SYCE3

SYCE3 is a central element protein of the meiotic synaptonemal complex (SC) that is essential for chromosome synapsis, meiotic recombination, and fertility in both sexes (PMID:21637789). It acts downstream of the transverse filament protein SYCP1 but upstream of the other central element proteins, and its loss blocks synapsis initiation, prevents chromosome loading of SYCE1, SYCE2, and TEX12, and abolishes meiotic recombination (loss of MLH1 foci), causing meiotic arrest (PMID:21637789). Structurally, SYCE3 adopts a dimeric four-helical bundle that serves as the building block for concentration-dependent hierarchical self-assembly into higher-order oligomers through staggered lateral and end-on interactions (PMID:31023827). Rather than passively stabilizing SYCP1, SYCE3 actively remodels the SC lattice: it converts SYCP1 tetramers into 2:1 SYCP1–SYCE3 heterotrimers, removing the original SYCP1 assembly interfaces, and then uses its own self-assembly to build a new lattice that tethers SYCP1 dimers while recruiting the SYCE1–SIX6OS1 and SYCE2–TEX12 complexes (PMID:36635604). SYCE3 directly contacts SYCE1 through an N-helix/C-helix interaction (PMID:25394919) and engages the SC-reinforcing element SCRE to maintain SC integrity (PMID:30949703), positioning SYCE3 at the core of the central element protein network at the transverse-filament–central-element junction (PMID:27103161).

1. 2011 High

   Established SYCE3 as an essential central element protein whose genetic position lies downstream of SYCP1 and upstream of all other CE proteins, defining its role as a gatekeeper for synapsis and recombination.

   Evidence Syce3 knockout mice with immunofluorescence and epistasis analysis of CE protein loading in spermatocytes

   PMID:21637789

   Open questions at the time

   • Does not define the biochemical mechanism by which SYCE3 enables loading of downstream CE proteins
   • Molecular nature of the SYCP1–SYCE3 functional dependency unresolved
2. 2014 High

   Localized SYCE3 to the bilayered central region together with the SYCP1 N-terminus and showed the four CE proteins interdependently stabilize the transverse-filament–central-element junction, building a spatial architecture for the SC.

   Evidence Immunoelectron microscopy with immuno-gold labeling and genetic disruption controls in mouse spermatocytes

   PMID:27103161

   Open questions at the time

   • Static ultrastructural snapshot does not reveal assembly dynamics
   • Direct binding interfaces between SYCE3 and SYCP1 not defined here
3. 2014 High

   Provided the first atomic view of SYCE3, showing it dimerizes and oligomerizes and contacts SYCE1 via a defined N-helix/C-helix interaction, identifying helical packing as the basis for CE protein association.

   Evidence X-ray crystallography of mouse SYCE3 plus biochemical SYCE3–SYCE1 interaction assay

   PMID:25394919

   Open questions at the time

   • Crystallographic oligomers may not reflect physiological assembly state
   • Functional consequence of the SYCE3–SYCE1 contact not tested in vivo
4. 2019 High

   Defined SYCE3's solution architecture as a dimeric four-helical bundle that self-assembles in a concentration-dependent, hierarchical manner, supplying the structural building block for SC lattice formation.

   Evidence Multi-angle light scattering and small-angle X-ray scattering of human SYCE3

   PMID:31023827

   Open questions at the time

   • Self-assembly characterized for isolated SYCE3 without SYCP1 or other CE partners
   • Domain-swapping model of end-on interactions inferred rather than directly visualized
5. 2019 Medium

   Embedded SYCE3 in a structural network with the SC-reinforcing element SCRE, showing physical interaction and a requirement for SC stability beyond initial assembly.

   Evidence Co-immunoprecipitation of SCRE–SYCE3 and SCRE–SYCP1 plus Scre knockout phenotyping in mice

   PMID:30949703

   Open questions at the time

   • Co-IP does not establish a direct binary SYCE3–SCRE contact
   • Structural basis of reinforcement not defined
6. 2019 Medium

   Tracked SYCE3 behavior during SC disassembly, showing it remains with lateral elements at axis separation and disappears as separation widens, implicating post-translational regulation in central region turnover.

   Evidence Immunolocalization and super-resolution microscopy in chicken oocytes at prophase I

   PMID:30793238

   Open questions at the time

   • Single avian model, not confirmed in mammals
   • Specific post-translational modifications driving disassembly not identified
7. 2022 Medium

   Reported a non-canonical role for SYCE3 in steroidogenesis, where its manipulation in somatic gonadal cells modulates steroidogenic gene expression and testosterone output and synergizes with SYCE1.

   Evidence Overexpression and siRNA knockdown of Syce3 in Sertoli and Leydig cell lines with steroidogenic gene and hormone readouts

   PMID:35697131

   Open questions at the time

   • Cell-line overexpression/knockdown without in vivo validation
   • Mechanism linking a meiotic CE protein to steroidogenesis unexplained
   • Physiological relevance in somatic cells uncertain
8. 2023 High

   Resolved how SYCE3 functions mechanistically, showing it actively remodels the SYCP1 lattice into 2:1 SYCP1–SYCE3 heterotrimers and builds a new SYCE3-based lattice that recruits downstream CE complexes, reframing SYCE3 from a stabilizer to an active architect of the SC.

   Evidence Biochemical reconstitution, separation-of-function mutagenesis in mice, and structural analysis

   PMID:36635604

   Open questions at the time

   • Atomic structure of the full SYCE3-based lattice not resolved
   • Temporal coordination of remodeling with recombination machinery unclear

• How SYCE3-driven lattice remodeling is temporally coordinated with recombination and disassembly, and the structural basis of its recruitment of SYCE1–SIX6OS1 and SYCE2–TEX12, remain open.
• No high-resolution structure of the mature SYCE3–SYCP1–CE lattice
• Regulation of disassembly by post-translational modification uncharacterized
• Recruitment interfaces for downstream CE complexes not structurally defined