DNAJC16 (ERdj8, "ER-resident protein ERdj8") is a large, single-pass type IV endoplasmic reticulum membrane protein of the DnaJ/HSP40 subfamily C. It has an N-terminal cleaved signal peptide, a cytoplasmically oriented N-terminal J domain and an adjacent thioredoxin (TRX) domain in its large cytoplasmic region, followed by a C-terminal transmembrane anchor. ERdj8 localizes to a meshwork-like ER subdomain together with phosphatidylinositol synthase and autophagy-related (Atg) proteins, and functions in autophagosome biogenesis where it regulates the size of forming autophagosomes, enabling engulfment of large targets; both its J domain and TRX domain are required for this activity. It is otherwise poorly characterized, and its direct molecular activity (presumed J-domain co-chaperone and/or redox function) has not been biochemically defined. An alternative splice isoform lacks the J and thioredoxin domains.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Automated SubCell localization to the ER membrane, redundant with the direct experimental (IDA) ER membrane localization from the same characterizing study. Reason: ERdj8 is a single-pass ER membrane protein; ER membrane is its core site of action and is supported by direct imaging. Supporting Evidence: PMID:32492081 ERdj8 localizes to a meshwork-like ER subdomain along with phosphatidylinositol synthase (PIS) and autophagy-related (Atg) proteins. |
| GO:0005789 endoplasmic reticulum membrane | IDA PMID:32492081 ERdj8 governs the size of autophagosomes during the formatio... | ACCEPT | Summary: Direct (IDA) evidence localizing ERdj8 to a meshwork-like ER membrane subdomain, co-distributing with phosphatidylinositol synthase and Atg proteins. Reason: Direct experimental localization to the ER membrane; the core cellular component for this single-pass ER membrane protein. Supporting Evidence: PMID:32492081 ERdj8 localizes to a meshwork-like ER subdomain along with phosphatidylinositol synthase (PIS) and autophagy-related (Atg) proteins. |
| GO:0016243 regulation of autophagosome size | IMP PMID:32492081 ERdj8 governs the size of autophagosomes during the formatio... | ACCEPT | Summary: IMP evidence that ERdj8 regulates autophagosome size during formation; overexpression enlarges autophagosomes (dependent on the J and TRX domains) and ablation impairs engulfment of larger targets. This is the central, experimentally established biological role of ERdj8. Reason: Directly supported by gain- and loss-of-function experiments (and conserved in C. elegans dnj-8); the core biological process for this gene. Supporting Evidence: PMID:32492081 ERdj8 overexpression extended the size of the autophagosome through its DnaJ and TRX domains. ERdj8 ablation resulted in a defect in engulfing larger targets. |
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Download this section (compressed HTML)Q: What is the direct molecular activity of ERdj8 - does its J domain recruit and stimulate an ER-luminal/cytosolic HSP70 (e.g. BiP/HSPA5), and is its thioredoxin domain redox-active?
Q: How does ERdj8 mechanistically couple the ER subdomain (with phosphatidylinositol synthase) to the control of autophagosome membrane size?
Experiment: Reconstitute and assay ERdj8 J-domain-stimulated HSP70 ATPase activity and test the redox activity of its thioredoxin domain in vitro, using the H57Q and C171A/C174A mutants as controls, to define the molecular function underlying autophagosome-size regulation.
Experiment: Proximity labeling (BioID/APEX) from ERdj8 at the ER-phagophore subdomain to identify its HSP70 partner(s) and Atg/lipid-synthesis machinery it engages during autophagosome formation.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The direct molecular activity of DNAJC16/ERdj8 is unresolved: it is not known whether its J domain directly recruits and stimulates a specific HSP70 partner, whether its TRX domain has redox activity, or whether these domains support autophagosome-size control through another effector mechanism.
OPEN BIOLOGYCURATION MF_DARK
What is known: ERdj8 is experimentally localized to an ER membrane subdomain and regulates autophagosome size; both the DnaJ/J domain and TRX domain are required for the overexpression phenotype. The missing step is the biochemical activity linking those domains to the autophagosome-size readout.
Significance: The gene has a solid biological-process annotation but no experimentally supported molecular-function annotation. Adding HSP70 binding or TRX activity by domain inference alone would overstate the evidence.
What would resolve it: In vitro ERdj8 J-domain assays for HSP70 ATPase stimulation and binding, direct identification of the relevant HSP70/effector partner, and biochemical testing of TRX-domain redox activity with H57Q and C171A/C174A controls.
Provenance (the field's own admissions):
Gap: The mechanism by which ERdj8-containing ER subdomains control autophagosome size and large-target engulfment is not defined.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: ERdj8 colocalizes with PIS and autophagy-initiation machinery at a specialized ER subdomain; overexpression enlarges autophagosomes, while knockdown restricts engulfment of larger targets. The unresolved part is how ERdj8 organizes lipid supply, Atg machinery, HSP70/effector proteins, or membrane dynamics to set phagophore size.
Significance: The process term captures the phenotype but not the causal mechanism. Closing this gap would explain how an ER membrane J-protein tunes autophagosome capacity for large selective-autophagy cargo.
What would resolve it: Proximity labeling or interaction proteomics at ERdj8-positive ER-phagophore subdomains, perturbation of candidate PIS/Atg/HSP70 effectors, and lipid-transfer or membrane-growth readouts during phagophore expansion.
Provenance (the field's own admissions):
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