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For most intracellular structures with larger than molecular dimensions, little is known about the connection between underlying molecular activities and higher order organization such as size and shape. Here, we show that both the size and shape of the amphibian oocyte nucleolus ultimately arise because nucleoli behave as liquid-like droplets of RNA and protein, exhibiting characteristic viscous fluid dynamics even on timescales of < 1 min. We use these dynamics to determine an apparent nucleolar viscosity, and we show that this viscosity is ATP-dependent, suggesting a role for active processes in fluidizing internal contents. Nucleolar surface tension and fluidity cause their restructuring into spherical droplets upon imposed mechanical deformations. Nucleoli exhibit a broad distribution of sizes with a characteristic power law, which we show is a consequence of spontaneous coalescence events. These results have implications for the function of nucleoli in ribosome subunit processing and provide a physical link between activity within a macromolecular assembly and its physical properties on larger length scales.
Fig. 5.
Dynamics of nucleolar substructures. (A) A GV exhibiting GFP∷fibrillarin/RFP∷NPM1�labeled nucleoli. Note that larger nucleoli contain many fibrillarin cores, whereas smaller nucleoli contain only one or two fibrillarin cores. (B) Image sequence showing fusion of several GFP∷fibrillarin/RFP∷NPM1-labeled nucleoli. Arrowheads track the positions of two fibrillar cores. (C) Image sequence showing the dynamics of GFP∷NO145 at the cortex of two fusing nucleoli. Note the GFP∷NO145 localization to the interior of nucleoli at the surface of nucleolar vacuole-like structures (*). (D) Intensity profile across the dotted line shown in C; peaks 1 and 2 reflect vacuolar surface, and peak 3 reflects nucleolar cortex.
Fig. 6.
The role of a surrounding actin scaffold in slowing internucleolar contact. (A) Pulling on one region of the GV with a microneedle displaces nucleoli in distal regions, suggesting the presence of an elastic scaffold. Four nucleoli are highlighted. (B) GV treated with Cyto-D to disrupt filamentous actin. Nucleoli freely diffuse throughout the GV and fuse with one another upon contact; the fates of three nucleoli are highlighted. One large nucleolar clump results; note, however, that under these drug conditions nucleoli do not appear to regain sphericity and fusion appears perturbed.
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