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Nat Cell Biol October 1, 2013; 15 (10): 1253-9.
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A nuclear F-actin scaffold stabilizes ribonucleoprotein droplets against gravity in large cells.

Feric M , Brangwynne CP .

The size of a typical eukaryotic cell is of the order of ∼10 μm. However, some cell types grow to very large sizes, including oocytes (immature eggs) of organisms from humans to starfish. For example, oocytes of the frog Xenopus laevis grow to a diameter ≥1 mm. They have a correspondingly large nucleus (germinal vesicle) of ∼450 μm in diameter, which is similar to smaller somatic nuclei, but contains a significantly higher concentration of actin. The form and structure of this nuclear actin remain controversial, and its potential mechanical role within these large nuclei is unknown. Here, we use a microrheology and quantitative imaging approach to show that germinal vesicles contain an elastic F-actin scaffold that mechanically stabilizes these large nuclei against gravitational forces, which are usually considered negligible within cells. We find that on actin disruption, ribonucleoprotein droplets, including nucleoli and histone locus bodies, undergo gravitational sedimentation and fusion. We develop a model that reveals how gravity becomes an increasingly potent force as cells and their nuclei grow larger than ∼10 μm, explaining the requirement for a stabilizing nuclear F-actin scaffold in large Xenopus oocytes. All life forms are subject to gravity, and our results may have broad implications for cell growth and size control.

PubMed ID: 23995731
PMC ID: PMC3789854
Article link: Nat Cell Biol
Grant support: [+]

Species referenced: Xenopus laevis
Genes referenced: actl6a actn1 coil fscn1 grap2 igf2bp3 lmnb3 npm1 xpo6

Article Images: [+] show captions
References [+] :
Aebi, The nuclear lamina is a meshwork of intermediate-type filaments. 1986, Pubmed, Xenbase