LHP1 interacts with ATRX through plant-specific domains at specific loci targeted by PRC2

Heterochromatin Protein 1 (HP1) is a major regulator of chromatin structure and function. In animals, the network of proteins interacting with HP1 is mainly associated with constitutive heterochromatin marked by H3K9me3. HP1 physically interacts with the putative orthologue of the SNF2 chromatin remodeler ATRX, which controls deposition of the histone variant H3.3 in mammals. In Arabidopsis thaliana, we show that the orthologue of ATRX participates in H3.3 deposition and characterize the function of conserved domains of plant ATRX. We show that the plant Like HP1 (LHP1) interacts with ATRX through domains that evolved specifically in land plants ancestors. Interaction between ATRX and LHP1 affects the expression of a limited subset of genes controlled by the POLYCOMB REPRESSIVE COMPLEX 2 (PRC2), including the flowering time regulator FLC. In the context of flowering time, ATRX function requires the novel LHP1-interacting domain and the ATPase of the ATRX SNF2 helicase. We conclude that distinct evolutionary pathways led to interaction between ATRX and HP1 in mammals or its counterpart LHP1 in plants, resulting in distinct modes of transcriptional regulation. 1 sample (mutant) + 1 reference sample (control)