Crowded chromatin is not sufficient for heterochromatin formation and not required for its maintenance

In contrast to cytoplasmic organelles, which are usually separated from the rest of the cell by phospholipid membranes, nuclear compartments are readily accessible to diffusing proteins and must rely on different mechanisms to maintain their integrity. Specific interactions between scaffolding proteins are known to have important roles for the formation and maintenance of nuclear structures. General physical mechanisms such as molecular crowding, phase separation or colloidal behavior have also been suggested, but their physiological significance remains uncertain. For macromolecular crowding, a role in the maintenance of nucleoli and promyelocytic leukemia (PML) nuclear bodies has been shown. Here, we tested whether a modulation of the compaction state of chromatin, which directly influences the local crowding state, has an impact on the formation and maintenance of densely packed heterochromatin. By osmotic perturbations, we could modify the packing state of chromatin in a controlled manner and show that chromatin compaction, which is associated with increased crowding conditions, is not, per se, sufficient to initiate the formation of new bona fide heterochromatin structures nor is it necessary to maintain already established heterochromatin domains. In consequence, if an increase in crowding induced by chromatin compaction maybe an early step in heterochromatin formation, specific protein-protein interactions are nevertheless required to make heterochromatin long lasting and independent of the crowding state.