An uncharacterized protein of the UPF0711 family containing a DUF4674 domain. Originally identified as upregulated by Hepatitis B virus X protein (hence alias XTP13), this 220 amino acid protein is ubiquitously expressed and conserved in vertebrates. Despite appearing in hundreds of CRISPR screens affecting cell fitness and having numerous potential protein interactions, its molecular function was long unknown. A strong but not-yet-verified lead (an OpenScientist Foldseek run plus recent 2025-2026 literature) identifies C18orf21 as RMP24, a subunit of the human RNase MRP ribonucleoprotein complex, assigning the DUF4674 fold to the RPP21 RNase P/MRP-subunit superfamily; this remains to be confirmed against the primary literature before the annotations are updated.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005634 nucleus | IEA | NEW | Summary: Nuclear localization predicted based on subcellular distribution studies but requires experimental validation Reason: C18orf21 shows predicted nuclear localization based on computational analyses and subcellular distribution studies, potentially concentrating in nucleolar substructures. While this is an uncharacterized protein, the nuclear localization prediction is more specific than generic cellular component annotations and may reflect its actual subcellular distribution. However, this annotation should be considered preliminary pending experimental validation. Supporting Evidence: file:human/C18orf21/C18orf21-deep-research.md Immunohistochemistry data from the Human Protein Atlas indicate that C18orf21 is present in both the **cytoplasm and nucleus** of cells in multiple tissues file:human/C18orf21/C18orf21-deep-research.md Notably, antibody-based staining showed enrichment in the **nucleolus** of certain cell types |
| GO:0005737 cytoplasm | IEA | NEW | Summary: Cytoplasmic localization predicted based on subcellular distribution studies but requires experimental validation Reason: C18orf21 shows predicted cytoplasmic localization based on computational analyses and subcellular distribution studies, indicating a dual nuclear-cytoplasmic distribution pattern. While this is an uncharacterized protein with unknown function, the cytoplasmic localization prediction is supported by subcellular distribution analyses. This annotation should be considered preliminary pending experimental validation of the actual subcellular localization. Supporting Evidence: file:human/C18orf21/C18orf21-deep-research.md Immunohistochemistry data from the Human Protein Atlas indicate that C18orf21 is present in both the **cytoplasm and nucleus** of cells in multiple tissues |
| GO:0003674 molecular_function | NAS | NEW | Summary: Historically unknown (DUF4674/UPF0711 orphan); a strong but not-yet-verified lead now points to RNase MRP subunit RMP24. Reason: STRONG LEAD requiring verification, not yet asserted as a confirmed annotation: an OpenScientist run assigned the DUF4674 fold by Foldseek to the RPP21 RNase P/MRP-subunit superfamily, and recent (2025-2026) literature identifies C18orf21 as RMP24, a subunit of the human RNase MRP ribonucleoprotein complex. The primary papers (PMID:40867056, 39974906, 41888142, 41136609) are not yet in the local cache and have not been independently verified here; once confirmed, the MF/CC annotations should be updated (e.g. ribonuclease MRP complex, GO:0000172). Until then the root molecular_function term is retained. Supporting Evidence: file:human/C18orf21/C18orf21-deep-research.md BioGRID reports ~180 candidate interactors of C18orf21 identified by affinity purification-mass spectrometry experiments file:human/C18orf21/C18orf21-deep-research.md Immunohistochemistry data from the Human Protein Atlas indicate that C18orf21 is present in both the cytoplasm and nucleus of cells in multiple tissues file:human/C18orf21/C18orf21-bioinformatics/RESULTS.md Isoelectric Point (pI): 10.27 ... Predicted Disordered Regions: Residues 22-199 (disorder-promoting residue enriched) file:human/C18orf21/C18orf21-hypotheses/duf4674-foldseek-only/openscientist.md (Ribonuclease MRP protein subunit p24), a constitutive and specific subunit of the human RNase MRP ribonucleoprotein complex file:human/C18orf21/C18orf21-hypotheses/duf4674-foldseek-only/openscientist.md RPP21 orthologues from three species appear as the closest non-self structural homologs |
Loading supporting content…
Download this section (compressed HTML)Q: What is the molecular function of C18orf21 and how does it contribute to cellular processes?
Q: How is C18orf21 expression regulated and what determines its tissue-specific distribution?
Q: What are the protein domains and structural features that define C18orf21 function?
Q: How does C18orf21 interact with other cellular proteins and what pathways does it regulate?
Experiment: Proteomics approaches using affinity purification and mass spectrometry to identify C18orf21 interacting partners
Experiment: Structural characterization using X-ray crystallography or NMR to determine the three-dimensional structure of C18orf21
Experiment: Functional genomics screens using CRISPR knockout to identify cellular processes dependent on C18orf21
Experiment: Subcellular localization studies using fluorescent protein fusions to determine C18orf21 cellular distribution
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The strongest current functional lead identifies C18orf21 as an RNase MRP-specific subunit, but this assignment still needs peer-reviewed consolidation before the review should replace the historical unknown-function framing with definitive GO molecular-function and complex annotations.
NARROWING BIOLOGYCURATION MF_DARK
What is known: The review records C18orf21 as an intracellular UPF0711/DUF4674 protein with unknown molecular function, while noting a strong RNase MRP/RMP24 lead. It does not yet assert RNase MRP complex membership as the settled core function.
Significance: Resolving this gap would determine whether C18orf21 should be annotated to RNase MRP complex membership and pre-rRNA processing/ribosome biogenesis, replacing broad root-function annotations with specific curated terms.
What would resolve it: Peer-reviewed publication or independently reproduced evidence should be checked against the primary data, followed by a focused update to molecular-function, complex, and biological-process annotations.
Provenance (the field's own admissions):
Gap: The precise biochemical contribution of C18orf21 within RNase MRP remains incompletely resolved.
OPEN BIOLOGYCURATION MF_DARK
What is known: Current evidence supports a non-catalytic, MRP-specific RNP subunit model. The unresolved part is how C18orf21 contacts RMRP RNA and other protein subunits to control RNA stability, assembly, cleavage specificity, and pre-rRNA processing.
Significance: Mechanistic resolution would determine whether the most informative GO representation is complex membership alone, an RNA-binding/RNP-stabilizing molecular function, or a more specific contribution to RNase MRP-mediated pre-rRNA cleavage.
What would resolve it: Structural studies of the metazoan RNase MRP complex, mutational rescue of predicted C18orf21-RMRP and C18orf21-RPP29 contacts, and direct cleavage assays should distinguish assembly, RNA-stabilization, and catalytic-site positioning roles.
Provenance (the field's own admissions):
Gap: The cellular and disease contexts in which C18orf21/RNase MRP activity is limiting remain unclear.
OPEN BIOLOGYCURATION BP_DARK
What is known: The review can support intracellular/nuclear localization and a strong nucleolar RNase MRP lead, but it does not yet connect C18orf21 directly to HBV response, human disease, or broader cell-fitness phenotypes beyond ribosome-biogenesis hypotheses.
Significance: Resolving this gap would prevent disease, viral-response, or generic essentiality annotations from being inferred from aliases, screens, or expression data, while preserving a route to specific process annotations if direct mechanisms are demonstrated.
What would resolve it: Endogenous localization, cell-type-specific depletion/rescue, viral perturbation assays, and disease-genetics follow-up should test whether C18orf21 has contexts beyond core RNase MRP-dependent ribosome biogenesis.
Provenance (the field's own admissions):
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)📊 View Pathway Visualization Interactive pathway diagram with detailed annotations