SYCE1

SYCE1 is a central element protein of the synaptonemal complex (SC) essential for meiotic chromosome synapsis, crossover recombination, and fertility in both sexes. SYCE1 homodimerizes through an anti-parallel α-helical coiled-coil core (residues 25–179) forming an elongated ~50 nm structural strut that tethers other SC components, including SYCP1 and SIX6OS1 (C14ORF39), via two distinct multivalent binding interfaces on SIX6OS1—the N-terminal interface disrupts SYCE1's dimeric core to form a 1:1 complex, while a second downstream interface provides additional essential contacts (PMID:30607510, PMID:32917591). Loss of SYCE1 function—through null mutation, splice-site disruption, or frameshift—results in absent protein (via nonsense-mediated decay of transcript), failure of homologous synapsis, meiotic arrest before pachytene with increased apoptosis, and complete infertility, establishing SYCE1 as a cause of premature ovarian insufficiency and non-obstructive azoospermia (PMID:19247432, PMID:32402064, PMID:39909494).

1. 2009 High

   Establishing that SYCE1 is required for meiotic synapsis answered the fundamental question of whether central element proteins are structurally essential or merely accessory: Syce1-null mice failed to synapse homologous chromosomes and revealed an unexpected biochemical link between SC component SYCE2 and recombination factor RAD51, placing SYCE1 at the interface of synapsis and recombination.

   Evidence Mouse Syce1 knockout with immunostaining and co-immunoprecipitation

   PMID:19247432

   Open questions at the time

   • Molecular structure of SYCE1 unknown
   • Direct binding partners and interaction interfaces not mapped
   • Whether SYCE1 loss affects transcript stability or only protein function was not addressed
2. 2019 High

   Determining that SYCE1 forms an anti-parallel α-helical coiled-coil homodimer (~20 nm core, >50 nm total) answered how a single SC central element protein achieves the physical dimensions needed to span SC lattice repeats and function as a structural strut.

   Evidence Multi-angle light scattering (MALS) and small-angle X-ray scattering (SAXS) on recombinant human SYCE1

   PMID:30607510

   Open questions at the time

   • No atomic-resolution structure obtained
   • How the C-terminal extensions engage other SC components was unresolved
   • Whether the homodimer is the functional unit in vivo or is remodeled by partners was unknown
3. 2020 High

   Mapping two distinct SYCE1–SIX6OS1 binding interfaces—one that disrupts SYCE1's dimeric core and one downstream—answered how multivalent interactions build the SC central element, and showed that disruption of either interface alone abolishes synapsis and fertility.

   Evidence Biochemical binding assays, CRISPR knock-in mouse models, co-immunoprecipitation, and immunofluorescence

   PMID:32917591

   Open questions at the time

   • Structural basis of each interface at atomic resolution not determined
   • Whether other central element proteins (SYCE2, SYCE3, TEX12) contact SYCE1 through additional interfaces was not resolved
   • Stoichiometry of the higher-order SC lattice assembly remains unknown
4. 2020 High

   Demonstrating that human pathogenic SYCE1 mutations (nonsense, frameshift, splice-site) cause loss of protein through nonsense-mediated decay rather than production of a dysfunctional protein answered the mechanistic basis of SYCE1-associated infertility and established that complete loss of function underlies the phenotype.

   Evidence CRISPR/Cas9 humanized mouse models carrying patient-equivalent mutations, RT-qPCR, immunostaining, and cytological analysis of meiocytes

   PMID:32402064 PMID:35023261 PMID:39909494

   Open questions at the time

   • Whether hypomorphic (partial loss) SYCE1 mutations can produce intermediate phenotypes is untested
   • Mechanism of meiotic checkpoint activation upon SYCE1 loss not characterized
   • Whether SYCE1 loss differentially affects male vs. female meiotic progression beyond arrest is unclear
5. 2020 Medium

   Identification that patient SYCE1 mutations disrupting interaction with SYCP1 or SIX6OS1 impair SC assembly linked specific clinical variants to defined molecular interfaces, bridging structural biology with human disease genetics.

   Evidence Exome sequencing of infertile patients with functional co-immunoprecipitation in cell lines

   PMID:34718620

   Open questions at the time

   • Small patient cohort without independent replication
   • SYCP1–SYCE1 interaction interface not structurally mapped
   • Whether these mutations affect protein stability versus interaction specificity was not fully distinguished
6. 2022 Low

   A report that SYCE1 overexpression in somatic testicular cells (Sertoli, Leydig) activates steroidogenic gene expression (Star, Hsd3b, Srd5a1) raised the possibility of non-canonical roles outside meiotic chromosomes, but this has not been validated in vivo.

   Evidence Transfection and siRNA knockdown in mouse Sertoli and Leydig cell lines with gene expression analysis

   PMID:35697131

   Open questions at the time

   • Ectopic overexpression in somatic cells may not reflect physiological function; no in vivo confirmation
   • No mechanism linking a structural SC protein to transcriptional regulation of steroidogenic genes
   • Not independently replicated

• Key unresolved questions include the atomic-resolution structure of SYCE1 and its complexes with SIX6OS1 and SYCP1, the precise stoichiometry and architecture of the assembled SC central element lattice, the identity of the meiotic checkpoint pathway triggered by SYCE1 loss, and whether SYCE1 has any function outside the synaptonemal complex.
• No high-resolution crystal or cryo-EM structure of SYCE1 or its complexes
• SC lattice assembly mechanism and stoichiometry remain unresolved
• Checkpoint pathway connecting failed synapsis to meiotic arrest is not characterized for SYCE1 loss