P3R3URF (PIK3R3 upstream open reading frame protein; UniProt A0A087WWA1, 95 aa; HGNC:53451) is a putative microprotein encoded by an upstream open reading frame located on the PIK3R3 locus. It 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. No P3R3URF-specific biochemical activity, cellular localization, binding partner, or pathway role has been established. Functional annotation should therefore not extrapolate from canonical PIK3R3 (p55gamma) PI3K/AKT/mTOR biology to P3R3URF.
| 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 family PTN008494619. This pathway role is plausible only by analogy to canonical PIK3R3 (p55gamma), which is a PI3K regulatory subunit acting downstream of cytokine receptors. There is no direct experimental evidence that P3R3URF itself participates in cytokine signaling; the deep-research review found no detectable P3R3URF transcripts in the one relevant experimental study and no demonstrated biochemical or signaling activity for the uORF product. Reason: P3R3URF is a putative upstream-ORF microprotein with no demonstrated protein product, localization, or signaling activity. The IBA propagation appears to conflate P3R3URF with canonical PIK3R3 biology. Pathway conclusions from the only relevant experimental study should be attributed to PIK3R3, not P3R3URF. 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. |
| GO:0003674 molecular_function | ND | NEW | Summary: Root molecular_function term reflects total absence of experimentally defined molecular activity for P3R3URF. The falcon deep-research confirms that no peptide-level mass spectrometry identification, no Ribo-seq translation evidence with proper handling of multi-mapping reads, and no biochemical activity assays specifically attributable to P3R3URF exist 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. |
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Download this section (compressed HTML)Q: Is the P3R3URF ORF translated into a stable peptide in any human tissue or cell condition, as judged by ribosome profiling with proper multi-mapping handling and by mass spectrometry with stringent FDR control?
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.
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