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 on assembly within a fibrous lattice. We combine these structures with mutagenesis, biophysics and electron microscopy to reveal the hierarchical mechanism of SYCE2-TEX12 fiber assembly. SYCE2-TEX12’s building blocks are 2:2 coiled coils that dimerize into 4:4 hetero-oligomers and interact end-to-end and laterally to form 10-nm fibers that intertwine within 40-nm bundled micrometer-long fibers 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 behavior typical of cytoskeletal proteins to provide an α-fibrous SC backbone that structurally underpins synaptic elongation along meiotic chromosomes.

联会复合体 (SC) 是一种超分子蛋白组装体，可在减数分裂期间介导同源染色体之间的突触。SC 沿染色体长度（最长 24 μm）的伸长取决于其中线 α-纤维成分 SYCE2-TEX12。在这里，我们报告了人类 SYCE2-TEX12 的 X 射线晶体结构作为单独的构建块并在纤维晶格内组装。我们将这些结构与诱变、生物物理学和电子显微镜相结合，以揭示 SYCE2-TEX12 纤维组装的分层机制。SYCE2-TEX12 的构建模块是 2:2 盘绕线圈，它们二聚成 4:4 异质低聚物并端到端和横向相互作用，形成 10-nm 纤维，这些纤维在 40-nm 捆绑的微米长纤维中交织在一起，定义了 SC 的中线结构。这种组装机制与中间丝蛋白波形蛋白、核纤层蛋白和角蛋白有着惊人的相似之处。因此，SYCE2-TEX12 表现出细胞骨架蛋白的典型行为，以提供在结构上支撑沿减数分裂染色体突触伸长的 α-纤维 SC 主链。