CTU2

CTU2 is an essential subunit of the cytoplasmic CTU1/CTU2 thiolase complex that catalyzes 2-thiolation of the wobble uridine (U34) in the anticodon loop of tRNA-Lys, tRNA-Gln, and tRNA-Glu, a modification required for accurate decoding (PMID:31301155). In patient-derived cells, biallelic loss-of-function variants in CTU2 specifically abolish 2-thiolation across all wobble uridine-containing tRNAs, establishing the protein's catalytic indispensability and linking its loss to a human Mendelian disorder (PMID:31301155). The requirement of the CTU2 ortholog (Ncs2) for wobble uridine thiolation is conserved in yeast, where loss of 2-thiolation abrogates pathogenic virulence and complex stability and interaction depend on nucleotide binding (PMID:37462076). Beyond its tRNA-modifying role, CTU2 expression is transcriptionally activated by Liver X Receptor through a promoter LXR response element and by USF1, and CTU2 promotes lipogenesis and tumor progression in hepatocellular carcinoma, where its knockdown reduces tumor burden and synergizes with LXR ligands to induce apoptosis (PMID:38630355, PMID:40375999).

1. 2019 High

   Established that CTU2 is the catalytic determinant of wobble uridine 2-thiolation in human tRNAs and that its loss causes a defined biochemical and clinical phenotype.

   Evidence Functional characterization of patient-derived cells carrying five CTU2 alleles with direct biochemical thiolation readout

   PMID:31301155

   Open questions at the time

   • Structural basis of the CTU1/CTU2 catalytic mechanism not resolved
   • Downstream translational consequences of thiolation loss not directly mapped
   • Stoichiometry and assembly of the complex in human cells not defined
2. 2023 High

   Showed the thiolation function is evolutionarily conserved and physiologically consequential, with complex stability dependent on nucleotide binding and thiolation required for fungal virulence.

   Evidence Genetic and biochemical analysis of NCS2* point mutant, complex interaction/stability assays, and mouse and Candida albicans virulence models

   PMID:37462076

   Open questions at the time

   • Direct extrapolation of yeast findings to human CTU2 not established
   • Nucleotide-binding site within the complex not structurally defined
3. 2024 Medium

   Linked CTU2 to lipid metabolism and cancer by identifying LXR-driven transcriptional control and a pro-lipogenic, pro-tumorigenic role in hepatocellular carcinoma.

   Evidence LXR agonist/knockout in HepG2 cells, promoter LXR response element analysis, CTU2 knockdown, and xenograft mouse model

   PMID:38630355

   Open questions at the time

   • Mechanism by which CTU2 enhances lipogenic protein synthesis not resolved
   • Whether tumor effects depend on tRNA thiolation activity is untested
4. 2025 Medium

   Extended transcriptional regulation to USF1 and connected CTU2 overexpression to genomic and epigenetic alterations in cancer while confirming tRNA-Lys-TTT modification.

   Evidence ChIP-seq analysis, dual-luciferase reporter, EMSA for USF1 binding, pan-cancer copy number and methylation analysis, and oncogenic validation

   PMID:40375999

   Open questions at the time

   • Single-lab USF1 regulation without independent replication
   • Causal link between copy number/methylation changes and CTU2 expression not established

• How CTU2-dependent wobble uridine thiolation mechanistically couples to lipogenesis and tumor progression remains unresolved.
• No structural model of the human CTU1/CTU2 complex
• Translational targets affected by thiolation loss not enumerated
• Whether metabolic and disease roles are catalysis-dependent is unknown