Truncated HP1 lacking a functional chromodomain induces heterochromatinization upon in vivo targeting

Packaging of the eukaryotic genome into higher order chromatin structures is tightly related to gene expression. Pericentromeric heterochromatin is typified by accumulations of heterochromatin protein 1 (HP1), methylation of histone H3 at lysine 9 (MeH3K9) and global histone deacetylation. HP1 interacts with chromatin by binding to MeH3K9 through the chromodomain (CD). HP1 dimerizes with itself and binds a variety of proteins through its chromoshadow domain. We have analyzed at the single cell level whether HP1 lacking its functional CD is able to induce heterochromatinization in vivo. We used a lac-operator array-based system in mammalian cells to target EGFP-lac repressor tagged truncated HP1α and HP1β to a lac operator containing gene-amplified chromosome region in living cells. After targeting truncated HP1α or HP1β we observe enhanced tri-MeH3K9 and recruitment of endogenous HP1α and HP1β to the chromosome region. We show that CD-less HP1α can induce chromatin condensation, whereas the effect of truncated HP1β is less pronounced. Our results demonstrate that after lac repressor-mediated targeting, HP1α and HP1β without a functional CD are able to induce heterochromatinization.