Self-crowding induced phase separation in protein dispersions

External field-driven property localization in liquids of responsive macromolecules

We explore theoretically the effects of external potentials on the spatial distribution of particle properties in a liquid of explicitly responsive macromolecules. In particular, we focus on the bistable particle size as a coarse-grained internal degree of freedom (DoF, or "property"), σ, that moves in a bimodal energy landscape, in order to model the response of a state-switching (big-to-small) macromolecular liquid to external stimuli. We employ a mean-field density functional theory (DFT) that provides the full inhomogeneous equilibrium distributions of a one-component model system of responsive colloids (RCs) interacting with a Gaussian pair potential. For systems confined between two parallel hard walls, we observe and rationalize a significant localization of the big particle state close to the walls, with pressures described by an exact RC wall theorem. Application of more complex external potentials, such as linear (gravitational), osmotic, and Hamaker potentials, promotes even stronger particle size segregation, in which macromolecules of different size are localized in different spatial regions. Importantly, we demonstrate how the degree of responsiveness of the particle size and its coupling to the external potential tune the position-dependent size distribution. The DFT predictions are corroborated by Brownian dynamics simulations. Our study highlights the fact that particle responsiveness can be used to localize liquid properties and therefore helps to control the property- and position-dependent function of macromolecules, e.g., in biomedical applications.

Active responsive colloids coupled to different thermostats

We introduce a model of active responsive colloids (ARCs) in which an internal degree of freedom (DoF) of a single colloidal particle is "activated" by coupling it to a different thermostat than for the translational DoFs. As for the responsive internal DoF, we consider specifically the size (diameter) of the spherical particles, which is confined by a harmonic parent potential being either entropic or energetic in nature. The ARCs interact via a repulsive Hertzian pair potential, appropriate to model hydrogels or elastic colloids, and are studied for various densities using Brownian dynamics simulations. We tune the internal activity in the nonequilibrium steady state by scanning through a wide range of internal temperatures, both smaller ("colder") and larger ("hotter") than the translational temperature. The results show a rich and intriguing behavior for the emergent property distributions, colloidal pair structure, and the diffusive translational dynamics controlled by the internal activity, substantially depending on whether the internal DoF moves in an entropic or energetic potential. We discuss theoretical thermal limits and phenomenological models which can explain some of the nonequilibrium trends qualitatively. Our study indicates that particle dynamical polydispersity as well as the structure and dynamics of dense macromolecular suspensions can be vastly tuned by internal activity in terms of internal "hot" or "cold" fluctuating states.

Active interaction switching controls the dynamic heterogeneity of soft colloidal dispersions

We employ Reactive Dynamical Density Functional Theory (R-DDFT) and Reactive Brownian Dynamics (R-BD) simulations to investigate the dynamics of a suspension of active soft Gaussian colloids with binary interaction switching, i.e., a one-component colloidal system in which every particle stochastically switches at predefined rates between two interaction states with different mobility. Using R-DDFT we extend a theory previously developed to access the dynamics of inhomogeneous liquids [Archer et al., Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2007, 75, 040501] to study the influence of the switching activity on the self and distinct part of the Van Hove function in bulk solution, and determine the corresponding mean squared displacement of the switching particles. Our results demonstrate that, even though the average diffusion coefficient is not affected by the switching activity, it significantly modifies the non-equilibrium dynamics and diffusion coefficients of the individual particles, leading to a crossover from short to long times, with a regime for intermediate times showing anomalous diffusion. In addition, the self-part of the van Hove function has a Gaussian form at short and long times, but becomes non-Gaussian at intermediates ones, having a crossover between short and large displacements. The corresponding self-intermediate scattering function shows the two-step relaxation patters typically observed in soft materials with heterogeneous dynamics such as glasses and gels. We also introduce a phenomenological Continuous Time Random Walk (CTRW) theory to understand the heterogeneous diffusion of this system. R-DDFT results are in excellent agreement with R-BD simulations and the analytical predictions of CTRW theory, thus confirming that R-DDFT constitutes a powerful method to investigate not only the structure and phase behavior, but also the dynamical properties of non-equilibrium active switching colloidal suspensions.

Modulating internal transition kinetics of responsive macromolecules by collective crowding

Packing and crowding are used in biology as mechanisms to (self-)regulate internal molecular or cellular processes based on collective signaling. Here, we study how the transition kinetics of an internal "switch" of responsive macromolecules is modified collectively by their spatial packing. We employ Brownian dynamics simulations of a model of Responsive Colloids, in which an explicit internal degree of freedom-here, the particle size-moving in a bimodal energy landscape self-consistently responds to the density fluctuations of the crowded environment. We demonstrate that populations and transition times for the two-state switching kinetics can be tuned over one order of magnitude by "self-crowding." An exponential scaling law derived from a combination of Kramers' and liquid state perturbation theory is in very good agreement with the simulations.

Osmolyte accumulation regulates the SUMOylation and inclusion dynamics of the prionogenic Cyc8-Tup1 transcription corepressor

Environmental stressors can severely perturb cellular homeostasis and compromise viability. To cope with environmental stressors, eukaryotes have developed distinct signaling programs that allow for adaptation during different stress conditions. These programs often require a host of post-translational modifications that alter proteins to elicit appropriate cellular responses. One crucial protein modifier during stress is the small ubiquitin-like modifier SUMO. In many cases, however, the functions of stress dependent protein SUMOylation remain unclear. Previously, we showed that the conserved Saccharomyces cerevisiae Cyc8-Tup1 transcriptional corepressor complex undergoes transient hyperosmotic stress-induced SUMOylation and inclusion formation, which are important for appropriate regulation of hyperosmotic-stress genes. Here, we show the osmostress-responsive MAP kinase Hog1 regulates Cyc8 SUMOylation and inclusion formation via its role in the transcriptional activation of glycerol biosynthesis genes. Mutations that ablate Cyc8 SUMOylation can partially rescue the osmosensitivity of hog1Δ cells, and this is facilitated by inappropriate derepression of glycerol-biosynthesis genes. Furthermore, cells specifically unable to synthesize the osmolyte glycerol cause transient Cyc8 SUMOylation and inclusions to persist, indicating a regulatory role for glycerol to reestablish the basal state of Cyc8 following adaptation to hyperosmotic stress. These observations unveil a novel intersection between phosphorylation and SUMOylation networks, which are critical for shifting gene expression and metabolic programs during stress adaptation.

The ability to sense and react to diverse environmental cues is a central aspect in the maintenance of cellular homeostasis. In response to harsh conditions, cells must rapidly deploy specific stress responses in order to adapt, survive, and proliferate. To ensure optimal spatial and temporal control over stress responses, many proteins undergo biophysical and biochemical alterations. More specifically, these alterations include conformational changes and post-translational modifications–such as phosphorylation, ubiquitination, and SUMOylation–that alter the function, localization, and interactome of target proteins. In this study, we show that the Hog1 MAPK regulates SUMOylation and biomolecular condensation of the yeast transcription corepressor complex Cyc8-Tup1 during exposure to hyperosmotic stress. In turn, this signaling relationship functions to effectively rewire yeast metabolism toward the biosynthesis of the compatible osmolyte glycerol, which serves as the ultimate signal to reset this genetic circuit.

Isotropic and nematic liquid crystalline phases of adaptive rotaxanes

We describe the thermodynamics of a solution of rotaxanes which can change their length from a short state of length L to a long state of length qL in response to their surrounding environment. We call these rotaxanes "adaptive." We show that such a system can exhibit both isotropic and nematic liquid crystalline phases. The system shows several interesting kinds of behaviour. First we predict that the fraction of short-length rotaxanes increases linearly with concentration and is a maximum at the critical concentration that marks the isotropic to nematic transition. Second, the critical concentration shows a minimum at a certain value of q. Our model suggests that the effect of adaptive length changes is most dramatic at small q and where the long state is slightly favoured.