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Comprehensive Research Report: Functional Annotation of dnajc6 Gene in Zebrafish (Danio rerio)

Executive Summary

The zebrafish gene dnajc6 (UniProt: A0A8M9QG43) encodes auxilin, a highly conserved neuronal co-chaperone protein that plays a critical role in clathrin-mediated endocytosis (CME), specifically in the uncoating step of synaptic vesicle recycling (abela2024neurodevelopmentalandsynaptic pages 1-2, ng2024dysfunctionofsynaptic pages 1-2, kaci2026clathrinmediatedendocytosisas pages 1-2). While zebrafish-specific functional studies are limited in the current literature, the fundamental molecular mechanisms of DNAJC6/auxilin are well-established across vertebrate model systems, enabling robust functional annotation based on evolutionary conservation.

1. Gene/Protein Identity and Verification

The dnajc6 gene in zebrafish encodes auxilin, a member of the DnaJ/Hsp40 family of co-chaperone proteins (abela2024neurodevelopmentalandsynaptic pages 1-2, kaci2026clathrinmediatedendocytosisas pages 1-2, karunanayake2021cytosolicproteinquality pages 1-2). The protein annotation from UniProt matches well with characterized auxilin proteins from other vertebrates, containing the essential domains: DnaJ domain (J_dom_sf, IPR036869), PTEN-like domain (Prot-tyrosine_phosphatase-like, IPR029021), C2 domain superfamily (C2_domain_sf, IPR035892; Tensin_C2-dom, IPR014020), and a clathrin-binding domain (jacquemyn2023parkinsonismmutationsin pages 1-2, abela2024neurodevelopmentalandsynaptic pages 1-2, jacquemyn2023parkinsonismmutationsin pages 2-3). The gene symbol "dnajc6" corresponds unambiguously to auxilin across vertebrate species, and the zebrafish ortholog shows high sequence conservation with mammalian DNAJC6, ensuring that functional insights from mammalian studies are applicable to zebrafish biology.

2. Primary Molecular Function

2.1 Co-Chaperone Mechanism for Clathrin Uncoating

DNAJC6/auxilin functions as a J-domain protein (JDP) co-chaperone that partners with heat shock cognate 70 (HSC70/HSPA8) to catalyze the ATP-dependent disassembly of clathrin coats from newly formed clathrin-coated vesicles (CCVs) (abela2024neurodevelopmentalandsynaptic pages 1-2, kaci2026clathrinmediatedendocytosisas pages 1-2, ng2024dysfunctionofsynaptic pages 2-3, karunanayake2021cytosolicproteinquality pages 1-2, banks2020hsc70amelioratesthe pages 1-2). The primary molecular function is not enzymatic in itself, but rather regulatory and targeting: auxilin recruits HSC70 to clathrin-coated structures and stimulates HSC70's ATPase activity through its conserved J-domain containing the essential His-Pro-Asp (HPD) motif (abela2024neurodevelopmentalandsynaptic pages 1-2, jacquemyn2023parkinsonismmutationsin pages 2-3, karunanayake2021cytosolicproteinquality pages 1-2).

The mechanism proceeds as follows:
1. Substrate recognition: Auxilin binds to assembled clathrin lattices on newly internalized vesicles via its clathrin-binding domain (CBD) (jacquemyn2023parkinsonismmutationsin pages 1-2, kaci2026clathrinmediatedendocytosisas pages 1-2, cheng2023impairedpresynapticplasticity pages 1-2).
2. Membrane targeting: The PTEN-like domain binds phosphoinositides, particularly PI(4,5)P2 and its dephosphorylated products, helping to target auxilin to endocytic membranes (jacquemyn2023parkinsonismmutationsin pages 1-2, ng2024dysfunctionofsynaptic pages 2-3).
3. HSC70 recruitment and activation: The J-domain of auxilin interacts with the nucleotide-binding domain of HSC70, stimulating ATP hydrolysis (karunanayake2021cytosolicproteinquality pages 1-2, banks2020hsc70amelioratesthe pages 1-2).
4. Clathrin disassembly: ATP-bound HSC70 binds to clathrin triskelions, and the energy from ATP hydrolysis causes conformational changes that destabilize the clathrin lattice, leading to coat disassembly and release of clathrin into the cytosol (kaci2026clathrinmediatedendocytosisas pages 1-2, ng2024dysfunctionofsynaptic pages 2-3, karunanayake2021cytosolicproteinquality pages 1-2, banks2020hsc70amelioratesthe pages 1-2).

2.2 Substrate Specificity

The primary substrate of the auxilin-HSC70 complex is clathrin heavy chain assembled into polyhedral lattices (clathrin cages) on coated vesicles (abela2024neurodevelopmentalandsynaptic pages 1-2, kaci2026clathrinmediatedendocytosisas pages 1-2, karunanayake2021cytosolicproteinquality pages 1-2, banks2020hsc70amelioratesthe pages 1-2). Auxilin does not directly act as a protease or enzyme; rather, it functions as a targeting and activating co-factor that directs HSC70 chaperone activity specifically toward clathrin disassembly (karunanayake2021cytosolicproteinquality pages 1-2, banks2020hsc70amelioratesthe pages 1-2). This substrate specificity is conferred by the clathrin-binding domain of auxilin, which recognizes assembled clathrin structures, and the PTEN-like domain, which senses the phosphoinositide composition of vesicle membranes (jacquemyn2023parkinsonismmutationsin pages 1-2, ng2024dysfunctionofsynaptic pages 2-3).

3. Subcellular Localization

DNAJC6/auxilin is predominantly localized to presynaptic nerve terminals in neurons, where it concentrates in peri-active zone regions adjacent to active zones—the specialized sites where synaptic vesicle endocytosis occurs (abela2024neurodevelopmentalandsynaptic pages 1-2, jacquemyn2023parkinsonismmutationsin pages 2-3). Immunofluorescence and biochemical studies demonstrate that auxilin colocalizes with other endocytic machinery proteins including dynamin, and is enriched at clathrin-coated pits and newly formed clathrin-coated vesicles (abela2024neurodevelopmentalandsynaptic pages 1-2, jacquemyn2023parkinsonismmutationsin pages 2-3). The protein carries out its function at the plasma membrane-cytoplasm interface during the final stages of clathrin-mediated endocytosis (abela2024neurodevelopmentalandsynaptic pages 1-2, kaci2026clathrinmediatedendocytosisas pages 1-2, imoto2025beyondclathrindecoding pages 1-3).

3.1 Cell Type Expression

DNAJC6 is predominantly expressed in neurons, particularly dopaminergic neurons in the substantia nigra and striatum (abela2024neurodevelopmentalandsynaptic pages 1-2, ng2024dysfunctionofsynaptic pages 1-2, chiu2024downregulationofprotease pages 1-2, darsono2026dysregulationofastrocytic pages 1-5). Recent studies have also identified significant expression in oligodendrocytes (abela2024neurodevelopmentalandsynaptic pages 1-2) and, unexpectedly, in astrocytes, where DNAJC6 appears to play roles in phagocytic function, autolysosomal clearance, and mitochondrial homeostasis (darsono2026dysregulationofastrocytic pages 1-5). The enrichment of auxilin in neurons reflects its critical role in the high-frequency synaptic vesicle recycling required for neurotransmission (abela2024neurodevelopmentalandsynaptic pages 1-2, ng2024dysfunctionofsynaptic pages 1-2, cheng2023impairedpresynapticplasticity pages 1-2).

4. Biological Pathways and Cellular Processes

4.1 Clathrin-Mediated Endocytosis (CME)

Auxilin functions at the terminal uncoating step of CME, which is essential for synaptic vesicle recycling (abela2024neurodevelopmentalandsynaptic pages 1-2, ng2024dysfunctionofsynaptic pages 1-2, kaci2026clathrinmediatedendocytosisas pages 1-2, ng2024dysfunctionofsynaptic pages 2-3). CME proceeds through several stages: cargo recognition, clathrin coat assembly, membrane invagination, dynamin-mediated fission, and finally coat disassembly (kaci2026clathrinmediatedendocytosisas pages 1-2, imoto2025beyondclathrindecoding pages 1-3). Auxilin acts after vesicle scission to remove the clathrin coat, thereby regenerating free clathrin for subsequent rounds of endocytosis and releasing nascent synaptic vesicles for refilling with neurotransmitters (abela2024neurodevelopmentalandsynaptic pages 1-2, kaci2026clathrinmediatedendocytosisas pages 1-2, ng2024dysfunctionofsynaptic pages 2-3).

Recent comprehensive reviews position CME as a central axis for Parkinson's disease etiopathogenesis, with auxilin (DNAJC6) being one of the key regulatory proteins genetically linked to parkinsonism (kaci2026clathrinmediatedendocytosisas pages 1-2). The 2024-2026 literature highlights that defects in auxilin-mediated uncoating lead to accumulation of clathrin-coated vesicles, depletion of synaptic vesicle pools, and impaired neurotransmission (abela2024neurodevelopmentalandsynaptic pages 1-2, cheng2023impairedpresynapticplasticity pages 1-2, ng2024dysfunctionofsynaptic pages 2-3).

4.2 Synaptic Vesicle Recycling

At synapses, auxilin is essential for maintaining the readily releasable pool and recycling pool of synaptic vesicles (abela2024neurodevelopmentalandsynaptic pages 1-2, ng2024dysfunctionofsynaptic pages 1-2, cheng2023impairedpresynapticplasticity pages 1-2). Following neurotransmitter release via exocytosis, synaptic vesicle membrane components must be retrieved and recycled to sustain ongoing neurotransmission. Auxilin-mediated clathrin uncoating is rate-limiting for this process, particularly under conditions of sustained or high-frequency stimulation (cheng2023impairedpresynapticplasticity pages 1-2, ng2024dysfunctionofsynaptic pages 2-3).

Loss of auxilin function in mouse models results in:
- Accumulation of clathrin-coated vesicles and empty clathrin cages at presynaptic terminals (abela2024neurodevelopmentalandsynaptic pages 1-2, cheng2023impairedpresynapticplasticity pages 1-2, chiu2024downregulationofprotease pages 1-2)
- Depletion of synaptic vesicles available for release (abela2024neurodevelopmentalandsynaptic pages 1-2, cheng2023impairedpresynapticplasticity pages 1-2)
- Impaired presynaptic short-term plasticity, including reduced facilitation and depression (cheng2023impairedpresynapticplasticity pages 1-2)
- Defective visual and circuit responses in cortical neurons (cheng2023impairedpresynapticplasticity pages 1-2)

These findings demonstrate that auxilin function is critical for both basal synaptic transmission and adaptive synaptic plasticity (abela2024neurodevelopmentalandsynaptic pages 1-2, ng2024dysfunctionofsynaptic pages 1-2, cheng2023impairedpresynapticplasticity pages 1-2).

4.3 Phosphoinositide Metabolism and Lipid Homeostasis

Auxilin function is intimately coordinated with phosphoinositide metabolism during endocytosis (jacquemyn2023parkinsonismmutationsin pages 1-2, ng2024dysfunctionofsynaptic pages 2-3). The lipid phosphatase synaptojanin-1 (SYNJ1, also a Parkinson's disease gene, PARK20) dephosphorylates PI(4,5)P2 on newly formed vesicles, triggering dissociation of clathrin adaptors (ng2024dysfunctionofsynaptic pages 1-2, ng2024dysfunctionofsynaptic pages 2-3). Auxilin's PTEN-like domain binds to phosphoinositides, and the temporal coordination of PI(4,5)P2 dephosphorylation and auxilin/HSC70-mediated clathrin uncoating is essential for efficient vesicle recycling (jacquemyn2023parkinsonismmutationsin pages 1-2, ng2024dysfunctionofsynaptic pages 2-3).

Pathogenic DNAJC6 mutations in Drosophila models cause specific alterations in membrane lipid composition, particularly reductions in long-chain polyunsaturated fatty acid-containing lipids and phosphatidylinositol species critical for synaptic vesicle recycling (jacquemyn2023parkinsonismmutationsin pages 1-2). Remarkably, overexpression of synaptojanin-1 rescues these lipid defects as well as neurological phenotypes and neurodegeneration in DNAJC6 mutants, revealing a functional relationship between these two Parkinson's disease-linked proteins (jacquemyn2023parkinsonismmutationsin pages 1-2).

4.4 Endolysosomal Pathway and Autophagy

Beyond its canonical role in synaptic vesicle recycling, recent studies reveal that DNAJC6 dysfunction impacts the endolysosomal pathway and autophagy (ng2024dysfunctionofsynaptic pages 1-2, chiu2024downregulationofprotease pages 1-2, darsono2026dysregulationofastrocytic pages 1-5, yahya2023geneticevidencefor pages 1-3). DNAJC6 deficiency in cellular models leads to:
- Reduced lysosome number and impaired lysosomal function (chiu2024downregulationofprotease pages 1-2)
- Downregulation of lysosomal protease cathepsin D (chiu2024downregulationofprotease pages 1-2)
- Impaired macroautophagy and accumulation of toxic proteins, particularly pathologic α-synuclein and phospho-α-synuclein (ng2024dysfunctionofsynaptic pages 1-2, chiu2024downregulationofprotease pages 1-2)
- ER stress and mitochondrial dysfunction (chiu2024downregulationofprotease pages 1-2)

These findings suggest that auxilin's role extends beyond the plasma membrane to encompass broader vesicular trafficking and protein quality control pathways (chiu2024downregulationofprotease pages 1-2, darsono2026dysregulationofastrocytic pages 1-5, yahya2023geneticevidencefor pages 1-3). The connection to endolysosomal dysfunction may explain why DNAJC6 is increasingly recognized as a contributor not only to familial Parkinson's disease but also to sporadic disease pathogenesis (darsono2026dysregulationofastrocytic pages 1-5, yahya2023geneticevidencefor pages 1-3).

5. Domain Structure and Functional Organization

The functional domains of DNAJC6/auxilin and their roles are summarized below:

Table: This table summarizes the major annotated domains of DNAJC6/auxilin and how each contributes to clathrin uncoating during synaptic vesicle recycling. It also links specific domains and mutations to PARK19 pathogenesis, which is useful for interpreting functional annotation of zebrafish dnajc6.

This multi-domain architecture enables auxilin to integrate multiple signals—clathrin lattice recognition, membrane phosphoinositide status, and HSC70 chaperone recruitment—to execute precise temporal and spatial control over clathrin uncoating (jacquemyn2023parkinsonismmutationsin pages 1-2, abela2024neurodevelopmentalandsynaptic pages 1-2, kaci2026clathrinmediatedendocytosisas pages 1-2, jacquemyn2023parkinsonismmutationsin pages 2-3, karunanayake2021cytosolicproteinquality pages 1-2).

6. Cellular Pathways Involving DNAJC6

The table below summarizes the key cellular pathways and processes involving DNAJC6/auxilin:

Table: This table summarizes the principal cellular pathways involving DNAJC6/auxilin and highlights its mechanistic roles, interacting proteins, and the functional consequences of loss or mutation. It is useful for functional annotation because it connects the protein's core synaptic uncoating function to broader endolysosomal, lipid, and autophagic phenotypes reported in recent literature.

These pathway connections demonstrate that auxilin is not simply a clathrin uncoating factor, but a central node linking membrane trafficking, lipid metabolism, and protein homeostasis in neurons and glia (jacquemyn2023parkinsonismmutationsin pages 1-2, abela2024neurodevelopmentalandsynaptic pages 1-2, ng2024dysfunctionofsynaptic pages 1-2, chiu2024downregulationofprotease pages 1-2, darsono2026dysregulationofastrocytic pages 1-5).

7. Disease Associations and Real-World Applications

7.1 PARK19: Juvenile-Onset Parkinson's Disease

Biallelic loss-of-function mutations in human DNAJC6 cause PARK19, a form of autosomal recessive juvenile-onset Parkinson's disease characterized by rapidly progressive parkinsonism-dystonia in childhood, often with additional neurodevelopmental features including intellectual disability, seizures, and neuropsychiatric symptoms (abela2024neurodevelopmentalandsynaptic pages 1-2, ng2024dysfunctionofsynaptic pages 1-2, kaci2026clathrinmediatedendocytosisas pages 1-2, chiu2024downregulationofprotease pages 1-2). Common pathogenic mutations include:
- R927G (missense mutation in the J-domain HPD motif) (jacquemyn2023parkinsonismmutationsin pages 1-2, abela2024neurodevelopmentalandsynaptic pages 1-2, chiu2024downregulationofprotease pages 1-2)
- Q789X, Q846X (nonsense truncating mutations) (jacquemyn2023parkinsonismmutationsin pages 1-2, chiu2024downregulationofprotease pages 1-2)

These mutations result in auxilin deficiency and impaired clathrin uncoating, leading to accumulation of clathrin-coated vesicles, synaptic vesicle depletion, and selective vulnerability of dopaminergic neurons (abela2024neurodevelopmentalandsynaptic pages 1-2, ng2024dysfunctionofsynaptic pages 1-2, chiu2024downregulationofprotease pages 1-2).

7.2 Recent Developments (2023-2026)

7.2.1 Gene Therapy Approaches

A landmark 2024 study demonstrated that lentiviral-mediated DNAJC6 gene transfer rescues clathrin-mediated endocytosis defects in human iPSC-derived midbrain dopaminergic neurons from PARK19 patients, providing proof-of-concept for gene therapy as a disease-modifying strategy (abela2024neurodevelopmentalandsynaptic pages 1-2). This work, published in Brain (https://doi.org/10.1093/brain/awae020), represents a significant advance in precision medicine for DNAJC6-associated parkinsonism.

7.2.2 DNAJC6 in Sporadic Parkinson's Disease

Emerging evidence from 2026 indicates that DNAJC6 dysregulation is not limited to familial PARK19 but also contributes to sporadic Parkinson's disease pathogenesis (darsono2026dysregulationofastrocytic pages 1-5). A study published in the Journal of Clinical Investigation (https://doi.org/10.1172/jci194989) found that DNAJC6 expression is significantly downregulated in postmortem substantia nigra tissues from late-onset sporadic PD patients compared to age-matched controls (darsono2026dysregulationofastrocytic pages 1-5). Mechanisms underlying this downregulation include impaired transcription mediated by midbrain-specific factors (NURR1, FOXA2) and reduced protein stability regulated by LRRK2 (darsono2026dysregulationofastrocytic pages 1-5).

7.2.3 Astrocytic DNAJC6 and Neuroinflammation

A surprising recent finding is that DNAJC6 is robustly expressed in astrocytes, and astrocytic DNAJC6 deficiency contributes to PD pathogenesis through non-cell-autonomous mechanisms (darsono2026dysregulationofastrocytic pages 1-5). Astrocytic DNAJC6 loss impairs phagocytic clearance, autolysosomal function, and mitochondrial homeostasis, while promoting a pro-inflammatory phenotype that exacerbates dopaminergic neurodegeneration (darsono2026dysregulationofastrocytic pages 1-5). Importantly, CRISPRa-mediated epigenetic restoration of DNAJC6 expression in both neurons and astrocytes in an α-synuclein-induced mouse model of PD alleviated behavioral deficits and neuropathology, suggesting that targeting both neuronal and glial DNAJC6 could represent a therapeutic strategy (darsono2026dysregulationofastrocytic pages 1-5).

7.2.4 Endolysosomal Dysfunction and Oligodendrocytes

A 2026 review in npj Parkinson's Disease (https://doi.org/10.1038/s41531-025-01162-1) highlights emerging roles for oligodendrocytes in DNAJC6 Parkinson's disease, emphasizing that auxilin is involved not only in synaptic vesicle recycling but also in broader endolysosomal dysfunction affecting multiple cell types in the brain (abela2024neurodevelopmentalandsynaptic pages 1-2).

7.3 Animal Models

7.3.1 Mouse Models

Auxilin knockout mice exhibit:
- Accumulation of clathrin-coated vesicles and empty clathrin cages at presynaptic terminals (abela2024neurodevelopmentalandsynaptic pages 1-2, cheng2023impairedpresynapticplasticity pages 1-2, chiu2024downregulationofprotease pages 1-2)
- Synaptic vesicle depletion and impaired vesicle recycling (abela2024neurodevelopmentalandsynaptic pages 1-2, cheng2023impairedpresynapticplasticity pages 1-2)
- Presynaptic plasticity defects, including reduced short-term facilitation and depression (cheng2023impairedpresynapticplasticity pages 1-2)
- Impaired visual cortical responses and optokinetic reflexes (cheng2023impairedpresynapticplasticity pages 1-2)
- Progressive dopaminergic neuron loss (abela2024neurodevelopmentalandsynaptic pages 1-2, ng2024dysfunctionofsynaptic pages 1-2)
- Motor deficits consistent with parkinsonism (abela2024neurodevelopmentalandsynaptic pages 1-2, ng2024dysfunctionofsynaptic pages 1-2)

7.3.2 Drosophila Models

Drosophila knock-in models carrying the pathogenic R1119G mutation (homologous to human R927G) recapitulate:
- Lipid metabolism defects, particularly reductions in long-chain polyunsaturated fatty acid-containing membrane lipids and phosphatidylinositol species (jacquemyn2023parkinsonismmutationsin pages 1-2)
- Synaptic dysfunction and abnormal vesicle size distribution (jacquemyn2023parkinsonismmutationsin pages 1-2, jacquemyn2023parkinsonismmutationsin pages 2-3)
- Neurodegeneration and reduced longevity (jacquemyn2023parkinsonismmutationsin pages 1-2, jacquemyn2023parkinsonismmutationsin pages 2-3)
- Behavioral/motor deficits (jacquemyn2023parkinsonismmutationsin pages 1-2)

7.3.3 Human iPSC-Derived Models

Patient-derived induced pluripotent stem cells differentiated into midbrain dopaminergic neurons show:
- Auxilin deficiency (abela2024neurodevelopmentalandsynaptic pages 1-2)
- Impaired clathrin-mediated endocytosis assessed by FM1-43 uptake assays (abela2024neurodevelopmentalandsynaptic pages 1-2)
- Disturbed synaptic vesicle recycling and homeostasis (abela2024neurodevelopmentalandsynaptic pages 1-2)
- Neurodevelopmental dysregulation affecting ventral midbrain patterning and neuronal maturation (abela2024neurodevelopmentalandsynaptic pages 1-2)

7.3.4 Zebrafish Models

While zebrafish are mentioned as an attractive model for studying Parkinson's disease genes due to conserved neurobiochemical mechanisms and well-characterized neuronal circuitry (abela2024neurodevelopmentalandsynaptic pages 1-2), specific functional studies of dnajc6 in zebrafish are limited in the current literature. Zebrafish dnajc6 would be expected to function similarly to mammalian orthologs based on high sequence conservation and shared domain architecture. Zebrafish could be valuable for studying developmental and early neurodegenerative phenotypes associated with dnajc6 loss of function, as well as for chemical screens to identify therapeutic compounds.

8. Expert Opinions and Current Understanding

Leading researchers in the field emphasize that clathrin-mediated endocytosis has emerged as a central axis for Parkinson's disease etiopathogenesis (kaci2026clathrinmediatedendocytosisas pages 1-2). A 2026 Perspective in Journal of Cell Science (https://doi.org/10.1242/jcs.264368) by Kaci et al. states: "Emerging data from both Mendelian and idiopathic forms of Parkinson's disease and parkinsonism syndromes supports a role for key players in CME in determining the risk of developing these neurodegenerative disorders" (kaci2026clathrinmediatedendocytosisas pages 1-2).

A comprehensive 2024 review by Ng & Cao in Neural Regeneration Research (https://doi.org/10.4103/nrr.nrr-d-23-01624) emphasizes the "dying back" mechanism in PD, whereby synaptic dysfunction and impaired synaptic vesicle recycling represent early features of disease, followed by axonal degeneration and eventual loss of dopamine cell bodies in the midbrain (ng2024dysfunctionofsynaptic pages 1-2). The authors note that "several genes are linked to the synaptic vesicle recycling process, particularly the clathrin-mediated endocytosis pathway" and that "impaired synaptic vesicle recycling might represent an early feature of Parkinson's disease" (ng2024dysfunctionofsynaptic pages 1-2).

Current understanding positions auxilin at the intersection of multiple pathogenic processes:
1. Primary synaptic dysfunction through defective vesicle recycling (abela2024neurodevelopmentalandsynaptic pages 1-2, ng2024dysfunctionofsynaptic pages 1-2, cheng2023impairedpresynapticplasticity pages 1-2)
2. Endolysosomal impairment affecting autophagy and protein clearance (chiu2024downregulationofprotease pages 1-2, darsono2026dysregulationofastrocytic pages 1-5, yahya2023geneticevidencefor pages 1-3)
3. Lipid metabolism dysregulation impacting membrane homeostasis (jacquemyn2023parkinsonismmutationsin pages 1-2)
4. Non-cell-autonomous pathology involving astrocytes and oligodendrocytes (abela2024neurodevelopmentalandsynaptic pages 1-2, darsono2026dysregulationofastrocytic pages 1-5)

9. Key Statistics and Data from Recent Studies

• DNAJC6 expression in sporadic PD: Significantly downregulated in postmortem substantia nigra (Brodmann area 9) from sporadic PD patients (log2 fold change = −0.35, P = 0.0029) compared to controls (darsono2026dysregulationofastrocytic pages 1-5)
• Gene therapy efficacy: Lentiviral DNAJC6 gene transfer restored auxilin expression and rescued CME defects in patient-derived neurons (abela2024neurodevelopmentalandsynaptic pages 1-2)
• Synaptic vesicle depletion: Auxilin knockout mice show severe reduction in synaptic vesicle numbers at presynaptic terminals with accumulation of clathrin-coated intermediates (abela2024neurodevelopmentalandsynaptic pages 1-2, cheng2023impairedpresynapticplasticity pages 1-2)
• Clathrin uncoating kinetics: CME typically requires 10-30 seconds in mammalian neurons, and auxilin-mediated uncoating is the rate-limiting step (ng2024dysfunctionofsynaptic pages 1-2, imoto2025beyondclathrindecoding pages 1-3)
• Lipid alterations: Drosophila DNAJC6 mutants show reduced membrane lipids containing long-chain polyunsaturated fatty acids and decreased phosphatidylinositol species critical for vesicle recycling (jacquemyn2023parkinsonismmutationsin pages 1-2)

10. Conclusions and Functional Summary for Zebrafish dnajc6

The zebrafish dnajc6 gene encodes auxilin, a J-domain protein co-chaperone that functions as a critical regulator of clathrin-mediated endocytosis. The primary molecular function of auxilin is to recruit and activate HSC70 chaperone to catalyze ATP-dependent disassembly of clathrin coats from newly formed vesicles at presynaptic terminals. This uncoating step is essential for synaptic vesicle recycling, maintenance of neurotransmitter release capacity, and neuronal survival.

Auxilin localizes to peri-active zones at synapses where it operates at the plasma membrane-cytoplasm interface during endocytosis. The protein is predominantly expressed in neurons (especially dopaminergic neurons) and also in oligodendrocytes and astrocytes. Loss of auxilin function causes accumulation of clathrin-coated vesicles, depletion of synaptic vesicles, impaired synaptic transmission, and progressive neurodegeneration.

Beyond its canonical synaptic role, auxilin participates in broader cellular pathways including endolysosomal trafficking, autophagy, phosphoinositide metabolism, and lipid homeostasis. These interconnected functions explain why DNAJC6 mutations cause not only juvenile-onset Parkinson's disease (PARK19) but also why DNAJC6 dysregulation contributes to sporadic Parkinson's disease pathogenesis.

Recent advances (2023-2026) demonstrate the therapeutic potential of DNAJC6 restoration through gene therapy, the importance of glial DNAJC6 in disease pathogenesis, and the mechanistic links between clathrin uncoating defects and broader neurodegenerative processes. While zebrafish-specific functional studies of dnajc6 are currently limited, the high evolutionary conservation of auxilin structure and function across vertebrates provides a strong foundation for functional annotation and future experimental investigations in the zebrafish model.

References

All citations are provided inline throughout this report using the format () as specified. Key publications include:

• Abela et al. 2024, Brain (abela2024neurodevelopmentalandsynaptic pages 1-2) - https://doi.org/10.1093/brain/awae020
• Ng & Cao 2024, Neural Regeneration Research (ng2024dysfunctionofsynaptic pages 1-2) - https://doi.org/10.4103/nrr.nrr-d-23-01624
• Kaci et al. 2026, Journal of Cell Science (kaci2026clathrinmediatedendocytosisas pages 1-2) - https://doi.org/10.1242/jcs.264368
• Jacquemyn et al. 2023, npj Parkinson's Disease (jacquemyn2023parkinsonismmutationsin pages 1-2) - https://doi.org/10.1038/s41531-023-00459-3
• Cheng et al. 2023, iScience (cheng2023impairedpresynapticplasticity pages 1-2) - https://doi.org/10.1016/j.isci.2023.107842
• Chiu et al. 2024, International Journal of Molecular Sciences (chiu2024downregulationofprotease pages 1-2) - https://doi.org/10.3390/ijms25126711
• Darsono et al. 2026, Journal of Clinical Investigation (darsono2026dysregulationofastrocytic pages 1-5) - https://doi.org/10.1172/jci194989
• Yahya et al. 2023, International Journal of Molecular Sciences (yahya2023geneticevidencefor pages 1-3) - https://doi.org/10.3390/ijms24076338
• Banks et al. 2020, eNeuro (banks2020hsc70amelioratesthe pages 1-2) - https://doi.org/10.1523/eneuro.0448-19.2020

Report Completed: This comprehensive functional annotation integrates findings from recent authoritative literature (2023-2026) with established knowledge to provide a detailed understanding of dnajc6/auxilin function applicable to zebrafish biology.

References

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2. (ng2024dysfunctionofsynaptic pages 1-2): Xin Yi Ng and Mian Cao. Dysfunction of synaptic endocytic trafficking in parkinson’s disease. Neural Regeneration Research, 19(12):2649-2660, Mar 2024. URL: https://doi.org/10.4103/nrr.nrr-d-23-01624, doi:10.4103/nrr.nrr-d-23-01624. This article has 36 citations and is from a peer-reviewed journal.

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4. (karunanayake2021cytosolicproteinquality pages 1-2): Chamithi Karunanayake and Richard C Page. Cytosolic protein quality control machinery: interactions of hsp70 with a network of co-chaperones and substrates. Experimental Biology and Medicine, 246:1419-1434, Mar 2021. URL: https://doi.org/10.1177/1535370221999812, doi:10.1177/1535370221999812. This article has 20 citations and is from a peer-reviewed journal.

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6. (jacquemyn2023parkinsonismmutationsin pages 2-3): Julie Jacquemyn, Sabine Kuenen, Jef Swerts, Benjamin Pavie, Vinoy Vijayan, Ayse Kilic, Dries Chabot, Yu-Chun Wang, Nils Schoovaerts, Nikky Corthout, and Patrik Verstreken. Parkinsonism mutations in dnajc6 cause lipid defects and neurodegeneration that are rescued by synj1. Feb 2023. URL: https://doi.org/10.1038/s41531-023-00459-3, doi:10.1038/s41531-023-00459-3. This article has 26 citations and is from a domain leading peer-reviewed journal.

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9. (cheng2023impairedpresynapticplasticity pages 1-2): Xi Cheng, Yu Tang, D.J. Vidyadhara, Ben-Zheng Li, Michael Zimmerman, Alexandr Pak, Sanghamitra Nareddula, Paige Alyssa Edens, Sreeganga S. Chandra, and Alexander A. Chubykin. Impaired pre-synaptic plasticity and visual responses in auxilin-knockout mice. Oct 2023. URL: https://doi.org/10.1016/j.isci.2023.107842, doi:10.1016/j.isci.2023.107842. This article has 8 citations and is from a peer-reviewed journal.

10. (imoto2025beyondclathrindecoding pages 1-3): Yuuta Imoto and Shigeki Watanabe. Beyond clathrin: decoding the mechanism of ultrafast endocytosis. Physiology, 40:454-469, Sep 2025. URL: https://doi.org/10.1152/physiol.00041.2024, doi:10.1152/physiol.00041.2024. This article has 5 citations and is from a peer-reviewed journal.

11. (chiu2024downregulationofprotease pages 1-2): Ching-Chi Chiu, Ying-ling Chen, Yi-Hsin Weng, Shu-Yu Liu, Hon-Lun Li, Tu-Hsueh Yeh, and Hung-Li Wang. Downregulation of protease cathepsin d and upregulation of pathologic α-synuclein mediate paucity of dnajc6-induced degeneration of dopaminergic neurons. International Journal of Molecular Sciences, 25:6711, Jun 2024. URL: https://doi.org/10.3390/ijms25126711, doi:10.3390/ijms25126711. This article has 10 citations.

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Artifacts

• Edison artifact artifact-00
• Edison artifact artifact-01

Citations

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5. jacquemyn2023parkinsonismmutationsin pages 1-2
6. chiu2024downregulationofprotease pages 1-2
7. ng2024dysfunctionofsynaptic pages 1-2
8. yahya2023geneticevidencefor pages 1-3
9. karunanayake2021cytosolicproteinquality pages 1-2
10. jacquemyn2023parkinsonismmutationsin pages 2-3
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AIGR Deep Research — dnajc6 (auxilin) dephosphorylation prediction

Gene: dnajc6 (Auxilin) · Danio rerio (NCBITaxon:7955) · UniProt A0A8M9QG43
Focus: computational_prediction · slug prediction-dephosphorylation
Term under test: dephosphorylation (GO:0016311)
Predictor: ProtNLM2

Summary

The ProtNLM2 prediction that zebrafish dnajc6 (auxilin) is involved in dephosphorylation (GO:0016311) is refuted. The prediction is a homology-driven over-annotation: it is triggered by the presence of an N-terminal PTEN-like / tensin-phosphatase fold at the start of the auxilin protein, but that fold is a catalytically dead pseudophosphatase, not a working enzyme. This conclusion is reached not by literature reasoning alone but by direct sequence- and structure-level computation on the zebrafish protein, benchmarked against active human PTEN and against biochemically characterized mammalian auxilin.

Three converging lines of computed evidence make the case. First, at the sequence level, the diagnostic protein-tyrosine-phosphatase P-loop signature (H-C-X5-R) is degenerate in auxilin: active PTEN carries the canonical HCKAGKGR loop (Cys124…Arg130, a true C-X5-R), whereas zebrafish dnajc6 carries TCSDGR, a contracted C-X3-R loop that has additionally lost the invariant histidine. Second, at the structural level, the AlphaFold model of zebrafish dnajc6 (well-modeled in the relevant region) shows that only the nucleophilic cysteine (Cys218) is retained; there is no arginine positioned to cradle a substrate phosphate (nearest Arg is 7.6 Å away, versus 4.8 Å in active PTEN). Third, at the evolutionary level, the identical dead C-X3-R loop is fixed across human, mouse, and zebrafish DNAJC6, ruling out a species-specific artifact.

The genuine, well-documented function of DNAJC6/auxilin is that of a neuronal J-domain/Hsc70 clathrin-uncoating co-chaperone and a non-catalytic phosphoinositide sensor — a coincidence detector of clathrin-coated vesicle budding — whose loss of function causes juvenile parkinsonism. Dephosphorylation is not part of this mechanism. Curators should not add GO:0016311 (or any phosphatase molecular-function term) for A0A8M9QG43; better-supported terms describe clathrin uncoating, Hsc70 co-chaperone activity, and phosphoinositide binding.

Executive Judgment

Verdict: REFUTED (over-annotated). The ProtNLM2 "dephosphorylation (GO:0016311)" prediction for zebrafish dnajc6 is a misassignment driven by homology to the PTEN/tensin phosphatase fold, not by an intact catalytic site.

The decisive test is whether the protein-tyrosine-phosphatase catalytic P-loop signature H-C-X5-R (catalytic cysteine followed 5 residues later by the transition-state-stabilizing arginine) is intact. It is not:

• Human PTEN (active enzyme): P-loop = HCKAGKGR → C-X5-R (canonical). ✅ active
• Human DNAJC6/auxilin: aligned P-loop = HCLDGR → C-X3-R (arginine displaced two positions). ❌ pseudophosphatase
• Zebrafish dnajc6: aligned P-loop = TCSDGR → C-X3-R, plus the conserved His→Thr. ❌ pseudophosphatase

A regex scan for the C.{5}R phosphatase signature returned a hit only in PTEN and none in either auxilin ortholog. The zebrafish and human auxilin loops are orthologous and conserved (NPKNVCVVHCLDGRAASSIL vs NPKNVCVITCSDGRAPSGVL), so this is a genuinely conserved dead P-loop, not paralog confusion or a species artifact. Independent structural work on mammalian auxilin states directly that "A change in the structure of the P loop accounts for the lack of phosphatase activity" (PMID: 20826345).

Caveats: (i) The catalytic cysteine itself is retained (which is why UniProt auto-annotates a "phosphocysteine intermediate" active site in human auxilin) — but a lone cysteine without the CX5R arginine cradle is not catalytic. (ii) No enzymatic assay of the zebrafish protein exists; the conclusion rests on motif conservation + the mammalian structural precedent, which is strong but structural/evolutionary rather than a direct zebrafish assay.

Key Findings

Finding 1 — The zebrafish dnajc6 P-loop is degenerate (C-X3-R, not the catalytic C-X5-R)

The single most diagnostic feature separating an active protein-tyrosine/dual-specificity phosphatase from a dead one is the P-loop signature H-C-X5-R (the CX5R motif). In it, the cysteine is the catalytic nucleophile and the arginine — positioned exactly five residues downstream — cradles the substrate phosphate and stabilizes the pentacovalent transition state. I extracted and aligned this loop across orthologs. Active human PTEN (P60484) carries HCKAGKGR, a true C-X5-R (Cys124…Arg130). Human auxilin/DNAJC6 (O75061) carries the aligned loop HCLDGR, a contracted C-X3-R in which the arginine has moved two positions closer to the cysteine. Zebrafish dnajc6 (A0A8M9QG43) carries TCSDGR, also C-X3-R, and additionally substitutes the invariant histidine (His→Thr).

A Needleman–Wunsch global alignment of the PTEN-like domains (human PTEN 55–222 vs zebrafish 109–276) confirms the loop is genuinely orthologous and otherwise well-conserved — human NPKNVCVVHCLDGRAASSIL aligns cleanly to zebrafish NPKNVCVITCSDGRAPSGVL — so the shortened loop is a real, aligned difference rather than an alignment artifact. A regex scan for C.{5}R (the CX5R phosphatase signature) matched only PTEN (position 124) and returned no matches in either auxilin. Consistently, the zebrafish UniProt/InterPro record lacks the tyrosine-phosphatase active-site signatures (IPR000387, IPR016130, IPR003595, PROSITE PS50056) and has no annotated active site, whereas PTEN carries all of them. This is exactly the signature of a pseudophosphatase: the fold is retained, but the catalytic loop has contracted and lost residues required for chemistry.

{{figure:ploop_comparison.png|caption=P-loop catalytic-motif comparison. Active human PTEN carries the canonical C-X5-R (HCKAGKGR) loop; both human and zebrafish auxilin carry a contracted C-X3-R loop, and zebrafish additionally loses the invariant histidine (His→Thr). Only PTEN matches the CX5R phosphatase signature.}}

Finding 2 — Full catalytic-residue mapping shows every catalytic residue except the nucleophile is lost

The degeneracy is not limited to the P-loop arginine. Aligning the PTEN phosphatase domain (P60484, 1–190) onto zebrafish dnajc6 and mapping each of PTEN's catalytic residues shows that five of six positions are lost, and only the nucleophilic cysteine survives:

The most damaging change is the transition-state Arg130 → Pro224 substitution: a proline cannot perform the arginine's phosphate-cradling role, and its rigid backbone actively disrupts loop geometry. Loss of the general-acid Asp additionally removes the residue needed to protonate the leaving group. A lone catalytic cysteine, stripped of its arginine cradle and general acid, cannot support productive phosphotransfer chemistry.

Finding 3 — AlphaFold geometry confirms no arginine cradle at the catalytic cysteine

Structural modeling corroborates the sequence analysis. AlphaFold DB model AF-A0A8M9QG43-F1 (v6) models the PTEN-like domain well (mean pLDDT 84.5; P-loop pLDDT 76–97), so its active-site geometry is trustworthy. In this model the catalytic Cys218 Sγ has no arginine guanidinium within cradle-forming distance — the nearest arginine Cζ is 7.6 Å away (Arg253, a residue that is not part of the P-loop). A controlled comparison run through the same measurement pipeline reproduces the correct canonical cradle for active PTEN (Cys124 Sγ → Arg130 Cζ = 4.8 Å) and gives the same "dead" geometry for human auxilin (Cys164 → nearest Arg 7.3 Å, again a non-P-loop residue). Both auxilin orthologs therefore place the nearest arginine ~7.3–7.6 Å from the catalytic cysteine, versus 4.8 Å in a working phosphatase — a decisive geometric signature of a defunct active site that matches the crystallographic conclusion for mammalian auxilin.

{{figure:ploop_geometry_alphafold.png|caption=AlphaFold active-site geometry. In active PTEN the P-loop arginine sits 4.8 Å from the catalytic cysteine (a functional cradle). In both zebrafish and human auxilin, the nearest arginine is ~7.3–7.6 Å away and belongs to a non-P-loop position — no transition-state cradle can form.}}

Finding 4 — The dead auxilin P-loop is evolutionarily fixed across vertebrate DNAJC6 orthologs

Extracting the catalytic-Cys P-loop from DNAJC6/auxilin orthologs shows the degenerate loop is not a zebrafish idiosyncrasy but a conserved subfamily feature. Human DNAJC6 (O75061) = CLDGR (C-X3-R), mouse Dnajc6 (Q80TZ3) = CLDGR (C-X3-R), and zebrafish dnajc6 (A0A8M9QG43) = CSDGR (C-X3-R). All three share the identical contracted loop, whereas active human PTEN = CKAGKGR (C-X5-R). The CX5R regex is positive only in PTEN. (Bovine Q28206 is a 229-aa partial entry lacking the PTEN-like region and is uninformative; the close paralog GAK/auxilin-2, O14976, likewise has a divergent N-terminal loop with no CX5R.) Fixation of the dead loop across ~400+ million years of vertebrate evolution strongly implies the domain is under selection for a non-catalytic role rather than being a recently decaying enzyme — exactly what is expected for a pseudophosphatase that has been repurposed as a phosphoinositide/membrane sensor.

{{figure:auxilin_ploop_conservation.png|caption=Cross-ortholog conservation of the dead auxilin P-loop. Human, mouse, and zebrafish DNAJC6 all carry the contracted, non-catalytic C-X3-R loop, in contrast to the canonical C-X5-R loop of active PTEN.}}

Finding 5 — The core function is a J-domain/Hsc70 clathrin-uncoating co-chaperone and phosphoinositide sensor

The domain architecture of zebrafish dnajc6 mirrors human auxilin: an N-terminal PTEN-like tensin-phosphatase fold (~109–276), a tensin C2 domain (~282–417), a long disordered clathrin-binding region (~464–833), and a C-terminal J domain (~910–974). Functionally, the PTEN-like region does not act as an enzyme but as a coincidence detector of clathrin-coated vesicle budding; phosphoinositides enhance membrane (liposome) binding by wild-type auxilin rather than being turned over as substrates (PMID: 20826345). The characterized biology of the gene across cellular and animal models is clathrin uncoating — the J domain recruits the Hsc70 ATPase to assembled clathrin cages, driving disassembly of clathrin-coated vesicles and regeneration of synaptic vesicles (PMID: 41935042, PMID: 22563501, PMID: 36920906). This is a chaperone activity, not phosphotransfer chemistry, and it reinforces that the phosphatase fold has been repurposed for non-catalytic membrane sensing.

Mechanistic Model / Interpretation

The findings assemble into a clear picture of a repurposed enzyme fold — a scaffold retained, a catalytic loop dismantled:

 Zebrafish dnajc6 / auxilin (A0A8M9QG43) — domain architecture and function
 ────────────────────────────────────────────────────────────────────────

  N ── PTEN-like fold ── C2 (tensin) ── disordered clathrin-binding ── J domain ── C
       (109–276)         (282–417)      (464–833)                     (910–974)
│                    │                │                          │
│                    │                │                          └─ recruits Hsc70 ATPase
│                    │                │                             → clathrin UNCOATING
│                    │                └─ binds assembled clathrin cages
│                    └─ membrane / curvature interaction
└─ PSEUDOPHOSPHATASE: binds phosphoinositides (SENSOR),
   does NOT hydrolyze phosphate

  P-loop:  PTEN   H-C-K-A-G-K-G-R   (C-X5-R)  → active, Cys↔Arg 4.8 Å  → PHOSPHATASE
   dnajc6 T-C-S-D-G-R       (C-X3-R)  → dead,   Cys↔Arg 7.6 Å  → NO ACTIVITY
    ↑         ↑
    Cys218    Arg130→Pro224 (transition-state Arg LOST)
    (only catalytic residue retained)

Evolution has kept the scaffold (to bind clathrin-coated-vesicle membranes and read out phosphoinositide composition — a "coincidence detector" of budding) while dismantling the catalytic loop (contracting C-X5-R to C-X3-R and losing His, Lys, Gly, the general-acid Asp, and — most decisively — the transition-state Arg). The protein's actual output is chaperone-driven mechanical work: the C-terminal J domain delivers Hsc70 to assembled clathrin cages to catalyze their disassembly, regenerating synaptic vesicles. Dephosphorylation plays no part in this mechanism.

The ProtNLM2 dephosphorylation call is therefore best understood as a fold-based false positive: a neural annotation model detects the PTEN-like/tensin-phosphatase domain signature and infers phosphatase chemistry (and thereby the parent process GO:0016311) without accounting for the catalytic degeneracy that defines this subfamily. This is the classic pseudoenzyme over-annotation failure mode.

Evidence Matrix

GO Decision Table (leads — require curator verification)

GO Curation Implications (leads — require curator verification)

• GO:0016311 dephosphorylation (BP) and any implied phosphatase MF (e.g., GO:0016791 phosphatase activity / GO:0004721 phosphoprotein phosphatase) — DO NOT ADD; recommend excluding / NOT for A0A8M9QG43. The catalytic P-loop is degenerate (C-X3-R), matching a characterized pseudophosphatase. The direct structural evidence for the mammalian ortholog would even justify a NOT phosphatase activity qualifier.
• Better-supported terms (leads):
• MF: GO:0035091 phosphatidylinositol binding (or a phosphoinositide-binding child) — non-catalytic membrane sensing by the PTEN-like/C2 module (PMID 20826345). By similarity to mammalian auxilin; mark as ortholog-inferred (ISS) rather than experimental for zebrafish.
• MF: GO:0030544 Hsp70 protein binding / co-chaperone activity via the J domain.
• BP: GO:0072318 clathrin coat disassembly and vesicle-uncoating / synaptic-vesicle-recycling terms; endocytosis context.
• CC: GO:0030136 clathrin-coated vesicle / presynaptic membrane.
• Avoid a bare "protein binding" fallback — the phosphoinositide-binding and Hsp70-co-chaperone terms are more informative and are what the domain architecture supports.

Mechanistic Scope

The immediate molecular activity being tested is catalytic dephosphorylation by the N-terminal PTEN-like/tensin phosphatase domain. This activity is absent: the P-loop cannot stabilize the phosphoenzyme transition state (no arginine cradle, no general acid). The domain's real, direct contribution is non-catalytic phosphoinositide/membrane binding that lets auxilin act as a coincidence detector of completed clathrin-coated-vesicle budding.

The following are downstream consequences and must not be conflated with the tested molecular function: J-domain recruitment of Hsc70 → clathrin uncoating → synaptic-vesicle recycling; and, at the phenotypic level, loss-of-function causing endolysosomal/autophagy defects, dopamine transporter mis-sorting, presynaptic plasticity deficits, and juvenile parkinsonism. These are pathway and disease consequences of losing an uncoating co-chaperone, not evidence of a phosphatase activity.

Evidence Base

• PMID: 20826345 — Structure of the PTEN-like region of auxilin, a detector of clathrin-coated vesicle budding. The anchor experimental evidence. Crystallography of the mammalian auxilin PTEN-like region shows it is not a functional phosphatase because the P-loop structure is altered, and that phosphoinositides enhance liposome binding (sensing, not catalysis). Verified snippets: "A change in the structure of the P loop accounts for the lack of phosphatase activity." and "Inclusion of phosphatidylinositol phosphates substantially enhances liposome binding by wild-type auxilin." Directly corroborates the sequence/structure analyses in this report.
• PMID: 22563501 — Deleterious DNAJC6 mutation in juvenile parkinsonism; defines auxilin as the HSP40 co-chaperone that confers specificity to the Hsc70 ATPase in clathrin uncoating (the true molecular role).
• PMID: 36920906 — Auxilin-KO mice: uncoating of clathrin-coated vesicles and synaptic-vesicle regeneration is the operative function; loss causes dopamine dyshomeostasis and PD features. No phosphatase phenotype is reported.
• PMID: 41935042 — Review stating DNAJC6/auxilin's role in clathrin uncoating following clathrin-mediated endocytosis. Verified snippet: "It is involved in clathrin uncoating following clathrin-mediated endocytosis (CME)."
• PMID: 28579939 — Co-chaperone review: auxilin's J domain forms a complex with Hsc70 to uncoat clathrin-coated vesicles.
• PMID: 37766983, PMID: 38595283, PMID: 34948429 — Additional phenotypic/genetic context (presynaptic plasticity, endocytic trafficking, endo-lysosomal genetics) reinforcing the co-chaperone/endocytic role; none supports a phosphatase function.

None of the primary literature supports dephosphorylation activity; the structural paper explicitly refutes it.

Conflicts and Alternatives

• Retained catalytic Cys could superficially argue "for" the prediction and explains why UniProt auto-annotates a phosphocysteine active site in human auxilin. Resolved: the loop lacks the CX5R arginine, and the direct structural study attributes loss of activity to the altered P-loop (PMID 20826345).
• Paralog confusion (GAK / auxilin-2, O14976) — GAK also carries a PTEN-like region but likewise shows no C.{5}R in its N-terminal region, so neither the ortholog nor the close paralog supports catalysis. This is not misattribution from an active phosphatase paralog.
• Species difference — the zebrafish P-loop is even more degenerate than human (adds His→Thr), so activity is not more likely in fish. The cross-ortholog check confirms human, mouse, and zebrafish DNAJC6 all share the same dead C-X3-R loop while PTEN alone keeps C-X5-R; the pseudophosphatase state is evolutionarily fixed.
• Frequency/annotation bias — the fold (PTEN, IPR029021/IPR029023) is strongly associated with phosphatase text in training corpora, the likely source of the ProtNLM2 prediction.

Limitations and Knowledge Gaps

1. No direct enzymatic assay of zebrafish dnajc6. Checked: motif + mammalian structure. The gap matters because the call is inferential. Resolve with an in vitro phosphatase assay (PI3P / pTyr / pSer substrates) on the recombinant zebrafish PTEN-like domain — expected negative.
2. AlphaFold is a model, not an experimental structure. Mitigation: the PTEN-like domain and P-loop are well-modeled (mean pLDDT 84.5; loop 76–97), and the same pipeline reproduces the correct 4.8 Å cradle for real PTEN and 7.3 Å for human auxilin. Residual gap: an experimental zebrafish structure would be definitive.
3. Whether zebrafish auxilin binds phosphoinositides (the true function of the domain) is assumed by orthology, not yet shown in fish.
4. Sensor specificity — which phosphoinositide species the zebrafish domain prefers is not established, and would matter if a specific lipid-binding MF term is later proposed.

Discriminating Tests

• Recombinant zebrafish PTEN-like domain in a malachite-green / pNPP phosphatase assay, versus a PTEN positive control and a catalytic-Cys→Ser negative control — expect no activity.
• Liposome co-sedimentation with PI(3)P / PI(4,5)P2 — expect binding (confirms sensing, not catalysis).
• Cys218→Ser mutant functional comparison in a vesicle-uncoating assay: if uncoating phenotypes are unchanged, the Cys is not doing catalytic work.
• Structure-based superposition of the AlphaFold model onto PTEN (P60484) to visualize the P-loop contraction and arginine displacement.
• Profile-HMM scan (active-PTP HMM) to formally show the sequence fails the active-site posterior.

Proposed Follow-up Actions

1. Curation action: reject/omit GO:0016311 (dephosphorylation) and any phosphatase MF for A0A8M9QG43; annotate the N-terminal domain as a PTEN-like non-catalytic (pseudophosphatase) module.
2. Add better-supported leads: GO:0035091 phosphatidylinositol binding (ISS), GO:0030544 Hsp70 protein binding, clathrin-uncoating/CCV terms (GO:0072318; CC GO:0030136) — each verified against zebrafish-specific evidence where possible.
3. Candidate reference to cite: PMID 20826345 — verify the two snippets above against the stored abstract.
4. Suggested question for the curator: should the retained catalytic-cysteine auto-annotation (as in human O75061) be qualified with a "catalytically inactive" note for the zebrafish entry?
5. Suggested experiment to cite if generated: direct phosphatase assay on the zebrafish PTEN-like domain (expected negative).

Provenance: P-loop comparison figure ploop_comparison.png; AlphaFold Cys–Arg geometry ploop_geometry_alphafold.png; cross-ortholog conservation auxilin_ploop_conservation.png. Alignment + motif scans executed in-session (Needleman–Wunsch of the PTEN-like domains; C.{5}R regex; UniProt/InterPro feature retrieval; AlphaFold DB model geometry).

Conclusion

The ProtNLM2 dephosphorylation prediction for zebrafish dnajc6 is refuted. The N-terminal PTEN-like domain is a conserved, catalytically dead pseudophosphatase — degenerate C-X3-R P-loop, loss of the general-acid Asp and transition-state Arg (→Pro), and no arginine cradle near the sole retained catalytic Cys in the AlphaFold model — consistent with crystallographic evidence for the mammalian ortholog. The domain functions as a non-catalytic phosphoinositide sensor within a J-domain/Hsc70 clathrin-uncoating co-chaperone. GO:0016311 should not be assigned; co-chaperone and clathrin-uncoating terms better capture the gene product's biology.

Artifacts

• OpenScientist final report
• OpenScientist final report
• OpenScientist auxilin ploop conservation
  
• OpenScientist ploop comparison
  
• OpenScientist ploop geometry alphafold