Stress granules regulate paraspeckles: RNP granule continuum at work.

Eukaryotic cells contain several types of RNA-protein membraneless macro-complexes - ribonucleoprotein (RNP) granules that form by liquid-liquid phase separation. These structures represent biochemical microreactors for a variety of cellular processes and also act as highly accurate sensors of changes in the cellular environment. RNP granules share multiple protein components, however, the connection between spatially separated granules remains surprisingly understudied. Paraspeckles are constitutive nuclear RNP granules whose numbers significantly increase in stressed cells. Our recent work using affinity-purified paraspeckles revealed that another type of RNP granule, cytoplasmic stress granule (SG), acts as an important regulator of stress-induced paraspeckle assembly. Our study demonstrates that despite their residency in different cellular compartments, the two RNP granules are closely connected. This study suggests that nuclear and cytoplasmic RNP granules are integral parts of the intracellular "RNP granule continuum" and that rapid exchange of protein components within this continuum is important for the temporal control of cellular stress responses. It also suggests that cells can tolerate and efficiently handle a certain level of phase separation, which is reflected in the existence of "bursts", or "waves", of RNP granule formation. Our study triggers a number of important questions related to the mechanisms controlling the flow of RNP granule components within the continuum and to the possibility of targeting these mechanisms in human disease.