Size scaling of phase-separated domains and mesoscale clusters that precede liquid-liquid phase separation (LLPS): theory and experiment

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• Samuel Safran (Weizmann Institute of Science)
• Monday 09 October 2023, 09:40-10:40
• Seminar Room 1, Newton Institute.

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SPLW03 - Biological condensates: cellular mechanisms governed by phase transitions

Gonen Golani, Maria Oranges, Manas Seal, Alexey Bogdanov, Daniella Goldfarb, Samuel Safran Department of Chemical and Biological Physics, Weizmann Institute of Science, Rehovot, Israel We review the theoretical and experimental understanding of how domain sizes are fixed in LLPS in the cases of (i) equilibrium phase separation in a confined system (chromatin in the nucleus [1]) and (ii) non-equilibrium phase separation (e.g., solute production and degradation, see poster by Amit Kumar).  We then discuss observations [2,3]  of large (10’s-100 nm’s) protein assemblies in the one-phase regime that precedes LLPS . We have formulated an analytical theory of these protein assemblies based on analogies with other mesoscale structures in amphiphilic (surfactant or lipid) systems where core-shell assemblies are observed. What is unique about intrinsically disordered proteins is that the same protein can – via its different conformations [4]—act as both the “inner phase” and “amphiphilic surface layer.”  This is consistent with Ref. 2 that identified two types of dynamics associated with the “clusters”.  Thus, relatively large assemblies can be stable for even a single protein in water with no amphiphile or “internal phase” required. We formulate a statistical mechanics model of such core-shell assemblies to predict the size distribution of the observed “clusters” in the one-phase region and compare it with the results of light scattering experiment [2, 3]. The data for relatively large clusters is well-fit by a model with interfacial tension, while the fits for smaller clusters must also account for the bending energy and geometric corrections. In addition, electron spin-resonance experiments [3] estimate the core-shell volume ratios, indicating that at the LLPS transition, there is no sharp change in the rotational time scales of the proteins in the core and shell. This may suggest that, in these cases, the LLPS may arise from attraction-induced phase separation of the “clusters,” similar to phase separations in some spherical microemulsions [5]. [1] Amiad-Pavlova et al., Sci. Adv. (2021) 7, eabf6251; Bajpai et al., eLife (2021) 10, e63976. [2] M. Kar et al., PNAS (2022) 119, e2202222119. [3] M. Seal et al., J. Phys. Chem. B (2021) 125, 12947. [4] Mugnai et al., BioArchiv (2023). [5] J. S. Huang et al., Phys. Rev. Lett. (1981) 47, 1462 and (1984) 53, 592.  

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