UTP4

UTP4 (Cirhin) is a WD40-repeat nucleolar protein that functions as a core component of the t-Utp/SSU processome subcomplex required for maturation of the small-subunit ribosomal RNA (PMID:20385600, PMID:28575120). Its crystal structure reveals two orthogonal, highly basic β-propellers, and UV RNA-crosslinking demonstrates that UTP4 binds directly to the 5'-ETS of pre-ribosomal RNA, while a Velcro-closure of the C-terminal propeller supports protein integrity and partner recognition (PMID:28575120). The intact C-terminus is essential for cell growth and 18S/25S rRNA processing, mediating interactions with Utp8 and with the metazoan-specific nucleolar protein NOL11 that drive SSU processome assembly (PMID:20385600, PMID:22916032). Beyond ribosome biogenesis, UTP4 forms the mitotic NWC complex with NOL11 and WDR43, which relocalizes from the nucleolus to perichromosomal regions to support centromeric enrichment of Aurora B, histone H3 threonine-3 phosphorylation, sister chromatid cohesion, and chromosome congression (PMID:32479628). UTP4 also interacts via its C-terminus with Cirip (the C-terminal domain of HIVEP1) to upregulate a canonical NF-κB element (PMID:19732766). The R565W missense mutation causes North American Indian childhood cirrhosis (NAIC); this allele leaves nucleolar localization intact but weakens C-terminal protein-protein interactions, and loss of the zebrafish ortholog cirh1a produces biliary defects that are fully suppressed by tp53 mutation, placing UTP4 upstream of a p53-mediated stress response in biliary development (PMID:12417987, PMID:16225863, PMID:24147052).

1. 2002 Medium

   Established UTP4/Cirhin as a disease gene by identifying the R565W missense mutation as the cause of North American Indian childhood cirrhosis, framing the protein as relevant to embryonic liver biology.

   Evidence Genetic mapping and patient sequencing with bioinformatic domain analysis

   PMID:12417987

   Open questions at the time

   • No functional mechanism for how R565W causes disease
   • WD-repeat function not tested
2. 2005 Medium

   Resolved the long-standing question of where Cirhin acts by showing it is nucleolar rather than mitochondrial, defining the NLS and nucleolar localization signal and demonstrating that R565W does not perturb localization.

   Evidence EGFP/His-tagged fusion expression in HeLa and HepG2 cells with fluorescence microscopy

   PMID:16225863

   Open questions at the time

   • Nucleolar function not yet identified
   • Disease mechanism still unexplained since localization is normal
3. 2009 Medium

   Linked Cirhin to transcriptional signaling by identifying the HIVEP1-derived Cirip as a C-terminal partner that together activate an NF-κB element, and showed R565W weakens this interaction.

   Evidence Yeast two-hybrid, co-IP from nuclear extracts, and NF-κB reporter assay

   PMID:19732766

   Open questions at the time

   • Relationship between NF-κB activity and ribosome biogenesis role unclear
   • In vivo relevance to disease untested
4. 2010 High

   Defined the core molecular function by showing the Utp4 C-terminus is required for cell growth and 18S/25S rRNA maturation and interacts with Utp8 for SSU processome assembly.

   Evidence Yeast C-terminal truncation mutagenesis, rRNA processing assays, and Y2H interaction mapping

   PMID:20385600

   Open questions at the time

   • Homologous R565W did not reproduce defects in yeast, leaving disease mechanism unresolved
   • Direct RNA binding not yet demonstrated
5. 2012 High

   Extended SSU processome membership to humans by identifying NOL11 as a Cirhin/UTP4 partner and SSU processome component, with R565W partially disrupting the interaction.

   Evidence Y2H cDNA screen, AP-MS, co-IP, and siRNA knockdown in human cells

   PMID:22916032

   Open questions at the time

   • Quantitative contribution of NOL11 disruption to disease not established
   • Structural basis of interaction unknown
6. 2016 Medium

   Identified UTP4 as a substrate of the arginine methyltransferase Hmt1, introducing a post-translational regulatory layer.

   Evidence Proteome array screen with ex vivo methylation and MS/MS validation in yeast

   PMID:26572822

   Open questions at the time

   • Specific methylation sites not mapped
   • Functional consequence of methylation unknown
7. 2013 High

   Placed UTP4 in a developmental pathway by showing zebrafish cirh1a loss causes biliary defects that are fully suppressed by tp53 mutation, linking impaired UTP4 function to a p53-mediated stress response.

   Evidence Morpholino knockdown in zebrafish, hepatobiliary reporter assays, and genetic epistasis with tp53 mutant

   PMID:24147052

   Open questions at the time

   • How ribosome biogenesis defect triggers p53 in biliary cells not defined
   • Tissue specificity of the phenotype unexplained
8. 2017 High

   Provided the structural and biochemical basis of UTP4 function, showing two basic β-propellers and direct binding to the 5'-ETS of pre-rRNA.

   Evidence X-ray crystallography at 2.15 Å, CRAC UV RNA-crosslinking, and cryo-EM fitting in Chaetomium thermophilum

   PMID:28575120

   Open questions at the time

   • Structure of the human protein and disease residue not determined
   • Mechanism of Utp8 recognition by the Velcro closure not directly shown
9. 2020 High

   Revealed a moonlighting mitotic role by defining the NWC complex (NOL11–WDR43–Cirhin/UTP4) that translocates to perichromosomal regions to support Aurora B enrichment, H3T3 phosphorylation, and faithful chromosome segregation.

   Evidence Reciprocal co-IP, live-cell imaging, and RNAi with multiple mitotic phenotype readouts

   PMID:32479628

   Open questions at the time

   • Whether the mitotic role contributes to NAIC unknown
   • Mechanism coupling nucleolar release to perichromosomal targeting undefined
10. 2023 Medium

    Distinguished UTP4 loss from loss of nucleolar structure by showing UTP4 knockdown does not trigger global nuclear reorganization, isolating its effect to ribosome synthesis rather than nucleolar architecture.

    Evidence siRNA knockdown in HeLa cells with fluorescence microscopy of nuclear organization

    PMID:37610836

    Open questions at the time

    • Negative result limits mechanistic conclusions
    • No assessment of subtler functional consequences
11. 2026 Low

    Implicated UTP4 in cancer cell behavior by showing knockdown inhibits gastric cancer cell proliferation, migration, and invasion.

    Evidence CRISPR screen prediction followed by siRNA/shRNA knockdown with proliferation, migration, and invasion assays

    PMID:41822801

    Open questions at the time

    • No molecular pathway placement or mechanistic follow-up
    • Connection to ribosome biogenesis or NWC functions not tested

• It remains unresolved how a single nucleolar SSU processome protein mechanistically produces a tissue-specific biliary disease via R565W when the mutation leaves localization intact and only partially weakens partner interactions.
• Quantitative link between weakened C-terminal interactions and human disease severity
• Whether the NF-κB or mitotic NWC functions contribute to NAIC
• Cell-type basis of the p53-dependent biliary phenotype