SYCE3

SYCE3 is a central element protein of the synaptonemal complex (SC) that drives the structural maturation required for homologous chromosome synapsis during meiosis. SYCE3 forms a dimeric four-helical bundle that self-assembles into higher-order oligomers through staggered lateral interactions and domain swapping, and it actively remodels the SYCP1 transverse filament lattice by converting SYCP1 tetramers into 2:1 SYCP1–SYCE3 heterotrimers, disrupting original SYCP1 assembly interfaces and establishing a new SYCE3-based lattice that tethers SYCP1 dimers (PMID:31023827, PMID:36635604). Genetic epistasis in knockout mice places SYCE3 downstream of SYCP1 but upstream of SYCE1, SYCE2, and TEX12, and biochemical reconstitution confirms that SYCE3 recruits the SYCE1–SIX6OS1 and SYCE2–TEX12 complexes to achieve long-range synapsis (PMID:21637789, PMID:36635604). Immuno-electron microscopy and structural studies show that SYCE3 and the SYCP1 N-terminus form a bilayered junction in the SC central region, with SYCE1 and SYCE2 positioned between the layers (PMID:27103161, PMID:25394919).

1. 2011 High

   The question of where SYCE3 acts in SC assembly was resolved: knockout mice revealed that SYCE3 is required after SYCP1 loading but before SYCE1, SYCE2, and TEX12, establishing its position as the first central element component to act downstream of transverse filaments.

   Evidence Syce3 knockout mouse with immunofluorescence and epistasis analysis of SC protein loading

   PMID:21637789

   Open questions at the time

   • Molecular mechanism by which SYCE3 interacts with SYCP1 was unknown
   • Whether SYCE3 functions as a monomer, dimer, or oligomer was unresolved
   • How SYCE3 enables loading of downstream CE proteins was unclear
2. 2014 High

   The first atomic structure of SYCE3 revealed a dimeric fold and showed that its N-helix directly engages the SYCE1 C-helix, providing a structural basis for CE protein–protein interactions.

   Evidence X-ray crystallography of mouse SYCE3; in vitro binding assays

   PMID:25394919

   Open questions at the time

   • Whether SYCE3 forms higher-order assemblies beyond dimers was unknown
   • Structural relationship between SYCE3 and SYCP1 was not resolved
3. 2016 High

   Immuno-EM placed SYCE3 and the SYCP1 N-terminus in a bilayered structure at the SC midline, with SYCE1 and SYCE2 between the layers, providing the first nanoscale architectural map of how central element proteins are spatially organized.

   Evidence Immuno-gold electron microscopy combined with protein interaction assays

   PMID:27103161

   Open questions at the time

   • The stoichiometry of the SYCP1–SYCE3 interaction was not determined
   • How this architecture is dynamically assembled was unknown
4. 2019 High

   Biophysical analysis showed that SYCE3 dimers self-assemble into discrete higher-order oligomers (predominantly dodecamers) through staggered lateral and domain-swapping interactions, explaining how SYCE3 can build an extended lattice within the central element.

   Evidence Multi-angle light scattering (MALS) and small-angle X-ray scattering (SAXS) in solution

   PMID:31023827

   Open questions at the time

   • Whether self-assembly is regulated in vivo was unknown
   • How SYCE3 oligomerization interfaces relate to SYCP1 binding was unresolved
5. 2019 Medium

   Discovery of SCRE as a binding partner of both SYCP1 and SYCE3 revealed an additional reinforcing layer in SC architecture; loss of SCRE caused synapsis instability, placing SYCE3 within a broader protein network that stabilizes the SC.

   Evidence Co-immunoprecipitation; Scre knockout mouse with immunofluorescence

   PMID:30949703

   Open questions at the time

   • Direct binding interface between SCRE and SYCE3 was not structurally characterized
   • Whether SCRE modulates SYCE3 self-assembly or SYCP1 remodeling is unknown
6. 2019 Medium

   Tracking SC disassembly in chicken oocytes showed that SYCE3 and SYCP1 persist together on lateral elements during early desynapsis and disappear coordinately, suggesting their removal is a regulated step in SC dismantling.

   Evidence Super-resolution immunolocalization in chicken oocytes during desynapsis

   PMID:30793238

   Open questions at the time

   • Post-translational modifications driving SYCE3 removal were not identified
   • Whether this disassembly order is conserved in mammals was not tested
7. 2023 High

   The central mechanistic question — how SYCE3 restructures the SC — was answered: SYCE3 actively remodels SYCP1 tetramers into 2:1 SYCP1–SYCE3 heterotrimers, breaking original SYCP1 assembly interfaces and building a new SYCE3-based lattice that tethers SYCP1 dimers and recruits SYCE1–SIX6OS1 and SYCE2–TEX12 for long-range synapsis.

   Evidence Biochemical reconstitution; separation-of-function mutagenesis in mice; structural analysis; co-immunoprecipitation

   PMID:36635604

   Open questions at the time

   • High-resolution cryo-EM or crystal structure of the SYCP1–SYCE3 heterotrimer is lacking
   • How the transition from SYCP1 tetramer to heterotrimer is temporally regulated during prophase I is unknown
   • Whether the SYCE3 lattice has a defined periodicity within the SC is unresolved

• Key open questions remain: the high-resolution structure of the full SYCP1–SYCE3 heterotrimer, the regulatory signals (e.g., post-translational modifications) controlling SYCE3-mediated lattice remodeling and subsequent disassembly, and whether SYCE3 has functional roles outside meiotic cells.
• No high-resolution structure of the assembled SYCP1–SYCE3 heterotrimer exists
• Regulatory mechanisms controlling timing of SYCE3 remodeling activity are unknown
• Claimed steroidogenic role of SYCE3 in somatic cells lacks independent confirmation