(623c) RNA/Protein Phase Transitions and Cell Growth

Ribonucleoprotein (RNP) bodies are non membrane bound macromolecular assemblies. They are crucial for many fundamental biological processes, including RNA splicing, translational control, embryonic development, and cell growth. Among dozens of examples include germ granules and P bodies in the cytoplasm, and Cajal bodies and nucleoli in the nucleus. We have shown that these structures behave as liquid droplet phases of RNA and protein. We are particularly interested in the nucleolus, an RNP droplet which functions as a micro-reactor to catalyze the RNA processing steps in ribosome biogenesis, critical for cell growth. Using quantitative live-cell imaging and molecular biology approaches, we show that the nucleolus assembles by an intracellular phase transition resulting from protein-protein and protein-RNA interactions; perturbing these interactions using RNA interference (RNAi) leads to altered nucleolar structure and function. Moreover, we demonstrate that this phase transition is sensitive to geometric cues within the cell. Finally, we use microfabrication approaches to investigate the consequences of these perturbations on cell growth within a development context, using the model organism C.elegans.