Abstract
It has become evident that chromatin in cell nuclei is organized at multiple scales. Significant effort has been devoted to understanding the connection between the nuclear environment and the diverse biological processes taking place therein. A fundamental question is how cells manage to orchestrate these reactions, both spatially and temporally. Recent insights into phase-separated membraneless organelles may be the key for answering this. Of the two models that have been proposed for phase-separated entities, one largely depends on chromatin–protein interactions and the other on multivalent protein–protein and/or protein–RNA ones. Each has its own characteristics, but both would be able to, at least in part, explain chromatin and transcriptional organization. Here, we attempt to give an overview of these two models and their studied examples to date, before discussing the forces that could govern phase separation and prevent it from arising unrestrainedly.
Highlights
The core concept of phase separation itself is not really new
The fact that nuclear compartmentalization cannot be static in order to accommodate and coordinate the huge variety of biochemical reactions that take place therein, a major question arising is: how might such phase-separated nuclear entities contribute to the organization and regulation of chromatin? In the light of recent data on phase separation-driven compartmentalization, this review aims at providing some insight on the key characteristics of nuclear phase-separated formations, on how phase separation may regulate chromatin organization and on the forces that restrain phase separation from occurring in a non-orchestrated manner
Different studies have proposed that phase separation might, at least in part, control transcription [8,9,10] and, as a result, genome architecture and accessibility [11,12] via the formation of a large variety of membraneless nuclear bodies
Summary
The core concept of phase separation itself is not really new. Already in 1899, the American biologist E. (c) Persistence of elevated nuclear ATP levels, in conjunction with chromatin/protein modifications and high local RNA titres, aid in the maintenance of supramolecular condensates (factories) by TFs and the general transcription machinery, while low ATP levels, Mg++ cations and additional insofar unknown factors will deter and/or reverse such phase separation in the nucleoplasm Along these lines, different studies have proposed that phase separation might, at least in part, control transcription [8,9,10] and, as a result, genome architecture and accessibility [11,12] via the formation of a large variety of membraneless nuclear bodies (figure 1a). In the case of PPPS, molecules need not actively bind to one another but are dependent on the availability of chromatin (and most probably of other contributing factors), while in LLPS, bridging interactions with nucleic acids are not a prerequisite for droplet formation compared to the interactions between disordered domains of the contributing proteins (for a comparison of the two, see table 1)
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