Société Française de Biochimie et Biologie Moléculaire

October 2022 - Audrey Cochard

Paris ENS -  Chemical Department "RNA at the surface of phase-separated condensates impacts their size and number"
Biophysical Journal, 121, Pages 1675-1690.
Audrey Cochard, Marina Garcia-Jove Navarro, Leonard Piroska, Shunnichi Kashida, Michel Kress, Dominique Weil, Zoher Gueroui


Audrey Cochard, 27 years old, is a Ph.D. student under the direction of Zoher Gueroui of the Department of Chemistry of the ENS Paris, and of Dominique Weil of the Institute of Biology Paris-Seine of Sorbonne University. Her Ph.D. project concerns biomolecular condensates, which are membraneless organelles taking part in the intracellular organization. Because of the biochemical complexity of endogenous condensates, reconstitution approaches in cells of artificial condensates with controlled composition are required. Audrey Cochard and her collaborators have developed a method to build in cells artificial condensates able to recruit a single RNA species. In this article published in Biophysical Journal, visualization via smFISH (single molecule FISH) of the single molecules of this target RNA, recruited at the surface of the artificial condensates, allowed to highlight the role that surface RNA can play in the control of condensates biophysical properties (size, number, morphology). Large condensates thus display only a few RNAs on their surface, whereas high RNA density always implied smaller and more numerous condensates. Authors propose a steric hindrance caused by surface RNA that would prevent the growth of condensates by both subunit addition and coalescence.


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Ecole Normale Supérieure

Département de chimie

Pôle de Chimie Physique et Biologique de la Matière Vivante

24, rue Lhomond

75005 Paris

Résumé de l'article

Although it is now recognized that specific RNAs and protein families are critical for the biogenesis of ribonucleoprotein (RNP) condensates, how these molecular constituents determine condensate size and morphology is unknown. To circumvent the biochemical complexity of endogenous RNP condensates, the use of programmable tools to reconstitute condensate formation with minimal constituents can be instrumental. Here we report a methodology to form RNA-containing condensates in living cells programmed to specifically recruit a single RNA species. Our bioengineered condensates are made of ArtiGranule scaffolds composed of an orthogonal protein that can bind to a specific heterologously expressed RNA. These scaffolds undergo liquid-liquid phase separation in cells and can be chemically controlled to prevent condensation or to trigger condensate dissolution. We found that the targeted RNAs localize at the condensate surface, either as isolated RNA molecules or as a homogenous corona of RNA molecules around the condensate. The recruitment of RNA changes the material properties of condensates by hardening the condensate body. Moreover, the condensate size scales with RNA surface density; the higher the RNA density is, the smaller and more frequent the condensates are. These results suggest a mechanism based on physical constraints, provided by RNAs at the condensate surface, that limit condensate growth and coalescence.