Shape Profile of Ribonucleoprotein Droplets Offer Insights into how Surface Tension and Dynamics Are Critical to Structure and Function

Abstract

Non-membrane bound nuclear organelles play essential roles in the organism, and the mechanisms behind their self-assembly and dynamics are still poorly understood. Nucleoli, especially, are indispensable in their role in ribosome biogenesis, acting as factories for the cell. They have also lately been discovered to exhibit characteristics of liquids, and this work further investigates the material properties, namely surface tension and viscosity, of these nucleoli. The sessile drop method, based on balancing the forces of gravity and surface energy on a drop, was used to determine the surface tension for different-sized nucleoli of immature oocytes of Xenopus laevis. These nucleoli were imaged with a right angle prism to obtain the XZ-plane profiles so that the necessary parameters could be extracted in order to use the sessile drop method. It was found that the surface tension is 4.9E-7 N/m ± 1E-6 N/m. Large nucleoli, however, deviated from expected results, and this is perhaps attributed to not allotting enough time for them to relax completely. To further investigate this, nucleoli were imaged while they settled to calculate a characteristic relaxation time. The data from this also allowed us to estimate an apparent viscosity of the nucleoli, calculated as on the order of 10\(^{-2}\) Pa·s. The results shed light on the dynamics within nucleoli and their interaction with the nucleoplasm. The relatively low surface tension and relatively high viscosity may allow for easier exchange of proteins between the nucleoli and nucleoplasm and slowed down nucleolar processes, respectively. Additional experiments were done to 1) observe dense fibrillar components within the nucleoli to further understand the interactions between these regions and the surrounding medium in the nucleoli, and 2) to test the effects of the actin-disrupting drug that was used for the surface tension experiments.