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Molecular Crowding Tunes Material States of Ribonucleoprotein Condensates.

ABSTRACT: Ribonucleoprotein (RNP) granules are membraneless liquid condensates that dynamically form,dissolve, and mature into a gel-like state in response to a changing cellular environment. RNP condensation islargely governed by promiscuous attractive inter-chain interactions mediated by low-complexity domains(LCDs). Using an archetypal disordered RNP, fused in sarcoma (FUS), here we study how molecular crowdingimpacts the RNP liquid condensation. We observe that the liquid?liquid coexistence boundary of FUS islowered by polymer crowders, consistent with an excluded volume model. With increasing bulk crowderconcentration, the RNP partition increases and the diffusion rate decreases in the condensed phase.Furthermore, we show that RNP condensates undergo substantial hardening wherein protein-dense dropletstransition from viscous fluid to viscoelastic gel-like states in a crowder concentration-dependent manner.Utilizing two distinct LCDs that broadly represent commonly occurring sequence motifs driving RNP phasetransitions, we reveal that the impact of crowding is largely independent of LCD charge and sequence patterns.These results are consistent with a thermodynamic model of crowder-mediated depletion interaction, whichsuggests that inter-RNP attraction is enhanced by molecular crowding. The depletion force is likely to play akey role in tuning the physical properties of RNP condensates within the crowded cellular space.

SUBMITTER: Kaur T

PROVIDER: S-EPMC6406554 | biostudies-literature | 2019 Feb

REPOSITORIES: biostudies-literature

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Molecular Crowding Tunes Material States of Ribonucleoprotein Condensates.

Kaur Taranpreet T Alshareedah Ibraheem I Wang Wei W Ngo Jason J Moosa Mahdi Muhammad MM Banerjee Priya R PR

Biomolecules 20190219 2

Ribonucleoprotein (RNP) granules are membraneless liquid condensates that dynamically form,dissolve, and mature into a gel-like state in response to a changing cellular environment. RNP condensation islargely governed by promiscuous attractive inter-chain interactions mediated by low-complexity domains(LCDs). Using an archetypal disordered RNP, fused in sarcoma (FUS), here we study how molecular crowdingimpacts the RNP liquid condensation. We observe that the liquid⁻liquid coexistence boundary o ...[more]

PMID: 30791483

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Project description:Living organisms typically store their genomic DNA in a condensed form. Mechanistically, DNA condensation can be driven by macromolecular crowding, multivalent cations, or positively charged proteins. At low DNA concentration, condensation triggers the conformational change of individual DNA molecules into a compacted state, with distinct morphologies. Above a critical DNA concentration, condensation goes along with phase separation into a DNA-dilute and a DNA-dense phase. The latter DNA-dense phase can have different material properties and has been reported to be rather liquid-like or solid-like depending on the characteristics of the DNA and the solvent composition. Here, we systematically assess the influence of DNA length on the properties of the resulting condensates. We show that short DNA molecules with sizes below 1 kb can form dynamic liquid-like assemblies when condensation is triggered by polyethylene glycol and magnesium ions, binding of linker histone H1, or nucleosome reconstitution in combination with linker histone H1. With increasing DNA length, molecules preferentially condense into less dynamic more solid-like assemblies, with phage λ-DNA with 48.5 kb forming mostly solid-like assemblies under the conditions assessed here. The transition from liquid-like to solid-like condensates appears to be gradual, with DNA molecules of roughly 1-10 kb forming condensates with intermediate properties. Titration experiments with linker histone H1 suggest that the fluidity of condensates depends on the net number of attractive interactions established by each DNA molecule. We conclude that DNA molecules that are much shorter than a typical human gene are able to undergo liquid-liquid phase separation, whereas longer DNA molecules phase separate by default into rather solid-like condensates. We speculate that the local distribution of condensing factors can modulate the effective length of chromosomal domains in the cell. We anticipate that the link between DNA length and fluidity established here will improve our understanding of biomolecular condensates involving DNA.

Project description:Biomolecular condensates are important contributors to the internal organization of the cell material. While initially described as liquid-like droplets, the term biomolecular condensates is now used to describe a diversity of condensed phase assemblies with material properties extending from low to high viscous liquids, gels, and even glasses. Because the material properties of condensates are determined by the intrinsic behavior of their molecules, characterizing such properties is integral to rationalizing the molecular mechanisms that dictate their functions and roles in health and disease. Here, we apply and compare three distinct computational methods to measure the viscoelasticity of biomolecular condensates in molecular simulations. These methods are the Green-Kubo (GK) relation, the oscillatory shear (OS) technique, and the bead tracking (BT) method. We find that, although all of these methods provide consistent results for the viscosity of the condensates, the GK and OS techniques outperform the BT method in terms of computational efficiency and statistical uncertainty. We thus apply the GK and OS techniques for a set of 12 different protein/RNA systems using a sequence-dependent coarse-grained model. Our results reveal a strong correlation between condensate viscosity and density, as well as with protein/RNA length and the number of stickers vs spacers in the amino acid protein sequence. Moreover, we couple the GK and the OS technique to nonequilibrium molecular dynamics simulations that mimic the progressive liquid-to-gel transition of protein condensates due to the accumulation of interprotein β-sheets. We compare the behavior of three different protein condensates, i.e., those formed by either hnRNPA1, FUS, or TDP-43 proteins, whose liquid-to-gel transitions are associated with the onset of amyotrophic lateral sclerosis and frontotemporal dementia. We find that both the GK and OS techniques successfully predict the transition from functional liquid-like behavior to kinetically arrested states once the network of interprotein β-sheets has percolated through the condensates. Overall, our work provides a comparison of different modeling rheological techniques to assess the viscosity of biomolecular condensates, a critical magnitude that provides information on the behavior of biomolecules inside condensates.

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Molecular Crowding Tunes Material States of Ribonucleoprotein Condensates.

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Molecular Crowding Tunes Material States of Ribonucleoprotein Condensates.

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