Membraneless nuclear organelles and the search for phases within phases

Abstract

Cells are segregated into two distinct compartment groups to optimize cellular function. The first is characterized by lipid membranes that encapsulate specific regions and regulate macromolecular flux. The second, known collectively as membraneless organelles (MLOs), lacks defining lipid membranes and exhibits self-organizing properties. MLOs are enriched with specific RNAs and proteins that catalyze essential cellular processes. A prominent sub-class of MLOs are known as nuclear bodies, which includes nucleoli, paraspeckles, and other droplets. These microenvironments contain specific RNAs, exhibit archetypal liquid-liquid phase separation characteristics, and harbor intrinsically disordered, multivalent hub proteins. We present an analysis of nuclear body protein disorder that suggests MLO proteomes are significantly more disordered than structured cellular features. We also outline common MLO ultrastructural features, exemplified by the three sub-compartments present inside the nucleolus. A core-shell configuration, or phase within a phase, is displayed by several nuclear bodies and may be functionally important. Finally, we summarize evidence indicating extensive RNA and protein sharing between distinct nuclear bodies, suggesting functional cooperation and similar nucleation principles. Considering the substantial accumulation of specific coding and noncoding RNA classes inside MLOs, evidence that RNA buffers specific phase transition events, and the absence of a clear correlation between total intrinsic protein disorder and MLO accumulation, we conclude that RNA biogenesis may play a key role in MLO formation, internal organization, and function. This article is categorized under: RNA Export and Localization > RNA Localization RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Keywords: intrinsically disordered regions; membraneless organelles; non-coding RNA; nuclear body; nucleolus; phase separation.