BTBD1

BTBD1 is a substrate adaptor for the CRL3 (Cullin 3/RING) E3 ubiquitin ligase complex that controls the abundance of topoisomerase I (TOP1) and its DNA-bound cleavage complexes (PMID:36490343). BTBD1 physically binds TOP1 through less than the C-terminal half of the protein, an interaction that depends on TOP1 residues 236 and 237 (PMID:11818025, PMID:21092135). Within the CRL3 complex, BTBD1 promotes ubiquitination and proteasomal degradation of TOP1-cleavage complexes in a manner activated by Cullin 3 NEDDylation, such that blocking NEDDylation or depleting Cullin 3 sensitizes cancer cells to TOP1 inhibitors (PMID:36490343). Consistent with this post-translational control of TOP1, BTBD1 maintains proper peripheral nuclear TOP1 localization and is required for myogenic differentiation: a dominant-negative N-terminal truncation reduces nuclear TOP1 content and blocks differentiation (PMID:15486563). BTBD1 also assembles with BTBD2 into cytoplasmic bodies organized by the TRIM5delta scaffold (PMID:12878161).

1. 2002 Medium

   Established the first molecular partner of BTBD1 by showing it binds topoisomerase I, defining the binding determinant within BTBD1.

   Evidence Yeast two-hybrid, co-precipitation, and GST-pulldown deletion mapping with HeLa TOP1

   PMID:11818025

   Open questions at the time

   • Functional consequence of the BTBD1-TOP1 interaction not determined
   • TOP1 residues required for binding not yet mapped
2. 2003 Medium

   Defined the subcellular context of BTBD1 by showing it assembles with BTBD2 into cytoplasmic bodies dependent on a TRIM5delta scaffold.

   Evidence Immunofluorescence colocalization with GFP fusions and TRIM5delta deletion/point mutants in mouse and human cells

   PMID:12878161

   Open questions at the time

   • Biochemical function of the cytoplasmic bodies unknown
   • Relationship between cytoplasmic localization and nuclear TOP1 regulation unresolved
3. 2004 Medium

   Connected BTBD1 to a cellular phenotype, showing it is required for myogenic differentiation and governs nuclear TOP1 localization.

   Evidence Stable dominant-negative truncated BTBD1 overexpression in C2C12 myoblasts with differentiation assays and TOP1 fractionation/imaging

   PMID:15486563

   Open questions at the time

   • Mechanism linking BTBD1 to TOP1 relocalization not established
   • Truncation may act dominant-negatively rather than reflecting loss of function
4. 2007 Low

   Suggested BTBD1 controls TOP1 at the protein level and balances myogenic versus adipogenic fates.

   Evidence Expression profiling across differentiation models with western blot for TOP1 protein versus mRNA

   PMID:17462629

   Open questions at the time

   • Correlative only, no direct manipulation of BTBD1
   • Post-translational mechanism inferred but not demonstrated
5. 2010 Medium

   Mapped the TOP1 binding interface to residues 236-237 and linked the BTBD module to host restriction of HIV-1.

   Evidence Co-precipitation with TOP1 point mutants plus RNAi knockdown and HIV-1 infectivity assays

   PMID:21092135

   Open questions at the time

   • HIV-1 phenotype tested for BTBD2 but not specifically BTBD1
   • Mechanistic basis of altered HIV permissiveness unknown
6. 2022 High

   Resolved the biochemical function of BTBD1 as a CRL3 substrate adaptor that targets TOP1-cleavage complexes for NEDDylation-activated ubiquitin-proteasome degradation.

   Evidence In vitro CUL3-BTBD1/2 ubiquitination reconstitution, NEDD8 pathway inhibition, siRNA depletion, and TOP1 genomic occupancy analysis

   PMID:36490343

   Open questions at the time

   • Relative contributions of BTBD1 versus BTBD2 not dissected
   • Link between this degradation pathway and the myogenic differentiation phenotype not directly tested

• How the cytoplasmic TRIM5delta-organized BTBD1 bodies relate to its nuclear CRL3-mediated TOP1 degradation function remains unresolved.
• No mechanism connecting cytoplasmic localization to nuclear TOP1 control
• Substrate repertoire of CRL3-BTBD1 beyond TOP1-cc unknown