Head-to-tail polymerization by VEL proteins underpins cold-induced Polycomb silencing in flowering control

Transcriptional silencing through the Polycomb silencing machinery utilizes a read-writemechanism involving histone tail modifications. However, nucleation of silencing and long-term stable transmission of the silenced state also requires P-olycomb Repressive Complex 2 (PRC2) accessory proteins, whose molec-ular role is poorly understood. The Arabidopsis VEL proteins are accessory proteins that interact with PRC2 to nucleate and propagate silencing at the FLOWERING LOCUS C (FLC) locus, enabling early flowering in spring. Here, we report that VEL proteins contain a domain related to an atypical four-helix bundle that en-gages in spontaneous concentration-dependent head-to-tail polymerization to assemble dynamic biomole-cular condensates. Mutations blocking polymerization of this VEL domain prevent Polycomb silencing at FLC. Plant VEL proteins thus facilitate assembly of dynamic multivalent Polycomb complexes required for inheritance of the silenced state.

Automated summary

Accessory proteins called VEL proteins in Arabidopsis facilitate the assembly of dynamic multivalent Polycomb complexes, which play a crucial role in the initiation and transmission of gene silencing, thereby regulating the flowering time in plants.