Affinage

POLR3D

DNA-directed RNA polymerase III subunit RPC4 · UniProt P05423

Round 2 corrected
Length
398 aa
Mass
44.4 kDa
Annotated
2026-04-28
45 papers in source corpus 9 papers cited in narrative 9 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

POLR3D encodes the RPC4 subunit of RNA polymerase III and is essential for transcription of tRNA, 7SK RNA, and other short non-coding RNAs. Within the Pol III complex, POLR3D forms a heterotrimer subcomplex with RPC5/C37 that is tethered to the polymerase core via an iron-sulfur cluster; this subcomplex physically contacts the RNA 3′ end and transcribed DNA strand at the active site, is required for promoter opening and efficient transcriptional termination, and its C-terminal domain carries the essential functional determinant of the subunit (PMID:1429657, PMID:19940126, PMID:33558764). As a component of cytosolic Pol III, POLR3D participates in innate immune sensing by enabling transcription of AT-rich dsDNA into 5′-triphosphate RNA that activates the RIG-I–IFN-β signaling axis (PMID:19631370). Biallelic pathogenic variants in POLR3D cause POLR3-related hypomyelinating leukodystrophy by disrupting PAQosome-mediated Pol III complex maturation and reducing tRNA and 7SK RNA transcription (PMID:37915380).

Mechanistic history

Synthesis pass · year-by-year structured walk · 6 steps
  1. 1992 High

    Establishing that the POLR3D ortholog (RPC53) is an essential Pol III subunit whose C-terminal domain is the functional determinant for tRNA transcription resolved the first structure–function boundary of this subunit and linked it genetically to the largest Pol III subunit RPC160.

    Evidence Gene disruption, truncation mutagenesis, thermolability and multicopy suppressor analysis in S. cerevisiae

    PMID:1429657

    Open questions at the time
    • Mechanism by which the C-terminal domain supports transcription was unknown
    • No information on the human ortholog at this stage
  2. 2002 High

    Identification of human RPC53 (POLR3D) within the purified human Pol III complex and demonstration that its partner RPC5 is required for type 2 and type 3 promoter transcription extended the yeast findings to the human system and defined the RPC4–RPC5 subcomplex as a conserved functional unit.

    Evidence Affinity purification of human Pol III, mass spectrometry, in vitro transcription with RPC5 depletion

    PMID:12391170

    Open questions at the time
    • Physical position of the RPC4–RPC5 subcomplex within the Pol III architecture was unresolved
    • Roles in specific transcription steps (initiation, elongation, termination) not yet dissected for the human proteins
  3. 2009 High

    Photochemical cross-linking placed the POLR3D ortholog at the Pol III active site in contact with the RNA 3′ end and transcribed DNA, and functional assays showed the C53/C37 subcomplex is required for both promoter opening and transcriptional termination, defining two distinct mechanistic roles.

    Evidence In vitro transcription with C53/C37-deleted Pol III, reconstituted elongation complexes, protein–RNA and protein–DNA photochemical cross-linking in yeast

    PMID:19940126

    Open questions at the time
    • Whether these dual roles are mechanistically coupled or separable was not determined
    • Cross-linking performed with yeast enzyme; direct demonstration in human Pol III pending
  4. 2009 High

    Discovery that Pol III (containing POLR3D) acts as a cytosolic DNA sensor by transcribing AT-rich dsDNA into 5′-triphosphate RNA that triggers RIG-I–IFN-β signaling revealed an unexpected innate immune function for the polymerase beyond housekeeping transcription.

    Evidence Biochemical purification of poly(dA-dT)-transcribing activity, pharmacological and siRNA Pol III inhibition, IFN-β reporter assays, Legionella infection model

    PMID:19631370

    Open questions at the time
    • Contribution of individual Pol III subunits including POLR3D to the immune-sensing function was not dissected
    • Whether this function operates in all cell types or is tissue-restricted was unclear
  5. 2021 High

    High-resolution cryo-EM structures of human Pol III in multiple functional states resolved the POLR3D-containing heterotrimer architecture, revealed its tethering to the core via an iron-sulfur cluster, and enabled structural mapping of disease mutations, providing the first atomic-level framework for understanding POLR3D function.

    Evidence Cryo-EM at 2.8–3.3 Å of human Pol III in unbound and transcribing states

    PMID:33335104 PMID:33558764

    Open questions at the time
    • Dynamic conformational changes of the POLR3D heterotrimer during the transition from initiation to termination remain structurally unresolved
    • Role of the iron-sulfur cluster beyond structural tethering (e.g., redox sensing) not tested
  6. 2023 High

    Identification of biallelic POLR3D variants as a cause of hypomyelinating leukodystrophy, with evidence that a missense variant disrupts PAQosome-mediated Pol III maturation rather than subunit assembly, established a direct genotype–phenotype link and revealed a chaperone-dependent quality control step in Pol III biogenesis.

    Evidence Exome sequencing, patient fibroblast RT-qPCR for Pol III transcripts, AP-MS of wild-type vs. p.P181S POLR3D

    PMID:37915380

    Open questions at the time
    • Structural basis of the POLR3D–PAQosome interaction is unknown
    • How reduced tRNA and 7SK levels lead specifically to hypomyelination rather than broader pathology is not explained
    • Whether other POLR3D variants have the same maturation defect or affect different steps is untested

Open questions

Synthesis pass · forward-looking unresolved questions
  • It remains unknown how POLR3D's dual roles in transcription (promoter opening and termination) are coordinated during the transcription cycle, whether its iron-sulfur cluster has a regulatory function, and how its involvement in innate immune sensing is regulated relative to its housekeeping role in tRNA transcription.
  • No time-resolved structural data capturing the POLR3D heterotrimer through a complete transcription cycle
  • Functional significance of the iron-sulfur cluster beyond structural anchoring untested
  • Cell-type-specific regulation of Pol III immune sensing vs. tRNA transcription not addressed

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0005198 structural molecule activity 3 GO:0003677 DNA binding 2 GO:0003723 RNA binding 1
Localization
GO:0005634 nucleus 4 GO:0005829 cytosol 1
Pathway
R-HSA-74160 Gene expression (Transcription) 4 R-HSA-1643685 Disease 1 R-HSA-168256 Immune System 1
Partners
DDX1POLR3APOLR3ERPAP3
Complex memberships
RNA Polymerase IIIRPC4-RPC5 (C53-C37) heterotrimer subcomplex

Evidence

Reading pass · 9 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
1992 The yeast RPC53 gene (ortholog of human POLR3D) encodes an essential subunit required for tRNA gene transcription by RNA polymerase III in vivo. The carboxyl-terminal region of C53 contains the essential functional domain of the subunit; a mutant RNA polymerase containing only this domain is thermolabile for its transcriptional function in vivo and in vitro. Thermolability of the C53 carboxyl-terminal domain is suppressed by multicopy RPC160, encoding the largest subunit of RNA polymerase C, establishing a genetic interaction with RPC160. Gene disruption, in vivo and in vitro transcription assays, multicopy suppressor screen, thermolability analysis of truncation mutants The Journal of biological chemistry High 1429657
2002 Human RPC53 (POLR3D) was identified as a subunit of the human RNA polymerase III complex and shown to associate with RPC5 (the human ortholog of yeast C37), paralleling the C53–C37 association in S. cerevisiae. RPC5 was required for transcription from type 2 (VAI) and type 3 (U6) promoters, placing the RPC53–RPC5 subcomplex as essential for human Pol III transcriptional activity. Affinity purification of human Pol III complex, mass spectrometry subunit identification, co-purification/interaction assays, in vitro transcription assays with RPC5 depletion Molecular and cellular biology High 12391170
2007 Systematic affinity purification/mass spectrometry of the human transcription machinery confirmed that POLR3D (RPC53) is a component of the human RNA Pol III complex and physically associates with other Pol III subunits within the purified transcription machinery network. Protein affinity purification coupled to mass spectrometry (AP-MS) of tagged transcription machinery components Molecular cell Medium 17643375
2009 The C53/C37 (RPC53/RPC5 in human; POLR3D ortholog/partner) subcomplex of yeast RNA polymerase III is required for efficient transcriptional termination, and additionally plays a role in forming the initiation-ready open promoter complex. In its absence, the transcription bubble fails to stably propagate to and beyond the transcriptional start site. Protein-RNA and protein-DNA photochemical cross-linking placed a segment of C53 (POLR3D ortholog) proximal to the RNA 3' end and transcribed DNA strand at the catalytic center of the Pol III elongation complex, demonstrating its physical proximity to the active site. In vitro transcription assays with C53/C37-deleted Pol III, supercoiled DNA template assays, artificially assembled elongation complex reconstitution, protein-RNA and protein-DNA photochemical cross-linking The Journal of biological chemistry High 19940126
2009 RNA polymerase III (containing POLR3D as a subunit) was identified as the cytosolic DNA sensor that transcribes AT-rich double-stranded DNA (poly(dA-dT)) into 5'-triphosphate RNA, which then activates the RIG-I innate immune pathway to induce IFN-β. Biochemical purification identified Pol III as the enzyme responsible; Pol III inhibition prevented IFN-β induction by transfected DNA, DNA viruses, and intracellular Legionella pneumophila. Biochemical purification of the poly(dA-dT)-transcribing activity, RNA Pol III inhibition (pharmacological and siRNA), IFN-β reporter assays, bacterial infection models Cell High 19631370
2015 In Arabidopsis thaliana, two genes encode homologs of the yeast C53 subunit (POLR3D ortholog), and either protein can assemble into the Pol III complex, demonstrating functional redundancy. Only specific variants of other subunits (C17, C31) assembled into Pol III, indicating subunit-specific assembly rules conserved across eukaryotes. Affinity purification of Pol I and Pol III complexes coupled to mass spectrometry in plants Nucleic acids research Medium 25813043
2020 Cryo-EM structure of human RNA Pol III at 4.0 Å resolution was determined, enabling mapping of disease-associated mutations onto the complex including subunits such as POLR3D (RPC4). Mutations causing neurodevelopmental defects cluster in hotspots affecting Pol III stability and/or biogenesis, while mutations affecting viral sensing are located near DNA binding regions. Cryo-EM reconstruction at 4.0 Å, X-ray crystallography, SAXS, cell-based assembly and stability assays Nature communications High 33335104
2021 Cryo-EM structures of human RNA Pol III in unbound and transcribing states at 2.8–3.3 Å resolution revealed the architecture of the complex including the POLR3D (RPC4)-containing heterotrimer subcomplex, which is tethered to the core via an iron-sulfur cluster. The structures resolved elements absent from yeast Pol III, enabled mapping of disease-related mutations, and provided mechanistic insights into transcription initiation and termination. Cryo-EM at 2.8–3.3 Å resolution of human Pol III in multiple functional states Nature structural & molecular biology High 33558764
2023 Biallelic pathogenic variants in POLR3D (encoding the RPC4 subunit of Pol III) cause POLR3-related hypomyelinating leukodystrophy. Functional studies in patient fibroblasts showed significantly decreased RNA-level expression of POLR3D and reduced expression of other Pol III subunit genes. Pol III transcription was aberrant, with significant decreases in 7SK RNA and several tRNA genes. Affinity purification/mass spectrometry of the POLR3D p.P181S missense variant showed normal assembly of Pol III subunits but altered interaction of Pol III with the PAQosome chaperone complex, indicating the variant alters complex maturation rather than subunit assembly. Exome sequencing, patient fibroblast functional studies (RT-qPCR for Pol III transcripts), affinity purification coupled to mass spectrometry (AP-MS) of variant vs. wild-type POLR3D Frontiers in neurology High 37915380

Source papers

Stage 0 corpus · 45 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2006 Global, in vivo, and site-specific phosphorylation dynamics in signaling networks. Cell 2861 17081983
2002 Generation and initial analysis of more than 15,000 full-length human and mouse cDNA sequences. Proceedings of the National Academy of Sciences of the United States of America 1479 12477932
2015 The BioPlex Network: A Systematic Exploration of the Human Interactome. Cell 1118 26186194
2017 Architecture of the human interactome defines protein communities and disease networks. Nature 1085 28514442
2015 A human interactome in three quantitative dimensions organized by stoichiometries and abundances. Cell 1015 26496610
2009 RNA polymerase III detects cytosolic DNA and induces type I interferons through the RIG-I pathway. Cell 977 19631370
2003 Complete sequencing and characterization of 21,243 full-length human cDNAs. Nature genetics 754 14702039
2021 Dual proteome-scale networks reveal cell-specific remodeling of the human interactome. Cell 705 33961781
2012 A census of human soluble protein complexes. Cell 689 22939629
2011 Phylogenetic-based propagation of functional annotations within the Gene Ontology consortium. Briefings in bioinformatics 656 21873635
2010 An atlas of combinatorial transcriptional regulation in mouse and man. Cell 573 20211142
2004 The status, quality, and expansion of the NIH full-length cDNA project: the Mammalian Gene Collection (MGC). Genome research 438 15489334
2022 OpenCell: Endogenous tagging for the cartography of human cellular organization. Science (New York, N.Y.) 432 35271311
2015 Panorama of ancient metazoan macromolecular complexes. Nature 407 26344197
2007 Systematic analysis of the protein interaction network for the human transcription machinery reveals the identity of the 7SK capping enzyme. Molecular cell 367 17643375
2011 A directed protein interaction network for investigating intracellular signal transduction. Science signaling 258 21900206
2007 hORFeome v3.1: a resource of human open reading frames representing over 10,000 human genes. Genomics 222 17207965
2015 A deep proteomics perspective on CRM1-mediated nuclear export and nucleocytoplasmic partitioning. eLife 198 26673895
2015 Cul3-KLHL20 Ubiquitin Ligase Governs the Turnover of ULK1 and VPS34 Complexes to Control Autophagy Termination. Molecular cell 186 26687681
2020 Synthetic Lethal and Resistance Interactions with BET Bromodomain Inhibitors in Triple-Negative Breast Cancer. Molecular cell 159 32416067
2009 Comparison of substrate specificity of the ubiquitin ligases Nedd4 and Nedd4-2 using proteome arrays. Molecular systems biology 129 19953087
2011 Prevalence, correlates and clinical usefulness of antibodies to RNA polymerase III in systemic sclerosis: a cross-sectional analysis of data from an Australian cohort. Arthritis research & therapy 125 22189167
2010 A novel C53/LZAP-interacting protein regulates stability of C53/LZAP and DDRGK domain-containing Protein 1 (DDRGK1) and modulates NF-kappaB signaling. The Journal of biological chemistry 109 20228063
2013 Genomic study in Mexicans identifies a new locus for triglycerides and refines European lipid loci. Journal of medical genetics 105 23505323
2022 EZH2 depletion potentiates MYC degradation inhibiting neuroblastoma and small cell carcinoma tumor formation. Nature communications 99 35013218
2017 Cell cycle-dependent phosphorylation regulates RECQL4 pathway choice and ubiquitination in DNA double-strand break repair. Nature communications 89 29229926
2021 Cryo-EM structures of human RNA polymerase III in its unbound and transcribing states. Nature structural & molecular biology 80 33558764
2014 Human-chromatin-related protein interactions identify a demethylase complex required for chromosome segregation. Cell reports 80 24981860
2005 Cdk5 activator-binding protein C53 regulates apoptosis induced by genotoxic stress via modulating the G2/M DNA damage checkpoint. The Journal of biological chemistry 71 15790566
2022 Scalable multiplex co-fractionation/mass spectrometry platform for accelerated protein interactome discovery. Nature communications 65 35831314
2002 Characterization of human RNA polymerase III identifies orthologues for Saccharomyces cerevisiae RNA polymerase III subunits. Molecular and cellular biology 64 12391170
2009 The C53/C37 subcomplex of RNA polymerase III lies near the active site and participates in promoter opening. The Journal of biological chemistry 60 19940126
2020 Structure of human RNA polymerase III. Nature communications 55 33335104
2012 Nuclear γ-tubulin associates with nucleoli and interacts with tumor suppressor protein C53. Journal of cellular physiology 48 21465471
2009 Tumor suppressor protein C53 antagonizes checkpoint kinases to promote cyclin-dependent kinase 1 activation. Cell research 48 19223857
2013 Caspase-mediated cleavage of C53/LZAP protein causes abnormal microtubule bundling and rupture of the nuclear envelope. Cell research 32 23478299
2019 C53: A novel particulate guanylyl cyclase B receptor activator that has sustained activity in vivo with anti-fibrotic actions in human cardiac and renal fibroblasts. Journal of molecular and cellular cardiology 29 30954448
2020 C53 is a cross-kingdom conserved reticulophagy receptor that bridges the gap betweenselective autophagy and ribosome stalling at the endoplasmic reticulum. Autophagy 28 33164651
2015 Subunit compositions of Arabidopsis RNA polymerases I and III reveal Pol I- and Pol III-specific forms of the AC40 subunit and alternative forms of the C53 subunit. Nucleic acids research 22 25813043
2022 C53 Interacting with UFM1-Protein Ligase 1 Regulates Microtubule Nucleation in Response to ER Stress. Cells 17 35159364
1992 Biochemical and genetic dissection of the Saccharomyces cerevisiae RNA polymerase C53 subunit through the analysis of a mitochondrially mis-sorted mutant construct. The Journal of biological chemistry 15 1429657
2011 Interaction of LHBs with C53 promotes hepatocyte mitotic entry: A novel mechanism for HBV-induced hepatocellular carcinoma. Oncology reports 14 21971960
2023 Biallelic pathogenic variants in POLR3D alter tRNA transcription and cause a hypomyelinating leukodystrophy: A case report. Frontiers in neurology 13 37915380
2005 Novel interaction between nuclear co-activator CBP and the CDK5 activator binding protein - C53. International journal of molecular medicine 12 16012757
2025 Construction and expression of multi-stage antigen fusion protein RPC4 vaccine for Mycobacterium tuberculosis and its immunogenicity analysis in combination with adjuvant DIMQ. Tuberculosis (Edinburgh, Scotland) 2 40168905