P3R3URF (PIK3R3 upstream open reading frame protein; UniProt A0A087WWA1, 95 aa; HGNC:53451) is a testis-enriched microprotein encoded by an upstream open reading frame located on the PIK3R3 locus. The 95-aa product has MANE/CCDS transcript support and peptide-detection repository cross-references, but no P3R3URF-specific biochemical activity, cellular localization, binding partner, or pathway role has been established. The locus has been hypothesized to give rise to a P3R3URF-PIK3R3 readthrough/fusion product, but the only directly relevant experimental study (Tidball et al. 2023, bioRxiv) failed to detect P3R3URF or P3R3URF-PIK3R3 transcripts by qRT-PCR in human iNeurons and instead favored a post-translational modification of canonical PIK3R3 as the explanation for a higher-molecular-weight immunoblot band.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0019221 cytokine-mediated signaling pathway | IBA GO_REF:0000033 | REMOVE | Summary: PANTHER IBA annotation to cytokine-mediated signaling pathway, propagated from PANTHER node PTN008494619 and mouse donor MGI:MGI:2385459. The same node supports cytokine-signaling annotations on human SOCS4 and SOCS5; no direct evidence places the 95-aa P3R3URF product in that SH2/SOCS-box cytokine-receptor branch. Reason: P3R3URF is an upstream-ORF microprotein with protein-detection support but no demonstrated localization, binding partner, perturbation phenotype, or signaling activity. The cytokine-signaling IBA is asserted on the SOCS4/SOCS5 branch of PTHR10155 rather than on the p85/p55 PI3K regulatory subunit branch; a 95-aa arginine/proline-rich peptide with no SH2 domain and no SOCS box cannot plausibly be placed in a clade whose cytokine role is defined by SH2-mediated receptor engagement. Pathway conclusions from the only relevant experimental study should be attributed to PIK3R3, not P3R3URF. Propagation Review Root cause: PROPAGATION BAD Failure modes: ROLE CONFLATION Sources checked: MGI:MGI:2385459 · MGI:MGI:2385459 SUPPORTS SOURCE BUT NOT TARGET The mouse experimental donor supports cytokine signaling for its native branch; the P3R3URF defect is propagation across the wrong node boundary, not weak source evidence. PANTHER:PTN008494619 · PTN008494619 SUPPORTS SOURCE BUT NOT TARGET PTN008494619 propagates GO:0019221 to human SOCS4 and SOCS5, whereas the actual p85/p55 PI3K regulatory-subunit node PTN008302071 carries GO:0005943, GO:0046935, and GO:0008286 but not GO:0019221. The 95-aa P3R3URF uORF microprotein lacks the SH2 domain and SOCS box that define the cytokine-receptor role of the SOCS clade. Supporting Evidence: UniProt:A0A087WWA1 GO; GO:0019221; P:cytokine-mediated signaling pathway; IBA:GO_Central. file:human/P3R3URF/P3R3URF-deep-research-falcon.md Tidball et al. report a higher-molecular-weight band on a PIK3R3 immunoblot that "fit with a known fusion readthrough protein from an open reading frame upstream of PIK3R3 called P3R3URF." ... the same study performed qRT-PCR assays intended to detect PIK3R3, P3R3URF-PIK3R3, and P3R3URF transcripts and reports that they only found detectable transcript for PIK3R3, leading them to interpret the second band as "possibly a post-translational modification." file:human/P3R3URF/P3R3URF-deep-research-falcon.md No pathway can be attributed to P3R3URF specifically with current evidence. ... because P3R3URF transcripts were not detected, these results should be interpreted as PIK3R3 biology rather than P3R3URF biology. file:human/P3R3URF/P3R3URF-hypotheses/kgap-p3r3urf-microprotein-expression-function/openscientist.md Only GO annotation is IBA phylogenetic (GO_REF:0000033, PANTHER PTN008494619); NCBI = "Predicted" |
| GO:0003674 molecular_function | ND | NEW | Summary: Root molecular_function term reflects total absence of experimentally defined molecular activity for P3R3URF. The Falcon and OpenScientist reviews find no P3R3URF-specific biochemical activity, localization, interactor, perturbation, or pathway evidence in the accessible literature. Evidence type ND (No biological Data available) per PR #775 review feedback — this is a curator-proposed root-term annotation for unknown function, not a computationally-inferred IEA. Reason: Molecular function genuinely unknown; no experimentally defined activity for this putative microprotein. Supporting Evidence: file:human/P3R3URF/P3R3URF-deep-research-falcon.md Primary function: Unknown in the accessible corpus. The only P3R3URF-relevant study investigated whether a PIK3R3 immunoblot band could represent a P3R3URF readthrough product but did not detect P3R3URF transcripts and did not establish any P3R3URF-specific activity. file:human/P3R3URF/P3R3URF-deep-research-falcon.md the study does not provide: Direct demonstration of an expressed P3R3URF peptide/protein (e.g., mass spectrometry identification of a P3R3URF-unique peptide). ... Cellular localization (cytoplasm, nucleus, membranes, organelles) for a P3R3URF product. ... A biochemical function (enzyme activity, binding partners) specifically attributable to P3R3URF. file:human/P3R3URF/P3R3URF-hypotheses/kgap-p3r3urf-microprotein-expression-function/openscientist.md P3R3URF is an established independently functional microprotein | **Not supported (no direct evidence)** | No interactors, localization, loss-of-function, or conservation-of-function data |
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Download this section (compressed HTML)Q: Is the P3R3URF ORF translated into a stable endogenous peptide, as judged by targeted mass spectrometry with P3R3URF-unique peptides and by ribosome profiling that controls for multi-mapping reads?
Q: Which germ-cell stage or other cell condition translates the putative 95-aa P3R3URF microprotein, and what is its stoichiometry relative to any P3R3URF-PIK3R3 readthrough product?
Q: If a P3R3URF peptide exists, does it act independently or only as the N-terminal extension of a P3R3URF-PIK3R3 readthrough fusion, and what is its subcellular localization?
Q: Does the P3R3URF uORF regulate translation or stability of canonical PIK3R3 mRNA, independent of producing a peptide product?
Experiment: Targeted Ribo-seq plus translation initiation site profiling at the PIK3R3 locus using a pipeline such as Rp3 that controls for multi-mapping short reads, in tissues with high PIK3R3 expression (e.g. brain, lymphoid tissue).
Experiment: Immunoprecipitation/mass spectrometry using antibodies or epitope tags specific to the P3R3URF N-terminal sequence (distinct from PIK3R3) to directly identify P3R3URF-unique tryptic peptides with at least two PSMs at <1% FDR.
Experiment: CRISPR base editing or precise deletion of the P3R3URF start codon and/or stop codon without altering the PIK3R3 coding sequence, followed by quantitative measurement of PIK3R3 mRNA and protein levels and downstream PI3K/AKT/mTOR signaling, to distinguish uORF-mediated translational regulation from microprotein production.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: P3R3URF has no established molecular function or pathway role despite protein-level evidence for synthesis of its 95-aa microprotein.
OPEN BIOLOGY MF_DARK
What is known: The UniProt PE1 and proteomics-repository evidence support protein detection, not biochemical activity, localization, a direct interactor set, or cytokine-signaling participation. The sole GOA pathway annotation is a PANTHER IBA transfer from the SOCS4/SOCS5 cytokine-signaling node rather than a direct P3R3URF assay.
Significance: Resolving this gap would distinguish a testis-enriched functional microprotein from detectable translation without a selected independent role, and would avoid transferring SOCS-clade cytokine-signaling functions onto a 95-aa uORF product with no SH2 domain or SOCS box.
What would resolve it: Use P3R3URF-ORF-specific ribosome profiling, targeted mass spectrometry, endogenous tagging/interactomics, and start-codon or frameshift edits that spare the PIK3R3 coding sequence.
Provenance (the field's own admissions):
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