# TXNDC16 (Q9P2K2) review notes

## Identity
- Thioredoxin domain-containing protein 16; alias ERp90, KIAA1344. HGNC:19965. 825 aa precursor (signal peptide 1-27; chain 28-825).
- Large soluble ER-luminal glycoprotein of the PDI family with multiple (five) potential thioredoxin (Trx)-like domains. UniProt annotates one Trx domain 392-495, N-glycosylation at N460, intramolecular disulfide 449-456.
- C-terminal sequence has a MASKED, NON-FUNCTIONAL KDEL-like ER-retrieval motif (816-819), which permits partial secretion.
- UniProt has NO FUNCTION block and NO redox-active CXXC feature: the catalytic status is uncertain/likely redox-inactive.

## Key functional evidence

### PMID:21359175 (Riemer et al. 2011, "Identification of the PDI-family member ERp90 as an interaction partner of ERFAD") — full text cached
- "we identify the previously uncharacterized PDI-family member ERp90. In cultured human cells, we find ERp90 to be a soluble ER-luminal glycoprotein that comprises five potential thioredoxin (Trx)-like domains."
- "Mature ERp90 contains 10 cysteine residues, of which at least some form intramolecular disulfides. **While none of the Trx domains contain a canonical Cys-Xaa-Xaa-Cys active-site motif**, other conserved cysteines could endow the protein with redox activity."
- "ERp90 co-immunoprecipitates with ERFAD, a flavoprotein involved in ER-associated degradation (ERAD), through what is most likely a direct interaction. We propose that the function of ERp90 is related to substrate recruitment or delivery to the ERAD retrotranslocation machinery by ERFAD."
- ERFAD = FOXRED2 (UniProt Q8IWF2). The IPI protein binding annotation (PMID:21359175, with FOXRED2) is the functionally meaningful interaction (ERAD context), unlike typical HT captures, but is still a bare protein binding term.

### PMID:25122923 (Harz et al. 2014, "Secretion and immunogenicity of the meningioma-associated antigen TXNDC16") — abstract only cached
- "TXNDC16 was previously found to be an endoplasmic reticulum (ER)-luminal glycoprotein."
- "we show an additional ER-associated localization of TXNDC16 in the cytosol ... We were able to show TXNDC16 secretion in different human cell lines due to masked and therefore nonfunctional ER retrieval motif."
- "A previously indicated exosomal TXNDC16 secretion could not be confirmed in HEK293 cells." -> casts doubt on the exosome (PMID:19199708 HDA) localization.
- Meningioma-associated antigen; autoantibodies in patient sera; secreted serum protein in circulating immune complexes. Basis of the GO:0005576 extracellular region (EXP/IEA Secreted) annotations.

### PMID:19199708 (parotid gland exosome MudPIT proteomics) — HDA extracellular exosome
- High-throughput proteomic detection in exosomes. Note PMID:25122923 could NOT confirm exosomal secretion in HEK293; treat as non-core / uncertain.

## Redox/catalytic status
- No canonical CXXC in any Trx domain -> likely a redox-inactive / pseudo-thioredoxin scaffold (like the b-type PDI domains), possibly contributing to substrate recruitment/delivery rather than direct thiol-disulfide catalysis. Do NOT assert protein disulfide isomerase activity as a confirmed core MF; the evidence does not support a catalytic CXXC.

## GOA assessment
- GO:0005788 ER lumen (IEA SubCell + EXP PMID:25122923 + IDA PMID:21359175) — ACCEPT (primary localization).
- GO:0005576 extracellular region (IEA Secreted + EXP PMID:25122923) — KEEP_AS_NON_CORE (secreted due to masked KDEL; real but secondary).
- GO:0005515 protein binding (IPI, PMID:21359175, FOXRED2/ERFAD) — KEEP_AS_NON_CORE (functionally meaningful ERAD interaction but bare protein binding term; suggest a more informative MF if ERAD adapter role confirmed).
- GO:0070062 extracellular exosome (HDA, PMID:19199708) — KEEP_AS_NON_CORE / uncertain; exosomal secretion was not confirmed in a later study (PMID:25122923).

## Core function summary
TXNDC16/ERp90 is an ER-luminal PDI-family glycoprotein lacking a canonical CXXC redox motif; its best-supported role is as an interaction partner of the ERAD flavoprotein ERFAD/FOXRED2, proposed to function in substrate recruitment/delivery to the ERAD retrotranslocation machinery. It is partly secreted (masked KDEL) and is a meningioma-associated antigen. Core MF is not a confirmed catalytic oxidoreductase activity; propose no strong catalytic MF. BP = likely ERAD-related (proposed). CC = ER lumen (primary).

## Falcon deep-research findings (incorporated 2026-06)

- Review-level corroboration of non-catalytic status: Patel et al. 2020 (Cells; PubMed-verified PMID:32971745) explicitly classify ERp90/TXNDC16 as non-catalytic, with non-canonical Trx-like motifs (reported as Trxl1-CX8C, Trxl2-CX9C, Trxl3-CX6C, Trxl4/5 inactive) rather than active CXXC [PMID:32971745]. This is review/family-level inference consistent with the primary finding (PMID:21359175) that none of the Trx domains carry a canonical CXXC; do NOT over-claim a defined catalytic reaction.
- The same review places ERp90/TXNDC16 in the ERAD (retrotranslocation) category and reports associations with the ERAD lectin OS-9 and adaptor SEL1L [PMID:32971745]. These are secondary-source associations (not gene-specific primary co-IP data in the retrieved corpus), so they are recorded as context only and not added as annotations.
- Biomarker context (review): Korte & Mathios 2024 (Int J Mol Sci; PubMed-verified PMID:38673779) review TXNDC16 as a meningioma-associated antigen and restate that a five-epitope autoantibody panel (from 163 overlapping peptides) discriminated meningioma vs controls with ~90% sensitivity and ~83.7% specificity, framing it as a proof-of-concept liquid-biopsy serology marker [PMID:38673779]. This restates the existing Harz 2014 (PMID:25122923) primary findings; contextual/disease relevance only.
- Falcon confirms (via Harz 2014, already cited as PMID:25122923) the signal-peptide dependence of ER targeting: deleting the N-terminal 27-aa signal peptide shifted TXNDC16 from ER-associated to cytosolic localization, and that ER retention is incomplete due to a masked DKEL retrieval motif [PMID:25122923 (Harz 2014); restated in PMID:38673779]. Already captured in the review.
- No new gene-specific mechanistic 2023-2025 primary studies of human TXNDC16 were surfaced; a 2024 PNAS mouse-testis WFDC-cluster knockout proteomics study (Kent et al., PMID/DOI:10.1073/pnas.2413195121) merely notes TXNDC16 upregulation among protein-degradation/quality-control proteins - directionally consistent with an ERAD/quality-control role but NOT human gene-specific mechanistic proof; notes-only, not added to YAML.
- A 2025 ER-redoxome review (Oliveira et al., DOI:10.1021/acs.biochem.5c00527) lists TXNDC16/ERp90 among ER PDI/thioredoxin-domain proteins without adding TXNDC16-specific mechanistic detail; background only.
- Net: the core ERFAD/FOXRED2-linked ERAD substrate-recruitment role and the non-catalytic (no canonical CXXC) classification already in the review remain the best-supported picture; Falcon adds review-level corroboration plus the meningioma biomarker context, no change to actions.