Distinct accessory roles of Arabidopsis VEL proteins in Polycomb silencing

Polycomb repressive complex 2 (PRC2) mediates epigenetic silencing of target genes in animals and plants. In Arabidopsis, PRC2 is required for the cold-induced epigenetic silencing of the FLC floral repressor locus to align flowering with spring. During this process, PRC2 relies on VEL accessory factors, including the constitutively expressed VRN5 and the cold-induced VIN3. The VEL proteins are physically associated with PRC2, but their individual functions remain unclear. Here, we show an intimate association between recombinant VRN5 and multiple components within a reconstituted PRC2, dependent on a compact conformation of VRN5 central domains. Key residues mediating this compact conformation are conserved among VRN5 orthologs across the plant kingdom. In contrast, VIN3 interacts with VAL1, a transcriptional repressor that binds directly to FLC These associations differentially affect their role in H3K27me deposition: Both proteins are required for H3K27me3, but only VRN5 is necessary for H3K27me2. Although originally defined as vernalization regulators, VIN3 and VRN5 coassociate with many targets in the Arabidopsis genome that are modified with H3K27me3. Our work therefore reveals the distinct accessory roles for VEL proteins in conferring cold-induced silencing on FLC, with broad relevance for PRC2 targets generally.

The C-terminal WD40 repeats on the TOPLESS co-repressor function as a protein-protein interaction surface

The two predicted WD40 propellers on TOPLESS function as protein-protein interaction domains. The 1st WD40 propeller mediates interaction with RAV1, and the 2nd WD40 propeller mediates interaction with VRN5. The TOPLESS/TOPLESS-RELATED (TPL/TPR) co-repressor family proteins are known to interact with a wide variety of proteins including transcription factors, Mediator subunits, histone deacetylases, and histone tails. Through these interactions, TPL/TPR act to repress transcription in an increasingly diverse array of plant pathways. Proteins that bind TPL/TPR typically contain one or more Repression Domains (RDs) that mediate the interaction. For example, the well-characterized Ethylene response factor-associated Amphiphilic Repression (EAR) motif is known to facilitate interaction by binding the TOPLESS Domain (TPD) located in the N-terminus. Here we show that in yeast two-hybrid assays, the non-EAR protein, Related to ABI3/VP1-1 (RAV1), binds a novel region located within the first nine WD40-repeats of TPL. Protein modeling and in silico analysis suggest that these nine WD40 repeats may form the first of two WD40 propellers located on C-terminus of TPL. The interaction between RAV1 and the 1st WD40 propeller is conserved with another RAV family member, TEMPRANILLO1 (TEM1) and is mediated by the B3 Repression Domain (BRD) located on both RAV1 and TEM1. Also, the predicted 2nd WD40 propeller was shown in yeast cells to bind Vernalization 5 (VRN5), which contains several unconfirmed partial RDs. Furthermore, we demonstrate that the 1st WD40 propeller of TPL can form a complex with RAV1 both in yeast and in Arabidopsis protoplasts.