TBL3

TBL3 (UTP13) is a WD-repeat protein that functions in the assembly of the 90S small subunit (SSU) processome and the early processing of pre-ribosomal RNA (PMID:15231838, PMID:39036955). Its yeast ortholog Utp13 forms a stable Pwp2-containing subcomplex (with Dip2, Utp6, Utp18, and Utp21) that binds directly to the 5' end of the 35S pre-rRNA independently of the U3 snoRNP, and this association is required for U3 snoRNP recruitment, 90S pre-ribosome assembly, and pre-rRNA cleavage at sites A0, A1, and A2 that generate 18S rRNA and the 40S subunit (PMID:15231838). Structural probing places the C-terminal domain of Utp13 at the junction organizing the 5'ETS and ITS1 regions of the pre-rRNA and the U3 snoRNA within the nascent processome (PMID:35076539). In human and zebrafish cells, TBL3/UTP13 concentrates in the dense fibrillar and granular components of the nucleolus, displaying very low mobility consistent with a scaffolding core for SSU processome complexes, and redistributes to nucleolar caps when rRNA transcription is blocked (PMID:24754225); its nucleolar delivery depends on UTP3/SAS10 acting through importin-α, and its loss causes accumulation of aberrantly processed 5'ETS products (PMID:39036955). Consistent with this role, zebrafish tbl3 loss-of-function slows the cell cycle in hematopoietic and retinal progenitors in a tissue-specific, p53-independent manner (PMID:22659140). Beyond ribosome biogenesis, TBL3 binds the N-terminal domains of TBL1 and TBLR1 of the SMRT/NCoR corepressor complex and self-oligomerizes through LisH-domain interactions (PMID:18202150).

1. 2004 High

   Established that the TBL3 ortholog Utp13 is an integral component of the 90S pre-ribosome and defined where it acts in pre-rRNA processing, answering whether it functions before or after U3 snoRNP recruitment.

   Evidence Conditional depletion, reciprocal immunoprecipitation, and gradient sedimentation in yeast

   PMID:15231838

   Open questions at the time

   • Does not define the human TBL3 contribution directly
   • Does not resolve atomic contacts between Utp13 and the pre-rRNA
2. 2008 Medium

   Identified a ribosome-biogenesis-independent interaction, showing human TBL3 binds the N-terminal domains of corepressor subunits TBL1/TBLR1 and oligomerizes via LisH domains, raising a possible role in transcriptional repression.

   Evidence Co-immunoprecipitation, Gal4 fusion repression assay, and LisH-domain mutagenesis

   PMID:18202150

   Open questions at the time

   • Functional consequence of TBL3 in the SMRT/NCoR complex not established
   • No demonstration this interaction operates in a native chromatin context
3. 2012 High

   Showed in a vertebrate that TBL3 loss produces a tissue-specific cell-cycle defect that, unlike yeast, is uncoupled from p53-dependent death, clarifying the in vivo phenotypic logic of TBL3 deficiency.

   Evidence Zebrafish forward-genetic mutant, morpholino knockdown, and p53-/- epistasis with cell cycle analysis

   PMID:22659140

   Open questions at the time

   • Does not establish the molecular basis of tissue specificity
   • Does not directly link the cell-cycle defect to a specific rRNA processing step
4. 2014 Medium

   Determined the subnuclear behavior of human TBL3, placing it in nucleolar dense fibrillar/granular components with low mobility and transcription-dependent redistribution, consistent with a scaffolding role for SSU processome complexes.

   Evidence Live-cell GFP-fusion microscopy, FRAP, and rRNA transcription inhibition

   PMID:24754225

   Open questions at the time

   • Scaffold role inferred from mobility, not from defined structural interactions
   • Single-lab observation
5. 2021 Medium

   Connected TBL3 to repeat-expansion disease by showing it binds expanded CAG-repeat ATXN2 and HTT RNAs in vitro alongside disrupted rRNA processing in SCA2 and HD brain, raising a sequestration hypothesis.

   Evidence RNA immunoprecipitation, in vitro binding, and qPCR of rRNA intermediates in human brain tissue

   PMID:34390268

   Open questions at the time

   • Link between TBL3 sequestration and rRNA defect is correlative
   • No demonstration that restoring TBL3 rescues processing in disease tissue
6. 2021 Low

   Nominated TBL3 as a candidate reader of the ac4C RNA modification, suggesting an epitranscriptomic input to its function.

   Evidence Bioinformatics prediction followed by RNA pulldown in HEK293T cells

   PMID:34395433

   Open questions at the time

   • Single pulldown without functional validation of the binding's consequence
   • No mapping of ac4C sites recognized
   • Prediction-assisted identification not orthogonally confirmed
7. 2022 Medium

   Provided spatial information on TBL3's position within the nascent processome, mapping how the 5'ETS, ITS1, and U3 snoRNA are organized around the C-terminal domain of Utp13.

   Evidence MNase-tethering structural probing in yeast

   PMID:35076539

   Open questions at the time

   • Spatial probing does not give atomic-resolution contacts
   • Conducted in yeast, not validated for human TBL3
8. 2024 High

   Defined how TBL3/UTP13 reaches its site of action and confirmed its essential processing role, showing UTP3/SAS10 ferries it to the nucleolus via importin-α and that its loss causes aberrant 5'ETS processing.

   Evidence Nucleolar localization screen of 50 SSU processome components, importin interaction assay, and zebrafish loss-of-function with pre-rRNA processing analysis

   PMID:39036955

   Open questions at the time

   • Direct importin-alpha/UTP13 binding interface not resolved
   • Does not reconcile the corepressor interaction with the nucleolar role

• How TBL3's nucleolar ribosome-biogenesis function relates to its proposed roles in the SMRT/NCoR corepressor complex and in ac4C recognition remains unresolved.
• No model integrating ribosome biogenesis with transcriptional corepression
• Functional significance of ac4C binding uncharacterized
• No structural model of human TBL3 within the processome