LPAL2 is a transcribed human pseudogene in the apolipoprotein(a)/plasminogen gene cluster. Its annotated transcripts contain truncated open reading frames and are candidates for nonsense-mediated decay. An early liver transcript defined a hypothetical 132-amino-acid product with a signal peptide and one kringle domain, represented by UniProt Q16609, but the splice-donor defect, frameshift, and premature stops underlying that model were identified at the RNA level; no stable endogenous LPAL2 protein has been demonstrated, and UniProt assigns protein-existence level 5. LPAL2 transcripts have been studied as pseudogene-derived long noncoding RNAs. In hepatocellular carcinoma models, LPAL2 RNA is reduced in tumors, its depletion promotes growth, migration, invasion, stem-like phenotypes, and doxorubicin resistance, and depletion increases MMP9 expression; the direct RNA mechanism remains unresolved. The single-kringle homology does not establish lipoprotein(a) particle assembly, lipid transport, plasminogen regulation, protease activity, or any other LPA- or plasminogen-derived protein function.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005576 extracellular region | IEA GO_REF:0000044 | MARK AS OVER ANNOTATED | Summary: Marked as over-annotated. A predicted signal peptide would route the historical 132-residue ORF to the secretory pathway if translated, but no stable endogenous LPAL2 protein has been demonstrated. Reason: The SL-0243 mapping faithfully propagates UniProt's predicted "Secreted" statement, and PMID:7749817 derives a secretion signal from a transcript ORF. However, current NCBI/HGNC classification treats LPAL2 as a pseudogene whose transcript ORFs are truncated and candidates for nonsense-mediated decay; PMID:7749817 reports a splice-donor defect, exon skipping, a frameshift, and premature stops rather than endogenous protein detection. UniProt assigns Q16609 protein-existence level 5 and cautions that it may be a pseudogene product. Thus extracellular localization is conditionally plausible for an experimentally expressed ORF but overstates the evidence for the endogenous gene product. It remains unsuitable as a core function and does not imply any LPA particle, plasminogen, or protease biology. Supporting Evidence: PMID:7749817 This results in "exon skipping" during maturation of the mRNA, causing a coding frame shift and the presence of premature stop codons. file:human/LPAL2/LPAL2-uniprot.txt Could be the product of a pseudogene. |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | MARK AS OVER ANNOTATED | Summary: Marked as over-annotated: the source is a proteome-scale binary-interaction map and the normalized row records VAC14 as the partner, but generic protein binding is uninformative and physiological relevance is not established. Reason: The IPI annotation is retained with curator deference to the experimental screen, but it should not be interpreted as a core activity. LPAL2 is a short, putatively secreted single-kringle precursor, whereas VAC14 is an intracellular phosphoinositide-regulatory scaffold; the cached paper does not discuss LPAL2 or validate this pair in a physiological compartment. A high-throughput binary contact neither supplies a more informative molecular-function term nor supports transfer of apo(a)/plasminogen biology. Supporting Evidence: PMID:25416956 Here, we describe a systematic map of ?14,000 high-quality human binary protein-protein interactions. |
| GO:0005515 protein binding | IPI PMID:32814053 Interactome Mapping Provides a Network of Neurodegenerative ... | MARK AS OVER ANNOTATED | Summary: Marked as over-annotated: five high-throughput candidate interactions are collapsed into this normalized protein-binding row, but none establishes a defining molecular activity of LPAL2. Reason: Live QuickGO records HIP1, CLSTN1 isoform 2, CCK, CASP6, and RAN as the five WITH/FROM partners. The source is a systematic yeast two-hybrid interactome focused on neurodegenerative-disease proteins, and the cached abstract does not discuss LPAL2 or provide pair-specific orthogonal validation. Several partners are intracellular and therefore topologically difficult to reconcile with a signal-peptide-bearing putative secreted product; CCK may share an extracellular route, but the screen alone does not establish a physiological interaction. Curator deference argues against removing the IPI result, while the generic GO:0005515 term and pooled partner set should remain non-core and must not be used to infer LPA-like function. Supporting Evidence: PMID:32814053 connects ∼5,000 human proteins via ∼30,000 candidate interactions and is generated by systematic yeast two-hybrid interaction screening |
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Download this section (compressed HTML)Q: Is any LPAL2 transcript translated into a stable endogenous protein, or is Q16609 solely a conceptual product of the historical 132-residue ORF?
Q: Which LPAL2 transcript isoform and RNA structural elements are necessary and sufficient for the hepatocellular-carcinoma phenotypes?
Q: Does LPAL2 RNA regulate MMP9 directly at chromatin or through an RNA-binding protein, microRNA, transcript-stability, or signaling intermediate?
Experiment: Perform transcript-resolved ribosome profiling and targeted parallel-reaction monitoring for peptides unique to the predicted LPAL2 ORF across liver and hepatocyte models, before and after NMD inhibition. Confirm any signal with an independently validated endogenous antibody and CRISPR disruption of the ORF without altering the candidate lncRNA promoter.
Hypothesis: Endogenous LPAL2 transcripts are not translated into a stable Q16609-like protein.
Type: endogenous translation and proteomics assay
Experiment: Resolve LPAL2 transcripts by long-read RNA sequencing, selectively deplete each isoform with junction-specific reagents, and rescue with matched wild-type and start-codon-disabled RNA constructs expressed at endogenous abundance. Measure growth, migration, invasion, MMP9 expression, and NMD sensitivity.
Hypothesis: A specific noncoding LPAL2 transcript, rather than translation of its short ORF, mediates the observed tumor-suppressive phenotype.
Type: isoform-resolved RNA loss-of-function and rescue
Experiment: Map endogenous LPAL2 RNA-protein contacts by orthogonal RNA pulldown and crosslinking methods and map RNA-chromatin occupancy at the MMP9 locus. Validate candidate partners with reciprocal binding, targeted depletion, and rescue, and measure nascent MMP9 transcription rather than steady-state RNA alone.
Hypothesis: LPAL2 RNA represses MMP9 transcription through a specific RNA-associated protein or chromatin interaction.
Type: lncRNA interactome and chromatin-mechanism analysis
Experiment: Combine LPAL2 depletion with MMP9 knockout or catalytic inhibition in multiple hepatoma lines, primary HCC organoids, and xenografts. Rescue LPAL2 with the validated noncoding transcript and test whether MMP9 epistasis separates invasion from proliferation, stemness, and doxorubicin-resistance phenotypes.
Hypothesis: MMP9 induction is a causal mediator of the growth and invasion phenotypes observed after LPAL2 depletion.
Type: genetic epistasis in HCC models
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: It is unknown whether any LPAL2 transcript produces a stable endogenous polypeptide corresponding to Q16609 or another open reading frame.
OPEN BIOLOGY MF_DARK
What is known: Liver transcription is established, and one historical transcript contains a predicted 132-residue signal-peptide-plus-kringle ORF. The same transcript study documents exon skipping, a frameshift, and premature stop codons; current transcript models are truncated and NMD candidates, and UniProt rates Q16609 protein existence as uncertain.
Significance: Product-existence evidence is prerequisite to assigning any protein molecular function, localization, or interaction to LPAL2.
What would resolve it: Demonstrate endogenous translation and a stable product with transcript- resolved ribosome profiling, targeted mass spectrometry using unique peptides, and an independently validated endogenous antibody, including NMD and pseudogene-negative controls.
Provenance (the field's own admissions):
Gap: The direct molecular mechanism by which LPAL2 RNA alters MMP9 transcription and hepatocellular-carcinoma phenotypes is unknown.
OPEN BIOLOGY MF_DARK
What is known: LPAL2 depletion increases MMP9 RNA and protein and promotes growth, migration, invasion, sphere formation, and doxorubicin resistance in hepatoma models, but no RNA-binding protein, RNA target, chromatin site, or causal intermediate has been established.
Significance: Without a direct RNA mechanism, the observed phenotypes cannot be represented as a precise core molecular activity or distinguished confidently from indirect perturbation effects.
What would resolve it: Combine endogenous LPAL2 perturbation and RNA-only rescue with RNA-protein and RNA-chromatin interaction mapping, then test candidate intermediates and MMP9 promoter effects by epistasis.
Provenance (the field's own admissions):
Gap: The transcript isoform and structural RNA elements responsible for the reported lncRNA activity have not been resolved.
OPEN BIOLOGY
What is known: LPAL2 has multiple truncated transcript variants, while the functional study perturbs LPAL2 RNA and attributes the phenotype to a pseudogene-derived lncRNA without establishing which transcript form or sequence element is sufficient.
Significance: Isoform resolution is necessary to separate a reproducible RNA function from NMD-linked transcript abundance and from the historical Q16609 ORF model.
What would resolve it: Define full-length LPAL2 transcripts by long-read RNA sequencing and map NMD sensitivity, then perform isoform-specific loss-of-function and rescue with nontranslatable RNA constructs.
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