LNPK encodes lunapark, an N-myristoylated double-pass endoplasmic-reticulum membrane protein that concentrates at three-way junctions of the peripheral ER tubular network. Its cytosolic coiled-coil and zinc-finger-containing regions support self-association and junction targeting, allowing LNPK to stabilize newly formed junctions and preserve the balance between ER tubules, junctions, and sheets. Mitotic phosphorylation reduces LNPK self-association and junction targeting. Loss of LNPK disrupts ER architecture in human cells and causes a recessive neurodevelopmental disorder characterized by severe developmental impairment and corpus-callosum hypoplasia. LNPK-marked ER junctions also organize a context-dependent site of secretome-mRNA translation near lysosomes.
Definition: A specialized region of the endoplasmic reticulum tubular-network membrane at which three endoplasmic reticulum tubules meet.
Justification: LNPK is repeatedly observed at ER three-way junctions, but GO:0098826 captures only the broader tubular-network membrane and cannot distinguish the defining junctional subcompartment.
Parent term: endoplasmic reticulum tubular network membrane
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0071786 endoplasmic reticulum tubular network organization | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference assigns the conserved Lunapark family role in organizing the endoplasmic reticulum tubular network. This agrees with direct mammalian LNPK loss-of-function and rescue experiments. Reason: This is the core cellular process of LNPK. Human U2OS knockout reduces tubules and three-way junctions in favor of sheets, and low-level LNPK re-expression restores the reticular network (PMID:27619977). Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN000499233 SUPPORTS TRANSFER SGD:S000001235 SUPPORTS TRANSFER UniProtKB:Q6DFJ8 SUPPORTS TRANSFER UniProtKB:Q9C0E8 SUPPORTS TRANSFER Supporting Evidence: PMID:27619977 Lnp is not essential for network formation, but its inactivation or absence causes the loss of three-way junctions and tubules in favor of sheets. |
| GO:0071782 endoplasmic reticulum tubular network | IBA GO_REF:0000033 | ACCEPT | Summary: IBA places LNPK in the endoplasmic reticulum tubular network, the compartment in which Lunapark concentrates at three-way tubular junctions. Reason: This is a core active compartment. Direct mammalian imaging localizes LNPK preferentially to three-way junctions and links that localization to its effect on ER morphology (PMID:27619977). Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN000499233 SUPPORTS TRANSFER SGD:S000001235 SUPPORTS TRANSFER UniProtKB:Q9C0E8 SUPPORTS TRANSFER Supporting Evidence: PMID:27619977 Lnp-mCherry localized preferentially to punctae at three-way junctions |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic localization to the endoplasmic reticulum membrane derives from the reviewed UniProt subcellular-location vocabulary and is independently supported by direct topology and imaging experiments. Reason: LNPK is a double-spanning integral ER membrane protein; the mapped location is correct and core (PMID:24223779). Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB-SubCell:SL-0097 SUPPORTS TRANSFER Supporting Evidence: PMID:24223779 protein Lunapark, the human ortholog of yeast protein Lnp1p that has recently been found to be involved in network formation of the endoplasmic reticulum (ER), is an N-myristoylated polytopic integral membrane protein. |
| GO:0071786 endoplasmic reticulum tubular network organization | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro maps the conserved Lunapark family to endoplasmic reticulum tubular network organization. Direct mammalian experiments support the same process. Reason: The family mapping is specific to the established core role of LNPK in maintaining ER tubules and three-way junctions; there is no propagation mismatch. Propagation Review Root cause: NO FAILURE CORE Sources checked: InterPro:IPR040115 SUPPORTS TRANSFER Supporting Evidence: PMID:27619977 Cells lacking Lnp exhibited a proliferation of peripheral sheets and a reduction of tubules and junctions |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | MARK AS OVER ANNOTATED | Summary: A proteome-scale affinity-purification screen reports association of LNPK with synaptoporin (SYNPR/Q8TBG9). Reason: Generic protein binding does not describe LNPK's molecular role. A single context-dependent interaction from a large-scale interactome does not establish a stable complex or a core mechanistic partner. |
| GO:0005515 protein binding | IPI Q9C0E8-2 PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | Summary: A high-throughput human binary-interactome screen reports eight partners for LNPK isoform 2. The isoform and complete partner union are retained in the machine-sourced fields. Reason: The generic protein-binding term is uninformative and the screened interactions do not by themselves establish physiological partners, a stable complex, or LNPK's core molecular function. |
| GO:0005783 endoplasmic reticulum | IDA GO_REF:0000052 | ACCEPT | Summary: Human Protein Atlas immunofluorescence localizes LNPK to the endoplasmic reticulum. Reason: Although broader than the ER tubular network membrane annotations, this experimentally observed parent location is correct and consistent with multiple direct LNPK studies. |
| GO:0005789 endoplasmic reticulum membrane | EXP PMID:22729086 ER network formation requires a balance of the dynamin-like ... | ACCEPT | Summary: This study primarily analyzes yeast Lnp1p but also expresses a human LNPK cDNA in COS-7 cells and observes localization at mammalian ER tubular junctions. Reason: The paper contains direct human-protein localization evidence, so the ER membrane annotation is valid and should not be dismissed as a yeast-only transfer. Supporting Evidence: PMID:22729086 Lnp1p binds to the reticulons and Yop1p and resides at ER tubule junctions in both yeast and mammalian cells. |
| GO:0005789 endoplasmic reticulum membrane | EXP PMID:25404289 A model for the generation and interconversion of ER morphol... | ACCEPT | Summary: Lunapark is experimentally localized within the peripheral ER while the paper develops a model in which it stabilizes negatively curved edge lines at tubular junctions. Reason: ER membrane localization is integral to the observed junctional curvature-stabilizing role and is concordant with independent human topology and imaging evidence. Supporting Evidence: PMID:25404289 Lunapark localization to three-way junctions, which we show are actually small triangular sheets with concave edges, suggests that it stabilizes the negative curvature of edge lines. |
| GO:0005789 endoplasmic reticulum membrane | EXP PMID:25548161 Lunapark stabilizes nascent three-way junctions in the endop... | ACCEPT | Summary: Live-cell imaging shows mammalian Lunapark on ER three-way junctions and relates its presence to junction stability. Reason: ER membrane localization is central to the reported behavior. The cached record is abstract-only and does not expose exact construct provenance, but the annotation is fully concordant with the established human protein location; defer to the experimental curator. Supporting Evidence: PMID:25548161 In mammalian cells, only approximately half the junctions contain Lnp1. |
| GO:0005789 endoplasmic reticulum membrane | EXP PMID:27619977 Cooperation of the ER-shaping proteins atlastin, lunapark, a... | ACCEPT | Summary: Human U2OS imaging and knockout/rescue experiments place LNPK in the ER membrane at three-way tubular junctions. Reason: This is direct and strong evidence for a core cellular location of LNPK. Supporting Evidence: PMID:27619977 Lnp-mCherry localized preferentially to punctae at three-way junctions |
| GO:0007029 endoplasmic reticulum organization | IMP PMID:30032983 Mutations in LNPK, Encoding the Endoplasmic Reticulum Juncti... | ACCEPT | Summary: Biallelic LNPK loss-of-function in affected individuals eliminates full-length Lunapark and produces aberrant ER structures with increased luminal mass density. Reason: The human cellular phenotype directly supports a role in ER organization. The broader term is appropriate because the patient-cell assay reports global ER structural abnormalities rather than only a quantified tubular-network feature. Supporting Evidence: PMID:30032983 Affected cells showed the absence of full-length lunapark, aberrant ER structures, and increased luminal mass density. |
| GO:0042802 identical protein binding | IDA PMID:27619977 Cooperation of the ER-shaping proteins atlastin, lunapark, a... | ACCEPT | Summary: LNPK self-association is mediated by its zinc-finger-containing region and is required for correct junction localization; phosphorylation weakens oligomerization during mitosis. Reason: Unlike generic protein binding, identical protein binding records a mechanistically important property of LNPK that is linked to its core junction-stabilizing function. Supporting Evidence: PMID:27619977 the region containing the Zn2+-finger is required for Lnp-Lnp interaction |
| GO:0071788 endoplasmic reticulum tubular network maintenance | IMP PMID:27619977 Cooperation of the ER-shaping proteins atlastin, lunapark, a... | ACCEPT | Summary: LNPK knockout in human U2OS cells reduces peripheral tubules and three-way junctions in favor of sheets, while low-level re-expression restores the network. Reason: These loss-of-function and rescue experiments directly establish tubular-network maintenance as a core LNPK process. Supporting Evidence: PMID:27619977 Cells lacking Lnp exhibited a proliferation of peripheral sheets and a reduction of tubules and junctions |
| GO:0098826 endoplasmic reticulum tubular network membrane | IDA PMID:27619977 Cooperation of the ER-shaping proteins atlastin, lunapark, a... | ACCEPT | Summary: Direct imaging places LNPK at puncta corresponding to three-way junctions in the ER tubular network membrane. Reason: The specific membrane subcompartment is a core LNPK location and is more informative than the parent ER and ER membrane terms. Supporting Evidence: PMID:27619977 Lnp-mCherry localized preferentially to punctae at three-way junctions |
| GO:0071788 endoplasmic reticulum tubular network maintenance | IMP PMID:25548161 Lunapark stabilizes nascent three-way junctions in the endop... | ACCEPT | Summary: Live-cell imaging shows that nascent mammalian ER junctions acquiring Lunapark persist, whereas those that fail to acquire it rapidly undergo ring closure. Reason: The observations directly support ER tubular-network maintenance. The cached record is abstract-only, so exact construct provenance cannot be independently recovered here, but the annotation is biologically concordant and should be retained. Supporting Evidence: PMID:25548161 Newly formed junctions that acquire mLnp1 remain stable within the ER network, whereas nascent junctions that fail to acquire mLnp1 undergo rapid ring closure. |
| GO:0098826 endoplasmic reticulum tubular network membrane | IDA PMID:25548161 Lunapark stabilizes nascent three-way junctions in the endop... | ACCEPT | Summary: Mammalian Lunapark is imaged at ER three-way junctions, the defining sites within the tubular network membrane. Reason: This specific location is consistent with the live-cell stabilization phenotype and with direct human imaging in other studies. The abstract-only cache does not expose exact construct provenance, so the experimental curator's assignment is retained. Supporting Evidence: PMID:25548161 In mammalian cells, only approximately half the junctions contain Lnp1. |
| GO:1903373 positive regulation of endoplasmic reticulum tubular network organization | IMP PMID:25404289 A model for the generation and interconversion of ER morphol... | MODIFY | Summary: Modeling and overexpression experiments support Lunapark as a direct curvature-stabilizing component of ER three-way junctions. Reason: LNPK executes junction stabilization rather than acting only as an upstream positive regulator. The direct structural role is better represented as maintenance of the ER tubular network. Proposed replacements: endoplasmic reticulum tubular network maintenance Supporting Evidence: PMID:25404289 In a tubular ER network, lunapark stabilizes three-way junctions, i.e., small triangular sheets with concave edges. |
| GO:0005789 endoplasmic reticulum membrane | IDA PMID:24223779 Protein N-myristoylation plays a critical role in the endopl... | ACCEPT | Summary: Biochemical topology experiments establish human LNPK as an N-myristoylated double-spanning integral ER membrane protein. Reason: The study directly tests human LNPK membrane integration, topology, and intracellular localization, providing strong evidence for this core location. Supporting Evidence: PMID:24223779 protein Lunapark, the human ortholog of yeast protein Lnp1p that has recently been found to be involved in network formation of the endoplasmic reticulum (ER), is an N-myristoylated polytopic integral membrane protein. |
| GO:0098826 endoplasmic reticulum tubular network membrane | IDA PMID:24223779 Protein N-myristoylation plays a critical role in the endopl... | ACCEPT | Summary: Overexpressed human LNPK localizes mainly to peripheral ER and induces large polygonal tubular structures. Reason: The direct imaging supports location in the ER tubular network membrane, consistent with later physiological junction-localization studies. Supporting Evidence: PMID:24223779 overexpressed protein Lunapark localized mainly to the peripheral ER and induced the formation of large polygonal tubular structures. |
| GO:1903373 positive regulation of endoplasmic reticulum tubular network organization | IMP PMID:24223779 Protein N-myristoylation plays a critical role in the endopl... | MODIFY | Summary: Human LNPK overexpression changes peripheral ER morphology, while loss of N-myristoylation or disruption of the zinc-finger motif inhibits that effect. Reason: The experiment shows direct participation in ER tubular-network morphology, not merely an upstream positive regulatory effect. The general organization term is appropriate because the overexpression assay does not specifically measure physiological maintenance. Proposed replacements: endoplasmic reticulum tubular network organization Supporting Evidence: PMID:24223779 Morphological changes in the ER induced by overexpressed protein Lunapark were significantly inhibited by the inhibition of protein N-myristoylation by means of replacing Gly2 with Ala. |
| GO:0098826 endoplasmic reticulum tubular network membrane | IDA PMID:25404289 A model for the generation and interconversion of ER morphol... | ACCEPT | Summary: Lunapark localizes at the negatively curved edges of ER three-way junctions and stabilizes those junctional membrane structures. Reason: The experimental observations underlying the model support the specific ER tubular-network membrane location and agree with independent human imaging. Supporting Evidence: PMID:25404289 Lunapark localization to three-way junctions, which we show are actually small triangular sheets with concave edges, suggests that it stabilizes the negative curvature of edge lines. |
| GO:0016020 membrane | NAS PMID:12732147 A global control region defines a chromosomal regulatory lan... | MODIFY | Summary: The NAS source is a mouse HoxD regulatory-landscape and expression study; it does not establish LNPK protein membrane localization. Independent human studies establish the more precise ER membrane location. Reason: The broad membrane location is biologically correct but insufficiently specific and miscited to PMID:12732147. Replace it with endoplasmic reticulum membrane, supported directly by human topology and imaging experiments. Proposed replacements: endoplasmic reticulum membrane |
| GO:0060173 limb development | NAS PMID:12732147 A global control region defines a chromosomal regulatory lan... | MARK AS OVER ANNOTATED | Summary: Mouse Lnp shares limb expression domains with neighboring Hoxd genes and Evx2 in a regulatory landscape study. Reason: Spatial expression similarity does not establish that LNPK executes limb development, and no causal limb phenotype is reported. This is not part of the core ER-junction function. Supporting Evidence: PMID:12732147 upstream from the cluster and found Lunapark (Lnp), a gene which shares limb and CNS expression specificities with both Hoxd genes and Evx2, another gene located nearby. |
| GO:0022038 corpus callosum development | IMP PMID:30032983 Mutations in LNPK, Encoding the Endoplasmic Reticulum Juncti... | NEW | Summary: Biallelic human LNPK loss-of-function is associated with corpus-callosum hypoplasia, supporting a developmental requirement downstream of LNPK's ER-shaping activity. Reason: All three affected individuals had corpus-callosum hypoplasia, and the authors explicitly implicate lunapark in establishing this structure. This is a clinically important downstream role rather than LNPK's core cell-biological function. Supporting Evidence: PMID:30032983 Together, our results implicate the ER junction stabilizer lunapark in establishing the corpus callosum. |
| GO:0045948 positive regulation of translational initiation | IMP PMID:41193816 Secretome translation shaped by lysosomes and lunapark-marke... | NEW | Summary: Human-cell LNPK depletion selectively reduces eIF2-dependent initiation and translation efficiency of secretome mRNAs at lysosome-proximal ER junctions. Reason: Knockout and depletion experiments in human U-2 OS cells show that LNPK loss raises the phospho-eIF2Ξ±/total-eIF2Ξ± ratio and that a non-canonical ISR-like pathway acts downstream of LNPK; ISRIB rescues the initiation defect. The regulatory effect is therefore signalling-mediated rather than evidence that LNPK directly participates in the initiation machinery, and remains a secondary junction-associated role. Supporting Evidence: PMID:41193816 Lunapark depletion reduced ribosome density and translation efficiency of secretome mRNAs near lysosomes, an effect that was dependent on eIF2-mediated initiation and was reversed by the integrated stress response inhibitor ISRIB. PMID:41193816 Together, these findings suggest that a non-canonical ISR-like pathway modulates secretome mRNA translation downstream of LNPK. |
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Download this section (compressed HTML)Q: Does endogenous human LNPK possess intrinsic ubiquitin-protein ligase activity, and if so, what are its physiological substrates and consequences at ER junctions?
Q: Is the secretome-mRNA initiation phenotype a direct function of LNPK or an indirect consequence of altered ER-junction architecture and lysosome positioning?
Q: Which neural cell type and LNPK-dependent cellular process make corpus-callosum development especially sensitive to loss of this broadly expressed ER-shaping protein?
Q: Do LNPK isoforms 2, 3, and 4 have endogenous expression, membrane topology, and junction-stabilizing functions distinct from the canonical product?
Experiment: Reconstitute ubiquitination with rigorously purified human LNPK, defined E1/E2 and ubiquitin components, and catalytic or N-terminal mutants; combine orthogonal product detection with endogenous substrate-trapping and knockout/rescue experiments.
Hypothesis: LNPK has intrinsic ubiquitin-protein ligase activity that modifies defined ER-junction substrates independently of copurifying E3 enzymes.
Type: purified-protein ubiquitination and cellular substrate validation
Experiment: Rescue LNPK-null human cells at endogenous expression with myristoylation-, coiled-coil-, zinc-finger-, and oligomerization-defective variants, then quantify ER topology, lysosome-junction proximity, spatial ribosome occupancy, and ISRIB-sensitive initiation in the same cells.
Hypothesis: LNPK supports secretome-mRNA initiation through a junction-specific property that can be separated from its global effect on ER morphology.
Type: separation-of-function rescue and spatial translation profiling
Experiment: Use isogenic LNPK-null human neural organoids with lineage-restricted wild-type and separation-of-function rescue, measuring ER ultrastructure, axon extension, glial differentiation, and secretome translation across relevant neural cell types.
Hypothesis: Corpus-callosum phenotypes arise from a lineage-restricted requirement for LNPK-mediated ER-junction maintenance during axon growth or glial support.
Type: neural-organoid lineage-rescue study
Experiment: Introduce isoform-specific endogenous tags and perform topology mapping, quantitative junction localization, and knockout-rescue assays for all four products at matched expression levels.
Hypothesis: Alternative LNPK products differ in membrane insertion and junction-stabilizing capacity because their N-terminal architectures are altered.
Type: isoform-resolved topology and functional rescue
What is not known β curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: Whether LNPK itself has physiologically relevant ubiquitin-protein ligase activity, and any endogenous substrate of that proposed activity, remain unresolved.
OPEN BIOLOGYCURATION MF_DARK
What is known: A single study reports in-vitro ubiquitin-ligase activity in mammalian-cell-purified LNPK and attributes activity both to an associated ligase and to LNPK itself. The cached record is abstract-only, the activity lacks independent replication, and it is not adopted in the current reviewed UniProt entry or GO annotation set.
Significance: Resolving this question would determine whether LNPK has a second catalytic molecular function or acts solely as an architectural ER-junction protein.
What would resolve it: Purified-protein reconstitution after excluding copurifying E3 enzymes, catalytic-site mutagenesis, endogenous ubiquitination assays, and substrate identification should test intrinsic activity and its cellular relevance.
Provenance (the field's own admissions):
Gap: The causal steps connecting LNPK-mediated ER-junction stabilization to selective secretome-mRNA initiation and to corpus-callosum development are not known.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: Human-cell loss experiments establish a spatially restricted translation-initiation phenotype at lysosome-proximal LNPK-marked ER junctions, while human biallelic loss establishes a requirement for normal ER structure and corpus-callosum development. Neither result shows that translation control or neural development is a second constitutive molecular activity of LNPK.
Significance: Defining these connections would explain how a broadly expressed membrane-architecture factor produces selective translational and neurodevelopmental outcomes.
What would resolve it: Separation-of-function rescue alleles should be tested jointly for ER topology, junction-proximal translation initiation, and neural-lineage phenotypes to establish whether the downstream effects require junction stabilization itself.
Provenance (the field's own admissions):
Gap: GO lacks a cellular-component term for the ER three-way junction membrane, so LNPK's defining active subcompartment can currently be represented only by the broader ER tubular-network membrane term.
OPEN ONTOLOGY CC_DARK
What is known: Multiple imaging studies place LNPK preferentially at three-way junctions, and the existing GO:0098826 term correctly captures the surrounding tubular-network membrane but not the junctional specialization.
Significance: A dedicated term would distinguish junction-resident architectural factors from proteins distributed throughout the ER tubular-network membrane.
What would resolve it: Add the proposed endoplasmic reticulum three-way junction membrane cellular-component term as a child of GO:0098826.
Provenance (the field's own admissions):
Proposed term (ontology gap):
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