- Abstract Heterochromatin protein 1 (HP1) controls heterochromatin formation in animal cells, at least partly through interaction with lysine 9 (Lys-9)-methylated histone H3. We aimed to determine whether a structurally conserved human HP1 protein exhibits conserved heterochromatin localization in plant cells and studied its relation to modified histone H3. We generated transgenic tobacco plants and cycling cells expressing the human HP1γ fused to green fluorescent protein (GFP) and followed its association with chromatin. Plants expressing GFP-HP1γ showed no phenotypic perturbations. We found that GFP-HP1γ is preferentially associated with the transcriptionally “inactive” heterochromatin fraction, a fraction enriched in Lys-9-methylated histone H3. During mitosis GFP-HP1γ is detached from chromosomes concomitantly with phosphorylation of histone H3 at serine 10 and reassembles as cells exit mitosis. However, this phosphorylation cannot directly account for the dissociation of GFP-HP1γ from mitotic chromosomes inasmuch as phosphorylation does not interfere with binding to HP1γ. It is, therefore, possible that phosphorylation at serine 10 creates a “code” that is read by as yet an unknown factor(s), eventually leading to detachment of GFP-HP1γ from mitotic chromosomes. Together, our results suggest that chromatin organization in plants and animals is conserved, being controlled at least partly by the association of HP1 proteins with methylated histone H3.