GET4

GET4 (mammalian TRC35; yeast Get4) is a core alpha-solenoid scaffolding subunit of the GET/TRC pathway for post-translational targeting of tail-anchored (TA) membrane proteins to the endoplasmic reticulum (PMID:20206626). It forms a tight, intimate heterodimer with Get5/UBL4A that dimerizes through the Get5 C-terminus and recruits the upstream co-chaperone Sgt2/SGTA, placing Get4/5 upstream of the Get3 ATPase (PMID:20106980, PMID:20554915, PMID:21832041). Get4/5 binds directly and with high affinity to ribosomes, positioning Sgt2 near the tunnel exit to capture nascent TA proteins, and competes with SRP for overlapping ribosomal contact sites to partition substrates between the GET and SRP pathways (PMID:33542241). GET4 then engages a conserved surface on the Get3 homodimer in a nucleotide-dependent manner, with two Get4/5 copies binding one Get3 dimer, and acts as a regulator of the substrate handover: it primes Get3 into the optimal ATP-bound conformation and controls its ATPase activity by remodeling Get3's lateral 'gate' to open the TA-binding chamber, enabling lateral, protected transfer of substrate from Sgt2 to Get3 (PMID:22190685, PMID:24727835, PMID:40902977). In the metazoan complex, TRC35/GET4 binds BAG6 on a face opposite the yeast Get4-Get5 interface, occluding the BAG6 nuclear localization sequence to retain BAG6 in the cytoplasm and protecting TRC35 from RNF126-mediated ubiquitylation; disruption of this interaction (for example by the circRNA-encoded protein SNX25-215 binding GET4 residues H207/E214) drives BAG6 nuclear translocation (PMID:29042515, PMID:41120269). Beyond TA targeting, GET4 participates as part of the nuclear BAG6-UBL4A-GET4 complex in DNA damage response signaling and BRCA1 recruitment, suppresses mitochondria-ER contact sites through interactions with IP3R and GRP75, and through cytoplasmic BAG6 regulates p53 acetylation and cell cycle progression (PMID:23723067, PMID:34704338, PMID:38467609). Compound heterozygous GET4 missense variants destabilize the entire TRC complex, reducing GET4, BAG6, and GET5 protein levels via proteasomal degradation and impairing TA-protein targeting (PMID:32395830).

1. 2010 High

   Established that Get4 is a structured alpha-solenoid scaffold that forms an intimate heterodimer with Get5 and bridges Sgt2 to Get3, defining its position upstream of the Get3 ATPase in TA targeting.

   Evidence Co-IP and X-ray crystallography of yeast Get4-Get5 and Get4 alone

   PMID:20106980 PMID:20206626 PMID:20554915

   Open questions at the time

   • Transient nature of the Get4-Get3 interaction left the substrate handover mechanism undefined
   • Stoichiometry of the Get3-Get4/5 assembly not yet resolved
2. 2011 High

   Defined the architecture and stoichiometry of the upstream targeting complex, showing a Get3 dimer engages two Get4/5 copies via an electrostatic interface and that an Sgt2 dimer binds a single Get5.

   Evidence ITC, SAXS, crystallography of the Sgt2 TPR domain, and Co-IP in yeast

   PMID:21832041 PMID:22190685

   Open questions at the time

   • How nucleotide state of Get3 couples to the handover step not established
   • Direct substrate transfer not visualized
3. 2012 High

   Extended the scaffold to the mammalian system, showing TRC35/GET4 forms a stable BAG6-UBL4A-TRC35 complex that recruits SGTA, establishing conservation of the targeting machinery and its link to ERAD.

   Evidence NMR, Co-IP, and biochemical binding assays in mammalian cells

   PMID:23246001

   Open questions at the time

   • Functional consequences of TRC35 within BAG6 complex beyond recruitment unclear
   • Role in ERAD substrate fate not mechanistically dissected
4. 2013 Medium

   Revealed a nuclear, non-targeting role for GET4 in the DNA damage response, where the BAG6-UBL4A-GET4 complex supports BRCA1 recruitment and cell survival after damage.

   Evidence siRNA knockdown, immunofluorescence, viability assays, and subcellular fractionation

   PMID:23723067

   Open questions at the time

   • Molecular mechanism by which GET4 promotes BRCA1 recruitment unknown
   • Whether GET4's nuclear and TA-targeting roles are mechanistically separable not addressed
5. 2014 High

   Provided the mechanistic basis for substrate handover, showing Get4 primes Get3 into the ATP-bound conformation optimal for capture and regulates Get3 ATPase activity essential for efficient targeting.

   Evidence Crystal structure of Get3-Get4-Get5, structure-guided mutagenesis, ATPase and TA-targeting assays in yeast

   PMID:24727835

   Open questions at the time

   • The conformational changes in Get3's substrate chamber not yet resolved
   • Coupling of Sgt2-to-Get3 transfer to this priming step not directly visualized
6. 2017 High

   Showed TRC35/GET4 controls BAG6 localization and complex stability by occluding the BAG6 nuclear localization sequence and shielding TRC35 from RNF126-mediated ubiquitylation.

   Evidence Crystal structure of Bag6-TRC35, binding and ubiquitylation assays

   PMID:29042515

   Open questions at the time

   • Signals that trigger release of BAG6 for nuclear entry not identified
   • Whether the metazoan Bag6 interface is mutually exclusive with Get5-type binding not fully resolved
7. 2020 Medium

   Linked GET4 to human disease, demonstrating that compound heterozygous missense variants destabilize the entire TRC complex via proteasomal degradation and impair TA-protein targeting.

   Evidence Patient fibroblast immunoblotting, proteasome inhibitor rescue, and TA-protein localization imaging

   PMID:32395830

   Open questions at the time

   • Single patient report; genotype-phenotype spectrum undefined
   • Causal link between TRC destabilization and specific clinical features not established
8. 2021 High

   Identified the ribosomal docking step, showing Get4/5 binds ribosomes directly to position Sgt2 at the tunnel exit and competes with SRP to partition TA versus secretory substrates.

   Evidence Ribosome binding, cross-linking, fluorescence anisotropy, and in vitro targeting assays in yeast

   PMID:33542241

   Open questions at the time

   • Structural basis of Get4/5-ribosome contact not resolved
   • How the SRP/GET partitioning decision is made in vivo not established
9. 2021 Medium

   Demonstrated a cancer-relevant consequence of GET4-mediated BAG6 retention, where GET4 loss drives BAG6 nuclear translocation, p53 acetylation, p21 reduction, and tumor growth.

   Evidence CRISPR-Cas9 knockout, immunofluorescence, in vitro and in vivo tumor assays in colorectal cancer cells

   PMID:34704338

   Open questions at the time

   • Direct mechanism linking nuclear BAG6 to p53 acetylation not defined
   • Generalizability beyond colorectal context unknown
10. 2024 Medium

    Uncovered a role in organelle contact biology, identifying GET4 (with BAG6) as a suppressor of mitochondria-ER contact sites that interacts with IP3R and GRP75 and modulates mitochondrial calcium and respiration.

    Evidence Genome-wide CRISPR screen, MERCS reporter, microscopy, Co-IP, calcium imaging, and a Drosophila Alzheimer's model

    PMID:38467609

    Open questions at the time

    • Whether GET4 acts at contacts via TA targeting or an independent mechanism unclear
    • Direct versus indirect nature of IP3R/GRP75 interactions not dissected
11. 2025 High

    Defined how Get4/5 remodels Get3 for protected substrate transfer, showing it unfolds a 'lateral gate' to open the TA-binding chamber positioned near the Sgt2-binding domain.

    Evidence Cryo-EM at 3.2 A, molecular dynamics, mutagenesis, ATPase and binding assays in yeast

    PMID:40902977

    Open questions at the time

    • The TA substrate itself not captured in the structure
    • Timing of gate closure and ATP hydrolysis during transfer not resolved
12. 2025 Medium

    Showed the GET4-BAG6 interaction is a regulatable node, with a circRNA-encoded protein binding GET4 at H207/E214 to disrupt the interface and release BAG6 to the nucleus.

    Evidence Co-IP, molecular docking, subcellular fractionation, and immunofluorescence

    PMID:41120269

    Open questions at the time

    • Co-IP and docking without structural validation of the binding site
    • Physiological contexts in which this regulation operates not established

• How GET4's well-defined cytoplasmic TA-targeting scaffold function mechanistically connects to its distinct nuclear DDR, p53/cell-cycle, and mitochondria-ER contact roles in metazoans remains unresolved.
• No unified mechanism linking targeting and nuclear/contact-site functions
• Signals controlling GET4 partitioning between cytoplasmic and nuclear complexes unknown
• Whether moonlighting roles depend on intact TRC complex or free GET4 not established