Curvature sensing by the epsin N-terminal homology domain measured on cylindrical lipid membrane tethers

Saddle curvature association of nsP1 facilitates the replication complex assembly of Chikungunya virus in cells

Positive-sense RNA viruses, including SARS-CoV-1 and -2, DENV, and CHIKV, replicate in curved membrane compartments within host cells. Non-structural proteins (nsPs) critically regulate these nanoscale membrane structures, yet their curvature-dependent assembly remains elusive due to the challenges of imaging nanoscale interaction on curved surfaces. Using vertically aligned nanostructures to generate pre-defined membrane curvatures, we here investigate the impact of curvature on nsPs assembly. Taking CHIKV as a model, we reveal that nsP1 preferentially binds and stabilizes on positively curved membranes, with stronger accumulation at radii ≤150 nm. This is driven by hydrophobic residues in the membrane association (MA) loops of individual nsP1. Molecular dynamics simulations further confirm the improved binding stability of nsP1 on curved membranes, particularly when it forms a dodecamer ring. Together, nsP1 supports a strong saddle curvature association, with flexible MA loops sensing a range of positive curvatures in the x-z plane while the rigid dodecamer stabilizing fixed negative curvature in the x-y plane - crucial for constraining the membrane spherule neck during replication progression. Moreover, CHIKV replication enriches on patterned nanoring structures, underscoring the curvature-guided assembly of the viral replication complex. Our findings highlight membrane curvature as a key regulator of viral nsPs organization, opening new avenues for studying membrane remodeling in viral replication.

Charge distribution and helicity tune the binding of septin's amphipathic helix domain to membranes

Amphipathic helices (AHs) are secondary structures that can facilitate binding of proteins to the membrane by folding into a helix with hydrophobic and hydrophilic faces that interact with the same surfaces in the lipid membrane. Septins are cytoskeletal proteins that preferentially bind to domains of micron-scale curvature on the cell membrane. Studies have shown that AH domains in septin are essential for curvature sensing. We present the first computational study of septin AH interactions with lipid bilayers. Using all-atom simulations and metadynamics-enhanced sampling, we study the effect of charge distribution at the flanking ends of septin AH on the energy for helical folding and its consequences on the binding configuration and affinity to the membrane. This is relevant to septins, since the net positive charge on the flanking C-terminal amino acids is a conserved property across several organisms. Simulations revealed that the energy barrier for folding in the neutral-capped AH is much larger than the charge-capped AH, leading to a small fraction of AH folding and integration to the membrane compared to a significantly folded configuration in the bound charge-capped AH. These observations are consistent with the binding measurements of synthetic AH constructs with variable helicity to lipid vesicles. Additionally, we examined an extended AH sequence including eight amino acids upstream and downstream of the AH to mimic the native protein. Again, simulations and experiments show that the extended peptide, with a net positive charge at C-terminus, adopts a strong helical configuration in solution, giving rise to a higher membrane affinity. Altogether, these results identify the energy cost for folding of AHs as a regulator of AH binding configuration and affinity and provide a basic template for parameterizing AH-membrane interactions as a starting point for the future multiscale simulations for septin-membrane interactions.

Membrane Remodeling Driven by Shallow Helix Insertions via a Cooperative Mechanism

Helix-membrane interactions are key to membrane deformation and play significant biological roles. However, systematic studies on the mechanisms behind these interactions are limited. This study uses a continuum membrane model to investigate how shallowly inserted helices interact with biological membranes, focusing on membrane deformation and the cooperative effects of multiple helices. Our findings show that even short helices (2 nm in length) can induce anisotropic membrane deformation. Longer helices and deeper insertions result in more significant deformations, and the spatial arrangement of helices affects the nature of these deformations. The perturbation area (PA) and perturbation extent (PE) are quantified to describe membrane deformation, revealing stronger cooperative effects in parallel insertions and more complex deformations in other arrangements. Additionally, membrane properties, such as lipid composition, influence the extent of deformation. In multi-helix systems, we observe local clustering behavior when perturbations are strong enough, with cooperativity varying based on helix length, insertion depth, and membrane composition. This study provides criteria for helix cooperativity, advancing our understanding of helix-membrane interactions and their biological significance in processes like membrane remodeling.

Biophysical modeling of membrane curvature generation and curvature sensing by the glycocalyx

Generation of membrane curvature is fundamental to cellular function. Recent studies have established that the glycocalyx, a sugar-rich polymer layer at the cell surface, can generate membrane curvature. While there have been some theoretical efforts to understand the interplay between the glycocalyx and membrane bending, there remain open questions about how the properties of the glycocalyx affect membrane bending. For example, the relationship between membrane curvature and the density of glycosylated proteins on its surface remains unclear. In this work, we use polymer brush theory to develop a detailed biophysical model of the energetic interactions of the glycocalyx with the membrane. Using this model, we identify the conditions under which the glycocalyx can both generate and sense curvature. Our model predicts that the extent of membrane curvature generated depends on the grafting density of the glycocalyx and the backbone length of the polymers constituting the glycocalyx. Furthermore, when coupled with the intrinsic membrane properties such as spontaneous curvature and a line tension along the membrane, the curvature generation properties of the glycocalyx are enhanced. These predictions were tested experimentally by examining the propensity of glycosylated transmembrane proteins to drive the assembly of highly curved filopodial protrusions at the plasma membrane of adherent mammalian cells. Our model also predicts that the glycocalyx has curvature-sensing capabilities, in agreement with the results of our experiments. Thus, our study develops a quantitative framework for mapping the properties of the glycocalyx to the curvature generation capability of the membrane.

Predicting protein curvature sensing across membrane compositions with a bilayer continuum model

Cytoplasmic proteins must recruit to membranes to function in processes such as endocytosis and cell division. Many of these proteins recognize not only the chemical structure of the membrane lipids, but the curvature of the surface, binding more strongly to more highly curved surfaces, or 'curvature sensing'. Curvature sensing by amphipathic helices is known to vary with membrane bending rigidity, but changes to lipid composition can simultaneously alter membrane thickness, spontaneous curvature, and leaflet symmetry, thus far preventing a systematic characterization of lipid composition on such curvature sensing through either experiment or simulation. Here we develop and apply a bilayer continuum membrane model that can tractably address this gap, quantifying how controlled changes to each material property can favor or disfavor protein curvature sensing. We evaluate both energetic and structural changes to vesicles upon helix insertion, with strong agreement to new in vitro experiments and all-atom MD simulations, respectively. Our membrane model builds on previous work to include both monolayers of the bilayer via representation by continuous triangular meshes. We introduce a coupling energy that captures the incompressibility of the membrane and the established energetics of lipid tilt. In agreement with experiment, our model predicts stronger curvature sensing in membranes with distinct tail groups (POPC vs DOPC vs DLPC), despite having identical head-group chemistry; the model shows that the primary driving force for weaker curvature sensing in DLPC is that it is thinner, and more wedge shaped. Somewhat surprisingly, asymmetry in lipid shape composition between the two leaflets has a negligible contribution to membrane mechanics following insertion. Our multi-scale approach can be used to quantitatively and efficiently predict how changes to membrane composition in flat to highly curved surfaces alter membrane energetics driven by proteins, a mechanism that helps proteins target membranes at the correct time and place.

Fission of double-membrane tubes under tension

The division of a cellular compartment culminates with the scission of a highly constricted membrane neck. Scission requires lipid rearrangements, topology changes, and transient formation of nonbilayer intermediate structures driven by curvature stress. Often, a side effect of this stress is pore-formation, which may lead to content leakage and thus breaching of the membrane barrier function. In single-membrane systems, leakage is avoided through the formation of a hemifusion (HF) intermediate, whose structure is still a subject of debate. The consequences of curvature stress have not been explored in double-membrane systems, such as the mitochondrion. Here, we combine experimental and theoretical approaches to study neck constriction and scission driven by tension in biomimetic lipid systems, namely single- and double-membrane nanotubes (sNTs and dNTs), respectively. In sNTs, constriction by high tension gives rise to a metastable HF intermediate (seen as stalk or worm-like micelle), whereas poration is universally slower in a simple neck. In dNTs, high membrane tension causes sequential rupture of each membrane. In contrast, low tension leads to the HF of both membranes, which may lead to a leaky fusion pathway, or may progress to further fusion of the two membranes along a number of transformation pathways. These findings provide a new mechanistic basis for fundamental cellular processes.

Transient pores in hemifusion diaphragms

Exchange of material across two membranes, as in the case of synaptic neurotransmitter release from a vesicle, involves the formation and poration of a hemifusion diaphragm (HD). The nontrivial geometry of the HD leads to environment-dependent control, regarding the stability and dynamics of the pores required for this kind of exocytosis. This work combines particle simulations, field-based calculations, and phenomenological modeling to explore the factors influencing the stability, dynamics, and possible control mechanisms of pores in HDs. We find that pores preferentially form at the HD rim, and that their stability is sensitive to a number of factors, including the three line tensions, membrane tension, HD size, and the ability of lipids to "flip-flop" across leaflets. Along with a detailed analysis of these factors, we discuss ways that vesicles or cells may use them to open and close pores and thereby quickly and efficiently transport material.

Membrane fission via transmembrane contact

Division of intracellular organelles often correlates with additional membrane wrapping, e.g., by the endoplasmic reticulum or the outer mitochondrial membrane. Such wrapping plays a vital role in proteome and lipidome organization. However, how an extra membrane impacts the mechanics of the division has not been investigated. Here we combine fluorescence and cryo-electron microscopy experiments with self-consistent field theory to explore the stress-induced instabilities imposed by membrane wrapping in a simple double-membrane tubular system. We find that, at physiologically relevant conditions, the outer membrane facilitates an alternative pathway for the inner-tube fission through the formation of a transient contact (hemi-fusion) between both membranes. A detailed molecular theory of the fission pathways in the double membrane system reveals the topological complexity of the process, resulting both in leaky and leakless intermediates, with energies and topologies predicting physiological events.

PMIpred: a physics-informed web server for quantitative protein-membrane interaction prediction

MOTIVATION

Many membrane peripheral proteins have evolved to transiently interact with the surface of (curved) lipid bilayers. Currently, methods to quantitatively predict sensing and binding free energies for protein sequences or structures are lacking, and such tools could greatly benefit the discovery of membrane-interacting motifs, as well as their de novo design.

RESULTS

Here, we trained a transformer neural network model on molecular dynamics data for >50 000 peptides that is able to accurately predict the (relative) membrane-binding free energy for any given amino acid sequence. Using this information, our physics-informed model is able to classify a peptide's membrane-associative activity as either non-binding, curvature sensing, or membrane binding. Moreover, this method can be applied to detect membrane-interaction regions in a wide variety of proteins, with comparable predictive performance as state-of-the-art data-driven tools like DREAMM, PPM3, and MODA, but with a wider applicability regarding protein diversity, and the added feature to distinguish curvature sensing from general membrane binding.

AVAILABILITY AND IMPLEMENTATION

We made these tools available as a web server, coined Protein-Membrane Interaction predictor (PMIpred), which can be accessed at https://pmipred.fkt.physik.tu-dortmund.de.

Epsin Endocytic Adaptor Proteins in Angiogenic and Lymphangiogenic Signaling

Circulating vascular endothelial growth factor (VEGF) ligands and receptors are central regulators of vasculogenesis, angiogenesis, and lymphangiogenesis. In response to VEGF ligand binding, VEGF receptor tyrosine kinases initiate the chain of events that transduce extracellular signals into endothelial cell responses such as survival, proliferation, and migration. These events are controlled by intricate cellular processes that include the regulation of gene expression at multiple levels, interactions of numerous proteins, and intracellular trafficking of receptor-ligand complexes. Endocytic uptake and transport of macromolecular complexes through the endosome-lysosome system helps fine-tune endothelial cell responses to VEGF signals. Clathrin-dependent endocytosis remains the best understood means of macromolecular entry into cells, although the importance of non-clathrin-dependent pathways is increasingly recognized. Many of these endocytic events rely on adaptor proteins that coordinate internalization of activated cell-surface receptors. In the endothelium of both blood and lymphatic vessels, epsins 1 and 2 are functionally redundant adaptors involved in receptor endocytosis and intracellular sorting. These proteins are capable of binding both lipids and proteins and are important for promoting curvature of the plasma membrane as well as binding ubiquitinated cargo. Here, we discuss the role of epsin proteins and other endocytic adaptors in governing VEGF signaling in angiogenesis and lymphangiogenesis and discuss their therapeutic potential as molecular targets.

Molecular Sensing and Manipulation of Protein Oligomerization in Membrane Nanotubes with Bolaamphiphilic Foldamers

Adaptive and reversible self-assembly of supramolecular protein structures is a fundamental characteristic of dynamic living matter. However, the quantitative detection and assessment of the emergence of mesoscale protein complexes from small and dynamic oligomeric precursors remains highly challenging. Here, we present a novel approach utilizing a short membrane nanotube (sNT) pulled from a planar membrane reservoir as nanotemplates for molecular reconstruction, manipulation, and sensing of protein oligomerization and self-assembly at the mesoscale. The sNT reports changes in membrane shape and rigidity caused by membrane-bound proteins as variations of the ionic conductivity of the sNT lumen. To confine oligomerization to the sNT, we have designed and synthesized rigid oligoamide foldamer tapes (ROFTs). Charged ROFTs incorporate into the planar and sNT membranes, mediate protein binding to the membranes, and, driven by the luminal electric field, shuttle the bound proteins between the sNT and planar membranes. Using Annexin-V (AnV) as a prototype, we show that the sNT detects AnV oligomers shuttled into the nanotube by ROFTs. Accumulation of AnV on the sNT induces its self-assembly into a curved lattice, restricting the sNT geometry and inhibiting the material uptake from the reservoir during the sNT extension, leading to the sNT fission. By comparing the spontaneous and ROFT-mediated entry of AnV into the sNT, we reveal how intricate membrane curvature sensing by small AnV oligomers controls the lattice self-assembly. These results establish sNT-ROFT as a powerful tool for molecular reconstruction and functional analyses of protein oligomerization and self-assembly, with broad application to various membrane processes.

A NanoCurvS platform for quantitative and multiplex analysis of curvature-sensing proteins

The cell membrane is characterized by a rich variety of topographical features such as local protrusions or invaginations. Curvature-sensing proteins, including the Bin/Amphiphysin/Rvs (BAR) or epsin N-terminal homology (ENTH) family proteins, sense the bending sharpness and the positive/negative sign of these topographical features to induce subsequent intracellular signaling. A number of assays have been developed to study curvature-sensing properties of proteins in vitro, but it is still challenging to probe low curvature regime with the diameter of curvature from hundreds of nanometers to micrometers. It is particularly difficult to generate negative membrane curvatures with well-defined curvature values in the low curvature regime. In this work, we develop a nanostructure-based curvature sensing (NanoCurvS) platform that enables quantitative and multiplex analysis of curvature-sensitive proteins in the low curvature regime, in both negative and positive directions. We use NanoCurvS to quantitatively measure the sensing range of a negative curvature-sensing protein IRSp53 (an I-BAR protein) and a positive curvature-sensing protein FBP17 (an F-BAR protein). We find that, in cell lysates, the I-BAR domain of IRSp53 is able to sense shallow negative curvatures with the diameter-of-curvature up to 1500 nm, a range much wider than previously expected. NanoCurvS is also used to probe the autoinhibition effect of IRSp53 and the phosphorylation effect of FBP17. Therefore, the NanoCurvS platform provides a robust, multiplex, and easy-to-use tool for quantitative analysis of both positive and negative curvature-sensing proteins.

Simulation study of membrane bending by protein crowding: a case study with the epsin N-terminal homology domain

The mechanisms by which peripheral membrane proteins generate curvature is currently an active area of research. One of the proposed mechanisms is amphipathic insertion or the 'wedge' mechanism in which the protein shallowly inserts an amphipathic helix inside the membrane to drive the curvature. However, recent experimental studies have challenged the efficiency of the 'wedge' mechanism as it requires unusual protein densities. These studies proposed an alternative mechanism, namely 'protein-crowding', in which the lateral pressure generated by the random collisions among the membrane bound proteins drives the bending. In this study, we employ atomistic and coarse-grained molecular dynamics simulations to investigate the effects of amphipathic insertion and protein crowding on the membrane surface. Considering epsin N-terminal homology (ENTH) domain as a model protein, we show that amphipathic insertion is not essential for membrane bending. Our results suggest that ENTH domains can aggregate on the membrane surface by employing another structured region (H₃ helix). And this protein crowding decreases the cohesive energy of the lipid tails which causes a significant decrease in the membrane bending rigidity. The ENTH domain can generate a similar degree of membrane curvature irrespective of the activity of its H₀ helix. Our results are consistent with the recent experimental results.

Formation of protein-mediated bilayer tubes is governed by a snapthrough transition

Plasma membrane tubes are ubiquitous in cellular membranes and in the membranes of intracellular organelles. They play crucial roles in trafficking, ion transport, and cellular motility. These tubes can be formed due to localized forces acting on the membrane or by the curvature induced by membrane-bound proteins. Here, we present a mathematical framework to model cylindrical tubular protrusions formed by proteins that induce anisotropic spontaneous curvature. Our analysis revealed that the tube radius depends on an effective tension that includes contributions from the bare membrane tension and the protein-induced curvature. We also found that the length of the tube undergoes an abrupt transition from a short, dome-shaped membrane to a long cylinder and this transition is characteristic of a snapthrough instability. Finally, we show that the snapthrough instability depends on the different parameters including coat area, bending modulus, and extent of protein-induced curvature. Our findings have implications for tube formation due to BAR-domain proteins in processes such as endocytosis, t-tubule formation in myocytes, and cristae formation in mitochondria.

Molecular Shape Solution for Mesoscopic Remodeling of Cellular Membranes

Cellular membranes self-assemble from and interact with various molecular species. Each molecule locally shapes the lipid bilayer, the soft elastic core of cellular membranes. The dynamic architecture of intracellular membrane systems is based on elastic transformations and lateral redistribution of these elementary shapes, driven by chemical and curvature stress gradients. The minimization of the total elastic stress by such redistribution composes the most basic, primordial mechanism of membrane curvature-composition coupling (CCC). Although CCC is generally considered in the context of dynamic compositional heterogeneity of cellular membrane systems, in this article we discuss a broader involvement of CCC in controlling membrane deformations. We focus specifically on the mesoscale membrane transformations in open, reservoir-governed systems, such as membrane budding, tubulation, and the emergence of highly curved sites of membrane fusion and fission. We reveal that the reshuffling of molecular shapes constitutes an independent deformation mode with complex rheological properties.This mode controls effective elasticity of local deformations as well as stationary elastic stress, thus emerging as a major regulator of intracellular membrane remodeling.

Quantification of Curvature Sensing Behavior of Curvature-Inducing Proteins on Model Wavy Substrates

Curvature-inducing proteins are involved in a variety of membrane remodeling processes in the cell. Several in vitro experiments have quantified the curvature sensing behavior of these proteins in model lipid systems. One such system consists of a membrane bilayer laid atop a wavy substrate (Hsieh in Langmuir 28:12838-12843, 2012). In these experiments, the bilayer conforms to the wavy substrate, and curvature-inducing proteins show preferential segregation on the wavy membrane. Using a mesoscale computational membrane model based on the Helfrich Hamiltonian, here we present a study which analyzes the curvature sensing characteristics of this membrane-protein system, and elucidates key physical principles governing protein segregation on the wavy substrate and other in vitro systems. In this article we compute the local protein densities from the free energy landscape associated with membrane remodeling by curvature-inducing proteins. In specific, we use the Widom insertion technique to compute the free energy landscape for an inhomogeneous system with spatially varying density and the results obtained with this minimal model show excellent agreement with experimental studies that demonstrate the association between membrane curvature and local protein density. The free energy-based framework employed in this study can be used for different membrane morphologies and varied protein characteristics to gain mechanistic insights into protein sorting on membranes.

Probing lipid membrane bending mechanics using gold nanorod tracking

Lipid bilayer membranes undergo rapid bending undulations with wavelengths from tens of nanometers to tens of microns due to thermal fluctuations. Here, we probe such undulations and the membranes' mechanics by measuring the time-varying orientation of single gold nanorods (GNRs) adhered to the membrane, using high-speed dark field microscopy. In a lipid vesicle, such measurements allow the determination of the membrane's viscosity, bending rigidity, and tension as well as the friction coefficient for sliding of the monolayers over one another. The in-plane rotation of the GNR is hindered by undulations in a tension dependent manner, consistent with simulations. The motion of single GNRs adhered to the plasma membrane of living cultured cells similarly reveals the membrane's complex physics and coupling to the cell's actomyosin cortex.

A continuum membrane model can predict curvature sensing by helix insertion

Protein domains, such as ENTH (epsin N-terminal homology) and BAR (bin/amphiphysin/rvs), contain amphipathic helices that drive preferential binding to curved membranes. However, predicting how the physical parameters of these domains control this 'curvature sensing' behavior is challenging due to the local membrane deformations generated by the nanoscopic helix on the surface of a large sphere. We here use a deformable continuum model that accounts for the physical properties of the membrane and the helix insertion to predict curvature sensing behavior, with direct validation against multiple experimental datasets. We show that the insertion can be modeled as a local change to the membrane's spontaneous curvature, cins0, producing excellent agreement with the energetics extracted from experiments on ENTH binding to vesicles and cylinders, and of ArfGAP helices to vesicles. For small vesicles with high curvature, the insertion lowers the membrane energy by relieving strain on a membrane that is far from its preferred curvature of zero. For larger vesicles, however, the insertion has the inverse effect, de-stabilizing the membrane by introducing more strain. We formulate here an empirical expression that accurately captures numerically calculated membrane energies as a function of both basic membrane properties (bending modulus κ and radius R) as well as stresses applied by the inserted helix (cins0 and area A_ins). We therefore predict how these physical parameters will alter the energetics of helix binding to curved vesicles, which is an essential step in understanding their localization dynamics during membrane remodeling processes.

Recent developments in membrane curvature sensing and induction by proteins

BACKGROUND

Membrane-bound intracellular organelles have characteristic shapes attributed to different local membrane curvatures, and these attributes are conserved across species. Over the past decade, it has been confirmed that specific proteins control the large curvatures of the membrane, whereas many others due to their specific structural features can sense the curvatures and bind to the specific geometrical cues. Elucidating the interplay between sensing and induction is indispensable to understand the mechanisms behind various biological processes such as vesicular trafficking and budding.

SCOPE OF REVIEW

We provide an overview of major classes of membrane proteins and the mechanisms of curvature sensing and induction. We then discuss the importance of membrane elastic characteristics to induce the membrane shapes similar to intracellular organelles. Finally, we survey recently available assays developed for studying the curvature sensing and induction by many proteins.

MAJOR CONCLUSIONS

Recent theoretical/computational modeling along with experimental studies have uncovered fascinating connections between lipid membrane and protein interactions. However, the phenomena of protein localization and synchronization to generate spatiotemporal dynamics in membrane morphology are yet to be fully understood.

GENERAL SIGNIFICANCE

The understanding of protein-membrane interactions is essential to shed light on various biological processes. This further enables the technological applications of many natural proteins/peptides in therapeutic treatments. The studies of membrane dynamic shapes help to understand the fundamental functions of membranes, while the medicinal roles of various macromolecules (such as proteins, peptides, etc.) are being increasingly investigated.

Protein-dependent membrane remodeling in mitochondrial morphology and clathrin-mediated endocytosis

Cellular membranes can adopt a plethora of complex and beautiful shapes, most of which are believed to have evolved for a particular physiological reason. The closely entangled relationship between membrane morphology and cellular physiology is strikingly seen in membrane trafficking pathways. During clathrin-mediated endocytosis, for example, over the course of a minute, a patch of the more or less flat plasma membrane is remodeled into a highly curved clathrin-coated vesicle. Such vesicles are internalized by the cell to degrade or recycle plasma membrane receptors or to take up extracellular ligands. Other, steadier, membrane morphologies can be observed in organellar membranes like the endoplasmic reticulum or mitochondria. In the case of mitochondria, which are double membrane-bound, ubiquitous organelles of eukaryotic cells, especially the mitochondrial inner membrane displays an intricated ultrastructure. It is highly folded and consequently has a much larger surface than the mitochondrial outer membrane. It can adopt different shapes in response to cellular demands and changes of the inner membrane morphology often accompany severe diseases, including neurodegenerative- and metabolic diseases and cancer. In recent years, progress was made in the identification of molecules that are important for the aforementioned membrane remodeling events. In this review, we will sum up recent results and discuss the main players of membrane remodeling processes that lead to the mitochondrial inner membrane ultrastructure and in clathrin-mediated endocytosis. We will compare differences and similarities between the molecular mechanisms that peripheral and integral membrane proteins use to deform membranes.

Clathrin senses membrane curvature

The ability of proteins to assemble at sites of high membrane curvature is essential to diverse membrane remodeling processes, including clathrin-mediated endocytosis. Multiple adaptor proteins within the clathrin pathway have been shown to sense regions of high membrane curvature, leading to local recruitment of the clathrin coat. Because clathrin triskelia do not bind to the membrane directly, it has remained unclear whether the clathrin coat plays an active role in sensing membrane curvature or is passively recruited by adaptor proteins. Using a synthetic tag to assemble clathrin directly on membrane surfaces, here we show that clathrin is a strong sensor of membrane curvature, comparable with previously studied adaptor proteins. Interestingly, this sensitivity arises from clathrin assembly rather than from the properties of unassembled triskelia, suggesting that triskelia have preferred angles of interaction, as predicted by earlier structural data. Furthermore, when clathrin is recruited by adaptors, its curvature sensitivity is amplified by 2- to 10-fold, such that the resulting protein complex is up to 100 times more likely to assemble on a highly curved surface compared with a flatter one. This exquisite sensitivity points to a synergistic relationship between the coat and its adaptor proteins, which enables clathrin to pinpoint sites of high membrane curvature, an essential step in ensuring robust membrane traffic. More broadly, these findings suggest that protein networks, rather than individual protein domains, are likely the most potent drivers of membrane curvature sensing.

ENTH domain-dependent membrane remodelling

Cellular membranes are anything but flat structures. They display a wide variety of complex and beautiful shapes, most of which have evolved for a particular physiological reason and are adapted to accommodate certain cellular demands. In membrane trafficking events, the dynamic remodelling of cellular membranes is apparent. In clathrin-mediated endocytosis for example, the plasma membrane undergoes heavy deformation to generate and internalize a highly curved clathrin-coated vesicle. This process has become a model system to study proteins with the ability to sense and induce membrane curvature and over the last two decades numerous membrane remodelling molecules and molecular mechanisms have been identified in this process. In this review, we discuss the interaction of epsin1 ENTH domain with membranes, which is one of the best-studied examples of a peripheral and transiently membrane bending protein important for clathrin-mediated endocytosis.

Coarse-Grained Simulations Suggest the Epsin N-Terminal Homology Domain Can Sense Membrane Curvature without Its Terminal Amphipathic Helix

Nanoscale membrane curvature is a common feature in cell biology required for functions such as endocytosis, exocytosis and cell migration. These processes require the cytoskeleton to exert forces on the membrane to deform it. Cytosolic proteins contain specific motifs which bind to the membrane, connecting it to the internal cytoskeletal machinery. These motifs often bind charged phosphatidylinositol phosphate lipids present in the cell membrane which play significant roles in signaling. These lipids are important for membrane deforming processes, such as endocytosis, but much remains unknown about their role in the sensing of membrane nanocurvature by protein domains. Using coarse-grained molecular dynamics simulations, we investigated the interaction of a model curvature active protein domain, the epsin N-terminal homology domain (ENTH), with curved lipid membranes. The combination of anionic lipids (phosphatidylinositol 4,5-bisphosphate and phosphatidylserine) within the membrane, protein backbone flexibility, and structural changes within the domain were found to affect the domain's ability to sense, bind, and localize with nanoscale precision at curved membrane regions. The findings suggest that the ENTH domain can sense membrane curvature without the presence of its terminal amphipathic α helix via another structural region we have denoted as H3, re-emphasizing the critical relationship between nanoscale membrane curvature and protein function.

Complimentary action of structured and unstructured domains of epsin supports clathrin-mediated endocytosis at high tension

Membrane tension plays an inhibitory role in clathrin-mediated endocytosis (CME) by impeding the transition of flat plasma membrane to hemispherical clathrin-coated structures (CCSs). Membrane tension also impedes the transition of hemispherical domes to omega-shaped CCSs. However, CME is not completely halted in cells under high tension conditions. Here we find that epsin, a membrane bending protein which inserts its N-terminus H0 helix into lipid bilayer, supports flat-to-dome transition of a CCS and stabilizes its curvature at high tension. This discovery is supported by molecular dynamic simulation of the epsin N-terminal homology (ENTH) domain that becomes more structured when embedded in a lipid bilayer. In addition, epsin has an intrinsically disordered protein (IDP) C-terminus domain which induces membrane curvature via steric repulsion. Insertion of H0 helix into lipid bilayer is not sufficient for stable epsin recruitment. Epsin's binding to adaptor protein 2 and clathrin is critical for epsin's association with CCSs under high tension conditions, supporting the importance of multivalent interactions in CCSs. Together, our results support a model where the ENTH and unstructured IDP region of epsin have complementary roles to ensure CME initiation and CCS maturation are unimpeded under high tension environments.

Murine Epsins Play an Integral Role in Podocyte Function

BACKGROUND

Epsins, a family of evolutionarily conserved membrane proteins, play an essential role in endocytosis and signaling in podocytes.

METHODS

Podocyte-specific Epn1, Epn2, Epn3 triple-knockout mice were generated to examine downstream regulation of serum response factor (SRF) by cell division control protein 42 homolog (Cdc42).

RESULTS

Podocyte-specific loss of epsins resulted in increased albuminuria and foot process effacement. Primary podocytes isolated from these knockout mice exhibited abnormalities in cell adhesion and spreading, which may be attributed to reduced activation of cell division control protein Cdc42 and SRF, resulting in diminished β1 integrin expression. In addition, podocyte-specific loss of Srf resulted in severe albuminuria and foot process effacement, and defects in cell adhesion and spreading, along with decreased β1 integrin expression.

CONCLUSIONS

Epsins play an indispensable role in maintaining properly functioning podocytes through the regulation of Cdc42 and SRF-dependent β1 integrin expression.

Reconstitution and real-time quantification of membrane remodeling by single proteins and protein complexes

Cellular membrane processes, from signal transduction to membrane fusion and fission, depend on acute membrane deformations produced by small and short-lived protein complexes working in conditions far from equilibrium. Real-time monitoring and quantitative assessment of such deformations are challenging; hence, mechanistic analyses of the protein action are commonly based on ensemble averaging, which masks important mechanistic details of the action. In this protocol, we describe how to reconstruct and quantify membrane remodeling by individual proteins and small protein complexes in vitro, using an ultra-short (80- to 400-nm) lipid nanotube (usNT) template. We use the luminal conductance of the usNT as the real-time reporter of the protein interaction(s) with the usNT. We explain how to make and calibrate the usNT template to achieve subnanometer precision in the geometrical assessment of the molecular footprints on the nanotube membrane. We next demonstrate how membrane deformations driven by purified proteins implicated in cellular membrane remodeling can be analyzed at a single-molecule level. The preparation of one usNT takes ~1 h, and the shortest procedure yielding the basic geometrical parameters of a small protein complex takes 10 h.

Listeria monocytogenes Exploits Host Caveolin for Cell-to-Cell Spreading

Listeria monocytogenes moves from one cell to another using actin-rich membrane protrusions that propel the bacterium toward neighboring cells. Despite cholesterol being required for this transfer process, the precise host internalization mechanism remains elusive. Here, we show that caveolin endocytosis is key to this event as bacterial cell-to-cell transfer is severely impaired when cells are depleted of caveolin-1. Only a subset of additional caveolar components (cavin-2 and EHD2) are present at sites of bacterial transfer, and although clathrin and the clathrin-associated proteins Eps15 and AP2 are absent from the bacterial invaginations, efficient L. monocytogenes spreading requires the clathrin-interacting protein epsin-1. We also directly demonstrated that isolated L. monocytogenes membrane protrusions can trigger the recruitment of caveolar proteins in a neighboring cell. The engulfment of these bacterial and cytoskeletal structures through a caveolin-based mechanism demonstrates that the classical nanometer-scale theoretical size limit for this internalization pathway is exceeded by these bacterial pathogens.IMPORTANCEListeria monocytogenes moves from one cell to another as it disseminates within tissues. This bacterial transfer process depends on the host actin cytoskeleton as the bacterium forms motile actin-rich membranous protrusions that propel the bacteria into neighboring cells, thus forming corresponding membrane invaginations. Here, we examine these membrane invaginations and demonstrate that caveolin-1-based endocytosis is crucial for efficient bacterial cell-to-cell spreading. We show that only a subset of caveolin-associated proteins (cavin-2 and EHD2) are involved in this process. Despite the absence of clathrin at the invaginations, the classical clathrin-associated protein epsin-1 is also required for efficient bacterial spreading. Using isolated L. monocytogenes protrusions added onto naive host cells, we demonstrate that actin-based propulsion is dispensable for caveolin-1 endocytosis as the presence of the protrusion/invagination interaction alone triggers caveolin-1 recruitment in the recipient cells. Finally, we provide a model of how this caveolin-1-based internalization event can exceed the theoretical size limit for this endocytic pathway.

Dynamic constriction and fission of endoplasmic reticulum membranes by reticulon

The endoplasmic reticulum (ER) is a continuous cell-wide membrane network. Network formation has been associated with proteins producing membrane curvature and fusion, such as reticulons and atlastin. Regulated network fragmentation, occurring in different physiological contexts, is less understood. Here we find that the ER has an embedded fragmentation mechanism based upon the ability of reticulon to produce fission of elongating network branches. In Drosophila, Rtnl1-facilitated fission is counterbalanced by atlastin-driven fusion, with the prevalence of Rtnl1 leading to ER fragmentation. Ectopic expression of Drosophila reticulon in COS-7 cells reveals individual fission events in dynamic ER tubules. Consistently, in vitro analyses show that reticulon produces velocity-dependent constriction of lipid nanotubes leading to stochastic fission via a hemifission mechanism. Fission occurs at elongation rates and pulling force ranges intrinsic to the ER, thus suggesting a principle whereby the dynamic balance between fusion and fission controlling organelle morphology depends on membrane motility.

Curving Cells Inside and Out: Roles of BAR Domain Proteins in Membrane Shaping and Its Cellular Implications

Many cellular processes rely on precise and timely deformation of the cell membrane. While many proteins participate in membrane reshaping and scission, usually in highly specialized ways, Bin/amphiphysin/Rvs (BAR) domain proteins play a pervasive role, as they not only participate in many aspects of cell trafficking but also are highly versatile membrane remodelers. Subtle changes in the shape and size of the BAR domain can greatly impact the way in which BAR domain proteins interact with the membrane. Furthermore, the activity of BAR domain proteins can be tuned by external physical parameters, and so they behave differently depending on protein surface density, membrane tension, or membrane shape. These proteins can form 3D structures that mold the membrane and alter its liquid properties, even promoting scission under various circumstances.As such, BAR domain proteins have numerous roles within the cell. Endocytosis is among the most highly studied processes in which BAR domain proteins take on important roles. Over the years, a more complete picture has emerged in which BAR domain proteins are tied to almost all intracellular compartments; examples include endosomal sorting and tubular networks in the endoplasmic reticulum and T-tubules. These proteins also have a role in autophagy, and their activity has been linked with cancer. Here, we briefly review the history of BAR domain protein discovery, discuss the mechanisms by which BAR domain proteins induce curvature, and attempt to settle important controversies in the field. Finally, we review BAR domain proteins in the context of a cell, highlighting their emerging roles in cell signaling and organelle shaping.

Single-Nanometer Changes in Nanopore Geometry Influence Curvature, Local Properties, and Protein Localization in Membrane Simulations

Nanoporous surfaces are used in many applications in intracellular sensing and drug delivery. However, the effects of such nanostructures on cell membrane properties are still far from understood. Here, we use coarse-grained molecular dynamics simulations to show that nanoporous substrates can stimulate membrane-curvature effects and that this curvature-sensing effect is much more sensitive than previously thought. We define a series of design parameters for inducing a nanoscale membrane curvature and show that nanopore taper plays a key role in membrane deformation, elucidating a previously unexplored fabrication parameter applicable to many nanostructured biomaterials. We report significant changes in the membrane area per lipid and thickness at regions of curvature. Finally, we demonstrate that regions of the nanopore-induced membrane curvature act as local hotspots for an increased bioactivity. We show spontaneous binding and localization of the epsin N-terminal homology (ENTH) domain to the regions of curvature. Understanding this interplay between the membrane curvature and nanoporosity at the biointerface helps both explain recent biological results and suggests a pathway for developing the next generation of cell-active substrates.

Molecular Mechanisms of Membrane Curvature Sensing by a Disordered Protein

The ability of proteins to sense membrane curvature is essential for the initiation and assembly of curved membrane structures. Established mechanisms of curvature sensing rely on proteins with specific structural features. In contrast, it has recently been discovered that intrinsically disordered proteins, which lack a defined three-dimensional fold, can also be potent sensors of membrane curvature. How can an unstructured protein sense the structure of the membrane surface? Many disordered proteins that associate with membranes have two key physical features: a high degree of conformational entropy and a high net negative charge. Binding of such proteins to membrane surfaces results simultaneously in a decrease in conformational entropy and an increase in electrostatic repulsion by anionic lipids. Here we show that each of these effects gives rise to a distinct mechanism of curvature sensing. Specifically, as the curvature of the membrane increases, the steric hindrance between the disordered protein and membrane is reduced, leading to an increase in chain entropy. At the same time, increasing membrane curvature increases the average separation between anionic amino acids and lipids, creating an electrostatic preference for curved membranes. Using quantitative imaging of membrane vesicles, our results demonstrate that long disordered amino acid chains with low net charge sense curvature predominately through the entropic mechanism. In contrast, shorter, more highly charged amino acid chains rely largely on the electrostatic mechanism. These findings provide a roadmap for predicting and testing the curvature sensitivity of the large and diverse set of disordered proteins that function at cellular membranes.

Membrane Curvature Sensing by Amphipathic Helices: Insights from Implicit Membrane Modeling

Sensing and generation of lipid membrane curvature, mediated by the binding of specific proteins onto the membrane surface, play crucial roles in cell biology. A number of mechanisms have been proposed, but the molecular understanding of these processes is incomplete. All-atom molecular dynamics simulations have offered valuable insights but are extremely demanding computationally. Implicit membrane simulations could provide a viable alternative, but current models apply only to planar membranes. In this work, the implicit membrane model 1 is extended to spherical and tubular membranes. The geometric change from planar to curved shapes is straightforward but insufficient for capturing the full curvature effect, which includes changes in lipid packing. Here, these packing effects are taken into account via the lateral pressure profile. The extended implicit membrane model 1 is tested on the wild-types and mutants of the antimicrobial peptide magainin, the ALPS motif of arfgap1, α-synuclein, and an ENTH domain. In these systems, the model is in qualitative agreement with experiments. We confirm that favorable electrostatic interactions tend to weaken curvature sensitivity in the presence of strong hydrophobic interactions but may actually have a positive effect when those are weak. We also find that binding to vesicles is more favorable than binding to tubes of the same diameter and that the long helix of α-synuclein tends to orient along the axis of tubes, whereas shorter helices tend to orient perpendicular to it. Adoption of a specific orientation could provide a mechanism for coupling protein oligomerization to tubule formation.

Intrinsically disordered proteins in synaptic vesicle trafficking and release

The past few years have resulted in an increased awareness and recognition of the prevalence and roles of intrinsically disordered proteins and protein regions (IDPs and IDRs, respectively) in synaptic vesicle trafficking and exocytosis and in overall synaptic organization. IDPs and IDRs constitute a class of proteins and protein regions that lack stable tertiary structure, but nevertheless retain biological function. Their significance in processes such as cell signaling is now well accepted, but their pervasiveness and importance in other areas of biology are not as widely appreciated. Here, we review the prevalence and functional roles of IDPs and IDRs associated with the release and recycling of synaptic vesicles at nerve terminals, as well as with the architecture of these terminals. We hope to promote awareness, especially among neuroscientists, of the importance of this class of proteins in these critical pathways and structures. The examples discussed illustrate some of the ways in which the structural flexibility conferred by intrinsic protein disorder can be functionally advantageous in the context of cellular trafficking and synaptic function.

Synergy between intrinsically disordered domains and structured proteins amplifies membrane curvature sensing

The ability of proteins to sense membrane curvature is essential to cellular function. All known sensing mechanisms rely on protein domains with specific structural features such as wedge-like amphipathic helices and crescent-shaped BAR domains. Yet many proteins that contain these domains also contain large intrinsically disordered regions. Here we report that disordered domains are themselves potent sensors of membrane curvature. Comparison of Monte Carlo simulations with in vitro and live-cell measurements demonstrates that the polymer-like behavior of disordered domains found in endocytic proteins drives them to partition preferentially to convex membrane surfaces, which place fewer geometric constraints on their conformational entropy. Further, proteins containing both structured curvature sensors and disordered regions are more than twice as curvature sensitive as their respective structured domains alone. These findings demonstrate an entropic mechanism of curvature sensing that is independent of protein structure and illustrate how structured and disordered domains can synergistically enhance curvature sensitivity.

Many proteins which sense membrane curvature contain intrinsically disordered domains. Here the authors use Monte Carlo simulations combined with experimental approaches and report that disordered domains are potent sensors of membrane curvature.

Biophysics of membrane curvature remodeling at molecular and mesoscopic lengthscales

At the micron scale, where cell organelles display an amazing complexity in their shape and organization, the physical properties of a biological membrane can be better-understood using continuum models subject to thermal (stochastic) undulations. Yet, the chief orchestrators of these complex and intriguing shapes are a specialized class of membrane associating often peripheral proteins called curvature remodeling proteins (CRPs) that operate at the molecular level through specific protein-lipid interactions. We review multiscale methodologies to model these systems at the molecular as well as at the mesoscopic and cellular scales, and also present a free energy perspective of membrane remodeling through the organization and assembly of CRPs. We discuss the morphological space of nearly planar to highly curved membranes, methods to include thermal fluctuations, and review studies that model such proteins as curvature fields to describe the emergent curved morphologies. We also discuss several mesoscale models applied to a variety of cellular processes, where the phenomenological parameters (such as curvature field strength) are often mapped to models of real systems based on molecular simulations. Much insight can be gained from the calculation of free energies of membranes states with protein fields, which enable accurate mapping of the state and parameter values at which the membrane undergoes morphological transformations such as vesiculation or tubulation. By tuning the strength, anisotropy, and spatial organization of the curvature-field, one can generate a rich array of membrane morphologies that are highly relevant to shapes of several cellular organelles. We review applications of these models to budding of vesicles commonly seen in cellular signaling and trafficking processes such as clathrin mediated endocytosis, sorting by the ESCRT protein complexes, and cellular exocytosis regulated by the exocyst complex. We discuss future prospects where such models can be combined with other models for cytoskeletal assembly, and discuss their role in understanding the effects of cell membrane tension and the mechanics of the extracellular microenvironment on cellular processes.

Lipid packing defects and membrane charge control RAB GTPase recruitment

Specific intracellular localization of RAB GTPases has been reported to be dependent on protein factors, but the contribution of the membrane physicochemical properties to this process has been poorly described. Here, we show that three RAB proteins (RAB1/RAB5/RAB6) preferentially bind in vitro to disordered and curved membranes, and that this feature is uniquely dependent on their prenyl group. Our results imply that the addition of a prenyl group confers to RAB proteins, and most probably also to other prenylated proteins, the ability to sense lipid packing defects induced by unsaturated conical-shaped lipids and curvature. Consistently, RAB recruitment increases with the amount of lipid packing defects, further indicating that these defects drive RAB membrane targeting. Membrane binding of RAB35 is also modulated by lipid packing defects but primarily dependent on negatively charged lipids. Our results suggest that a balance between hydrophobic insertion of the prenyl group into lipid packing defects and electrostatic interactions of the RAB C-terminal region with charged membranes tunes the specific intracellular localization of RAB proteins.

The glycolipid GM1 reshapes asymmetric biomembranes and giant vesicles by curvature generation

The ganglioside GM1 is present in neuronal membranes at elevated concentrations with an asymmetric spatial distribution. It is known to generate curvature and can be expected to strongly influence the neuron morphology. To elucidate these effects, we prepared giant vesicles with GM1 predominantly present in one leaflet of the membrane, mimicking the asymmetric GM1 distribution in neuronal membranes. Based on pulling inward and outward tubes, we developed a technique that allowed the direct measurement of the membrane spontaneous curvature. Using vesicle electroporation and fluorescence intensity analysis, we were able to quantify the GM1 asymmetry across the membrane and to subsequently estimate the local curvature generated by the molecule in the bilayer. Molecular-dynamics simulations confirm the experimentally determined dependence of the membrane spontaneous curvature as a function of GM1 asymmetry. GM1 plays a crucial role in connection with receptor proteins. Our results on curvature generation of GM1 point to an additional important role of this ganglioside, namely in shaping neuronal membranes.

Pulling Membrane Nanotubes from Giant Unilamellar Vesicles

The reshaping of the cell membrane is an integral part of many cellular phenomena, such as endocytosis, trafficking, the formation of filopodia, etc. Many different proteins associate with curved membranes because of their ability to sense or induce membrane curvature. Typically, these processes involve a multitude of proteins making them too complex to study quantitatively in the cell. We describe a protocol to reconstitute a curved membrane in vitro, mimicking a curved cellular structure, such as the endocytic neck. A giant unilamellar vesicle (GUV) is used as a model of a cell membrane, whose internal pressure and surface tension are controlled with micropipette aspiration. Applying a point pulling force on the GUV using optical tweezers creates a nanotube of high curvature connected to a flat membrane. This method has traditionally been used to measure the fundamental mechanical properties of lipid membranes, such as bending rigidity. In recent years, it has been expanded to study how proteins interact with membrane curvature and the way they affect the shape and the mechanics of membranes. A system combining micromanipulation, microinjection, optical tweezers, and confocal microscopy allows measurement of membrane curvature, membrane tension, and the surface density of proteins, concurrently. From these measurements, many important mechanical and morphological properties of the protein-membrane system can be inferred. In addition, we lay out a protocol of creating GUVs in the presence of physiological salt concentration, and a method of quantifying the surface density of proteins on the membrane from fluorescence intensities of labeled proteins and lipids.

Tension Independence of Lipid Diffusion and Membrane Viscosity

The diffusion of biomolecules at lipid membranes is governed by the viscosity of the underlying two-dimensionally fluid lipid bilayer. For common three-dimensional fluids, viscosity can be modulated by hydrostatic pressure, and pressure-viscosity data have been measured for decades. Remarkably, the two-dimensional analogue of this relationship, the dependence of molecular mobility on tension, has to the best of our knowledge never been measured for lipid bilayers, limiting our understanding of cellular mechanotransduction as well as the fundamental fluid mechanics of membranes. Here we report both molecular-scale and mesoscopic measures of fluidity in giant lipid vesicles as a function of mechanical tension applied using micropipette aspiration. Both molecular-scale data, from fluorescence recovery after photobleaching, and micron-scale data, from tracking the diffusion of phase-separated domains, show a surprisingly weak dependence of viscosity on tension, in contrast to predictions of recent molecular dynamics simulations, highlighting fundamental gaps in our understanding of membrane fluidity.

Single Lipid Molecule Dynamics on Supported Lipid Bilayers with Membrane Curvature

The plasma membrane is a highly compartmentalized, dynamic material and this organization is essential for a wide variety of cellular processes. Nanoscale domains allow proteins to organize for cell signaling, endo- and exocytosis, and other essential processes. Even in the absence of proteins, lipids have the ability to organize into domains as a result of a variety of chemical and physical interactions. One feature of membranes that affects lipid domain formation is membrane curvature. To directly test the role of curvature in lipid sorting, we measured the accumulation of two similar lipids, 1,2-Dihexadecanoyl-sn-glycero-3-phosphoethanolamine (DHPE) and hexadecanoic acid (HDA), using a supported lipid bilayer that was assembled over a nanopatterned surface to obtain regions of membrane curvature. Both lipids studied contain 16 carbon, saturated tails and a head group tag for fluorescence microscopy measurements. The accumulation of lipids at curvatures ranging from 28 nm to 55 nm radii was measured and fluorescein labeled DHPE accumulated more than fluorescein labeled HDA at regions of membrane curvature. We then tested whether single biotinylated DHPE molecules sense curvature using single particle tracking methods. Similar to groups of fluorescein labeled DHPE accumulating at curvature, the dynamics of single molecules of biotinylated DHPE was also affected by membrane curvature and highly confined motion was observed.

Curvature-Driven Migration of Colloids on Tense Lipid Bilayers

Inspired by proteins that generate membrane curvature, sense the underlying membrane geometry, and migrate driven by curvature gradients, we explore the question: Can colloids, adhered to lipid bilayers, also sense and respond to membrane geometry? We report the migration of Janus microparticles adhered to giant unilamellar vesicles elongated to present spatially varying curvatures. In our experiments, colloids migrate only when the membranes are tense, suggesting that they migrate to minimize membrane area. By determining the energy dissipated along a trajectory, the energy field is inferred to depend on the local deviatoric curvature, like curvature driven capillary migration on interfaces between immiscible fluids. In this latter system, energy gradients are larger, so colloids move deterministically, whereas the paths traced by colloids on vesicles have significant fluctuations. By addressing the role of Brownian motion, we show that the observed migration is analogous to curvature driven capillary migration, with membrane tension playing the role of interfacial tension. Since this motion is mediated by membrane shape, it can be turned on and off by dynamically deforming the vesicle. While particle-particle interactions on lipid membranes have been considered in many contributions, we report here an exciting and previously unexplored modality to actively direct the migration of colloids to desired locations on lipid bilayers.

The N-Terminal Amphipathic Helix of Endophilin Does Not Contribute to Its Molecular Curvature Generation Capacity

N-BAR proteins such as endophilin are thought to bend lipid membranes via scaffolding (the molding of membranes through the crescent protein shape) and membrane insertion (also called wedging) of amphipathic helices. However, the contributions from these distinct mechanisms to membrane curvature generation and sensing have remained controversial. Here we quantitatively demonstrate that the amphipathic N-terminal H0 helix of endophilin is important for recruiting this protein to the membrane, but does not contribute significantly to its intrinsic membrane curvature generation capacity. These observations elevate the importance of the scaffolding mechanism, rather than H0 insertion, for the membrane curvature generation by N-BAR domains. Furthermore, consistent with the thermodynamically required coupling between curvature generation and sensing, we observed that the H0-truncated N-BAR domain is capable of sensing membrane curvature. Overall, our contribution clarifies an important mechanistic controversy in the function of N-BAR domain proteins.

Regulation of membrane-shape transitions induced by I-BAR domains

I-BAR proteins are well-known actin-cytoskeleton adaptors and have been observed to be involved in the formation of plasma membrane protrusions (filopodia). I-BAR proteins contain an all-helical, crescent-shaped IRSp53-MIM domain (IMD) dimer that is believed to be able to couple with a membrane shape. This coupling could involve the sensing and even the generation of negative plasma membrane curvature. Indeed, the in vitro studies have shown that IMDs can induce inward tubulation of liposomes. While N-BAR domains, which generate positive membrane curvature, have received a considerable amount of attention from both theory and experiments, the mechanisms of curvature coupling through IMDs are comparatively less studied and understood. Here we used a membrane-shape stability assay developed recently in our lab to quantitatively characterize IMD-induced membrane-shape transitions. We determined a membrane-shape stability diagram for IMDs that reveals how membrane tension and protein density can comodulate the generation of IMD-induced membrane protrusions. From comparison to analytical theory, we determine three key parameters that characterize the curvature coupling of IMD. We find that the curvature generation capacity of IMDs is significantly stronger compared to that of endophilin, an N-BAR protein known to be involved in plasma membrane shape transitions. Contrary to N-BAR domains, where amphipathic helix insertion is known to promote its membrane curvature generation, for IMDs we find that amphipathic helices inhibit membrane shape transitions, consistent with the inverse curvature that IMDs generate. Importantly, in both of these types of BAR domains, electrostatic interactions affect membrane-binding capacity, but do not appear to affect the curvature generation capacity of the protein. These two types of BAR domain proteins show qualitatively similar membrane shape stability diagrams, suggesting an underlying ubiquitous mechanism by which peripheral proteins regulate membrane curvature.

When Physics Takes Over: BAR Proteins and Membrane Curvature

Cell membranes become highly curved during membrane trafficking, cytokinesis, infection, immune response, or cell motion. Bin/amphiphysin/Rvs (BAR) domain proteins with their intrinsically curved and anisotropic shape are involved in many of these processes, but with a large spectrum of modes of action. In vitro experiments and multiscale computer simulations have contributed in identifying a minimal set of physical parameters, namely protein density on the membrane, membrane tension, and membrane shape, that control how bound BAR domain proteins behave on the membrane. In this review, we summarize the multifaceted coupling of BAR proteins to membrane mechanics and propose a simple phase diagram that recapitulates the effects of these parameters.

Biophysics of α-synuclein induced membrane remodelling

α-Synuclein is an intrinsically disordered protein whose aggregation is a hallmark of Parkinson's disease. In neurons, α-synuclein is thought to play important roles in mediating both endo- and exocytosis of synaptic vesicles through interactions with either the lipid bilayer or other proteins. Upon membrane binding, the N-terminus of α-synuclein forms a helical structure and inserts into the hydrophobic region of the outer membrane leaflet. However, membrane structural changes induced by α-synuclein are still largely unclear. Here we report a substantial membrane area expansion induced by the binding of α-synuclein monomers. This measurement is accomplished by observing the increase of membrane area during the binding of α-synuclein to pipette-aspirated giant vesicles. The extent of membrane area expansion correlates linearly with the density of α-synuclein on the membrane, revealing a constant area increase induced by the binding per α-synuclein molecule. The area expansion per synuclein is found to exhibit a strong dependence on lipid composition, but is independent of membrane tension and vesicle size. Fragmentation or tubulation of the membrane follows the membrane expansion process. However, contrary to BAR domain proteins, no distinct tubulation-transition density can apparently be identified for α-synuclein, suggesting a more complex membrane curvature generation mechanism. Consideration of α-synuclein's membrane binding free energy and biophysical properties of the lipid bilayer leads us to conclude that membrane expansion by α-synuclein results in thinning of the bilayer. These membrane thinning and tubulation effects may underlie α-synuclein's role in mediating cell trafficking processes such as endo- and exocytosis.

Membrane tension controls the assembly of curvature-generating proteins

Proteins containing a Bin/Amphiphysin/Rvs (BAR) domain regulate membrane curvature in the cell. Recent simulations have revealed that BAR proteins assemble into linear aggregates, strongly affecting membrane curvature and its in-plane stress profile. Here, we explore the opposite question: do mechanical properties of the membrane impact protein association? By using coarse-grained molecular dynamics simulations, we show that increased surface tension significantly impacts the dynamics of protein assembly. While tensionless membranes promote a rapid formation of long-living linear aggregates of N-BAR proteins, increase in tension alters the geometry of protein association. At high tension, protein interactions are strongly inhibited. Increasing surface density of proteins leads to a wider range of protein association geometries, promoting the formation of meshes, which can be broken apart with membrane tension. Our work indicates that surface tension may play a key role in recruiting proteins to membrane-remodelling sites in the cell.

BAR domain proteins are known to reshape cell membranes. Using coarse-grained molecular dynamics simulations, Simunovic and Voth demonstrate that membrane tension strongly affects the association of BAR proteins, in turn controlling their recruitment to membrane-remodelling sites.

Spatial Control of Epsin-induced Clathrin Assembly by Membrane Curvature

Epsins belong to the family of highly conserved clathrin-associated sorting proteins that are indispensable for clathrin-mediated endocytosis, but their precise functions remain unclear. We have developed an assay system of budded supported membrane tubes displaying planar and highly curved membrane surfaces to analyze intrinsic membrane curvature preference shown by clathrin-associated sorting proteins. Using real-time fluorescence microscopy, we find that epsin preferentially partitions to and assembles clathrin on highly curved membrane surfaces. Sorting of epsin to regions of high curvature strictly depends on binding to phosphatidylinositol 4,5-bisphosphate. Fluorescently labeled clathrins rapidly assemble as foci, which in turn cluster epsin, while maintaining tube integrity. Clathrin foci grow in intensity with a typical time constant of ∼75 s, similar to the time scales for coated pit formation seen in cells. Epsin therefore effectively senses membrane curvature to spatially control clathrin assembly. Our results highlight the potential role of membrane curvature in orchestrating the myriad molecular interactions necessary for the success of clathrin-mediated membrane budding.

Membrane morphology is actively transformed by covalent binding of the protein Atg8 to PE-lipids

Autophagy is a cellular degradation pathway involving the shape transformation of lipid bilayers. During the onset of autophagy, the water-soluble protein Atg8 binds covalently to phosphatdylethanolamines (PEs) in the membrane in an ubiquitin-like reaction coupled to ATP hydrolysis. We reconstituted the Atg8 conjugation system in giant and nm-sized vesicles with a minimal set of enzymes and observed that formation of Atg8-PE on giant vesicles can cause substantial tubulation of membranes even in the absence of Atg12-Atg5-Atg16. Our findings show that ubiquitin-like processes can actively change properties of lipid membranes and that membrane crowding by proteins can be dynamically regulated in cells. Furthermore we provide evidence for curvature sorting of Atg8-PE. Curvature generation and sorting are directly linked to organelle shapes and, thus, to biological function. Our results suggest that a positive feedback exists between the ubiquitin-like reaction and the membrane curvature, which is important for dynamic shape changes of cell membranes, such as those involved in the formation of autophagosomes.