Many plants that grow in temperate regions require a phase of cold exposure before they can switch to flowering. This allows them to align reproductive growth with spring, a warmer phase that follows cold exposure. FLC is a key repressor of reproductive growth, and is only silenced after having been exposed to cold — a phenomenon known as vernalization. Silencing occurs epigenetically in two steps. During cold exposure, few histone H3 proteins of FLC-bound nucleosomes are trimethylated at a conserved lysine residue (H3K37me3) by the PRC2 complex (nucleation). Then, when temperatures rise again in spring, this repressive mark spreads across the entire locus, which again requires PRC2. It is not clear how PRC2 can extend the methylation pattern past the nucleation sites, not only on one strand of DNA but also on the daughter strand after replication in dividing cells.
These researchers show that VEL domain proteins that were previously known to associate with PRC2 complexes in plants have the capacity to form biomolecular condensates by head-to-tail fusion of monomers. Heterologously expressed VEL domain proteins can self-assemble depending on their VEL domain. In fact, the VEL domain alone — which adopts a globular fold with four α-helices — can polymerize in vitro to short homo-oligomers. Fiedler et al. found that the VEL domains of VERNALIZATION5/VIN3-LIKE 1 (VEL1) can self-assemble into fibrous superstructures that consist of three intertwined right-handed helical filaments of head-to-tail assembled monomers. VEL domains of VERNALIZATION INSENSITIVE 3 (VIN3) assemble into lattice superstructures, which involves domain swapping between two monomers. Such biomolecular condensates could allow for the concentration of PRC2 complexes at specific loci such as FLC, facilitating not only the spreading of previously nucleated repressive marks but also their inheritance to daughter strands after replication.
This is a preview of subscription content, access via your institution