| Aspect | Key finding | Evidence type (review/primary) | System/condition | Quantitative/statistics (if any) | Citation (context id) | Publication (authors, year, journal) | URL/DOI |
|---|---|---|---|---|---|---|---|
| Identity/domains | SSA4 is verified as **Ssa4**, a **cytosolic Hsp70** of *Saccharomyces cerevisiae*, encoded by **YER103W** and corresponding to **UniProt P22202**; canonical Hsp70 architecture includes an NBD and SBD linked by a flexible linker, with inter-isoform variation enriched in the SBD lid and an NBD surface implicated in J-protein interactions. | Review | Budding yeast cytosolic Hsp70 family | Ssa1 shares ~85% identity with Ssa4 | (pqac-00000000) | Lotz et al., 2019, *Current Genetics* | https://doi.org/10.1007/s00294-019-00978-8 |
| Identity/regulation | Ssa1–4 are the four cytosolic Ssa Hsp70s; **SSA3/SSA4 are stress-inducible**, whereas **SSA1/SSA2 are constitutive**. Any single Ssa can support viability, but **ssa1Δ ssa2Δ** cells are slow-growing and thermosensitive, indicating inducible paralogs do not fully replace constitutive ones. | Review | Yeast heat-shock/proteostasis network | No explicit SSA4 fold-change in excerpt | (pqac-00000014) | Verghese et al., 2012, *Microbiology and Molecular Biology Reviews* | https://doi.org/10.1128/MMBR.05018-11 |
| Regulation | **SSA4 is an Hsf1-induced gene**; Hsf1-mediated induction of **SSA3/SSA4** is central to the Hsp70–Hsf1 negative-feedback loop. A strain with all four SSA genes decoupled from Hsf1 regulation failed to induce Hsp70 during heat shock despite elevated basal Hsp70. | Primary | Heat shock response; feedback-severed DFBL yeast strain | Qualitative result: no heat-induced Hsp70 induction in DFBL | (pqac-00000006) | Krakowiak et al., 2018, *eLife* | https://doi.org/10.7554/eLife.31668 |
| Regulation | SSA4 was assayed as an **Hsf1 target** by RNA-seq and qRT-PCR; disruption of bipartite Hsp70–Hsf1 contacts caused constitutive Hsf1 activation, slow growth, and dysregulated target-gene expression including **SSA4**. | Primary | 37°C heat shock; Hsf1 mutant backgrounds | RNA-seq at 37°C for 15 min; qRT-PCR statistics reported but numeric SSA4 values not given in excerpt | (pqac-00000005) | Peffer et al., 2019, *Journal of Biological Chemistry* | https://doi.org/10.1074/jbc.RA119.008822 |
| Regulation/localization | Recent systems analysis places cytosolic Hsp70 paralogs (**SSA1, SSA3, SSA4**) in the heat-shock feedback architecture, where Hsp70 rebinding represses Hsf1 after stress and auxiliary loops regulate Hsp70 partitioning between nucleus and cytosolic condensates. | Primary preprint | Yeast heat shock response dynamics | Hsf1 regulon: 42 genes; induction magnitudes range from <10% to >8-fold across targets (not SSA4-specific) | (pqac-00000004) | Garde et al., 2024, *bioRxiv* | https://doi.org/10.1101/2024.01.09.574867 |
| Quantitative data/regulation | **SSA4 mRNA** is uniquely enriched in low-frequency codons that promote ribosome stalling during heat shock; **Asc1/Hel2 and RQC factors** tune SSA4 translation and recovery-phase decay rather than triggering standard NGD. | Primary | Heat shock and recovery | WT SSA4 mRNA t1/2 ~30 min; Asc1 M1X t1/2 84 min; ASC1 deletion increased half-life ~2.5× in one comparison; M1X produced ~7-fold more Ssa4 protein after heat shock | (pqac-00000022, pqac-00000023, pqac-00000025) | Boopathy et al., 2023, *Nucleic Acids Research* | https://doi.org/10.1093/nar/gkad338 |
| Localization | Ssa4 is a **cytosolic** Hsp70; inducible Ssa3/4 are low at 30°C, likely biasing interactome studies toward constitutive isoforms under non-stress conditions. | Review | Standard growth vs stress conditions | Reported physical interactors: Ssa4 57 vs Ssa1 717, Ssa2 375, Ssa3 69; only ~1.5% shared across isoforms | (pqac-00000001, pqac-00000016) | Lotz et al., 2019, *Current Genetics* | https://doi.org/10.1007/s00294-019-00978-8 |
| Function/pathways | Cytosolic Ssa Hsp70s including Ssa4 function in protein folding/refolding, prevention of aggregation, disaggregation with Hsp104, and triage of damaged proteins for degradation. | Review | General proteostasis network | Ssa4 half-life reported as >100 h | (pqac-00000000, pqac-00000010) | Lotz et al., 2019, *Current Genetics* | https://doi.org/10.1007/s00294-019-00978-8 |
| Function/pathways | The Ssa family broadly supports folding, translocation, and degradation; depletion/inactivation causes client-folding defects, while stress-inducible **SSA3/SSA4** are induced by heat shock or **SSA1/2** loss. | Review | General yeast proteostasis | Any single Ssa supports viability, but inducible paralogs incompletely complement Ssa1/2 loss | (pqac-00000014) | Verghese et al., 2012, *Microbiology and Molecular Biology Reviews* | https://doi.org/10.1128/MMBR.05018-11 |
| Function/pathways | Cells expressing stress-inducible **Ssa3 or Ssa4** as the sole Ssa isoform show reduced **α-synuclein toxicity** and protection against other aggregation-prone proteins; mechanism implicated is promotion of **autophagic degradation**, not simply general stress induction. | Primary | Yeast models of α-synuclein/polyQ proteotoxicity | No exact fold-change in excerpt | (pqac-00000002) | Gupta et al., 2018, *PLoS Genetics* | https://doi.org/10.1371/journal.pgen.1007751 |
| Function/pathways | Ssa1–4 are essential cytosolic Hsp70s; Hsp70 capacity must be balanced with the **ubiquitin–proteasome system (UPS)** and sequestration pathways. Hsp70 cooperates with Hsp104 for disaggregation and with Hsp90 for client folding. | Primary review-style research | Proteostasis mutants with low Hsp70 capacity | Simultaneous deletion of SSA1–SSA4 is lethal; reducing 26S proteasome levels improved growth/refolding in low-Hsp70-capacity mutants | (pqac-00000012) | Jawed et al., 2023, *Frontiers in Molecular Biosciences* | https://doi.org/10.3389/fmolb.2022.1106477 |
| Function/pathways | The Ssa family is described as managing irretrievably misfolded proteins—including aggregates and prions—through **proteasomal or lysosomal/autophagic** routes; however, Farley et al. chiefly establish Ssa1/Ssa2 roles and do not provide a distinct SSA4 mechanism in the excerpt. | Primary | Ste5 scaffold quality-control context; mating MAPK pathway | Qualitative family-level statement; no SSA4-specific statistic | (pqac-00000009, pqac-00000015) | Farley et al., 2023, *PLOS ONE* | https://doi.org/10.1371/journal.pone.0289339 |
| Comparative pathway evidence | During arsenite stress, aggregate clearance depends on Hsp104/Hsp70/Hsp40 systems; the excerpt directly supports **Ssa1/2** rather than Ssa4, but it places **Ssa1–Ssa4** within the cytoplasmic Hsp70 disaggregation framework. | Primary | Arsenite-induced proteotoxic stress | ssa1Δ ssa2Δ mutants accumulated more Sis1-GFP foci and cleared aggregates more slowly | (pqac-00000011) | Hua et al., 2022, *Journal of Biological Chemistry* | https://doi.org/10.1016/j.jbc.2022.102680 |
| Regulation/redox sensing | Heat-inducible **Ssa3/4** are distinguished from constitutive Ssa1/2 by cysteine content; Ssa3/4 lack the Ssa1/Ssa2 C264 but contain **C303**, supporting biochemical divergence among paralogs in stress sensing/regulation. | Primary | Thiol-reactive compound response; Hsf1 regulation context | No SSA4-specific fold-change in excerpt | (pqac-00000008) | Wang et al., 2012, *Molecular Biology of the Cell* | https://doi.org/10.1091/mbc.E12-06-0447 |


*Table: This table summarizes identity, regulation, localization, function, and quantitative findings for yeast SSA4/Ssa4 (YER103W; UniProt P22202) from key reviews and primary studies. It highlights where evidence is SSA4-specific versus family-level, and emphasizes recent 2023-2024 mechanistic work on SSA4 mRNA regulation.*