, and persistence of stress responsiveness in Ire1 mutants lacking the Kar2-binding site. KAR2 promoter contains unfolded-protein-responsive cis-elements. (pqac-00000002, pqac-00000016, pqac-00000018, pqac-00000008) | Ishiwata-Kimata & Kimata 2023, *Journal of Fungi*, https://doi.org/10.3390/jof9100989, Oct 2023; Hernández-Elvira et al. 2018, *Cells*, https://doi.org/10.3390/cells7080106, Aug 2018 |
| ERAD/quality control | Kar2p binds unfolded or unassembled ER proteins and contributes to quality control, maintaining client solubility and promoting retrotranslocation/degradation through ERAD. Kar2 availability influences proteotoxic stress outcomes and extracellular escape of ER residents when retention/retrieval fails. | Reviews state Kar2 associates with unfolded ER clients destined for proteasomal degradation; ER chaperones maintain solubility for retrotranslocation; Kar2 secretion assays reveal defects in ER quality-control/retrieval pathways. (pqac-00000002, pqac-00000000, pqac-00000007) | Ishiwata-Kimata & Kimata 2023, *Journal of Fungi*, https://doi.org/10.3390/jof9100989, Oct 2023; Hernández-Elvira et al. 2018, *Cells*, https://doi.org/10.3390/cells7080106, Aug 2018; Dorrington Quinones 2011, https://doi.org/10.7916/d85b08f5, Jan 2011 |
| Co-chaperones/NEFs | Kar2p works with Sec63 (J-domain cochaperone) and Sls1p, which stimulates Sec63-mediated activation of Kar2p. Lhs1 and Sil1 are identified as nucleotide exchange factors/functionally linked ER lumenal factors for BiP/Kar2; Scj1 is an Hsp40 cochaperone. | Sls1p binds Kar2p, shows synthetic lethality with kar2/sec63 mutants, and stimulates Kar2–Sec63 interaction/ATPase activation; Young et al. list Lhs1 as BiP NEF, Sil1 as required for ER translocation, and Scj1 as cochaperone. (pqac-00000010, pqac-00000009, pqac-00000018) | Kabani et al. 2000, *Molecular and Cellular Biology*, https://doi.org/10.1128/MCB.20.18.6923-6934.2000, Sep 2000; Young et al. 2013, *Traffic*, https://doi.org/10.1111/tra.12041, Apr 2013; Ishiwata-Kimata & Kimata 2023, *Journal of Fungi*, https://doi.org/10.3390/jof9100989, Oct 2023 |
| Quantitative data points | Specific values available from gathered evidence include: BiP-Strep-tag II-HDEL expected MW ~76.9 kDa; ~54% recovery of total BiP from lysates after affinity purification; Kar2 secretion index 9.0 in *erd1Δ*; UPRE-LacZ screen cutoff 1.5-fold over WT for constitutive UPR activation. | Tagged BiP construct and purification values from Young et al.; Kar2 secretion index and UPR-screen threshold from Dorrington Quinones. (pqac-00000013, pqac-00000007) | Young et al. 2013, *Traffic*, https://doi.org/10.1111/tra.12041, Apr 2013; Dorrington Quinones 2011, https://doi.org/10.7916/d85b08f5, Jan 2011 |
| Recent 2023-2024 developments | 2023 review synthesizes updated UPR control in yeast: Kar2/BiP acts as an Ire1 adjustor rather than a simple on/off inhibitor; Ire1 activation includes additional restraints beyond Kar2 release, including an N-terminal intrinsically disordered segment, and efficient clustering also depends on the actin/myosin cytoskeleton. 2023-2024 biotechnology reviews emphasize engineering UPR/ER folding capacity, where Kar2-centered pathways remain key leverage points for secretion. | Recent review highlights BiP-bound/nonclustered Ire1 producing weak but sustained UPR and outlines newer HAC1 splicing regulators; 2024 secretion-engineering review discusses ER folding/secretory pathway engineering in *S. cerevisiae*. (pqac-00000016, pqac-00000018) | Ishiwata-Kimata & Kimata 2023, *Journal of Fungi*, https://doi.org/10.3390/jof9100989, Oct 2023; Zhao et al. 2024, *Microbial Cell Factories*, https://doi.org/10.1186/s12934-024-02299-z, Jan 2024 |
| Key applications | Kar2p is a practical engineering and assay node in yeast biotechnology: used as a marker/reporting target of UPR and ER stress, as a benchmark ER-resident chaperone in localization/purification methods, and as a pathway target for improving recombinant protein secretion through UPR/folding engineering. | Young et al. provide validated tagging/purification tools for BiP/Kar2; reviews describe leveraging UPR and ER folding capacity in yeast protein production. (pqac-00000013, pqac-00000014, pqac-00000005, pqac-00000016) | Young et al. 2013, *Traffic*, https://doi.org/10.1111/tra.12041, Apr 2013; Ishiwata-Kimata & Kimata 2023, *Journal of Fungi*, https://doi.org/10.3390/jof9100989, Oct 2023; Zhao et al. 2024, *Microbial Cell Factories*, https://doi.org/10.1186/s12934-024-02299-z, Jan 2024 |


*Table: This table summarizes the verified identity, mechanism, localization, pathway roles, quantitative evidence, and recent applications of Saccharomyces cerevisiae KAR2/Kar2p-BiP. It is designed as a compact evidence map for functional annotation with direct citation IDs and source metadata.*