Abstrakt
The interphase nucleus of eukaryotes is a highly compartmentalized organelle. Its inner content can be categorized into a chromatin compartment and an interchromatin compartment.
The interchromatin compartment harbors numerous distinct protein/ RNA-based subcompartments that contain little or no DNA. Such nuclear subcompartments offer a mechanism by which to concentrate and spatially segregate nuclear activities.
For example, nuclear bodies represent the local accumulations of a set of resident proteins and RNAs in the interchromatin compartment and offer a microenvironment that can improve the efficiency and fidelity of protein-DNA or protein-RNA interactions that are important for gene expression. Specifically, a Polycomb (PcG) body is considered to be the accumulation of PcG proteins, and it has been proposed that the PcG body acts as a gene silencing factory in the interchromatin compartment.
However, the structural basis of the PcG body is still under debate. Diverging reports on protein-based versus chromatin-based nature of the PcG body were published.
In order to expand the information on the nature of the PcG bodies in U-2 OS cells, we focused on the behaviour of the PcG proteins encompassed in fluorescent PcG bodies under conditions of hyperosmotically induced molecular crowding that allows to differentiate between interchromatin and chromatin compartments. We observed under these conditions that the PcG bodies disappeared, but persisted as nuclear domains characterized by accumulations of DNA.
We found that the disappearance of PcG proteins from the original PcG bodies was associated with their hyper-phosphorylation. Importantly, fluorescence microscopy results showed that the changes observed in hyperosmotically treated cells were quickly reversible after reincubation of cells in normal medium.
We conclude that the PcG body in U-2 OS cells is not a nuclear body, but a chromatin domain.