TBL2

TBL2 is a WD40-repeat protein that functions principally as an adaptor in the PERK arm of the unfolded protein response, coupling ER stress sensing to selective translational control of the stress effector ATF4 (PMID:25393282, PMID:26239904). As an ER-localized type-I transmembrane protein, TBL2 binds preferentially to the phosphorylated form of PERK—but not GCN2 or IRE1—through its N-terminal region, while its WD40 domain engages eIF2α, the 60S ribosomal subunit, and ATF4 mRNA, thereby promoting ATF4 translation under ER stress and nutrient deprivation; loss of TBL2 impairs ATF4 induction and sensitizes cells to stress comparably to PERK depletion (PMID:25393282, PMID:25976671, PMID:26239904). The polycystin PKD2 interacts with TBL2 in the ER and is required for eIF2α recruitment to TBL2, linking this adaptor function to downstream amino acid biosynthesis (PMID:34015761). Beyond UPR signaling, TBL2 has additional context-dependent roles: it acts as a scaffold promoting PRMT5–WDR77 complex formation to enhance PRMT5 methyltransferase activity and AKT phosphorylation in breast cancer proliferation (PMID:39499734), and it binds the prenyltransferase TERE1 (UBIAD1) at mitochondrial membranes, an interaction disrupted by disease-associated TERE1 substitutions and tied to mitochondrial membrane potential, ROS/RNS production, and SXR target gene activation (PMID:23564352).

1. 1999 Medium

   Initial cloning established TBL2 as a WD40-repeat protein with a defined tissue expression pattern and conserved mouse ortholog, providing the molecular starting point before any function was known.

   Evidence cDNA cloning, genomic sequencing, RT-PCR, and comparative sequence analysis across human and mouse

   PMID:10575226

   Open questions at the time

   • No molecular function or interacting partners identified
   • Significance of the alternatively spliced 75-aa fragment unknown
   • Subcellular localization not determined
2. 2013 Medium

   TBL2 was shown to be a high-affinity direct binding partner of TERE1/UBIAD1, the first identified molecular interaction, linking it to mitochondrial redox and lipid/SXR signaling.

   Evidence Biochemical pulldown/Co-IP with disease-mutant mapping, subcellular fractionation, and functional ROS/NO/SXR readouts

   PMID:23564352

   Open questions at the time

   • Mechanism by which the TERE1-TBL2 complex alters mitochondrial potential and ROS not resolved
   • Mitochondrial localization here is at odds with later ER assignment
   • No structural model of the binding interface
3. 2014 High

   TBL2 was defined as an ER-localized adaptor that selectively binds phospho-PERK and eIF2α, establishing its central role in the PERK-eIF2α-ATF4 UPR branch.

   Evidence Reciprocal Co-IP with deletion-mutant domain mapping, subcellular fractionation, and siRNA knockdown phenotype

   PMID:25393282

   Open questions at the time

   • How phospho-PERK selectivity over GCN2/IRE1 is achieved structurally is unknown
   • Does not reconcile ER localization with earlier mitochondrial assignment
   • Direct effect on eIF2α phosphorylation status not established
4. 2015 Medium

   TBL2 was found to associate constitutively with the 60S but not 40S ribosomal subunit via its WD40 domain, connecting the adaptor to the translational machinery independent of stress.

   Evidence Co-IP of ribosomal subunit fractions with TBL2 deletion mutants

   PMID:25976671

   Open questions at the time

   • Functional consequence of 60S association for general or selective translation unclear
   • Whether binding is to free 60S or assembled ribosomes not distinguished
   • No identification of the 60S contact site
5. 2016 High

   TBL2 was shown to bind ATF4 mRNA through its WD40 domain and to be required for ATF4 translation, providing the mechanistic link between TBL2 and selective stress-induced gene expression.

   Evidence RNA-immunoprecipitation with deletion mutants plus knockdown and WD40-defective dominant-negative validation

   PMID:26239904

   Open questions at the time

   • The mRNA element/feature recognized by the WD40 domain is undefined
   • How 60S association and ATF4 mRNA binding are coordinated mechanistically is unknown
   • Direct RNA binding vs. bridging via other factors not distinguished
6. 2021 Medium

   PKD2 was identified as a TBL2 partner required for eIF2α recruitment, integrating TBL2 adaptor function into ADPKD-relevant amino acid biosynthesis.

   Evidence Co-IP, siRNA knockdown in renal tubular epithelial cells, and transcriptomics of Pkd2-knockout mouse kidneys

   PMID:34015761

   Open questions at the time

   • Whether PKD2 acts as a stable complex member or transient regulator unclear
   • Direct vs. indirect PKD2-TBL2 binding not mapped to domains
   • Physiological role in cyst formation not directly demonstrated
7. 2024 Medium

   TBL2 was assigned a UPR-independent scaffolding role promoting PRMT5-WDR77 assembly and AKT activation, expanding its function into oncogenic signaling.

   Evidence Proteomics, Co-IP, knockdown/overexpression, and in vivo xenograft assays in breast cancer cells

   PMID:39499734

   Open questions at the time

   • Whether the WD40 domain mediates PRMT5-WDR77 bridging not mapped
   • Relationship to TBL2's ER/UPR function unclear
   • Mechanism linking PRMT5 methylation to AKT phosphorylation not defined
8. 2025 Low

   TBL2 silencing was placed upstream of AMPK/mTOR-mediated autophagy in glioblastoma, linking it to tumor cell phenotypes through autophagy regulation.

   Evidence siRNA knockdown with pharmacological AMPK activation and chloroquine autophagy inhibition plus proliferation/migration/invasion assays

   PMID:41101043

   Open questions at the time

   • Pharmacological epistasis without direct molecular interaction data
   • No reconstitution and mechanism of AMPK/mTOR engagement undefined
   • Single lab, single cancer context

• How TBL2's distinct activities — ER/UPR adaptor, mitochondrial TERE1 partner, and cytoplasmic PRMT5 scaffold — are partitioned across compartments and reconciled into one protein's biology remains unresolved.
• No structural basis for the multiple WD40-mediated interactions
• Mechanism partitioning TBL2 between ER and mitochondria unknown
• Whether oncogenic scaffolding roles depend on or are independent of UPR function untested