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

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.

Higher‐order oligomerization promotes localization of SPOP to liquid nuclear speckles

“…Protein/protein and protein/RNA interactions promote assembly of and recruitment to these cellular bodies (Tourrière et al , 2003). Their liquid nature is thought to be generated via the process of liquid demixing phase separation (Brangwynne et al , 2009; Li et al , 2012b; Molliex et al , 2015; Nott et al , 2015; Patel et al , 2015), which leads to large, micrometer‐sized assemblies above threshold macromolecular concentrations. We show that nuclear speckles have liquid droplet properties and that they, together with nucleoli (Brangwynne et al , 2011), P granules (Brangwynne et al , 2009), and stress granules (Molliex et al , 2015), belong to the category of liquid membrane‐less organelles.…”

“…These bodies, in contrast to classic organelles, locally enrich components within defined boundaries despite the lack of an outer membrane. Nucleoli (Brangwynne et al , 2011), P granules (Brangwynne et al , 2009), and stress granules (Molliex et al , 2015; Patel et al , 2015), three membrane‐less organelles, were recently reported to have liquid droplet properties (Brangwynne, 2013), supporting a role for liquid–liquid phase separation in their formation (Li et al , 2012b; Nott et al , 2015; Patel et al , 2015). The recruitment of additional components to these bodies is not well understood but must involve protein/protein interactions (Tourrière et al , 2003).…”

Tau protein liquid–liquid phase separation can initiate tau aggregation

“…These reversible droplets represent physiologically active protein or protein‐nucleic acid “bioreactors” (Hyman et al , 2014), which form through a process known as LLPS (Brangwynne et al , 2015). Such transient membrane‐less organelles have multiple cellular functions, such as p‐granule formation to establish intracellular gradients of RNA transcription (Brangwynne et al , 2009), the enrichment of RNA binding proteins in stress granules (Lin et al , 2015; Molliex et al , 2015), concentrating transcription factors in nucleoli (Berry et al , 2015), and the initiation of microtubule spindle formation (Jiang et al , 2015). However, the functional phase separation of nuclear proteins was shown to be disrupted by C9orf72 GR/PR dipeptide repeats (Lee et al , 2016) and is related to protein aggregation in neurodegeneration (Schmidt & Rohatgi, 2016).…”

Microfluidic Formation of Monodisperse Coacervate Organelles in Liposomes

“…Protocells based on simple and complex coacervates have been shown to display interesting properties, including enhanced enzyme catalysis,3 selective partitioning of biomolecules,4 and model crowded environments 3e, 5. Importantly, coacervates can also be found in living cells, for example P granules,6 stress granules,7 and Cajal bodies 8. Increasing evidence suggests that these compartments originate via liquid–liquid phase separation (LLPS) of (intrinsically disordered) proteins and RNA9 and play an important role in cell structure and functions involving RNA metabolism 9.…”