Membrane reshaping by protein condensates

Inverse Bicontinuous and Discontinuous Phases of Lipids, and Membrane Curvature

In this review article I briefly describe lipid self-assembly, interfacial curvature, and lyotropic phase diagrams. I then go on to describe how the phase behaviour can be controlled, and the structure of lyotropic phases can be tuned, by various parameters such as temperature, hydrostatic pressure, or the addition of amphiphilic molecules such as fatty acids, diacylglycerols, and cholesterol. I then give a few illustrations of how such structures/phases may play roles in lipid-based biotechnologies, and in biomembrane systems.

Lipid packing and cholesterol content regulate membrane wetting and remodeling by biomolecular condensates

Biomolecular condensates play a central role in cellular processes by interacting with membranes driving wetting transitions and inducing mutual remodeling. While condensates are known to locally alter membrane properties such as lipid packing and hydration, it remains unclear how membrane composition and phase state in turn affect condensate affinity. Here, we show that it is not only the membrane phase itself, but rather the degree of lipid packing that determines the condensate affinity for membranes. Increasing lipid chain length, saturation, or cholesterol content, enhances lipid packing, thereby decreasing condensate interaction. This regulatory mechanism is consistent across various condensate-membrane systems, highlighting the critical role of the membrane interface. In addition, protein adsorption promotes extensive membrane remodeling, including the formation of tubes and double-membrane sheets. Our findings reveal a mechanism by which membrane composition fine-tunes condensate wetting, highlighting its potential impact on cellular functions and organelle interactions.

The mechanobiology of biomolecular condensates

The central goal of mechanobiology is to understand how the mechanical forces and material properties of organelles, cells, and tissues influence biological processes and functions. Since the first description of biomolecular condensates, it was hypothesized that they obtain material properties that are tuned to their functions inside cells. Thus, they represent an intriguing playground for mechanobiology. The idea that biomolecular condensates exhibit diverse and adaptive material properties highlights the need to understand how different material states respond to external forces and whether these responses are linked to their physiological roles within the cell. For example, liquids buffer and dissipate, while solids store and transmit mechanical stress, and the relaxation time of a viscoelastic material can act as a mechanical frequency filter. Hence, a liquid-solid transition of a condensate in the force transmission pathway can determine how mechanical signals are transduced within and in-between cells, affecting differentiation, neuronal network dynamics, and behavior to external stimuli. Here, we first review our current understanding of the molecular drivers and how rigidity phase transitions are set forth in the complex cellular environment. We will then summarize the technical advancements that were necessary to obtain insights into the rich and fascinating mechanobiology of condensates, and finally, we will highlight recent examples of physiological liquid-solid transitions and their connection to specific cellular functions. Our goal is to provide a comprehensive summary of the field on how cells harness and regulate condensate mechanics to achieve specific functions.

Quantification of Biomolecular Condensate Volume Reveals Network Swelling and Dissolution Regimes during Phase Transition

Accurate determination of biomolecular condensate volume reveals that destabilization of condensates can lead to either swelling or shrinking of condensates, giving fundamental insights into the regulation of the volume of cellular condensates. Determination of the volume of biomolecular condensates and coacervate protocells is essential to investigate their precise composition and impact on (bio)chemical reactions that are localized inside the condensates. It is not a straightforward task, as condensates have tiny volumes, are highly viscous, and are prone to wetting. Here, we examine different strategies to determine condensate volume and introduce two new methods, with which condensate volumes of 1 μL or less (volume fraction 0.4%) can be determined with a standard deviation of 0.03 μL. Using these methods, we show that the swelling or shrinking of condensates depends on the degree of physical cross-linking. These observations are supported by Flory-Huggins theory and can have profound effects on condensates in cell biology.

Exploring liquid-liquid phase separation in the organisation of Golgi matrix proteins

The Golgi apparatus is a critical organelle in protein sorting and lipid metabolism. Characterized by its stacked, flattened cisternal structure, the Golgi exhibits distinct polarity with its cis- and trans-faces orchestrating various protein maturation and transport processes. At the heart of its structural integrity and organisation are the Golgi Matrix Proteins (GMPs), predominantly comprising Golgins and GRASPs. These proteins contribute to this organelle's unique stacked and polarized structure and ensure the precise localization of Golgi-resident enzymes, which is crucial for accurate protein processing. Despite over a century of research since its discovery, the Golgi architecture's intricate mechanisms still need to be fully understood. Here, we discuss that GMPs across different Eukaryotic lineages present a significant tendency to form biomolecular condensates. Moreover, we validated experimentally that members of the GRASP family also exhibit a strong tendency. Our findings offer a new perspective on the possible roles of protein disorder and condensation of GMPs in the Golgi organisation.

Biomolecular Condensates in Contact with Membranes

Biomolecular condensates are highly versatile membraneless organelles involved in a plethora of cellular processes. Recent years have witnessed growing evidence of the interaction of these droplets with membrane-bound cellular structures. Condensates' adhesion to membranes can cause their mutual molding and regulation, and their interaction is of fundamental relevance to intracellular organization and communication, organelle remodeling, embryogenesis, and phagocytosis. In this article, we review advances in the understanding of membrane-condensate interactions, with a focus on in vitro models. These minimal systems allow the precise characterization and tuning of the material properties of both membranes and condensates and provide a workbench for visualizing the resulting morphologies and quantifying the interactions. These interactions can give rise to diverse biologically relevant phenomena, such as molecular-level restructuring of the membrane, nano- to microscale ruffling of the condensate-membrane interface, and coupling of the protein and lipid phases.

Protein-membrane interactions: sensing and generating curvature

Biological membranes are integral cellular structures that can be curved into various geometries. These curved structures are abundant in cells as they are essential for various physiological processes. However, curved membranes are inherently unstable, especially on nanometer length scales. To stabilize curved membranes, cells can utilize proteins that sense and generate membrane curvature. In this review, we summarize recent research that has advanced our understanding of interactions between proteins and curved membrane surfaces, as well as work that has expanded our ability to study curvature sensing and generation. Additionally, we look at specific examples of cellular processes that require membrane curvature, such as neurotransmission, clathrin-mediated endocytosis (CME), and organelle biogenesis.

Probing the surface charge of condensates using microelectrophoresis

Biomolecular condensates play an important role in cellular organization. Coacervates are commonly used models that mimic the physicochemical properties of biomolecular condensates. The surface of condensates plays a key role in governing molecular exchange between condensates, accumulation of species at the interface, and the stability of condensates against coalescence. However, most important surface properties, including the surface charge and zeta potential, remain poorly characterized and understood. The zeta potential of coacervates is often measured using laser doppler electrophoresis, which assumes a size-independent electrophoretic mobility. Here, we show that this assumption is incorrect for liquid-like condensates and present an alternative method to study the electrophoretic mobility of coacervates and in vitro condensate models by microelectrophoresis and single-particle tracking. Coacervates have a size-dependent electrophoretic mobility, originating from their fluid nature, from which a well-defined zeta potential is calculated. Interestingly, microelectrophoresis measurements reveal that polylysine chains are enriched at the surface of polylysine/polyaspartic acid complex coacervates, which causes the negatively charged protein ɑ-synuclein to adsorb and accumulate at the interface. Addition of ATP inverts the surface charge, displaces ɑ-synuclein from the surface and may help to suppress its interface-catalyzed aggregation. Together, these findings show how condensate surface charge can be measured and altered, making this microelectrophoresis platform combined with automated single-particle tracking a promising characterization technique for both biomolecular condensates and coacervate protocells.

Adhesion energy controls lipid binding-mediated endocytosis

Several bacterial toxins and viruses can deform membranes through multivalent binding to lipids for clathrin-independent endocytosis. However, it remains unclear, how membrane deformation and endocytic internalization are mechanistically linked. Here we show that many lipid-binding virions induce membrane deformation and clathrin-independent endocytosis, suggesting a common mechanism based on multivalent lipid binding by globular particles. We create a synthetic cellular system consisting of a lipid-anchored receptor in the form of GPI-anchored anti-GFP nanobodies and a multivalent globular binder exposing 180 regularly-spaced GFP molecules on its surface. We show that these globular, 40 nm diameter, particles bind to cells expressing the receptor, deform the plasma membrane upon adhesion and become endocytosed in a clathrin-independent manner. We explore the role of the membrane adhesion energy in endocytosis by using receptors with affinities varying over 7 orders of magnitude. Using this system, we find that once a threshold in adhesion energy is overcome to allow for membrane deformation, endocytosis occurs reliably. Multivalent, binding-induced membrane deformation by globular binders is thus sufficient for internalization to occur and we suggest it is the common, purely biophysical mechanism for lipid-binding mediated endocytosis of toxins and pathogens.

Interfacing Coacervates with Membranes: From Artificial Organelles and Hybrid Protocells to Intracellular Delivery

Compartmentalization is crucial for the functioning of cells. Membranes enclose and protect the cell, regulate the transport of molecules entering and exiting the cell, and organize cellular machinery in subcompartments. In addition, membraneless condensates, or coacervates, offer dynamic compartments that act as biomolecular storage centers, organizational hubs, or reaction crucibles. Emerging evidence shows that phase-separated membraneless bodies in the cell are involved in a wide range of functional interactions with cellular membranes, leading to transmembrane signaling, membrane remodeling, intracellular transport, and vesicle formation. Such functional and dynamic interplay between phase-separated droplets and membranes also offers many potential benefits to artificial cells, as shown by recent studies involving coacervates and liposomes. Depending on the relative sizes and interaction strength between coacervates and membranes, coacervates can serve as artificial membraneless organelles inside liposomes, as templates for membrane assembly and hybrid artificial cell formation, as membrane remodelers for tubulation and possibly division, and finally, as cargo containers for transport and delivery of biomolecules across membranes by endocytosis or direct membrane crossing. Here, recent experimental examples of each of these functions are reviewed and the underlying physicochemical principles and possible future applications are discussed.

Adhesion energy controls lipid binding-mediated endocytosis

Several bacterial toxins and viruses can deform membranes through multivalent binding to lipids for clathrin-independent endocytosis. However, it remains unclear, how membrane deformation and endocytic internalization are mechanistically linked. Here we show that many lipid-binding virions induce membrane deformation and clathrin-independent endocytosis, suggesting a common mechanism based on multivalent lipid binding by globular particles. We create a synthetic cellular system consisting of a lipid-anchored receptor in the form of GPI-anchored anti-GFP nanobodies and a multivalent globular binder exposing 180 regularly-spaced GFP molecules on its surface. We show that these globular, 40 nm diameter, particles bind to cells expressing the receptor, deform the plasma membrane upon adhesion and become endocytosed in a clathrin-independent manner. We explore the role of the membrane adhesion energy in endocytosis by using receptors with affinities varying over 7 orders of magnitude. Using this system, we find that once a threshold in adhesion energy is overcome to allow for membrane deformation, endocytosis occurs reliably. Multivalent, binding-induced membrane deformation by globular binders is thus sufficient for internalization to occur and we suggest it is the common, purely biophysical mechanism for lipid-binding mediated endocytosis of toxins and pathogens.