Abstract
The interphase eukaryotic nucleus is a highly organized organelle consisting of various structurally and functionally different subcompartments or domains. Generally, the nuclear subcompartments can be divided into chromatin or interchromatin domains.
In contrast to chromatin domains, interchromatin subcompartments contain little or no DNA and are of a proteinaceous or ribonucleoproteinaceous nature; and can be divided into nuclear factories, speckles and bodies. In our work, we were interested in Polycomb (PcG) bodies that were thought to be formed by accumulations of Polycomb group (PcG) proteins.
PcG proteins are important epigenetic factors that regulate gene expression by chromatin modification followed by chromatin compaction. The aim of our work was to established the fine structure of the PcG body and to follow its behaviour within the frame of the experimental model of molecular crowding, an important factor for formation and stability of proteinacoeus subcompartments.
We found out that the essence of the fluorescent PcG foci is associated with the local accumulation of heterochromatin fascicles with the same labeling density against polycomb BMI1 protein per area of dense chromatin throughout the nucleus. We defined a "PcG body" as a chromatin domain rather than a nuclear body.
Further, we observed that the behaviour of the fluorescent PcG foci in cells grown in hyperosmotic medium vastly differs from the behaviour of nuclear bodies. Whereas a typical nuclear bodies are formed under condition of experimental crowding, we found that PcG foci reversibly disappeared.
Importantly, "PcG bodies" as condensed chromatin accumulations persisted. Thus, we revealed that the nature of so-called "PcG body" is in condensed chromatin that shows no dependance on polycomb BMI1 and RING1a proteins to stay condensed under (not only) molecular crowding conditions.
Importantly, by using the correlative microscopy, our research was performed at the single cell level.