DNAJC16 (ERdj8, "ER-resident protein ERdj8") is a large, single-pass endoplasmic reticulum membrane protein of the DnaJ/HSP40 subfamily C. It has an N-terminal cleaved signal peptide, an N-terminal J domain and an adjacent thioredoxin (TRX) domain, followed by a C-terminal transmembrane anchor; the orientation of the J/TRX region is unresolved because UniProt predicts a cytoplasmic type IV topology while the sole primary study describes these domains on the ER luminal side. 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 - if its J domain is ER-luminal, does it recruit and stimulate BiP/HSPA5, and if it is cytoplasmic, does it recruit and stimulate HSPA8/HSPA1A or another cytosolic HSP70; is its thioredoxin-like domain redox-active or scaffolding?
Q: How does ERdj8 mechanistically couple the ER subdomain (with phosphatidylinositol synthase) to the control of autophagosome membrane size?
Experiment: Determine the membrane orientation of the ERdj8 J and thioredoxin-like domains directly, using protease protection of microsomes, glycosylation-site insertion mapping, or a split-fluorescent-protein topology reporter to resolve the type-I versus type-IV topology conflict.
Experiment: Reconstitute and assay ERdj8 J-domain-stimulated ATPase activity with topology-appropriate HSP70 candidates, comparing ER-luminal BiP/HSPA5 and cytosolic HSPA8/HSPA1A if the J domain is accessible, and test the redox activity of its thioredoxin-like domain in vitro, using 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 primary study places those domains on the ER luminal side and leaves BiP among the possible effectors, whereas UniProt and the OpenScientist report place them on the cytoplasmic side. The missing steps are the experimentally mapped topology and 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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