(470g) RNA/Protein Droplet Assembly by Intracellular Phase Transitions

Ribonucleoprotein (RNP) bodies are non membrane bound macromolecular assemblies that form from the dynamic interactions of RNA and protein. They are crucial for cell growth, embryonic development, and many other fundamental biological processes. Among dozens of examples include germ granules and processing bodies in the cytoplasm, and Cajal bodies and nucleoli in the nucleus. We show that these structures behave as liquid droplets of RNA and protein. RNP droplets assemble from components in the surrounding cytoplasm by a de-mixing phase transition, whose set-point is biologically controlled in space and time. For several types of RNP bodies, this phase transition appears to be controlled during different developmental stages to accommodate growth-dependent changes in RNA metabolism. To understand the biophysical properties of these structures, we use a micro-rheological approach to characterize their viscoelastic moduli, which appear to depend on their intrinsic biological activity. These findings shed light on how RNP bodies can self-assemble from soluble RNA/protein components, and simultaneously function as micro-reactors for these components, speeding up rate-limiting steps in RNA metabolism by localizing high concentrations of reactants.