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Germline P Granules Are Liquid Droplets That Localize by Controlled Dissolution/Condensation

Science  26 Jun 2009:
Vol. 324, Issue 5935, pp. 1729-1732
DOI: 10.1126/science.1172046

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P Granule Conundrum

In many organisms, the presumptive germ cells can be distinguished from somatic cells by the presence of distinctive cytoplasmic granules. In Caenorhabditis elegans, these P granules are more or less uniformly distributed in the oocyte and one-cell stage of the fertilized egg. By the end of the first cleavage, however, the anterior cell is essentially free of P granules, whereas the posterior cell still displays a prominent population of granules. Exactly how this process occurs and whether it involves directed migration of the granules is unclear. Now Brangwynne et al. (p. 1729, published online 21 May; see the Perspective by Le Goff and Lecuit) provide evidence that localization occurs by a quite different mechanism, controlled dissolution and condensation of granule components. This type of cytoplasmic remodeling by physicochemical mechanisms can now be looked for in other cellular and developmental systems.

Abstract

In sexually reproducing organisms, embryos specify germ cells, which ultimately generate sperm and eggs. In Caenorhabditis elegans, the first germ cell is established when RNA and protein-rich P granules localize to the posterior of the one-cell embryo. Localization of P granules and their physical nature remain poorly understood. Here we show that P granules exhibit liquid-like behaviors, including fusion, dripping, and wetting, which we used to estimate their viscosity and surface tension. As with other liquids, P granules rapidly dissolved and condensed. Localization occurred by a biased increase in P granule condensation at the posterior. This process reflects a classic phase transition, in which polarity proteins vary the condensation point across the cell. Such phase transitions may represent a fundamental physicochemical mechanism for structuring the cytoplasm.

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