TXNDC11

TXNDC11 is an ER-resident, thioredoxin domain-containing protein that initiates the N-glycan mannose-trimming step required to commit misfolded glycoproteins to ER-associated degradation (ERAD) (PMID:27283361, PMID:34698634). It functions as an obligate partner of the mannosidase EDEM2, forming a stable intermolecular disulfide bond between C692 in its Trx5 domain (its sole CXXC-containing domain) and C558 of EDEM2; this covalent linkage is essential for the complex's α1,2-mannosidase activity, which converts Man9GlcNAc2 to the Man8GlcNAc2 isomer B (PMID:32065582). The purified EDEM2-TXNDC11 complex catalyzes this first trimming step, while subsequent trimming is carried out by EDEM3 and EDEM1, placing the TXNDC11-EDEM2 complex at the entry point of the gpERAD oligosaccharide-processing route (PMID:34698634); TXNDC11 also associates with and stimulates the glycoprotein mannosidase activity of EDEM1 and EDEM2, particularly toward unfolded substrates (PMID:30374462). TXNDC11 is itself an ER stress-inducible gene, transcriptionally upregulated through a functional UPRE in its promoter via the IRE1-sXBP1 pathway, and it promotes turnover of the ER-resident transcription factors ATF6α, CREB3, and CREB3L2, with substrate selectivity that excludes NFE2L1 and LOX (PMID:36152228, PMID:41656087, PMID:41518417). An earlier identification described TXNDC11 (EFP1) as a thioredoxin-related interactor of the thyroid oxidases Duox1/Duox2 and TPO in thyrocytes (PMID:15561711).

1. 2004 Medium

   Before any ERAD role was known, the question was what protein TXNDC11 (EFP1) physically engages; identifying it as a thioredoxin-related binding partner of thyroid oxidases established it as a redox-active interactor in the ER/thyroid context.

   Evidence Yeast two-hybrid with Duox1 EF-hand bait, reciprocal Co-IP in COS cells and primary thyrocytes, immunofluorescence co-localization

   PMID:15561711

   Open questions at the time

   • No catalytic activity or redox mechanism demonstrated
   • Functional consequence of Duox/TPO binding not established
   • ER glycoprotein role not yet recognized
2. 2016 Medium

   Unbiased forward genetics asked which genes are required for glycoprotein ERAD and identified TXNDC11 as an EDEM2/3-associated factor needed to degrade MHC class I, linking it to the canonical gpERAD pathway.

   Evidence Parallel genome-wide CRISPR/Cas9 and haploid gene-trap screens for ERAD of MHC class I

   PMID:27283361

   Open questions at the time

   • Disulfide reductase activity asserted but not reconstituted in vitro
   • Direct biochemical partnership with EDEM2 not yet mapped
   • Substrate scope undefined
3. 2018 Medium

   To define what TXNDC11 contributes biochemically, this work showed it associates with EDEM1/EDEM2 and enhances their mannosidase activity on glycoprotein (not free-glycan) substrates, identifying it as a functional oxidoreductase cofactor of the EDEMs.

   Evidence In vitro mannosidase assays with native vs. denatured glycoprotein substrates, Co-IP

   PMID:30374462

   Open questions at the time

   • The molecular nature of the EDEM-TXNDC11 linkage not resolved
   • Specific cysteines and bond not identified
   • Step in the trimming cascade not assigned
4. 2020 High

   The mechanistic basis of the partnership was resolved by showing a stable intermolecular disulfide bond (EDEM2 C558–TXNDC11 Trx5 C692) is required for mannosidase activity, converting an associative cofactor model into a defined covalent catalytic complex.

   Evidence HCT116 knockouts, cysteine site-directed mutagenesis, purification of the EDEM2-TXNDC11 complex, in vitro α1,2-mannosidase assay with pyridylamine-labeled substrates

   PMID:32065582

   Open questions at the time

   • Mechanism by which the disulfide bond activates EDEM2 catalysis not structurally defined
   • Whether TXNDC11 redox chemistry actively forms/maintains the bond not shown
5. 2021 High

   To place the complex in the pathway, epistasis showed EDEM2-TXNDC11 performs the first trimming step (M9→M8B) while EDEM3/EDEM1 act subsequently, identifying TXNDC11 as the initiator of the gpERAD oligosaccharide route.

   Evidence Knockout epistasis and in vitro mannosidase assays with purified proteins and pyridylamine-labeled oligosaccharides

   PMID:34698634

   Open questions at the time

   • Determinants of substrate selection at the first step unknown
   • How trimming output is handed to downstream lectins/ERAD machinery not addressed
6. 2022 Medium

   This work asked how TXNDC11 is regulated and what it degrades, showing it is an IRE1-sXBP1/UPRE-induced gene that promotes turnover of ER-resident transcription factors ATF6α, CREB3, and CREB3L2, embedding it in UPR feedback.

   Evidence RT-PCR, Western blot, UPRE luciferase reporter, cycloheximide chase in TXNDC11-deficient vs. WT HEK293 cells

   PMID:36152228

   Open questions at the time

   • Whether these substrates are degraded via the EDEM2 mannose-trimming arm specifically not separated
   • Direct physical engagement with the transcription factors not shown
7. 2026 Low

   To test the breadth of TXNDC11-dependent ERAD, two studies showed NFE2L1 and LOX are NOT stabilized by TXNDC11 (or EDEM2) loss, unlike SEL1L/Hrd1 loss, establishing that the EDEM2-TXNDC11 arm is substrate-selective rather than a general ERAD requirement.

   Evidence Western blot of NFE2L1 and LOX in TXNDC11/EDEM2 KO vs. SEL1L/Hrd1 KO HEK293 cells with proteasome inhibitor controls

   PMID:41518417 PMID:41656087

   Open questions at the time

   • Single-method (Western blot) negative findings without orthogonal confirmation
   • Rules governing which substrates require the mannose-trimming arm undefined
   • Mechanistic basis of selectivity not addressed

• How TXNDC11's thioredoxin/CXXC redox chemistry mechanistically activates EDEM2 mannosidase catalysis, what determines substrate selectivity of the trimming arm, and how its earlier Duox/TPO association relates to its ERAD function remain unresolved.
• No structure of the EDEM2-TXNDC11 complex
• Catalytic redox cycle of TXNDC11 not demonstrated
• Connection between thyroid oxidase binding and gpERAD role unexplained