Different polycomb group CBX family proteins associate with distinct regions of chromatin using nonhomologous protein sequences

Abstract

Polycomb group proteins are transcriptional repressors recruited to many developmental control genes. The specificity of polycomb group protein targeting is incompletely understood. Subunits of polycomb repressive complexes (PRC) are encoded by multigene families in vertebrates. Five chromodomain-containing CBX family proteins are thought to mediate chromatin association by PRC1 complexes. We visualized the recruitment of CBX proteins to chromatin using bimolecular fluorescence complementation (BiFC) analysis, wherein fragments of fluorescent proteins fused to CBX family members and histone H3 form a fluorescent complex when the CBX proteins bind to nucleosomes. Different CBX family proteins associated with nucleosomes in different subnuclear regions in both ES cells and fibroblasts. The total populations of most CBX proteins had distributions distinct from those of the chromatin-associated complexes, indicating that most of these CBX proteins were not bound to nucleosomes. The conserved chromodomain and chromobox regions of CBX proteins were dispensable for chromatin association. The absence of H3 K27 trimethylation in EED null ES cells had minimal effects on chromatin association by CBX proteins. The BiFC complexes did not colocalize with anti-trimethyl-K27 immunofluorescence, with the exception of inactive X. Metaphase spreads derived from stable cell lines with inducible CBX fusion expression revealed reciprocal patterns of chromosome association by CBX2 and CBX6 BiFC complexes. H3.2 purified from CBX2-H3.2 BiFC complexes was enriched in trimethyl-K27, dimethyl-K4, and acetyl-K9 modifications. We conclude that different CBX proteins are recruited to distinct chromatin regions through nonconserved interactions, expanding the regulatory diversity of polycomb group proteins.