| Category | ERp29 summary | Evidence |
|---|---|---|
| Protein identity and synonyms | Human **ERP29** encodes **endoplasmic reticulum resident protein 29**; reported aliases include **ERp29**, **ERp28**, **ERp31**, and **PDIA9**. It is treated in the literature as a noncanonical member of the PDI family. | (pqac-00000007, pqac-00000015) |
| Domain architecture | ERp29 contains an **N-terminal thioredoxin-like domain** and a **C-terminal D domain** unique to ERp29; the D domain is the principal interface for binding lectin chaperones in the calnexin/calreticulin system. | (pqac-00000002, pqac-00000013) |
| Key structural features | Unlike classical PDI enzymes, ERp29 **lacks a functional CXXC active-site motif** and is therefore not considered a typical oxidoreductase; one study/reviewed summary notes a **single cysteine (Cys157)** instead. ERp29 also carries a **C-terminal KEEL ER-retention motif**, a KDEL-like sequence linked to relatively weak ER retention and recycling through the early secretory pathway. | (pqac-00000011, pqac-00000015) |
| Oligomeric/structural behavior | ERp29 is described as a **dimer** whose N-terminal thioredoxin-like region mediates homodimerization, while the **D domain** forms the binding surface for partner chaperones. Structural work identified helices **α8/α9** and residues including **R223, L227, L241** as critical for P-domain binding. | (pqac-00000002, pqac-00000013, pqac-00000014) |
| Primary molecular function | ERp29 functions primarily as an **ER luminal chaperone/co-chaperone** that assists folding, maturation, and trafficking of secretory and membrane proteins. Its role is best understood as **general protein-folding assistance** rather than catalysis of disulfide exchange. | (pqac-00000004, pqac-00000005, pqac-00000015) |
| Role in calnexin/calreticulin cycle | In the calnexin/calreticulin cycle, ERp29 is recruited by the **P domains of calnexin (CNX) and calreticulin (CRT)** as a function-specific chaperone, alongside ERp57 and cyclophilin B, to help mature monoglucosylated glycoprotein clients. | (pqac-00000002, pqac-00000009, pqac-00000015) |
| Binding affinity data | The **ERp29 D domain binds CNX/CRT P domains with micromolar affinity**; reported values are **better than 20 μM** by NMR for CNX and about **13 μM** for full-length ERp29–CRT by surface plasmon resonance. | (pqac-00000001, pqac-00000013) |
| Known substrates / client classes | Experimentally implicated clients include **thyroglobulin** (folding/secretion), **collagen**, **ENaC** during channel biogenesis, and broader classes of **glycoproteins** handled through the calnexin/calreticulin pathway. | (pqac-00000002, pqac-00000009, pqac-00000011, pqac-00000012) |
| Thyroglobulin-related role | ERp29 associates with **thyroglobulin (Tg)** in congenital hypothyroid disorders featuring ER-retained mutant Tg, supporting a role in Tg quality control and secretion. | (pqac-00000012) |
| ENaC-related role | ERp29 regulates **epithelial sodium channel (ENaC)** biogenesis; its **KEEL motif** and interaction with the **KDEL receptor (KDEL-R)** influence forward trafficking/processing during passage through the early secretory pathway. | (pqac-00000011) |
| Binding partners | Reported interacting or functionally associated partners include **calnexin**, **calreticulin**, **ERp57**, **cyclophilin B (CypB)**, **KDEL receptor**, and stress/folding chaperones such as **BiP/GRP78** and **GRP94**. | (pqac-00000002, pqac-00000004, pqac-00000009, pqac-00000011) |
| Subcellular localization | ERp29 is primarily an **ER lumen** resident protein. Because its **KEEL** motif is a weaker KDEL variant, it can function dynamically in the **early secretory pathway**, including ER-to-Golgi trafficking steps and likely **ERGIC/proximal Golgi** recycling behavior. | (pqac-00000004, pqac-00000011, pqac-00000015) |
| Associated pathways | Main pathways/processes linked to ERp29 are the **calnexin/calreticulin cycle**, **ER protein folding/quality control**, **unfolded protein response (UPR)/ER stress adaptation**, **ER-associated degradation (ERAD)-linked quality control**, and **protein secretion/ER-to-Golgi trafficking**. | (pqac-00000009, pqac-00000010, pqac-00000012) |
| ER stress biology | ERp29 is stress responsive and can modulate ER-stress outputs: in RPE cells, ERp29 overexpression increased **GRP78**, **p58IPK**, and **Nrf2**, while reducing **p-eIF2α** and **CHOP**, consistent with a protective role during ER stress. | (pqac-00000004) |
| Current functional interpretation | Overall, ERp29 is best understood as a **specialized, noncatalytic PDI-family chaperone** that couples **client folding** to **lectin-chaperone recruitment** and **early secretory-pathway trafficking**, rather than as a classical disulfide isomerase enzyme. | (pqac-00000002, pqac-00000005, pqac-00000007, pqac-00000009) |


*Table: This table summarizes the main structural features, molecular functions, interaction partners, substrates, localization, and pathway context of human ERp29. It is useful as a compact evidence-based reference for functional annotation of ERP29.*