Biology subjects

Bartolucci, G.

Publications and source records attributed to Bartolucci, G..

4 recordsLinked to original sources

bioRxiv2025

Theory of non-dilute binding and surface phase separation applied to membrane-binding proteins

Surface binding and surface phase separation of cytosolic scaffold proteins on lipid membranes are involved in many cellular processes, such as cell signaling, cell adhesion, and cortex regulation. However, the interplay between surface binding and surface phase separation is poorly understood. In this work, we study this interplay by deriving a general thermodynamic model and applying it to in vitro reconstitution experiments of membrane-binding proteins involved in tight junction initiation. Our theory extends the classical surface binding isotherm to account for non-dilute and heterogeneous conditions where components can phase separate. We use our theory to demonstrate how surface phase separation is governed by the interaction strength among membrane-bound scaffold proteins and their binding affinity to the membrane surface. Comparing the theory to reconstitution experiments, we show that tuning the oligomerization state of the adhesion receptors in the membrane controls surface phase transition and patterning of the scaffold protein ZO1. These findings suggest a fundamental role of the interplay between non-dilute surface binding and surface phase separation in forming the tight junction. More broadly, our work highlights non-dilute surface binding and surface phase separation as a common organizational principle for membrane-associated structures in living cells.

biophysics↗

bioRxiv2023

Assembly of tight junction belts by surface condensation and actin elongation

Formation of biomolecular condensates via phase separation enables compartmentation of many cellular processes. However, how cells can control condensation at specific locations to create complex cellular structures remains poorly understood. Here, we investigated the mechanism of tight junction formation, which involves condensation of scaffold proteins at cell-cell contacts and elongation of the condensates into a belt around the cellular perimeter. Using cell biology, reconstitution, and thermodynamic theory, we discovered that cells use surface phase transitions to control local condensation at the membrane far below bulk saturation. Surface condensation of junctional ZO-scaffold proteins is mediated by receptor binding and regulated by the receptors oligomerization state. Functionally, ZO surface condensation is directly coupled to actin polymerization and bundling, which drives elongation of receptor-ZO-actin condensates similar to tight junction belt formation in cells. We conclude that surface phase transitions provide a robust mechanism to control the position and shape of protein condensates. One-Sentence SummaryLocal surface binding of cytosolic scaffold proteins provides spatial control of protein condensation to assemble adhesion junctions.

biophysics↗

bioRxiv2023

The interplay between molecular assembly and phase separation

Many biological functions and dysfunctions rely on two fundamental processes, molecular assembly and the formation of condensed phases such as biomolecular condensates. Condensed phases generally form via phase separation, while molecular assemblies are clusters of molecules of various sizes, shapes, and functionality. We developed a theory that relies on thermodynamic principles to understand the interplay between molecular assembly and phase separation. We propose two prototypical classes of protein interactions and characterize their different equilibrium states and relaxation dynamics. We obtain results consistent with recent in vitro experimental observations of reconstituted proteins, including anomalous size distribution of assemblies, the gelation of condensed phases, and the change in condensate volume during ageing. Our theory provides the framework to unravel the mechanisms underlying physiological assemblies essential for cellular function, and aberrant assemblies which are associated with several neurodegenerative disorders.

biophysics↗

bioRxiv2023

Liquid spherical shells are a non-equilibrium steady state

Liquid-liquid phase separation is the process in which two immiscible liquids demix. This spontaneous phenomenon yields spherical droplets that eventually coarsen to one large, stable droplet governed by the principle of minimal free energy. In chemically fueled phase separation, the formation of phase-separating molecules is coupled to a fuel-driven, nonequilibrium reaction cycle. Chemically fueled phase separation yields dissipative structures sustained by a continuous fuel conversion. Such dissipative structures are ubiquitous in biology but poorly understood as they are governed by non-equilibrium thermodynamics. Here, we bridge the gap between passive, close-to-equilibrium, and active, dissipative structures with chemically fueled phase separation. We observe that spherical, active droplets can transition into a new morphology--a liquid, spherical shell of droplet material. A spherical shell would be highly unstable at equilibrium. Only by continuously converting chemical energy, this dissipative structure can be sustained. We demonstrate the transition mechanism, which is related to the activation of a product outside of the droplet, and the deactivation within the droplets leading to gradients of droplet material. We characterize how far out of equilibrium the spherical shell state is and the chemical power necessary to sustain it. Our work suggests new avenues for assembling complex stable morphologies, which might already be exploited to form membraneless organelles by cells.

biophysics↗