Membraneless organelles: P granules in Caenorhabditis elegans

Abstract

Membraneless organelles are distinct compartments within a cell that are not enclosed by a traditional lipid membrane and instead form through a process called liquid-liquid phase separation. Examples of these non-membrane-bound organelles include nucleoli, stress granules, P bodies, pericentriolar material and germ granules. Many recent studies have used Caenorhabditis elegans germ granules, known as P granules, to expand our understanding of the formation of these unique cellular compartments. From this work, we know that proteins with intrinsically disordered regions (IDRs) play a critical role in the process of phase separation. IDR phase separation is further tuned through their interactions with RNA and through protein modifications such as phosphorylation and methylation. These findings from C elegans, combined with work done in other model organisms, continue to provide insight into the formation of membraneless organelles and the important role they play in compartmentalizing cellular processes.

Keywords: P granules; intrinsically disordered regions; liquid phase separation; non-membrane-bound organelles.

FIGURE 1

P granules in the Caenorhabditis elegans germline. P granules (green) are shown in the germline (blue) of an adult. Germline stem cells (GSCs) proliferate in distal end of both gonad arms and are pushed from the stem cell niche where they enter meiosis. Each germ cell at this stage is attached to a common cytoplasm, and while similar but distinct P bodies are found in the syncytium, P granules remain at the periphery of each germ cell nucleus. As germ cells mature into oocytes and cellularize, P granules detach from the nuclear periphery and redistribute in the cytoplasm. During ovulation, oocytes pass through the spermatheca, become fertilized and enter the uterus. Fertilization initiates a cellular asymmetry cascade that culminates with P-granule condensation at the posterior of the cell before the first cell division. P granules segregate with germline blastomeres through four cell divisions, reattaching to the nucleus between the 2 and 8 cell stages. On the bottom left, representative IDRs from P-granule and NPC proteins are shown. A cartoon shows the complex heterogeneity of P granules, Z granules and Mutator foci at they sit at the nuclear periphery

FIGURE 2

Liquid-phase separation as symmetry breaks in the one-cell embryo. Embryos in the top panels demonstrate the symmetry of maternally inherited proteins just after fertilization and before maternal (left) and paternal (right) pronuclei meeting. The paternal pronucleus then migrates toward what will become the cell's posterior to initiate a cascade of events that result in PAR-1 enrichment at the posterior cortex. Embryos in the bottom panel are shown at a later point where maternal and paternal pronuclei migrate toward each other and meet. Posterior PAR-1 increases the diffusion rate of MEX-5, while slow diffusing MEX-5 in complex with PLK-1 and RNA phase separates in the cell's anterior. MEX-5's high affinity for RNA creates a potential RNA-sink, and this combined with the kinase activity of PLK-1 increases the rate of MEG-3, PIE-1 and POS-1 diffusion in the anterior, while slower diffusing MEG-3, PIE-1 and POS-1 phase separate in the posterior before the first cell division