Structural basis of meiotic chromosome synaptic elongation through hierarchical fibrous assembly of SYCE2-TEX12

Abstract

The synaptonemal complex (SC) is a supramolecular protein assembly that mediates synapsis between homologous chromosomes during meiosis. SC elongation along the chromosome length (up to 24 μm) depends on its midline α-fibrous component SYCE2-TEX12. Here, we report X-ray crystal structures of human SYCE2-TEX12 as an individual building-block and upon assembly within a fibrous lattice. We combine these structures with mutagenesis, biophysics and electron microscopy to reveal the hierarchical mechanism of SYCE2-TEX12 fibre assembly. SYCE2-TEX12’s building-blocks are 2:2 coiled-coils which dimerise into 4:4 hetero-oligomers and interact end-to-end and laterally to form 10-nm fibres, which intertwine within 40-nm bundled micrometre-long fibres that define the SC’s midline structure. This assembly mechanism bears striking resemblance with intermediate filament proteins vimentin, lamin and keratin. Thus, SYCE2-TEX12 exhibits behaviour typical of cytoskeletal proteins to provide an α-fibrous SC backbone that structurally underpins synaptic elongation along meiotic chromosomes.

Parallel recruitment pathways contribute to synaptonemal complex assembly during mammalian meiosis

“…These SYCE2 microfoci were present in both asynapsed and potentially synapsed regions of axes (Figure 1C). In accordance with their known existence in a cofolded SYCE2-TEX12 complex (Davies et al, 2012;Dunce et al, 2021), and despite the higher background and diffuse off-axis staining of anti-TEX12 antibodies, we also detected TEX12 microfoci associated with asynapsed and potentially synapsed regions of axes in both Syce3 Δ/Δ and Syce3 WY/WY mutants (Figure 1C). Thus, somewhat unexpectedly, weak foci containing central element components SYCE2 and TEX12, but not SYCE3 or SYCE1, are recruited to meiotic chromosomes in both Syce3 Δ/Δ and Syce3 WY/WY spermatocytes.…”

“…In some cases, SYCP1 or SYCE2 staining extended linearly away from these foci, along chromosome axes, to link and coalesce adjacent foci (Figure 7E). These SYCP1 and SYCE2 extensions may correspond to the growth of SYCP1-containing lattices and SYCE2-TEX12 fibres, respectively (Dunce et al, 2021, 2018). The formation of these self-assembling structures is promoted by SYCE3 in vitro (Crichton et al, 2022), explaining their absence in Syce3 and Sycp1 mutants.…”

“…SYCP1, SYCE3, SYCE1, SYCE1-SIX6OS1, TEX12 and SYCE2-TEX12 protein constructs and complexes were purified as previously described (Davies et al, 2012; Dunce et al, 2021, 2018; Dunne and Davies, 2019a, 2019b; Sánchez-Sáez et al, 2020). In general, proteins were expressed as His 6 - or His 6 -MBP fusions in BL21(DE3) E. coli cells (Novagen ® ), and purified from lysate through Ni-NTA (Qiagen) or amylose (NEB) affinity, with removal of the tag by TEV protease treatment, followed by anion exchange chromatography (HiTrap Q HP, Cytiva) and size exclusion chromatography (HiLoad™ 16/600 Superdex™ 200, Cytiva) in 20 mM Tris pH 8.0, 150 mM KCl, 2 mM DTT.…”

“…Hence, SYCE2-TEX12 may directly recruit and nucleate assembly of integrated SYCP1-SYCE3 lattices at recombination sites. It is uncertain whether SYCE2 at recombination sites adopts the same fibrous SYCE2-TEX12 assemblies that are formed within the CE (Dunce et al, 2021). The difference in staining intensity between SYCE2 microfoci and the CE means that it is not possible to determine whether SYCE2 structures at recombination sites remain intact upon full SC assembly.…”

AF2Complex predicts direct physical interactions in multimeric proteins with deep learning

“…Figure 2 displays the results of AF2Complex on some challenging targets. The first example, H1072, is a heterotetramer (stoichiometry A 2 B 2 ) consisting of two copies of two coiled-coil protein sequences 38 . Despite the simple topologies and the availability of an experimental structure for one monomer, H1072 is a difficult target.…”