| Aspect | Evidence summary | Key references with year + DOI URL | Notes/quantitative data |
|---|---|---|---|
| Primary molecular function | Ydj1/Mas5 is the **Saccharomyces cerevisiae** DnaJ/Hsp40 **J-domain co-chaperone** that presents non-native clients to Hsp70, stimulates Hsp70 ATPase activity through its J-domain/HPD motif, and supports client folding, transfer, and triage. Classic genetics and biochemistry also show it facilitates translocation of precursor proteins across mitochondrial and ER membranes. (pqac-00000000, pqac-00000003, pqac-00000040) | Caplan et al., 1992, Cell, https://doi.org/10.1016/S0092-8674(05)80063-7; Omkar et al., 2024, PLOS Genetics, https://doi.org/10.1371/journal.pgen.1011338 | Purified Hsp70 ATPase assays in Caplan et al. used 0.5 µM Hsp70 and 0.5 µM Ydj1 in 20 µl reactions for 10 min; mutant ydj1-151 had greatly reduced ATPase stimulation activity. (pqac-00000007) |
| Key domains | Recent domain schematics and summaries describe an N-terminal **J-domain** with the essential HPD motif, a **G/F-rich region** linked to client specificity, a **CTDI client-binding region containing a zinc-finger-like/cysteine-rich region**, CTDII, a **dimerization domain**, and a C-terminal extension. These features match the expected DnaJ family architecture for a type I Hsp40/J-protein. (pqac-00000002, pqac-00000005, pqac-00000016) | Omkar et al., 2024, PLOS Genetics, https://doi.org/10.1371/journal.pgen.1011338; Kampinga et al., 2019, Cell Stress and Chaperones, https://doi.org/10.1007/s12192-018-0948-4 | Figure evidence explicitly shows J-domain, G/F region, zinc-finger-like region, and C-terminal extension with CaaX motif. (pqac-00000016) |
| PTMs | Ydj1 carries a C-terminal **CaaX motif (CASQ)** and is **farnesylated**; unlike canonical CaaX proteins, it usually avoids downstream proteolysis and carboxylmethylation via a **“shunt” pathway**. Recent work also identifies multiple **J-domain lysine acetylation** sites whose acetyl-mimic mutations impair proteostasis-related functions and remodel Ydj1 interactions. (pqac-00000010, pqac-00000026, pqac-00000032) | Hildebrandt et al., 2016, eLife, https://doi.org/10.7554/eLife.15899; Kim et al., 2023, G3, https://doi.org/10.1093/g3journal/jkad094; Omkar et al., 2024, PLOS Genetics, https://doi.org/10.1371/journal.pgen.1011338 | Farnesylation is required for optimal growth at elevated temperature and for certain Hsp90-client interactions. Ydj1 runs as a doublet reflecting unfarnesylated/farnesylated forms; acetylation and farnesylation appear independently regulated. (pqac-00000010, pqac-00000030) |
| Localization | Ydj1 is mainly **cytosolic**, but farnesylation confers **partial membrane association**. Review and experimental evidence place it at the **cytosol, ER/perinuclear membrane, and mitochondrial membranes**, consistent with roles in organellar protein targeting/biogenesis. (pqac-00000003, pqac-00000008, pqac-00000034) | Bykov et al., 2020, Trends Biochem Sci, https://doi.org/10.1016/j.tibs.2020.04.001; Hildebrandt et al., 2016, eLife, https://doi.org/10.7554/eLife.15899 | Alternative CaaX motifs alter localization and can cause punctate accumulation; normal distribution is largely restored when downstream CaaX processing is blocked. (pqac-00000012, pqac-00000013) |
| Key biological processes | The strongest supported processes are **Hsp70-dependent proteostasis**, **mitochondrial protein import/biogenesis**, and broader **protein quality control**. Ydj1 acts with cytosolic Hsp70s to maintain import-competent precursor states for mitochondrial substrates including β-barrel proteins, and also influences translation-associated proteostasis in recent acetylation studies. (pqac-00000000, pqac-00000015, pqac-00000032) | Caplan et al., 1992, Cell, https://doi.org/10.1016/S0092-8674(05)80063-7; Jores et al., 2018, J Cell Biol, https://doi.org/10.1083/jcb.201712029; Omkar et al., 2024, PLOS Genetics, https://doi.org/10.1371/journal.pgen.1011338 | Bykov et al. summarize Ydj1 roles in import of ER-destined α-factor and aggregation-prone mitochondrial precursors such as Atp2 and porin. (pqac-00000008) |
| Key interaction partners | Ydj1 functionally and physically partners with **Ssa-class Hsp70s** and participates in client relay to **Hsp90/Hsp82**. In mitochondrial protein biogenesis, Ydj1/Sis1 cooperate with cytosolic Hsp70 and connect functionally to **Tom70/TOM receptor-dependent** import pathways; recent work also shows acetylation-sensitive changes in association with Ssa1 and Hsc82. (pqac-00000015, pqac-00000031, pqac-00000033) | Jores et al., 2018, J Cell Biol, https://doi.org/10.1083/jcb.201712029; Gaur et al., 2022, PLOS Genetics, https://doi.org/10.1371/journal.pgen.1010442; Omkar et al., 2024, PLOS Genetics, https://doi.org/10.1371/journal.pgen.1011338 | Proteomics identified **327 high-confidence interactors** in the 6KQ vs 6KR comparison; ~63% were unchanged, 21% preferred 6KR, and 16% preferred 6KQ. (pqac-00000030, pqac-00000031) |
| Phenotypes | Loss or perturbation of Ydj1 causes **temperature-sensitive growth defects**, sensitivity to **cell-wall stressors** (e.g., caffeine, CFW, SDS), and protein biogenesis/import defects. Farnesylation-defective or misprocessed CaaX variants show altered thermotolerance and localization, while acetyl-mimic mutants—especially **K23Q, K37Q, and 6KQ**—display strong functional defects. (pqac-00000011, pqac-00000012, pqac-00000031) | Hildebrandt et al., 2016, eLife, https://doi.org/10.7554/eLife.15899; Omkar et al., 2024, PLOS Genetics, https://doi.org/10.1371/journal.pgen.1011338 | In CaaX-processing experiments, overexpression of CASQ or SASQ caused a ~**2-fold increase in doubling time**, whereas CTLM/CVIA caused stronger growth defects; in acetylation work, K23Q, K37Q, and 6KQ showed complete loss of growth at high temperature. (pqac-00000012, pqac-00000031) |


*Table: This table summarizes the experimentally supported functional annotation of Saccharomyces cerevisiae Ydj1/Mas5 (UniProt P25491), covering molecular function, domains, PTMs, localization, processes, partners, and phenotypes. It is useful as a compact evidence map for literature-backed gene annotation.*