Hyperosmotic phase separation: Condensates beyond inclusions, granules and organelles

Abstract

Biological liquid-liquid phase separation has gained considerable attention in recent years as a driving force for the assembly of subcellular compartments termed membraneless organelles. The field has made great strides in elucidating the molecular basis of biomolecular phase separation in various disease, stress-response and developmental contexts. Many important biological consequences of such “condensation” are now emerging from in vivo studies. Here we review recent work from our group and others showing that many proteins undergo rapid, reversible condensation in the cellular response to ubiquitous environmental fluctuations such as osmotic changes. We discuss molecular crowding as an important driver of condensation in these responses and suggest that a significant fraction of the proteome is poised to undergo phase separation under physiological conditions. In addition, we review methods currently emerging to visualize, quantify and modulate the dynamics of intracellular condensates in live cells. Finally, we propose a metaphor for rapid phase separation based on cloud formation, reasoning that our familiar experiences with the readily reversible condensation of water droplets help understand the principle of phase separation. Overall, we provide an account of how biological phase separation supports the highly intertwined relationship between the composition and dynamic internal organization of cells, thus facilitating extremely rapid reorganization in response to internal and external fluctuations.

Folding Stability and Self‐Association of a Triplet‐Repeat (CAG)20 RNA Hairpin in Cytomimetic Media

“…Jalihal et al. report a physiological osmotic range of 285–295 mOsmol/kg [43] . Compared to the cryo‐osmometry results reported here, this would indicate a dilution factor between 1 : 2 and 1 : 3 from the cell lysis procedure.…”

A hydro-osmotic coarsening theory of biological cavity formation

“…Here, we have shown that active ion pumping may be an additional source of spatial bias for coarsening. Active osmotic volume control emerges hence as a novel mode of symmetry breaking for tissue patterning, that could be relevant also to oogenesis Alsous et al ( 2021 ); Lamiré et al ( 2020 ); Chartier et al ( 2020 ), liquid phase transition of biological condensates Jalihal et al ( 2021 ) and plant tissue growth Cheddadi et al ( 2019 ); Long et al ( 2020 ). Active ion pumping can also largely affect the collective lumen dynamics, with the emergence of a novel coarsening regime dominated by coalescence.…”

New Perspectives on the Biogenesis of Viral Inclusion Bodies in Negative-Sense RNA Virus Infections

“…Owing to their biophysical properties that they share with liquids (like droplet fusion, surface tension, etc. ), these biochemical functional compartments have been referred to as liquid (droplet) organelles or, more common in cellular biology, as biomolecular condensates [16][17][18][19][27][28][29][30]. Biomolecular condensates have been observed in different cells among eukaryots, bacteria, yeast, and archae [23,[31][32][33][34].…”

“…In cell biology, LLPS originates from protein:protein, protein:RNA, or RNA:RNA interactions that lead to the remodeling of a soluble phase into a condensated, dense phase. A key factor here is the multivalency of the molecules itself: multiple inter-and intramolecular connections that can lead to the formation of condensates with multiple interaction partners [28,29]. Certain properties of protein:protein interfaces have already been shown to drive protein phase separation: arginine-glycine-glycine/arginine-glycine (RGG/RG) motifs [48], chargecharge interactions and intrinsically disordered regions (IDRs) [49][50][51].…”

“…Interestingly, many IDR-containing proteins also have RNA-interaction interfaces. Under high concentration and molecular crowding, structured protein domains have also been described to drive LLPS [29,52]. Accordingly, posttranslational modifications such as phosphorylation or methylation also have a big impact on the formation of biomolecular condensates [53][54][55][56].…”