Data from: Heterochromatin Protein 1β (HP1β) has distinct functions and distinct nuclear distribution in pluripotent versus differentiated cells

Background: Pluripotent embryonic stem cells (ESCs) have the unique
ability to differentiate into every cell type and to self-renew. These
characteristics correlate with a distinct nuclear architecture, epigenetic
signatures enriched for active chromatin marks and hyperdynamic binding of
structural chromatin proteins. Recently, several chromatin-related
proteins have been shown to regulate ESC pluripotency and/or
differentiation, yet the role of the major heterochromatin proteins in
pluripotency is unknown. Results: Here we identify Heterochromatin Protein
1β (HP1β) as an essential protein for proper differentiation, and,
unexpectedly, for the maintenance of pluripotency in ESCs. In pluripotent
and differentiated cells HP1β is differentially localized and
differentially associated with chromatin. Deletion of HP1β, but not HP1α,
in ESCs provokes a loss of the morphological and proliferative
characteristics of embryonic pluripotent cells, reduces expression of
pluripotency factors and causes aberrant differentiation. However, in
differentiated cells, loss of HP1β has the opposite effect, perturbing
maintenance of the differentiation state and facilitating reprogramming to
an induced pluripotent state. Microscopy, biochemical fractionation and
chromatin immunoprecipitation reveal a diffuse nucleoplasmic distribution,
weak association with chromatin and high expression levels for HP1β in
ESCs. The minor fraction of HP1β that is chromatin-bound in ESCs is
enriched within exons, unlike the situation in differentiated cells, where
it binds heterochromatic satellite repeats and chromocenters. Conclusions:
We demonstrate an unexpected duality in the role of HP1β: it is essential
in ESCs for maintaining pluripotency, while it is required for proper
differentiation in differentiated cells. Thus, HP1β function both depends
on, and regulates, the pluripotent state.