In vivo labeling 1 (original) (raw)

Changes in chromatin structure are a key aspect in the epigenetic regulation of gene expression. We have used a lac operator array system to visualize by light microscopy the effect of heterochromatin protein 1 (HP1) ␣ (HP1␣) and HP1␤ on large-scale chromatin structure in living mammalian cells. The structure of HP1, containing a chromodomain, a chromoshadow domain, and a hinge domain, allows it to bind to a variety of proteins. In vivo targeting of an enhanced green fluorescent protein-tagged HP1-lac repressor fusion to a lac operator-containing, gene-amplified chromosome region causes local condensation of the higher-order chromatin structure, recruitment of the histone methyltransferase SETDB1, and enhanced trimethylation of histone H3 lysine 9. Polycomb group proteins of both the HPC/HPH and the EED/EZH2 complexes, which are involved in the heritable repression of gene activity, are not recruited to the amplified chromosome region by HP1␣ and HP1␤ in vivo targeting. HP1␣ targeting causes the recruitment of endogenous HP1␤ to the chromatin region and vice versa, indicating a direct interaction between the two HP1 homologous proteins. Our findings indicate that HP1␣ and HP1␤ targeting is sufficient to induce heterochromatin formation.