Functional specialization of Arabidopsis VEL polymerization domains in the switch to Polycomb silencing

Epigenetic gene silencing by Polycomb Repressive Complex 2 (PRC2) is essential for development in eukaryotes, yet what initiates silencing is still unclear. Polycomb silencing at Arabidopsis FLOWERING LOCUS C (FLC) requires PRC2 and accessory proteins VIN3 and VRN5, both containing a structurally conserved polymerization module. Here, we show that polymerization of the VIN3 VEL domain increases the size of nuclear assemblies, with enhanced local concentration promoting chromatin association. This increased avidity enables effective H3K27me3 nucleation, underlying the digital switch to an epigenetically silenced state. However, VRN5 VEL is not essential for this PRC2 nucleation and VRN5 VEL is unable to replace VIN3 VEL function. This work thus defines a specific role for polymerization-mediated multivalency in initiating and maintaining PRC2 nucleation and extends our understanding of the principles that initiate epigenetic silencing.