BUD23

BUD23 (WBSCR22) is an S-adenosylmethionine-dependent methyltransferase that catalyzes N7-methylguanosine (m7G) modification of G1575 in 18S rRNA during biogenesis of the small (40S) ribosomal subunit (PMID:18332120). This catalytic activity is dispensable for cell growth: methyltransferase-dead point mutants and the G1575A rRNA substrate mutant both support normal proliferation, establishing that the BUD23 protein scaffold — not its methylation chemistry — is the essential contribution to 40S maturation and nuclear export (PMID:18332120). BUD23 functions as an obligate heterodimer with TRMT112, which is required for BUD23 stability and methylation activity; the two proteins associate through a β-zipper interface, and failure of BUD23 to load onto nascent pre-ribosomes triggers a TRAMP-dependent nucleolar surveillance pathway that degrades the defective particle (PMID:22493060, PMID:25489090). Within ribosome assembly, BUD23 acts at the transition from the SSU Processome to the pre-40S subunit: it interacts directly with the DEAH-box helicase Dhr1/Ecm16 and promotes final disassembly of the SSU Processome, including release of U3 snoRNA-associated factors and the GTPase Bms1, enabling efficient A2 pre-rRNA cleavage and nuclear export (PMID:25489090, PMID:24710271, PMID:33306676). BUD23 is subsequently displaced from the maturing pre-40S by the atypical kinase Rio2, which occupies the same binding site and evicts BUD23 in a manner independent of nucleotide hydrolysis (PMID:34934010). At the organismal level, BUD23-programmed ribosomes selectively translate mRNAs with low 5′ UTR GC content — enriched for mitochondrial and lipogenic transcripts — linking this ribosome modification to bioenergetics; tissue-specific deletion causes cardiomyopathy with reduced mitochondrial content in mouse heart (PMID:31939735). In C. elegans, BUD-23-dependent m7G rRNA methylation mediates intergenerational hormesis, transmitting stress resistance and extended longevity to progeny following parental starvation (PMID:37689068).

1. 2008 High

   Established BUD23 as the enzyme writing m7G1575 on 18S rRNA, but separation-of-function mutagenesis revealed the surprising result that 40S biogenesis and export depend on the protein itself rather than its catalytic activity.

   Evidence Genetic deletion, methyltransferase-dead point mutants, rRNA modification mapping, and Rps2/Rps3-GFP nuclear export reporters in yeast

   PMID:18332120

   Open questions at the time

   • Did not define which molecular step of 40S maturation the protein scaffold supports
   • Mechanism by which the protein, separate from methylation, drives export unresolved
2. 2008 High

   Pinpointed the maturation defect by showing loss of BUD23 blocks 20S-to-18S processing and traps pre-40S subunits in the nucleus, defining BUD23 as required for small-subunit nuclear export.

   Evidence Northern blotting of rRNA intermediates plus GFP reporter and 5'-ITS1 localization in bud23Δ yeast

   PMID:18332120

   Open questions at the time

   • Did not identify the assembly-factor partners that mediate processing/export
   • Direct vs indirect role in export not distinguished
3. 2012 High

   Identified TRMT112 as the obligate partner stabilizing BUD23 and enabling methylation, and connected failed BUD23 loading to a quality-control pathway that destroys defective pre-ribosomes.

   Evidence In vitro direct binding, in vivo stability and modification assays, and genetic analysis with TRAMP complex in yeast

   PMID:22493060

   Open questions at the time

   • Structural basis of the BUD23–TRMT112 interaction not yet resolved
   • Trigger sensed by the surveillance pathway not defined
4. 2014 High

   Resolved the atomic architecture of the BUD23–TRMT112 complex and its SAM/substrate coordination, dated m7G methylation to a late maturation step despite early recruitment, and revealed a direct link to the Dhr1 helicase.

   Evidence X-ray crystallography (apo and SAM-bound), interface/active-site mutagenesis, complementation, Dhr1 co-IP, and pulse-chase methylation timing

   PMID:25489090

   Open questions at the time

   • Why early recruitment precedes late catalysis mechanistically unexplained
   • Functional consequence of the Dhr1 interaction at structural resolution not shown
5. 2014 High

   Placed BUD23 entry into the assembly pathway at the time of Dhr1/Ecm16 helicase function, using suppressor genetics to show ECM16 mutations rescue bud23Δ growth and A2 cleavage defects.

   Evidence ECM16 suppressor genetics, yeast two-hybrid binding-site mapping, and affinity purification of Bud23-containing 45S particles from ATPase-dead ecm16 mutants

   PMID:24710271

   Open questions at the time

   • Order of helicase action versus BUD23 catalysis not fully resolved
   • Direct enzymatic interplay between Dhr1 unwinding and BUD23 loading not reconstituted
6. 2020 High

   Defined the protein-scaffold function as promoting SSU Processome disassembly, identifying a suppressor network linking BUD23 to U3 snoRNA factors and the GTPase Bms1.

   Evidence Genome-wide bud23Δ suppressor screen (67 alleles) plus mass-spec composition of pre-40S particles in yeast

   PMID:33306676

   Open questions at the time

   • Biochemical mechanism by which BUD23 triggers factor release not established
   • Whether disassembly is direct or downstream of Dhr1 action unclear
7. 2021 High

   Explained how BUD23 leaves the maturing particle, showing Rio2 shares the BUD23 binding site and displaces it independently of nucleotide hydrolysis, with the Rps0 cluster required for release.

   Evidence Factor depletion plus affinity purification and in vitro reconstitution of Bud23 displacement by recombinant Rio2 with ATPase-dead controls in yeast

   PMID:34934010

   Open questions at the time

   • Structural overlap of Rio2 and BUD23 footprints not directly visualized
   • What licenses the timing of displacement in vivo unknown
8. 2020 High

   Connected BUD23 ribosome modification to translational selectivity and physiology, showing it promotes ribosome engagement with low-GC 5′ UTRs and is required for mitochondrial function in the heart.

   Evidence siRNA knockdown with ribosome profiling in human A549 cells and cardiomyocyte-specific Bud23 knockout mouse with mitochondrial and cardiac phenotyping

   PMID:31939735

   Open questions at the time

   • Whether translational selectivity requires m7G catalysis or the protein scaffold not dissected
   • Molecular basis for 5′ UTR GC-content preference unresolved
9. 2023 High

   Demonstrated a heritable epigenetic role for BUD23-dependent m7G rRNA methylation in mediating intergenerational stress hormesis, distinguishing it from other rRNA methyltransferases.

   Evidence Metabolic methyl-labeling, bud-23 deletion, rRNA modification mass spectrometry, and transgenerational phenotype assays in C. elegans

   PMID:37689068

   Open questions at the time

   • Mechanism by which rRNA methylation is inherited and read out not defined
   • Link between the heritable modification and specific translational outputs not shown
10. 2021 Medium

    Extended BUD23–TRMT112 function to a viral context, implicating it in chromosomal tethering of Borna disease virus ribonucleoproteins in a methyltransferase-dependent manner.

    Evidence BioID identification of BoDV-1 L-protein interactors, co-IP, and knockdown/mutant-rescue microscopy of vRNP localization

    PMID:34324219

    Open questions at the time

    • Single lab and not independently replicated
    • Direct substrate of methylation in the viral tethering process unknown
    • Physiological relevance to host ribosome biogenesis unclear
11. 2025 Medium

    Strengthened the metabolic role by showing adipocyte-specific BUD23 loss confers leanness and obesity resistance via altered translation of short-5′-UTR, GC-rich-codon transcripts characteristic of mitochondrial and lipogenic proteins.

    Evidence Adipocyte-specific Bud23 knockout mouse with metabolic phenotyping, ribosome profiling, 5′ UTR feature analysis, and human cardiometabolic Mendelian randomization (preprint)

    PMID:bio_10.1101_2025.05.16.654455

    Open questions at the time

    • Preprint, not yet peer-reviewed
    • Causal molecular link between specific rRNA modification and the translational feature preference not established
12. 2024 Low

    Reported a candidate oncogenic role in prostate cancer, with BUD23 knockdown reducing proliferation and PPAR-α/β/γ levels.

    Evidence siRNA knockdown, CCK-8/EdU proliferation assays, and Western/qPCR for PPARs in PC-3 and LNCaP cells

    PMID:39636451

    Open questions at the time

    • No mechanistic link between BUD23 methyltransferase activity and PPAR levels established
    • Single lab, single method set
13. 2025 Low

    Reported a proliferative/oncogenic role in clear cell renal carcinoma, with knockdown impairing growth and overexpression correlating with reduced tumor-suppressor expression.

    Evidence siRNA knockdown, proliferation, colony formation, and migration assays in KIRC cell lines

    PMID:40879514

    Open questions at the time

    • No direct molecular mechanism linking BUD23 to tumor-suppressor regulation
    • Correlative clinical association without causal experiment

• How m7G methylation versus the BUD23 protein scaffold each contributes to selective translation of low-GC/short-5′-UTR mRNAs, and whether the cancer-associated phenotypes derive from this translational program, remains unresolved.
• Catalytic vs scaffold contributions to translational selectivity undissected
• No mechanistic bridge between ribosome modification and tumor phenotypes
• Structural basis of the 5′ UTR GC preference unknown