HSPA13

HSPA13 (STCH) is a constitutively expressed, calcium-inducible Hsp70-family ATPase that localizes to the microsome/ER membrane and, uniquely among Hsp70 proteins, contains only the N-terminal ATPase domain while lacking the C-terminal peptide-binding domain — consequently its ATPase activity is independent of peptide stimulation (PMID:8131751). This truncated 'core ATPase' architecture, together with a conserved Hip-like co-chaperone-binding region, is preserved across human, rat, and C. elegans homologues (PMID:9358068), and its ATPase activity is functionally essential: ATP-binding-deficient mutants fail to recapitulate its activities (PMID:18793616, PMID:36244454). At the ER membrane HSPA13 acts on the secretory pathway by interacting with the Sec61 translocon, where its overexpression — exacerbated by ATPase-inactive mutants — impairs co-translational translocation and maturation of secretory proteins, causing cytosolic aggregation, while its loss destabilizes ER proteostasis (PMID:36244454). Through this proteostatic role it governs the surface delivery, stability, and degradation of membrane transporters, promoting NBCe1-B/NHE1-dependent intracellular pH recovery (PMID:23303189) and routing ER-resident NKCC2 to proteasomal and ER-to-lysosome-associated degradation (PMID:33672238). HSPA13 additionally functions as a signaling modulator that influences cell-fate and immune outcomes: it binds TNFR1 and RIP1 to stabilize complex I and bias TNFα signaling toward NF-κB and away from apoptosis/necroptosis (PMID:34613781), and it engages innate-immune and inflammasome components (RIG-I, ASC) to amplify antiviral and IL-1β responses (PMID:37776769). In B cells it supports plasma cell differentiation and antibody secretion via interaction with BCAP31-dependent ER-to-cytosol transport (PMID:32547538) and activates IL-10 transcription through direct promoter binding (PMID:39737854).

1. 1994 High

   Established HSPA13/STCH as a distinct Hsp70 family member by showing it is a microsomal, calcium-inducible ATPase whose activity, unlike BiP/DnaK, is peptide-independent and which structurally retains only the N-terminal ATPase domain.

   Evidence Subcellular fractionation, immunofluorescence, in vitro ATPase assay, and cDNA cloning

   PMID:8131751

   Open questions at the time

   • Physiological substrates or partners of the ATPase activity not identified
   • Functional consequence of lacking the peptide-binding domain unresolved
2. 1997 Medium

   Demonstrated the truncated core-ATPase architecture and a conserved Hip-like internal sequence are evolutionarily conserved, implying the domain truncation is functionally selected rather than an artifact.

   Evidence Sequence analysis and cloning of rat and C. elegans orthologs with expression analysis

   PMID:9358068

   Open questions at the time

   • No direct demonstration that the Hip-like region binds Hip in vivo
   • Functional role of the unique C-terminal cluster III undefined
3. 2000 Medium

   Identified ubiquitin-like proteins Chap1/Dsk2 and Chap2/scythe as binders of the STCH ATPase domain, linking it to ubiquitin-pathway co-chaperones.

   Evidence Peptide pulldown, yeast two-hybrid, and genetic complementation in S. cerevisiae

   PMID:10675567

   Open questions at the time

   • Biological consequence of these interactions in mammalian cells not established
   • Single-lab finding without reciprocal validation in human cells
4. 2008 Medium

   Showed ATPase activity is mechanistically required for STCH function by demonstrating an ATP-binding-deficient deletion mutant fails to sensitize cells to TRAIL-induced apoptosis.

   Evidence In vitro ATP-binding assay, site-specific mutagenesis, and overexpression with TRAIL cell-death readout

   PMID:18793616

   Open questions at the time

   • Molecular intermediates linking STCH to TRAIL apoptosis not defined
   • Overexpression context only; endogenous relevance unclear
5. 2013 High

   Defined a transporter-regulatory role by showing STCH binds NBCe1-B and NHE1 and is required for their surface delivery and intracellular pH recovery from acidification.

   Evidence Yeast two-hybrid, Xenopus oocyte surface expression, siRNA knockdown with pH measurement, and co-IP

   PMID:23303189

   Open questions at the time

   • Whether ATPase activity is required for transporter trafficking not tested
   • Mechanism of surface delivery (chaperoning vs. translocation) unresolved
6. 2020 Medium

   Connected ER-to-cytosol protein transport to humoral immunity by showing Hspa13 interacts with BCAP31 and is required for plasma cell differentiation, antibody production, and affinity maturation.

   Evidence Co-IP and B cell-specific conditional knockout mouse with immunization and ELISA readouts

   PMID:32547538

   Open questions at the time

   • Direct cargo transported via the BCAP31 axis not identified
   • Whether ATPase activity drives transport not tested
7. 2021 Medium

   Revealed a cell-fate switch function: HSPA13 binds TNFR1 and RIP1 to stabilize complex I, promoting NF-κB signaling and preventing RIP1 transition to death-inducing complex II.

   Evidence Reciprocal Co-IP, HSPA13 knockout cells, NF-κB reporter, and apoptosis/necroptosis assays

   PMID:34613781

   Open questions at the time

   • Whether HSPA13 acts catalytically or as a scaffold at complex I unclear
   • Single-lab finding
8. 2021 Medium

   Extended transporter regulation to degradation by showing HSPA13 binds ER-resident NKCC2 and promotes its turnover via proteasome and ERLAD pathways.

   Evidence Co-IP, knockdown/overexpression, cycloheximide chase, and proteasome/lysosome inhibitor treatment

   PMID:33672238

   Open questions at the time

   • How HSPA13 selects substrates for degradation vs. delivery unknown
   • Direct involvement of ATPase activity not tested
9. 2022 High

   Established the core ER mechanism: HSPA13 interacts with the Sec61 translocon and, in an ATPase-dependent manner, modulates co-translational translocation and maturation of secretory proteins, with loss destabilizing proteostasis.

   Evidence Mass-spectrometry interactome, ATPase-inactive mutagenesis with translocation assays, and knockout ER-stress sensitivity assay

   PMID:36244454

   Open questions at the time

   • Whether HSPA13 facilitates or gates translocation under physiological conditions unresolved
   • Structural basis of Sec61 engagement unknown
10. 2023 Medium

    Documented additional signaling roles in cancer and antiviral immunity: HSPA13 stabilizes TANK by inhibiting its ubiquitination and engages RIG-I and ASC to enhance type I IFN, ISG, and IL-1β responses.

    Evidence Co-IP, ubiquitination assays, and knockdown/overexpression with proliferation, cytokine, and viral replication readouts

    PMID:37776769 PMID:38062023

    Open questions at the time

    • Mechanism by which HSPA13 protects partners from ubiquitination undefined
    • Direct vs. indirect engagement of inflammasome components unclear
11. 2024 Medium

    Identified nuclear and regulatory functions: Hspa13 binds the IL-10 promoter to activate transcription and Treg differentiation, while a TGFβ1/Ca2+/PI3K-Akt axis drives EMT, and YTHDF3/m6A controls HSPA13 mRNA stability and downstream PD-L1.

    Evidence Promoter binding/ChIP, knockout IL-10/Treg assays, PI3K-Akt phosphorylation and in vivo PVR model, and m6A mRNA degradation/rescue assays

    PMID:38811341 PMID:39226050 PMID:39737854

    Open questions at the time

    • How a membrane ATPase localizes to the nucleus to bind DNA unexplained
    • Relationship between ER and transcriptional functions unresolved

• How a single truncated, peptide-independent ATPase coordinates its diverse ER-translocon, transporter, signaling, and transcriptional activities through a unified molecular mechanism remains unresolved.
• No structural model of HSPA13 bound to Sec61 or transporters
• No defined co-chaperone/nucleotide-exchange partner explaining ATPase cycling
• Mechanism reconciling cytosolic/ER and nuclear localizations not established