A molecular model for self-assembly of synaptonemal complex protein SYCE3

Abstract

The synaptonemal complex (SC) is a supramolecular protein assembly that mediates homologous chromosome synapsis during meiosis. This zipper-like structure assembles in a continuous manner between homologous chromosome axes, enforcing a 100-nm separation along their entire length, and providing the necessary three-dimensional framework for crossover formation. The mammalian SC is formed of eight components - SYCP1-3, SYCE1-3, TEX12 and SIX6OS1 - arranged in transverse and longitudinal structures. These largely α-helical coiled-coil proteins undergo heterotypic interactions, coupled with recursive self-assembly of SYCP1, SYCE2-TEX12, and SYCP2-SYCP3, to achieve the vast supramolecular structure of the SC. Here, we report a novel self-assembly mechanism of SC central element component SYCE3, identified through multi-angle light scattering and small-angle X-ray scattering. SYCE3 adopts a dimeric four-helical bundle structure that acts as the building block for concentration-dependent self-assembly into a series of discrete higher order oligomers. This is achieved through staggered lateral interactions between self-assembly surfaces of SYCE3 dimers, and their end-on interaction through intermolecular domain-swap between dimer folds. These mechanisms combine to achieve potentially limitless SYCE3 assembly, which particularly favours formation of dodecamers of three laterally associated domain-swap tetramers. Our findings extend the family of self-assembling proteins within the SC and provide novel means for structural stabilisation of the SC central element.