(314h) Micro-Rheology of Intracellular RNA/Protein Emulsions

Ribonucleoprotein (RNP) granules are membrane-less organelles assembled from the dynamic interactions of RNA and protein. We have shown that these structures behave as liquid phase droplets, which appear to assemble from soluble RNA and protein by an intracellular phase transition. The nucleolus is an RNP droplet that functions in ribosome biogenesis, and thus plays a central role in cell growth and size control. The growth of large cells such as X.laevis oocytes requires hundreds of extra-chromosomal nucleoli, which are distributed throughout the nuclear volume. We show that nuclear F-actin forms a network that spatially organizes the nucleus by kinetically stabilizing an emulsion of these nucleoli; in actin-disrupted nuclei, embedded droplets undergo gravitational sedimentation and fusion at the bottom of the nucleus.  Using a combination of active and passive micro-rheology approaches, together with quantitative imaging, we probe the length-scale dependent architecture and mechanics of these stabilized biological emulsions. We show how these measurements provide important insights into mechanical and geometric aspects of cell organization, and suggest biophysical constraints play a role in cell growth and size control.