Macromolecular regulators have matching effects on the phase equilibrium and interfacial tension of biomolecular condensates

Abstract

The interfacial tension of phase‐separated biomolecular condensates affects their fusion and multiphase organization, and yet how this important property depends on the composition and interactions of the constituent macromolecules is poorly understood. Here we use molecular dynamics simulations to determine the interfacial tension and phase equilibrium of model condensate‐forming systems. The model systems consist of binary mixtures of Lennard‐Jones particles or chains of such particles. We refer to the two components as drivers and regulators; the former has stronger self‐interactions and hence a higher critical temperature (Tc) for phase separation. In previous work, we have shown that, depending on the relative strengths of driver‐regulator and driver‐driver interactions, regulators can either promote or suppress phase separation (i.e., increase or decrease Tc). Here we find that the effects of regulators on Tc quantitatively match the effects on interfacial tension (γ). This important finding means that, when a condensate‐forming system experiences a change in macromolecular composition or a change in intermolecular interactions (e.g., by mutation or posttranslational modification, or by variation in solvent conditions such as temperature, pH, or salt), the resulting change in Tc can be used to predict the change in γ and vice versa. We also report initial results showing that disparity in intermolecular interactions drives multiphase coexistence. These findings provide much needed guidance for understanding how biomolecular condensates mediate cellular functions.

RNA chain length and stoichiometry govern surface tension and stability of protein-RNA condensates

“…Thus, we seek alternative sampling methods for studying mixed chain condensates with heterogeneous distributions and attractive long-range forces. These challenges have also been discussed ( Mazarakos and Zhou, 2021 ) in a recent study on mixed Lennard-Jones chain vapor-liquid phase equilibrium. Therefore, in the following section, we employ a multi-replica NVT sampling of protein-RNA droplet formation and NPT grid sampling of liquid state equilibrium.…”

“…However, one encounters many challenges when quantitatively studying liquid-vapor phase equilibrium of mixed protein-RNA condensates with stoichiometries that are strongly deviating from the 1:1 polyelectrolyte charge ratio, rendering the equilibrium density estimation of smaller components unreliable. In this section, we start from an observation that for mixed chain systems, the density of the chain packing at a given temperature and pressure is a natural proxy for estimating the location with respect to the binodal line ( Mazarakos and Zhou, 2021 ).…”

“…This work has employed a single-residue and single-base resolution coarse-grained polyelectrolyte model for protein and RNA chains which is commonly adopted in computational studies of biomolecular condensates ( Dignon et al., 2018b ; Alshareedah et al., 2020 ; Mazarakos and Zhou, 2021 ). The model and a few of its variations have been used in the previous studies of binary and ternary systems ( Kaur et al., 2021 ; Alshareedah et al., 2019 , 2020 ).…”

“…In this study, we have set the energetic scale of short range-attractions to be uniform which is done to ensure the formation of well-mixed droplets without sub-compartments in agreement with experimental findings presented in the next section ( Figure 6 A). Prior studies have shown that the interplay of scales in short-range attraction can lead to the formation of multiphasic condensattion and different degrees of wetting of multicomponent droplets ( Kaur et al., 2021 ; Hazra and Levy, 2021 ; Mazarakos and Zhou, 2021 ). We note, however, that when it comes to nucleic-acids the breakdown of interactions into short vs long range contributions may not be as trivial.…”

“…The thermodynamics of protein-RNA phase separation was further elucidated using minimalist patchy particle and residue-level models, showing that fine-tuning the interaction strengths and stoichiometries of components can indeed enhance or downregulate condensation ( Joseph et al., 2021 ). It is important to note that most of these pioneering computational studies of biomolecular phase transitions, with a few exceptions ( Mazarakos and Zhou, 2021 ), have largely focused on computing sequence-dependent phase-coexistence diagrams ( Choi et al., 2020a ; Regy et al., 2020 ) and have not investigated the sequence and composition dependence of interfacial properties. However, it is expected that interfacial tension and viscosity of condensates govern the dynamics of droplet fusion, extent of wetting, and topology with respect to other coexisting condensates ( Kaur et al., 2021 ; Ghosh et al., 2021 ; Alshareedah et al., 2021a ).…”