TAF1C

TAF1C (TAFI110/TAFI95) is an integral subunit of the RNA polymerase I-specific promoter selectivity factor SL1/TIF-IB, where it controls the assembly of the rDNA preinitiation complex (PMID:7801123, PMID:7491500). Within SL1, TAF1C binds TBP directly and in a manner mutually exclusive with the TFIID-specific TAFs, thereby committing TBP to Pol I rather than Pol II promoter recognition, and it makes direct contacts with the rDNA promoter (PMID:7801123, PMID:7491500). TAF1C serves as the docking site that recruits initiation-competent Pol I: hRRN3/TIF-IA engages TAF1C (via a conserved LARAK motif in TIF-IA) to deliver Pol Iβ to the promoter (PMID:11250903, PMID:12393749). This recruitment function is the principal node at which rRNA synthesis is regulated. During mitosis, Cdk1/cyclin B phosphorylates TAF1C at threonine 852 to repress transcription, a modification reversed by Cdc14B at mitotic exit and acting cooperatively with SIRT1-dependent deacetylation of the partner subunit TAFI68 (PMID:10339547, PMID:26023773). CK2 phosphorylation of TAF1C blocks SL1 binding to rDNA (PMID:16880508), and p53 represses Pol I by binding TBP and TAF1C to prevent the SL1–UBF interaction required for preinitiation complex assembly (PMID:10913176). SL1, through TAF1C, is also the target of growth- and oncogenic signals, being recruited by SV40 large T antigen to stimulate Pol I transcription and repressed by GSK3β association (PMID:9203586, PMID:18490923). In humans, homozygous loss-of-function and missense TAF1C variants cause early-onset neurodegeneration, with one missense allele misdirecting the protein from nucleoli into nucleoplasmic aggregates (PMID:40371665, PMID:32779182). A non-canonical chromatin role has also been described, in which TAF1C interacts with the H3K4 methyltransferase SETD1A and regulates enhancer H3K27ac and lipid metabolism gene expression (PMID:42031105).

1. 1994 High

   Established TAF1C as a defining SL1 subunit that imposes Pol I promoter selectivity by binding TBP mutually exclusively with TFIID TAFs, answering how a shared TBP is partitioned between polymerase systems.

   Evidence Recombinant subunit binding assays and reconstitution of SL1 from purified components

   PMID:7801123

   Open questions at the time

   • No structural model of the TAF1C–TBP interface
   • Stoichiometry of the assembled SL1 complex not resolved here
2. 1995 High

   Distinguished TAF1C's role in direct rDNA promoter contact from TAFI48's role as the UBF target, dividing labor among SL1 subunits during promoter recognition.

   Evidence In vitro transcription and protein–DNA contact assays with purified SL1 subunits

   PMID:7491500

   Open questions at the time

   • Precise promoter sequence contacted by TAF1C not mapped
   • How TAF1C–DNA contacts couple to UBF responsiveness unclear
3. 1997 High

   Showed SL1 (via TBP, TAFI48, and TAF1C) is the target through which SV40 large T antigen activates Pol I, linking SL1 recruitment to viral stimulation of rRNA synthesis.

   Evidence Co-IP in vitro and from infected cells, reconstituted transcription, and large T mutants

   PMID:9203586

   Open questions at the time

   • Whether the TAF1C contact alone is sufficient for activation not isolated
   • Functional consequence on Pol I recruitment kinetics not measured
4. 1999 High

   Identified mitotic Cdc2/cyclin B phosphorylation of TAF1C as the switch that represses Pol I in mitosis and is reversed in G1, establishing cell-cycle control of rRNA synthesis at the SL1 level.

   Evidence Synchronized cell extracts, in vitro transcription, phosphatase inhibitor analysis

   PMID:10339547

   Open questions at the time

   • Phosphorylation site not defined in this study
   • Phosphatase responsible for reactivation not identified here
5. 1999 Medium

   Connected developmental downregulation of rRNA synthesis to reduced TAF1C protein abundance, showing SL1 subunit availability as a regulatory parameter during differentiation.

   Evidence F9 embryonal carcinoma differentiation model, western blot, in vitro transcription

   PMID:9933634

   Open questions at the time

   • Mechanism reducing TAF1C levels (transcriptional vs degradation) not determined
   • Single differentiation model
6. 2000 High

   Defined p53 as a direct repressor of Pol I acting through binding to TBP and TAF1C to block the SL1–UBF interaction, placing tumor-suppressor control on rDNA preinitiation complex assembly.

   Evidence Cell-free transcription, protein interaction and template commitment assays, recombinant p53

   PMID:10913176

   Open questions at the time

   • TAF1C residues contacted by p53 not mapped
   • Relative contribution of TAF1C vs TBP contacts not separated
7. 2000 Medium

   Mapped the human TAF1C gene to 16q24 and detected multiple transcripts, raising the possibility of variant SL1 isoforms.

   Evidence Hybrid panel mapping, FISH, Northern analysis

   PMID:10894955

   Open questions at the time

   • Functional distinctness of transcript variants not tested
   • No protein-level evidence for isoform diversity
8. 2001 High

   Identified TAF1C (with TAFI63) as the SL1 surface that recruits hRRN3-bound initiation-competent Pol Iβ, defining the bridge between promoter recognition and polymerase delivery.

   Evidence Reciprocal Co-IP, direct interaction assays, ChIP, in vitro transcription

   PMID:11250903

   Open questions at the time

   • Structure of the TAF1C–hRRN3–Pol I interface unknown
   • Order of assembly relative to UBF not fully resolved
9. 2002 High

   Localized the TIF-IA contact to a conserved LARAK motif and showed the interaction is regulated by nutrient and density signals, linking growth state to SL1-dependent preinitiation.

   Evidence TIF-IA deletion mapping, Co-IP, in vitro interaction assays

   PMID:12393749

   Open questions at the time

   • Reciprocal TAF1C motif binding TIF-IA not mapped
   • Signaling pathway transmitting nutrient status to this interaction not defined here
10. 2006 High

    Established CK2 phosphorylation of TAF1C as a mechanism that blocks SL1–rDNA binding, adding a second kinase-controlled regulatory layer on preinitiation complex nucleation.

    Evidence Co-IP, in vitro kinase assay, ChIP, in vitro transcription with CK2 inhibitor

    PMID:16880508

    Open questions at the time

    • CK2 target residues on TAF1C not identified
    • Interplay with Cdk1-mediated phosphorylation not addressed
11. 2008 Medium

    Implicated GSK3β as a TAF1C-associated repressor of rRNA synthesis in transformed cells, extending oncogenic signaling control to SL1.

    Evidence Co-IP, ChIP, BrUTP run-on, active GSK3β mutant overexpression

    PMID:18490923

    Open questions at the time

    • Whether GSK3β phosphorylates TAF1C directly not shown
    • Single-lab interaction evidence
12. 2015 High

    Pinpointed Cdk1/cyclin B phosphorylation at TAF1C T852 (reversed by Cdc14B) and showed it cooperates with SIRT1-dependent deacetylation of TAFI68 to enforce mitotic Pol I silencing, resolving the molecular detail of cell-cycle repression.

    Evidence Phospho-site mutagenesis, Cdk1 kinase assay, SIRT1 inhibition, in vitro transcription, Co-IP

    PMID:26023773

    Open questions at the time

    • Structural effect of T852 phosphorylation on SL1 not defined
    • Whether other mitotic kinases contribute not tested
13. 2020 Medium

    Linked TAF1C to human disease, showing homozygous missense variants reduce TAF1C mRNA and protein in patient fibroblasts and cause early-onset neurological disease through loss of function.

    Evidence RT-PCR, western blot on patient fibroblasts, Sanger sequencing

    PMID:32779182

    Open questions at the time

    • Causality not confirmed by rescue
    • Only two patients; mechanism linking reduced TAF1C to neurodegeneration unestablished
14. 2025 Medium

    Demonstrated that proper nucleolar localization of TAF1C is required for function, with a p.Ser589Leu variant mislocalizing the protein to nucleoplasmic aggregates despite normal expression levels.

    Evidence Immunofluorescence and western blot in patient-derived cells

    PMID:40371665

    Open questions at the time

    • Single patient
    • Direct link between aggregation and Pol I transcription defect not measured
15. 2025 Medium

    Established an essential in vivo developmental role for Taf1c, with disease-orthologous and deletion alleles causing embryonic lethality at organogenesis in mice.

    Evidence CRISPR knock-in mouse models, Mendelian ratio analysis across developmental stages

    PMID:40953792

    Open questions at the time

    • Tissue-specific requirement not dissected
    • Molecular cause of lethality (rRNA synthesis failure) not directly demonstrated
16. 2026 Medium

    Revealed a non-canonical chromatin function in which TAF1C interacts with SETD1A and regulates enhancer H3K27ac and lipid metabolism genes, expanding TAF1C beyond Pol I transcription.

    Evidence Genome-wide CRISPR screen, ATAC-seq, Co-IP, ChIP-seq, shRNA in cell and mouse models

    PMID:42031105

    Open questions at the time

    • Whether this function is independent of SL1/Pol I role unclear
    • Single lab; mechanism of TAF1C recruitment to enhancers not defined

• How loss of TAF1C-dependent Pol I activity produces selective neurodegeneration, and how its canonical rDNA role relates to the newly described enhancer/SETD1A function, remain unresolved.
• No mechanistic link between reduced rRNA synthesis and neuronal vulnerability
• Relationship between nucleolar and chromatin functions unestablished
• No structural model of the assembled SL1 complex