TCF25

TCF25 (NULP1) is a multifunctional protein that operates in ribosome-associated quality control (RQC), transcriptional regulation, and lysosomal nutrient sensing. In the RQC pathway, TCF25 imposes K48-ubiquitin linkage specificity on nascent chains ubiquitinated by the E3 ligase Listerin by directly binding the Listerin RING domain and the acceptor ubiquitin to orient K48 for nucleophilic attack on the E2~Ub thioester; TCF25 itself is a K48-ubiquitinated Listerin substrate subject to proteasomal degradation (PMID:30244831, PMID:40169231, PMID:38228636). As a nuclear bHLH/DUF654-containing transcriptional repressor, TCF25 suppresses SRF transcriptional activity through HDAC-dependent mechanisms and inhibits NFAT3 signaling via direct interaction with the NFAT3 transactivation domain, with Nulp1 knockout exacerbating pressure-overload cardiac hypertrophy in mice (PMID:16574069, PMID:32805187). TCF25 also functions as a lysosomal nutrient sensor that targets V-ATPase to enhance lysosomal acidification and autophagy under glucose starvation, and its sustained activation drives ferritinophagy and lysosome-dependent cell death, with TCF25 deficiency protecting against hepatic ischemia-reperfusion injury in vivo (PMID:40844875).

1. 2002 Medium

   Establishing TCF25 as a nuclear protein resolved its likely compartment of action and was consistent with a transcriptional role for this uncharacterized bHLH-containing factor.

   Evidence EGFP-fusion of mouse Nulp1 expressed in HEK cells, fluorescence microscopy

   PMID:12107429

   Open questions at the time

   • Overexpression of fusion protein; endogenous localization not confirmed
   • No functional assay beyond localization
2. 2006 Medium

   Demonstrating that the DUF654 domain mediates transcriptional repression through HDAC activity and that TCF25 inhibits SRF signaling established its first mechanistic role as a transcriptional repressor.

   Evidence GAL4-fusion reporter assays, trichostatin A derepression, SRF reporter co-transfection in COS-7 cells

   PMID:16574069

   Open questions at the time

   • No direct HDAC binding or recruitment shown
   • Endogenous target genes not identified
   • Reporter-based assays only
3. 2007 Medium

   Identifying a physical interaction between TCF25 and the anti-apoptotic protein XIAP, enhanced during cell death, linked TCF25 to apoptotic regulation beyond transcription.

   Evidence Co-immunoprecipitation, immunostaining, overexpression-induced DNA fragmentation in Saos2 cells

   PMID:18068114

   Open questions at the time

   • Single co-IP direction reported; no reciprocal validation described
   • Mechanism by which XIAP binding modulates cell death not determined
   • Overexpression artifacts not excluded
4. 2018 High

   Reconstitution of the mammalian RQC pathway revealed a completely unexpected role for TCF25 as the factor that ensures K48-linkage specificity on nascent chains ubiquitinated by Listerin on stalled 60S complexes, establishing TCF25 as a core RQC component.

   Evidence In vitro reconstitution with purified mammalian RQC components, ubiquitin linkage analysis

   PMID:30244831

   Open questions at the time

   • Structural basis for K48 specificity not resolved
   • In vivo relevance in mammalian cells not directly tested
5. 2018 Medium

   Drosophila genetic studies showed that the TCF25 ortholog dNulp1 is required for limb development and acts as a Wnt/Wingless pathway cofactor, extending TCF25 function to developmental signaling.

   Evidence CRISPR/Cas9 knockout, overexpression rescue, qRT-PCR of Wg targets, TopFlash reporter in Drosophila

   PMID:29437009

   Open questions at the time

   • Conservation of Wnt cofactor role in mammals not tested
   • Direct binding partner in Wnt pathway not identified
   • Mechanism of transcriptional activation versus known repressor activity unresolved
6. 2020 High

   Mapping the direct TCF25–NFAT3 interaction to the NFAT3 transactivation domain and demonstrating that Nulp1 knockout worsens cardiac hypertrophy (rescued by NFAT inhibition) established TCF25 as a physiologically relevant negative regulator of NFAT signaling in the heart.

   Evidence Reciprocal co-IP, domain mapping, Nulp1 KO and transgenic mice, aortic banding, VIVIT peptide rescue, NFAT reporter assays

   PMID:32805187

   Open questions at the time

   • Whether TCF25 represses NFAT3 through HDAC recruitment or a distinct mechanism not resolved
   • Cardiac-specific versus systemic functions not dissected
7. 2024 Medium

   Confirming that endogenous TCF25 protein is maintained at low levels by proteasomal degradation validated the in vitro finding that TCF25 is itself a ubiquitin-proteasome substrate.

   Evidence Proteasome inhibitor treatment, immunoblotting, mRNA-protein level comparison across cell lines

   PMID:38228636

   Open questions at the time

   • Which E3 ligase(s) mediate endogenous TCF25 turnover not fully established in vivo
   • Functional consequence of TCF25 stabilization not tested
8. 2025 High

   Structural and biochemical dissection revealed the precise mechanism by which TCF25 enforces K48 specificity: it simultaneously contacts the Listerin RING domain and the acceptor ubiquitin to position K48 for thioester attack, and TCF25 itself undergoes the same K48-specific modification by Listerin.

   Evidence In vitro reconstitution, AlphaFold3 structural modeling, site-directed mutagenesis, ubiquitin linkage assays

   PMID:40169231

   Open questions at the time

   • No experimentally determined high-resolution structure yet
   • Relative contributions of TCF25 auto-ubiquitination versus substrate ubiquitination to RQC flux unknown
9. 2025 High

   A genome-wide CRISPR screen uncovered TCF25 as a lysosomal nutrient sensor that targets V-ATPase to control lysosomal acidification, autophagy, and glucose-starvation-induced lysosome-dependent cell death, with in vivo validation in hepatic ischemia-reperfusion injury.

   Evidence Genome-wide CRISPR-Cas9 screen, TCF25 KO in cell lines and mice, lysosomal acidification assays, V-ATPase interaction studies, autophagy flux, hepatic I/R model

   PMID:40844875

   Open questions at the time

   • How TCF25 senses glucose versus other nutrient signals not defined
   • Molecular interface between TCF25 and V-ATPase subunits not mapped
   • Relationship between lysosomal and RQC functions of TCF25 unexplored

• It remains unknown how the three major functions of TCF25 — RQC ubiquitin-linkage specification, transcriptional repression, and lysosomal nutrient sensing — are coordinated in cells, and whether distinct pools or post-translational modifications partition TCF25 among these pathways.
• No integrative study connecting RQC, transcriptional, and lysosomal roles
• No high-resolution experimental structure of TCF25 in any complex
• Tissue-specific and developmental functions in mammals remain largely uncharacterized