Lifetime of actin-dependent protein nanoclusters

Coarsening dynamics of aster defects in model polar active matter

We numerically study the dynamics of topological defects in 2D polar active matter coupled to a conserved density field, which shows anomalous kinetics and defect distribution. The initial many-defect state relaxes by pair-annihilation of defects, which behave like Ostwald ripening on short timescales. However, defect coarsening is arrested at long timescales, and the relaxation kinetics becomes anomalously slow compared to the equilibrium state. Specifically, the number of defects in the active system approaches a steady state, following a power-law dependence in the rate of change of the inverse density. In contrast, in thermal equilibrium, the decay is exponential. Finally, we show that the anomalous coarsening of defects leads to unique patterns in the coupled density field, which is consistent with patterns observed in experiments on the actin cytoskeleton. These patterns can act as cell signaling platforms and may have important biological consequences.

Self-diffusion is temperature independent on active membranes

Molecular transport maintains cellular structures and functions. For example, lipid and protein diffusion sculpts the dynamic shapes and structures on the cell membrane that perform essential cellular functions, such as cell signaling. Temperature variations in thermal equilibrium rapidly change molecular transport properties. The coefficient of lipid self-diffusion increases exponentially with temperature in thermal equilibrium, for example. Hence, maintaining cellular homeostasis through molecular transport is hard in thermal equilibrium in the noisy cellular environment, where temperatures can fluctuate widely due to local heat generation. In this paper, using both molecular and lattice-based modeling of membrane transport, we show that the presence of active transport originating from the cell's cytoskeleton can make the self-diffusion of the molecules on the membrane robust to temperature fluctuations. The resultant temperature-independence of self-diffusion keeps the precision of cellular signaling invariant over a broad range of ambient temperatures, allowing cells to make robust decisions. We have also found that the Kawasaki algorithm, the widely used model of lipid transport on lattices, predicts incorrect temperature dependence of lipid self-diffusion in equilibrium. We propose a new algorithm that correctly captures the equilibrium properties of lipid self-diffusion and reproduces experimental observations.

The ubiquitous nanocluster: A molecular scale organizing principle that governs cellular information flow

The language of biology at the scale of the cell is constituted of alphabets represented by biomolecules. These are stitched together in a variety of ways to create meaning. We argue that the phrases of this language are nanoscale molecular assemblies or nano-hubs for the purpose of information flow. At the cell surface information is sensed and processed via membrane receptors, often configured as multimers. These nano-assemblies serve as receiver nano-hubs, which are flexibly configured with additional nano-hubs that we term modifiers and transducers. This framework serves to process information that is transmitted for execution inside the cell. Here, we explore some examples about how nano-hubs are built and how they may contribute to cellular information flow.