PgmNr D1084: To get more for less: Thermodynamic versus active mechanisms of the nucleolus assembly.

Authors:
H. Falahati; E. Wieschaus


Institutes
Princeton University, Princeton, NJ.


Keyword: organelle maintenance & replication

Abstract:

Cells are unique in their ability to avert thermodynamic equilibrium by spending energy, and yet to exploit the laws of thermodynamic to minimize their energetic costs. Although biochemical assays can determine whether a single reaction is active or thermodynamically driven, addressing this question becomes particularly challenging for complex in vivo phenomena such as the formation of multicomponent membrane-less organelles. There is growing evidence supporting the possibility that such organelles form as a new phase, separating from the cytoplasm or nucleoplasm. Any in vivo assessment of this hypothesis, however, requires a robust and unambiguous manipulation of the factors affecting phase transition processes. To tackle this problem we have developed an in vivo approach for unambiguous disentanglement of a thermodynamically driven phase separation from an active process. This approach is based on the different predictions of each of the two models in response to changes in temperature. To employ this approach we used a microfluidics device for accurate temperature control during confocal imaging, and quantitative image analysis to study the mechanism by which six different nucleolar proteins localize to the nucleolus in D. melanogaster embryos. Our results indicate that the nucleolus assembly in vivo is an interplay between thermodynamically driven phase separations and active assembly processes, with the localization of the individual proteins following one of these two distinct mechanisms.