TOP6BL

TOP6BL (TOPOVIBL) is the regulatory subunit of the meiotic DNA double-strand break (DSB) machinery, partnering with SPO11 to form the TOPOVIL complex that initiates meiotic recombination. TOP6BL derives from the archaeal TopoVIB topoisomerase subunit but has lost ATP-binding and hydrolysis capacity; instead it exists as a monomer that senses DNA geometry, preferentially engaging supercoiled or branched substrates, and stabilizes DSB ends with high affinity after SPO11-mediated cleavage, which requires Mg²⁺-dependent SPO11 dimerization (2:2) but is ATP-independent (PMID:39972129, PMID:39972130, PMID:38966985). TOP6BL additionally serves as a regulatory scaffold by recruiting REC114 through conserved interacting domains—an interaction that is mutually exclusive with IHO1 and ANKRD31 binding to REC114—thereby controlling the efficiency, timing, and genomic distribution of meiotic DSBs (PMID:36396648, PMID:37431931). Autosomal recessive loss-of-function mutations in TOP6BL cause meiotic arrest and non-obstructive azoospermia in humans (PMID:36732965, PMID:41211863).

1. 2016 High

   Identification of TOPOVIBL as the long-sought partner of SPO11 established that meiotic DSB formation requires a TopoVI-like heteromeric complex, resolving how SPO11 is activated in vivo.

   Evidence Co-IP of TOPOVIBL–SPO11, structural homology to archaeal TopoVIB, and mouse knockout abolishing meiotic DSBs

   PMID:26917764

   Open questions at the time

   • Whether TOPOVIBL retains enzymatic (ATPase) activity was unknown
   • How TOPOVIBL connects to upstream DSB-promoting factors was unresolved
   • Whether the complex is sufficient for DNA cleavage in vitro was untested
2. 2022 High

   Demonstration that TOPOVIBL directly recruits REC114 through conserved domains revealed the molecular link between the catalytic core and the DSB-regulatory axis, explaining how DSB number and distribution are controlled genome-wide.

   Evidence Structural domain mapping, Co-IP, and point-mutation knock-in mice with genome-wide DSB profiling in oocytes and spermatocytes

   PMID:36396648

   Open questions at the time

   • Whether REC114 binding is mutually exclusive with other regulatory factors was not yet known
   • The precise stoichiometry of the TOPOVIBL–REC114 interaction in vivo was not determined
3. 2022 Medium

   Phylogenetic analysis revealed that TOPOVIBL has diverged far more than SPO11, with ATPase-inactivating mutations across eukaryotes, framing the functional shift from topoisomerase to meiotic DSB catalyst.

   Evidence Phylogenetic reconstruction and model structure comparisons across eukaryotic lineages

   PMID:36256608

   Open questions at the time

   • Computational inference without direct biochemical confirmation of ATP independence at that time
   • Whether structural divergence correlates with gain of novel regulatory interactions was untested
4. 2023 High

   Discovery that REC114's PH domain mediates mutually exclusive binding of TOPOVIBL, IHO1, and ANKRD31 established a competitive-interaction model for stepwise DSB complex assembly.

   Evidence AlphaFold2 modeling validated by co-purification, pull-downs, and SEC-MALS stoichiometry measurements

   PMID:37431931

   Open questions at the time

   • In vivo dynamics of the competitive exchange between IHO1 and TOPOVIBL on REC114 were not captured
   • Whether additional post-translational modifications regulate the switch was not addressed
5. 2024 High

   Biochemical characterization showed TOPOVIBL is monomeric, does not bind ATP, preferentially engages supercoiled/branched DNA, and adopts a dynamic conformation, establishing it as a DNA-geometry sensor rather than an ATPase.

   Evidence Protein purification, SEC, ATPase assays, SAXS, and DNA-binding assays with varied substrates

   PMID:38966985

   Open questions at the time

   • How DNA-geometry sensing translates to site selection in vivo was not determined
   • Whether conformational dynamics are regulated by accessory factors was unknown
6. 2024 High

   Cryo-EM structures of the yeast Spo11 core complex (containing Top6BL homologs Rec102/Rec104) bound to DNA defined the molecular contacts underlying DNA end recognition, providing the first high-resolution view of the meiotic DSB machinery on its substrate.

   Evidence Cryo-EM at up to 3.3 Å resolution with functional yeast assays

   PMID:39304764

   Open questions at the time

   • Structures captured post-cleavage complexes; the pre-cleavage DNA-engaged state was not resolved
   • Whether mammalian TOPOVIBL makes equivalent DNA contacts was not established
7. 2025 High

   In vitro reconstitution of the SPO11–TOP6BL complex demonstrated that the heterodimer is catalytically competent for DNA cleavage via covalent 5′-phosphotyrosyl intermediates, requires Mg²⁺ and SPO11 dimerization but not ATP, and that TOP6BL enhances DNA-end binding post-cleavage — three independent studies converged on this mechanism.

   Evidence Purified recombinant protein reconstitution, covalent complex assays, active-site mutagenesis, AlphaFold 3 modeling, deep sequencing of cleavage products, knock-in mouse validation

   PMID:39972125 PMID:39972129 PMID:39972130

   Open questions at the time

   • How accessory factors (REC114, IHO1, ANKRD31) modulate catalytic activity in vitro was not tested
   • The structural basis for DNA bending prior to cleavage lacks experimental atomic-resolution data
   • Whether the rotationally symmetric sequence bias fully explains in vivo hotspot selection is unclear
8. 2025 High

   Identification of distinct human TOP6BL pathogenic variants causing non-obstructive azoospermia through separable mechanisms — loss of REC114/SPO11 binding versus impaired self-dimerization — demonstrated that TOP6BL is essential for human fertility and has multiple functional surfaces.

   Evidence Whole-exome sequencing of infertile patients, Co-IP/binding assays for REC114 and SPO11, self-dimerization assays, mouse central-region deletion model

   PMID:36732965 PMID:41211863

   Open questions at the time

   • The functional role of TOP6BL self-dimerization in the cleavage mechanism is not understood
   • Whether heterozygous carriers have subtle meiotic defects has not been assessed

• Key unresolved questions include the structural basis of the full mammalian SPO11–TOP6BL complex on DNA, how accessory factors modulate catalytic activity, and the mechanism by which TOP6BL self-dimerization contributes to DSB formation.
• No high-resolution structure of the mammalian SPO11–TOP6BL complex exists
• In vitro reconstitution with the full accessory factor complement has not been achieved
• The functional significance of TOP6BL self-dimerization versus the obligate 2:2 SPO11 dimer is unresolved