Reconstitution of a flexible SYCP3-DNA fibre suggests a mechanism for SYCP3 coating of the meiotic chromosome axis

Abstract

The synaptonemal complex (SC) keeps homologous chromosomes in close alignment during meiotic crossover. A hallmark of SC formation is the presence of its protein component SYCP3 on the chromosome axis. As SC assembly progresses, SYCP3 is deposited on both axes of the homologue pair, forming the lateral element (LE) in the tripartite structure of the mature SC. We have used cryo-electron tomography and atomic force microscopy to study the mechanism of assembly and DNA binding of the SYCP3 fibre. We find that the three-dimensional architecture of the fibre is built on a highly irregular arrangement of SYCP3 molecules displaying very limited local geometry. Interaction between SYCP3 molecules is driven by the intrinsically disordered tails of the protein, with no contact between the helical cores, resulting in a flexible fibre assembly. We demonstrate that the SYCP3 fibre can engage in extensive interactions with DNA, indicative of an efficient mechanism for incorporation of DNA within the fibre. Taken together, our findings suggest that, upon deposition on the chromosome axis, SYCP3 spreads by polymerising into a fibre that is fastened to the chromosome surface via DNA binding. The resulting layer of SYCP3 coating the chromosome axis might provide a structural basis for LE assembly in meiotic prophase.

A conserved filamentous assembly underlies the structure of the meiotic chromosome axis

“…Several recent studies have reported that in vitro, mammalian SYCP3 forms coiled-coil homotetramers that further assemble into large oligomeric structures with a 22 nm periodicity (Syrjänen et al, 2014). Further, SYCP3 can bind DNA through two short patches of basic residues near the N-terminus of this protein’s coiled-coil domain (Syrjänen et al, 2014), and large SYCP3 oligomers appear to bind and condense plasmid DNA (Bollschweiler et al, 2018). These data have led to a model whereby homotypic SYCP3 oligomers, interacting directly with DNA, form a major part of the mammalian chromosome axis (Syrjänen et al, 2014).…”

A conserved mechanism for meiotic chromosome organization through self-assembly of a filamentous chromosome axis core

“…Several recent studies have reported that in vitro, mammalian SYCP3 forms coiled-coil homotetramers that further assemble into large oligomeric structures with a 22 nm periodicity (49,51,52). Further, SYCP3 can bind DNA through two short patches of basic residues near the N-terminus of this protein's coiled-coil domain (49), and large SYCP3 oligomers appear to bind and condense plasmid DNA (52). These data have led to a model whereby homotypic SYCP3 oligomers, interacting directly with DNA, form a major part of the mammalian chromosome axis (49,52).…”

“…Further, SYCP3 can bind DNA through two short patches of basic residues near the N-terminus of this protein's coiled-coil domain (49), and large SYCP3 oligomers appear to bind and condense plasmid DNA (52). These data have led to a model whereby homotypic SYCP3 oligomers, interacting directly with DNA, form a major part of the mammalian chromosome axis (49,52). While we cannot rule out the formation of homotypic SYCP3 assemblies in meiotic cells, our data shows that the SYCP2:SYCP3 heterotetramer is more stable in solution than the SYCP3 homotetramer, and is therefore likely to be the preferred assembly in meiotic cells.…”

“…More significantly, the oligomeric structure of the axis core proteins, whether this structure is conserved, and how this structure contributes to the axis's roles in chromosome organization, inter-homolog recombination, and SC architecture are important open questions. Mammalian SYCP3 is known to form coiled-coil homotetramers (49) that self-associate into larger structures that can be observed both in cell culture (31,50) and in vitro (49,51,52), but how SYCP3 cooperates with SYCP2 to mediate chromosome localization and axis assembly is not known. Neither fungal Red1 nor plant ASY3/ASY4 has been characterized biochemically, leaving open the question of how these proteins self-assemble, and whether these assemblies resemble those of mammalian SYCP3.…”