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

Abstract The supramolecular structure of the synaptonemal complex (SC) mediates homologous chromosome synapsis and facilitates the formation of genetic crossovers during meiosis. The mammalian SC is formed of eight coiled-coil proteins which interact in specific subcomplexes and self-assemble into distinct macromolecular arrays that fulfil specific aspects of the SC’s dynamic functional architecture. Here, we report the structure of SC subcomplex SYCE2-TEX12 and its mechanism of self-assembly into fibres that define the SC’s midline longitudinal structure. X-ray crystal structures, electron microscopy, biophysical and mutational analyses reveal that SYCE2-TEX12 is assembled from 2:2 complexes in which TEX12 chains are positioned at either end of a rod-like SYCE2 dimer. These molecular building-blocks assemble laterally into 4:4 complexes, and longitudinally into fibres of potentially limitless length, which acts as string-like threads that wind around one another in intertwined fibres of up to 40-nm in width and several micrometres in length. This hierarchical assembly mechanism is reminiscent of intermediate filament proteins and results in SYCE2-TEX12 fibres that can span the entire chromosome length, thereby providing the underpinning structural support for SC elongation during meiotic chromosome synapsis.