Calcium-dependent lateral organization in phosphatidylinositol 4,5-bisphosphate (PIP2)- and cholesterol-containing monolayers

Germ cell development: Polar granules and PIPs - best buds forever

Germ granules are specialized RNA-protein condensates that drive germ cell development. A new study reveals that germ granules promote Drosophila germ cell formation by altering membrane mechanics through PIP2 and actin.

Effects of Ca2+ on the Structure and Dynamics of PIP3 in Model Membranes Containing PC and PS

Phosphatidylinositol phosphates (PIPs) are a family of seven different eukaryotic membrane lipids that have a large role in cell viability, despite their minor concentration in eukaryotic cellular membranes. PIPs tightly regulate cellular processes, such as cellular growth, metabolism, immunity, and development through direct interactions with partner proteins. Understanding the biophysical properties of PIPs in the complex membrane environment is important to understand how PIPs selectively regulate a partner protein. Here, we investigate the structure and dynamics of PIP3 in lipid bilayers that are simplified models of the natural membrane environment. We probe the effects of the anionic lipid phosphatidylserine (PS) and the divalent cation Ca2+ by using full-length lipids in well-formed bilayers. We used solution and solid-state NMR on naturally abundant 1H, 31P, and 13C atoms combined with molecular dynamics (MD) simulations to characterize the structure and dynamics of PIPs. 1H and 31P 1D spectra show good resolution at temperatures above the phase transition with isolated peaks in the headgroup, interfacial, and bilayer regions. Site-specific assignment of the chemical shifts of these reporters enables the measurement of the effects of Ca2+ and PS at the single atom level. In particular, the resolved 31P signals of the PIP3 headgroup allow for extremely well-localized information about PIP3 phosphate dynamics, which the MD simulations can further explain. A quantitative assessment of cross-polarization kinetics provides additional dynamics measurements for the PIP3 headgroups.

A Two-Step Intermolecular Interaction Of Molecular Macroions With Multivalent Counterions

Macroion-counterion interaction is essential for regulating the solution behaviors of hydrophilic macroions, as simple models for polyelectrolytes. Here, we explore the interaction between uranyl peroxide molecular cluster Li68K12(OH)20[UO2(O2)OH]60 (U60) and multivalent counterions. Different from interaction with monovalent counterions that shows a simple one-step process, isothermal titration calorimetry, combined with light/X-ray scattering measurements and electron microscopy, confirm a two-step process for their interaction with multivalent counterions: an ion-pairing between U60 and the counterion with partial breakage of hydration shells followed by strong U60-U60 attraction, leading to the formation of large nanosheets with severe breakage and reconstruction of hydration shells. The detailed studies on macroion-counterion interaction can be nicely correlated to the microscopic (self-assembly) and macroscopic (gelation or phase separation) phase transitions in the dilute U60 aqueous solutions induced by multivalent counterions.

Thermal stability of bivalent cation/phosphoinositide domains in model membranes

As key mediators in a wide array of signaling events, phosphoinositides (PIPs) orchestrate the recruitment of proteins to specific cellular locations at precise moments. This intricate spatiotemporal regulation of protein activity often necessitates the localized enrichment of the corresponding PIP. We investigate the extent and thermal stabilities of phosphatidylinositol-4-phosphate (PI(4)P), phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2 and phosphatidylinositol-3,4,5-trisphosphate (PI(3,4,5)P3) clusters with calcium and magnesium ions. We observe negligible or minimal clustering of all examined PIPs in the presence of Mg2+ ions. While PI(4)P shows in the presence of Ca2+ no clustering, PI(4,5)P2 forms with Ca2+ strong clusters that exhibit stablity up to at least 80°C. The extent of cluster formation for the interaction of PI(3,4,5)P3 with Ca2+ is less than what was observed for PI(4,5)P2, yet we still observe some clustering up to 80°C. Given that cholesterol has been demonstrated to enhance PIP clustering, we examined whether bivalent cations and cholesterol synergistically promote PIP clustering. We found that the interaction of Mg2+ or Ca2+ with PI(4)P remains extraordinarily weak, even in the presence of cholesterol. In contrast, we observe synergistic interaction of cholesterol and Ca2+ with PI(4,5)P2. Also, in the presence of cholesterol, the interaction of Mg2+ with PI(4,5)P2 remains weak. PI(3,4,5)P3 does not show strong clustering with cholesterol for the experimental conditions of our study and the interaction with Ca2+ and Mg2+ was not influenced by the presence of cholesterol.

The impact of volatile anesthetics and propofol on phosphatidylinositol 4,5-bisphosphate signaling

Phosphatidylinositol 4,5-bisphosphate (PIP2), as well as other anionic phospholipids, play a pivotal role in various cellular processes, including ion channel regulation, receptor trafficking, and intracellular signaling pathways. The binding of volatile anesthetics and propofol to PIP2 leads to alterations in PIP2-mediated signaling causing modulation of ion channels such as ɣ-aminobutyric acid type A (GABAA) receptors, voltage-gated calcium channels, and potassium channels through various mechanisms. Additionally, the interaction between anionic phospholipids and G protein-coupled receptors plays a critical role in various anesthetic pathways, with these anesthetic-induced changes impacting PIP2 levels which cause cascading effects on receptor trafficking, including GABAA receptor internalization. This comprehensive review of various mechanisms of interaction provides insights into the intricate interplay between PIP2 signaling and anesthetic-induced changes, shedding light on the molecular mechanisms underlying anesthesia.

Effects of Ca 2+ on the Structure and Dynamics of PIP3 in Model Membranes Containing PC and PS

Phosphatidylinositol phosphates (PIPs) are a family of seven different eukaryotic membrane lipids that have a large role in cell viability, despite their minor concentration in eukaryotic cellular membranes. PIPs tightly regulate cellular processes such as cellular growth, metabolism, immunity, and development through direct interactions with partner proteins. Understanding the biophysical properties of PIPs in the complex membrane environment is important to understand how PIPs selectively regulate a partner protein. Here we investigate the structure and dynamics of PIP3 in lipid bilayers that are simplified models of the natural membrane environment. We probe the effects of the anionic lipid phosphatidylserine (PS) and the divalent cation Ca 2+ . We use solution and solid-state 1 H, 31 P, and 13 C NMR all at natural abundance combined with MD simulations to characterize the structure and dynamics of PIPs. 1 H and 31 P 1D spectra show good resolution at high temperatures with isolated peaks in the headgroup, interfacial, and bilayer regions. Site specific assignment of these 1D reporters were made and used to measure the effects of Ca 2+ and PS. In particular, the resolved 31 P signals of the PIP3 headgroup allowed for extremely well localized information about PIP3 phosphate dynamics, which the MD simulations were able to help explain. Cross polarization kinetics provided additional site-specific dynamics measurements for the PIP3 headgroups.

Ion-Induced PIP2 Clustering with Martini3: Modification of Phosphate-Ion Interactions and Comparison with CHARMM36

Phosphatidylinositol 4,5-bisphosphate (PIP2) is a critical lipid for cellular signaling. The specific phosphorylation of the inositol ring controls protein binding as well as clustering behavior. Two popular models to describe ion-mediated clustering of PIP2 are Martini3 (M3) and CHARMM36 (C36). Molecular dynamics simulations of PIP2-containing bilayers in solutions of potassium chloride, sodium chloride, and calcium chloride, and at two different resolutions are performed to understand the aggregation and the model parameters that drive it. The average M3 clusters of PIP2 in bilayers of 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine and PIP2 bilayers in the presence of K+, Na+, or Ca2+ contained 2.2, 2.6, and 6.4 times more PIP2 than C36 clusters, respectively. Indeed, the Ca2+-containing systems often formed a single large aggregate. Reparametrization of the M3 ion-phosphate Lennard-Jones interaction energies to reproduce experimental osmotic pressure of sodium dimethyl phosphate (DMP), K[DMP], and Ca[DMP]2 solutions, the same experimental target as C36, yielded comparably sized PIP2 clusters for the two models. Furthermore, C36 and the modified M3 predict similar saturation of the phosphate groups with increasing Ca2+, although the coarse-grained model does not capture the cooperativity between K+ and Ca2+. This characterization of the M3 behavior in the presence of monovalent and divalent ions lays a foundation to study cation/protein/PIP2 clustering.

Calcium transport and sensing in TRPC channels - New insights into a complex feedback regulation

Canonical TRP (TRPC) channels are a still enigmatic family of signaling molecules with multimodal sensing features. These channels enable Ca2+ influx through the plasma membrane to control a diverse range of cellular functions. Based on both regulatory- and recently uncovered structural features, TRPC channels are considered to coordinate Ca2+ and other divalent cations not only within the permeation path but also at additional sensory sites. Analysis of TRPC structures by cryo-EM identified multiple regulatory ion binding pockets. With this review, we aim at an overview and a critical discussion of the current concepts of divalent sensing by TRPC channels.

Phosphatidylinositol 4,5-bisphosphate and calcium at ER-PM junctions - Complex interplay of simple messengers

Endoplasmic reticulum-plasma membrane contact sites (ER-PM MCS) are a specialised domain involved in the control of Ca²⁺ dynamics and various Ca²⁺-dependent cellular processes. Intracellular Ca²⁺ signals are broadly supported by Ca²⁺ release from intracellular Ca²⁺ channels such as inositol 1,4,5-trisphosphate receptors (IP₃Rs) and subsequent store-operated Ca²⁺ entry (SOCE) across the PM to replenish store content. IP₃Rs sit in close proximity to the PM where they can easily access newly synthesised IP₃, interact with binding partners such as actin, and localise adjacent to ER-PM MCS populated by the SOCE machinery, STIM1-2 and Orai1-3, to possibly form a locally regulated unit of Ca²⁺ influx. PtdIns(4,5)P₂ is a multiplex regulator of Ca²⁺ signalling at the ER-PM MCS interacting with multiple proteins at these junctions such as actin and STIM1, whilst also being consumed as a substrate for phospholipase C to produce IP₃ in response to extracellular stimuli. In this review, we consider the mechanisms regulating the synthesis and turnover of PtdIns(4,5)P₂ via the phosphoinositide cycle and its significance for sustained signalling at the ER-PM MCS. Furthermore, we highlight recent insights into the role of PtdIns(4,5)P₂ in the spatiotemporal organization of signalling at ER-PM junctions and raise outstanding questions on how this multi-faceted regulation occurs.

Cholesterol Regulation of Membrane Proteins Revealed by Two-Color Super-Resolution Imaging

Cholesterol and phosphatidyl inositol 4,5-bisphosphate (PIP₂) are hydrophobic molecules that regulate protein function in the plasma membrane of all cells. In this review, we discuss how changes in cholesterol concentration cause nanoscopic (<200 nm) movements of membrane proteins to regulate their function. Cholesterol is known to cluster many membrane proteins (often palmitoylated proteins) with long-chain saturated lipids. Although PIP₂ is better known for gating ion channels, in this review, we will discuss a second independent function as a regulator of nanoscopic protein movement that opposes cholesterol clustering. The understanding of the movement of proteins between nanoscopic lipid domains emerged largely through the recent advent of super-resolution imaging and the establishment of two-color techniques to label lipids separate from proteins. We discuss the labeling techniques for imaging, their strengths and weakness, and how they are used to reveal novel mechanisms for an ion channel, transporter, and enzyme function. Among the mechanisms, we describe substrate and ligand presentation and their ability to activate enzymes, gate channels, and transporters rapidly and potently. Finally, we define cholesterol-regulated proteins (CRP) and discuss the role of PIP₂ in opposing the regulation of cholesterol, as seen through super-resolution imaging.

Ca2+ and Annexins - Emerging Players for Sensing and Transferring Cholesterol and Phosphoinositides via Membrane Contact Sites

Maintaining lipid composition diversity in membranes from different organelles is critical for numerous cellular processes. However, many lipids are synthesized in the endoplasmic reticulum (ER) and require delivery to other organelles. In this scenario, formation of membrane contact sites (MCS) between neighbouring organelles has emerged as a novel non-vesicular lipid transport mechanism. Dissecting the molecular composition of MCS identified phosphoinositides (PIs), cholesterol, scaffolding/tethering proteins as well as Ca²⁺ and Ca²⁺-binding proteins contributing to MCS functioning. Compelling evidence now exists for the shuttling of PIs and cholesterol across MCS, affecting their concentrations in distinct membrane domains and diverse roles in membrane trafficking. Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) at the plasma membrane (PM) not only controls endo-/exocytic membrane dynamics but is also critical in autophagy. Cholesterol is highly concentrated at the PM and enriched in recycling endosomes and Golgi membranes. MCS-mediated cholesterol transfer is intensely researched, identifying MCS dysfunction or altered MCS partnerships to correlate with de-regulated cellular cholesterol homeostasis and pathologies. Annexins, a conserved family of Ca²⁺-dependent phospholipid binding proteins, contribute to tethering and untethering events at MCS. In this chapter, we will discuss how Ca²⁺ homeostasis and annexins in the endocytic compartment affect the sensing and transfer of cholesterol and PIs across MCS.

PI(4,5)P2 and Cholesterol: Synthesis, Regulation, and Functions

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) is the most abundant membrane phosphoinositide and cholesterol is an essential component of the plasma membrane (PM). Both lipids play key roles in a variety of cellular functions including as signaling molecules and major regulators of protein function. This chapter provides an overview of these two important lipids. Starting from a brief description of their structure, synthesis, and regulation, the chapter continues to describe the primary functions and signaling processes in which PI(4,5)P₂ and cholesterol are involved. While PI(4,5)P₂ and cholesterol can act independently, they often act in concert or affect each other's impact. The chapters in this volume on "Cholesterol and PI(4,5)P₂ in Vital Biological Functions: From Coexistence to Crosstalk" focus on the emerging relationship between cholesterol and PI(4,5)P₂ in a variety of biological systems and processes. In this chapter, the next section provides examples from the ion channel field demonstrating that PI(4,5)P₂ and cholesterol can act via common mechanisms. The chapter ends with a discussion of future directions.

Unravelling the orientation of the inositol-biphosphate ring and its dependence on phosphatidylinositol 4,5-bisphosphate cluster formation in model membranes

HYPOTHESIS

Inositol phospholipids are well known to form clusters in the cytoplasmic leaflet of the plasma membrane that are responsible for the interaction and recruitment of proteins involved in key biological processes like endocytosis, ion channel activation and secondary messenger production. Although their phosphorylated inositol ring headgroup plays an important role in protein binding, its orientation with respect to the plane of the membrane and its lateral packing density has not been previously described experimentally.

EXPERIMENTS

Here, we study phosphatidylinositol 4,5-bisphosphate (PIP2) planar model membranes in the form of Langmuir monolayers by surface pressure-area isotherms, Brewster angle microscopy and neutron reflectometry to elucidate the relation between lateral (in-plane) and perpendicular (out-of-plane) molecular organization of PIP2.

FINDINGS

Different surface areas were explored through monolayer compression, allowing us to correlate the formation of transient PIP2 clusters with the change in orientation of the inositol-biphosphate headgroup, which was experimentally determined by neutron reflectometry.

PI(4,5)P2 Clustering and Its Impact on Biological Functions

Located at the inner leaflet of the plasma membrane (PM), phosphatidyl-inositol 4,5-bisphosphate [PI(4,5)P₂] composes only 1-2 mol% of total PM lipids. With its synthesis and turnover both spatially and temporally regulated, PI(4,5)P₂ recruits and interacts with hundreds of cellular proteins to support a broad spectrum of cellular functions. Several factors contribute to the versatile and dynamic distribution of PI(4,5)P₂ in membranes. Physiological multivalent cations such as Ca²⁺ and Mg²⁺ can bridge between PI(4,5)P₂ headgroups, forming nanoscopic PI(4,5)P₂-cation clusters. The distinct lipid environment surrounding PI(4,5)P₂ affects the degree of PI(4,5)P₂ clustering. In addition, diverse cellular proteins interacting with PI(4,5)P₂ can further regulate PI(4,5)P₂ lateral distribution and accessibility. This review summarizes the current understanding of PI(4,5)P₂ behavior in both cells and model membranes, with emphasis on both multivalent cation- and protein-induced PI(4,5)P₂ clustering. Understanding the nature of spatially separated pools of PI(4,5)P₂ is fundamental to cell biology.

A positive feedback loop between Flower and PI(4,5)P2 at periactive zones controls bulk endocytosis in Drosophila

Synaptic vesicle (SV) endocytosis is coupled to exocytosis to maintain SV pool size and thus neurotransmitter release. Intense stimulation induces activity-dependent bulk endocytosis (ADBE) to recapture large quantities of SV constituents in large endosomes from which SVs reform. How these consecutive processes are spatiotemporally coordinated remains unknown. Here, we show that Flower Ca²⁺ channel-dependent phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) compartmentalization governs control of these processes in Drosophila. Strong stimuli trigger PI(4,5)P₂ microdomain formation at periactive zones. Upon exocytosis, Flower translocates from SVs to periactive zones, where it increases PI(4,5)P₂ levels via Ca²⁺ influxes. Remarkably, PI(4,5)P₂ directly enhances Flower channel activity, thereby establishing a positive feedback loop for PI(4,5)P₂ microdomain compartmentalization. PI(4,5)P₂ microdomains drive ADBE and SV reformation from bulk endosomes. PI(4,5)P₂ further retrieves Flower to bulk endosomes, terminating endocytosis. We propose that the interplay between Flower and PI(4,5)P₂ is the crucial spatiotemporal cue that couples exocytosis to ADBE and subsequent SV reformation.

Membrane Heterogeneity Beyond the Plasma Membrane

The structure and organization of cellular membranes have received intense interest, particularly in investigations of the raft hypothesis. The vast majority of these investigations have focused on the plasma membrane of mammalian cells, yielding significant progress in understanding membrane heterogeneity in terms of lipid composition, molecular structure, dynamic regulation, and functional relevance. In contrast, investigations on lipid organization in other membrane systems have been comparatively scarce, despite the likely relevance of membrane domains in these contexts. In this review, we summarize recent observations on lipid organization in organellar membranes, including endoplasmic reticulum, Golgi, endo-lysosomes, lipid droplets, and secreted membranes like lung surfactant, milk fat globule membranes, and viral membranes. Across these non-plasma membrane systems, it seems that the biophysical principles underlying lipid self-organization contribute to lateral domains.

Review of PIP2 in Cellular Signaling, Functions and Diseases

Phosphoinositides play a crucial role in regulating many cellular functions, such as actin dynamics, signaling, intracellular trafficking, membrane dynamics, and cell-matrix adhesion. Central to this process is phosphatidylinositol bisphosphate (PIP2). The levels of PIP2 in the membrane are rapidly altered by the activity of phosphoinositide-directed kinases and phosphatases, and it binds to dozens of different intracellular proteins. Despite the vast literature dedicated to understanding the regulation of PIP2 in cells over past 30 years, much remains to be learned about its cellular functions. In this review, we focus on past and recent exciting results on different molecular mechanisms that regulate cellular functions by binding of specific proteins to PIP2 or by stabilizing phosphoinositide pools in different cellular compartments. Moreover, this review summarizes recent findings that implicate dysregulation of PIP2 in many diseases.

Microscopic Picture of Calcium-Assisted Lipid Demixing and Membrane Remodeling Using Multiscale Simulations

The specificity of anionic phospholipids-calcium ion interaction and lipid demixing has been established as a key regulatory mechanism in several cellular signaling processes. The mechanism and implications of this calcium-assisted demixing have not been elucidated from a microscopic point of view. Here, we present an overview of atomic interactions between calcium and phospholipids that can drive nonideal mixing of lipid molecules in a model lipid bilayer composed of zwitterionic (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)) and anionic (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (POPS)) lipids with computer simulations at multiple resolutions. Lipid nanodomain formation and growth were driven by calcium-enabled lipid bridging of the charged phosphatidylserine (PS) headgroups, which were favored against inter-POPS dipole interactions. Consistent with several experimental studies of calcium-associated membrane sculpting, our analyses also suggest modifications in local membrane curvature and cross-leaflet couplings as a response to such induced lateral heterogeneity. In addition, reverse mapping to a complementary atomistic description revealed structural insights in the presence of anionic nanodomains, at timescales not accessed by previous computational studies. This work bridges information across multiple scales to reveal a mechanistic picture of calcium ion's impact on membrane biophysics.

Structure and Lateral Organization of Phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) is a minor but ubiquitous component of the inner leaflet of the plasma membrane of eukaryotic cells. However, due to its particular complex biophysical properties, it stands out from its neighboring lipids as one of the most important regulators of membrane-associated signaling events. Despite its very low steady-state concentration, PI(4,5)P₂ is able to engage in a multitude of simultaneous cellular functions that are temporally and spatially regulated through the presence of localized transient pools of PI(4,5)P₂ in the membrane. These pools are crucial for the recruitment, activation, and organization of signaling proteins and consequent regulation of downstream signaling. The present review showcases some of the most important PI(4,5)P2 molecular and biophysical properties as well as their impact on its membrane dynamics, lateral organization, and interactions with other biochemical partners.

Roles of Membrane Domains in Integrin-Mediated Cell Adhesion

The composition and organization of the plasma membrane play important functional and regulatory roles in integrin signaling, which direct many physiological and pathological processes, such as development, wound healing, immunity, thrombosis, and cancer metastasis. Membranes are comprised of regions that are thick or thin owing to spontaneous partitioning of long-chain saturated lipids from short-chain polyunsaturated lipids into domains defined as ordered and liquid-disorder domains, respectively. Liquid-ordered domains are typically 100 nm in diameter and sometimes referred to as lipid rafts. We posit that integrin β senses membrane thickness and that mechanical force on the membrane regulates integrin activation through membrane thinning. This review examines what we know about the nature and mechanism of the interaction of integrins with the plasma membrane and its effects on regulating integrins and its binding partners.

Rendezvous at Plasma Membrane: Cellular Lipids and tRNA Set up Sites of HIV-1 Particle Assembly and Incorporation of Host Transmembrane Proteins

The HIV-1 structural polyprotein Gag drives the virus particle assembly specifically at the plasma membrane (PM). During this process, the nascent virion incorporates specific subsets of cellular lipids and host membrane proteins, in addition to viral glycoproteins and viral genomic RNA. Gag binding to the PM is regulated by cellular factors, including PM-specific phospholipid PI(4,5)P2 and tRNAs, both of which bind the highly basic region in the matrix domain of Gag. In this article, we review our current understanding of the roles played by cellular lipids and tRNAs in specific localization of HIV-1 Gag to the PM. Furthermore, we examine the effects of PM-bound Gag on the organization of the PM bilayer and discuss how the reorganization of the PM at the virus assembly site potentially contributes to the enrichment of host transmembrane proteins in the HIV-1 particle. Since some of these host transmembrane proteins alter release, attachment, or infectivity of the nascent virions, the mechanism of Gag targeting to the PM and the nature of virus assembly sites have major implications in virus spread.

Profilin: many facets of a small protein

Profilin is a ubiquitously expressed protein well known as a key regulator of actin polymerisation. The actin cytoskeleton is involved in almost all cellular processes including motility, endocytosis, metabolism, signal transduction and gene transcription. Hence, profilin's role in the cell goes beyond its direct and essential function in regulating actin dynamics. This review will focus on the interactions of Profilin 1 and its ligands at the plasma membrane, in the cytoplasm and the nucleus of the cells and the regulation of profilin activity within those cell compartments. We will discuss the interactions of profilin in cell signalling pathways and highlight the importance of the cell context in the multiple functions that this small essential protein has in conjunction with its role in cytoskeletal organisation and dynamics. We will review some of the mechanisms that control profilin expression and the implications of changed expression of profilin in the light of cancer biology and other pathologies.

A multiscale biophysical model for the recruitment of actin nucleating proteins at the membrane interface

The dynamics and organization of the actin cytoskeleton are crucial to many cellular events such as motility, polarization, cell shaping, and cell division. The intracellular and extracellular signaling associated with this cytoskeletal network is communicated through cell membranes. Hence the organization of membrane macromolecules and actin filament assembly are highly interdependent. Although the actin-membrane linkage is known to happen through many routes, the major class of interactions is through the direct interaction of actin-binding proteins with the lipid class containing poly-phosphatidylinositols (PPIs). Among the PPIs, phosphatidylinositol bisphosphate (PI(4,5)P₂) acts as a significant factor controlling actin polymerization in the proximity of the membrane by binding to actin-associated proteins. The molecular interactions between these actin-binding proteins and the membrane lipids remain elusive. Here, using molecular modeling, analytical theory, and experimental methods, we investigate the binding of three different actin-binding proteins, mDia2, NWASP, and gelsolin, to membranes containing PI(4,5)P₂ lipids. We perform molecular dynamics simulations on the protein-bilayer system and analyze the membrane binding in the form of hydrogen bonds and salt bridges at various PI(4,5)P₂ and cholesterol concentrations. Our experimental study with PI(4,5)P₂-containing large unilamellar vesicles mimics the computational experiments. Using the multivalencies of the proteins obtained in molecular simulations and the cooperative binding mechanisms of the proteins, we also propose a multivalent binding model that predicts the actin filament distributions at various PI(4,5)P₂ and protein concentrations.

Divalent cations bind to phosphoinositides to induce ion and isomer specific propensities for nano-cluster initiation in bilayer membranes

We report all-atom molecular dynamics simulations of asymmetric bilayers containing phosphoinositides in the presence of monovalent and divalent cations. We have characterized the molecular mechanism by which these divalent cations interact with phosphoinositides. Ca²⁺ desolvates more readily, consistent with single-molecule calculations, and forms a network of ionic-like bonds that serve as a 'molecular glue' that allows a single ion to coordinate with up to three phosphatidylinositol-(4,5)-bisphosphate (PI(4, 5)P₂) lipids. The phosphatidylinositol-(3,5)-bisphosphate isomer shows no such effect and neither does PI(4, 5)P₂ in the presence of Mg²⁺. The resulting network of Ca²⁺-mediated lipid-lipid bonds grows to span the entire simulation space and therefore has implications for the lateral distribution of phosophoinositides in the bilayer. We observe context-specific differences in lipid diffusion rates, lipid surface densities and bilayer structure. The molecular-scale delineation of ion-lipid arrangements reported here provides insight into similar nanocluster formation induced by peripheral proteins to regulate the formation of functional signalling complexes on the membrane.

Leaflet-Dependent Distribution of PtdIns[4,5]P2 in Supported Model Membranes

Supported planar lipid bilayers (SLBs) prepared by spreading of unilamellar vesicles on hydrophilic substrates such as silicon dioxide are frequently used to investigate lipid-protein interactions by means of surface-sensitive methods. In recent years, the receptor lipid phosphatidylinositol-4,5-bisphosphate (PtdIns[4,5]P₂) became particularly important as a significant number of proteins bind to this lipid at the inner leaflet of the plasma membrane. Here, we investigated how the lipid PtdIns[4,5]P₂ distributes between the two leaflets of an SLB on SiO₂ surfaces. We prepared SLBs on SiO₂ by spreading small unilamellar vesicles and quantified the adsorption of PtdIns[4,5]P₂ binding proteins providing information about the accessibility of PtdIns[4,5]P₂. We compared protein binding to PtdIns[4,5]P₂ in SLBs with that in lipid monolayers on a 1,1,1-trimethyl-N-(trimethylsilyl)silanamine-functionalized SiO₂ surface using reflectometric interference spectroscopy and atomic force microscopy. Our results clearly demonstrate that the accessibility of PtdIns[4,5]P₂ for protein binding is reduced in SLBs compared to that in supported hybrid membranes, which is discussed in terms of PtdIns[4,5]P₂ distribution in the two leaflets of SLBs.

Effect of H-Bond Donor Lipids on Phosphatidylinositol-3,4,5-Trisphosphate Ionization and Clustering

The phosphoinositide, phosphatidylinositol-3,4,5-trisphosphate (PI(3,4,5)P₃), is a key signaling lipid in the inner leaflet of the cell plasma membrane, regulating diverse signaling pathways including cell growth and migration. In this study we investigate the impact of the hydrogen-bond donor lipids phosphatidylethanolamine (PE) and phosphatidylinositol (PI) on the charge and phase behavior of PI(3,4,5)P₃. PE and PI can interact with PI(3,4,5)P₃ through hydrogen-bond formation, leading to altered ionization behavior and charge distribution within the PI(3,4,5)P₃ headgroup. We quantify the altered PI(3,4,5)P₃ ionization behavior using a multistate ionization model to obtain micro-pK_a values for the ionization of each phosphate group. The presence of PE leads to a decrease in the pK_a values for the initial deprotonation of PI(3,4,5)P₃, which describes the removal of the first proton of the three protons remaining at the phosphomonoester groups at pH 4.0. The decrease in these micro-pK_a values thus leads to a higher charge at low pH. Additionally, the charge distribution changes lead to increased charge on the 3- and 5-phosphates. In the presence of PI, the final deprotonation of PI(3,4,5)P₃ is delayed, leading to a lower charge at high pH. This is due to a combination of hydrogen-bond formation between PI and PI(3,4,5)P₃, and increased surface charge due to the addition of the negatively charged PI. The interaction between PI and PI(3,4,5)P₃ leads to the formation of PI and PI(3,4,5)P₃-enriched domains within the membrane. These domains may have a critical impact on PI(3,4,5)P₃-signaling. We also reevaluate results for all phosphatidylinositol bisphosphates as well as for PI(4,5)P₂ in complex lipid mixtures with the multistate ionization model.

Multivalent Cation-Bridged PI(4,5)P2 Clusters Form at Very Low Concentrations

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂ or PIP2), is a key component of the inner leaflet of the plasma membrane in eukaryotic cells. In model membranes, PIP2 has been reported to form clusters, but whether these locally different conditions could give rise to distinct pools of unclustered and clustered PIP2 is unclear. By use of both fluorescence self-quenching and Förster resonance energy transfer assays, we have discovered that PIP2 self-associates at remarkably low concentrations starting below 0.05 mol% of total lipids. Formation of these clusters was dependent on physiological divalent metal ions, such as Ca²⁺, Mg²⁺, Zn²⁺, or trivalent ions Fe³⁺ and Al³⁺. Formation of PIP2 clusters was also headgroup-specific, being largely independent of the type of acyl chain. The similarly labeled phospholipids phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol exhibited no such clustering. However, six phosphoinositide species coclustered with PIP2. The degree of PIP2 cation clustering was significantly influenced by the composition of the surrounding lipids, with cholesterol and phosphatidylinositol enhancing this behavior. We propose that PIP2 cation-bridged cluster formation, which might be similar to micelle formation, can be used as a physical model for what could be distinct pools of PIP2 in biological membranes. To our knowledge, this study provides the first evidence of PIP2 forming clusters at such low concentrations. The property of PIP2 to form such clusters at such extremely low concentrations in model membranes reveals, to our knowledge, a new behavior of PIP2 proposed to occur in cells, in which local multivalent metal ions, lipid compositions, and various binding proteins could greatly influence PIP2 properties. In turn, these different pools of PIP2 could further regulate cellular events.

Membrane Lipid Nanodomains

Lipid membranes can spontaneously organize their components into domains of different sizes and properties. The organization of membrane lipids into nanodomains might potentially play a role in vital functions of cells and organisms. Model membranes represent attractive systems to study lipid nanodomains, which cannot be directly addressed in living cells with the currently available methods. This review summarizes the knowledge on lipid nanodomains in model membranes and exposes how their specific character contrasts with large-scale phase separation. The overview on lipid nanodomains in membranes composed of diverse lipids (e.g., zwitterionic and anionic glycerophospholipids, ceramides, glycosphingolipids) and cholesterol aims to evidence the impact of chemical, electrostatic, and geometric properties of lipids on nanodomain formation. Furthermore, the effects of curvature, asymmetry, and ions on membrane nanodomains are shown to be highly relevant aspects that may also modulate lipid nanodomains in cellular membranes. Potential mechanisms responsible for the formation and dynamics of nanodomains are discussed with support from available theories and computational studies. A brief description of current fluorescence techniques and analytical tools that enabled progress in lipid nanodomain studies is also included. Further directions are proposed to successfully extend this research to cells.

ATP competes with PIP2 for binding to gelsolin

Gelsolin is a severing and capping protein that targets filamentous actin and regulates filament lengths near plasma membranes, contributing to cell movement and plasma membrane morphology. Gelsolin binds to the plasma membrane via phosphatidylinositol 4,5-bisphosphate (PIP2) in a state that cannot cap F-actin, and gelsolin-capped actin filaments are uncapped by PIP2 leading to filament elongation. The process by which gelsolin is removed from PIP2 at the plasma membrane is currently unknown. Gelsolin also binds ATP with unknown function. Here we characterize the role of ATP on PIP2-gelsolin complex dynamics. Fluorophore-labeled PIP2 and ATP were used to study their interactions with gelsolin using steady-state fluorescence anisotropy, and Alexa488-labeled gelsolin was utilized to reconstitute the regulation of gelsolin binding to PIP2-containing phospholipid vesicles by ATP. Under physiological salt conditions ATP competes with PIP2 for binding to gelsolin, while calcium causes the release of ATP from gelsolin. These data suggest a cycle for gelsolin activity. Firstly, calcium activates ATP-bound gelsolin allowing it to sever and cap F-actin. Secondly, PIP2-binding removes the gelsolin cap from F-actin at low calcium levels, leading to filament elongation. Finally, ATP competes with PIP2 to release the calcium-free ATP-bound gelsolin, allowing it to undergo a further round of severing.

Cofilin and profilin: partners in cancer aggressiveness

This review covers aspects of cofilin and profilin regulations and their influence on actin polymerisation responsible for cell motility and metastasis. The regulation of their activity by phosphorylation and nitration, miRs, PI(4,5)P₂ binding, pH, oxidative stress and post-translational modification is described. In this review, we have highlighted selected similarities, complementarities and differences between the two proteins and how their interplay affects actin filament dynamics.

Quantifying spatial and temporal variations of the cell membrane ultra-structure by bimFCS

It has been long recognized that the cell membrane is heterogeneous on scales ranging from a couple of molecules to micrometers in size and hence diffusion of receptors is length scale dependent. This heterogeneity modulates many cell-membrane-associated processes requiring transient spatiotemporal separation of components. The transient increase in local concentration of interacting signal components enables robust signaling in an otherwise thermally noisy system. Understanding how lipids and proteins self-organize and interact with the cell cortex requires quantifying the motion of the components. Multi-length scale diffusion measurements by single particle tracking, fluorescence correlation spectroscopy (FCS) or related techniques are able to identify components being transiently trapped in nanodomains, from freely moving one and from ones with reduced long-scale diffusion due to interaction with the cell cortex. One particular implementation of multi-length scale diffusion measurements is the combination of FCS with a spatially resolved detector, such as a camera and two-dimensional extended excitation profile. The main advantages of this approach are that all length scales are interrogated simultaneously, uniquely permits quantifying changes to the membrane structure caused by extrenal or internal perturbations. Here, we review how combining total internal reflection microscopy (TIRF) with FC resolves the membrane organization in living cells. We show how to implement the method, which requires only a few seconds of data acquisition to quantify membrane nanodomains, or the spacing of membrane fences caused by the actin cortex. The choice of diffusing fluorescent probe determines which membrane heterogeneity is detected. We review the instrument, sample preparation, experimental and computational requirements to perform such measurements, and discuss the potential and limitations. The discussion includes examples of spatial and temporal comparisons of the membrane structure in response to perturbations demonstrating the complex cell physiology.

K-Ras4B Remains Monomeric on Membranes over a Wide Range of Surface Densities and Lipid Compositions

Ras is a membrane-anchored signaling protein that serves as a hub for many signaling pathways and also plays a prominent role in cancer. The intrinsic behavior of Ras on the membrane has captivated the biophysics community in recent years, especially the possibility that it may form dimers. In this article, we describe results from a comprehensive series of experiments using fluorescence correlation spectroscopy and single-molecule tracking to probe the possible dimerization of natively expressed and fully processed K-Ras4B in supported lipid bilayer membranes. Key to these studies is the fact that K-Ras4B has its native membrane anchor, including both the farnesylation and methylation of the terminal cysteine, enabling detailed exploration of possible effects of cholesterol and lipid composition on K-Ras4B membrane organization. The results from all conditions studied indicate that full-length K-Ras4B lacks intrinsic dimerization capability. This suggests that any lateral organization of Ras in living cell membranes likely stems from interactions with other factors.

Microfluidic device as a facile in vitro tool to generate and investigate lipid gradients

This work describes a method that utilizes a microfluidic gradient generator to develop lateral lipid gradients in supported lipid bilayers (SLB). The new methodology provides freedom of choice with respect to the lipid composition of the SLB. In addition, the device has the ability to create a protein or bivalent cation gradient in the aqueous phase above the lipid bilayer which can elicit a gradient specific response in the SLB. To highlight these features we demonstrate that we can create a phosphoinositide gradient on various length scales, ranging from 2mm to 50μm. We further show that a Ca²⁺ gradient in the aqueous phase above the SLB causes anionic lipid clustering mirroring the cation gradient. We demonstrate this effect for mixed phosphatidylcholine/phosphatidylinositol-4,5-bisphosphate bilayers and fora mixed phosphatidylcholine/phosphatidylserine bilayers. The biomimetic platform can be combined with a Total Internal Reflection Fluorescence (TIRF) microscopy setup, which allows for the convenient observation of the time evolution of the gradient and the interaction of ligands with the lipid bilayer. The method provides unprecedented access to study the dynamics and mechanics of protein-lipid interactions on membranes with micron level gradients, mimicking plasma membrane gradients observed in organisms such as Dictyostelium discodeum and neutrophils.

Membrane Order Is a Key Regulator of Divalent Cation-Induced Clustering of PI(3,5)P2 and PI(4,5)P2

Although the evidence for the presence of functionally important nanosized phosphorylated phosphoinositide (PIP)-rich domains within cellular membranes has accumulated, very limited information is available regarding the structural determinants for compartmentalization of these phospholipids. Here, we used a combination of fluorescence spectroscopy and microscopy techniques to characterize differences in divalent cation-induced clustering of PI(4,5)P₂ and PI(3,5)P₂. Through these methodologies we were able to detect differences in divalent cation-induced clustering efficiency and cluster size. Ca²⁺-induced PI(4,5)P₂ clusters are shown to be significantly larger than the ones observed for PI(3,5)P₂. Clustering of PI(4,5)P₂ is also detected at physiological concentrations of Mg²⁺, suggesting that in cellular membranes, these molecules are constitutively driven to clustering by the high intracellular concentration of divalent cations. Importantly, it is shown that lipid membrane order is a key factor in the regulation of clustering for both PIP isoforms, with a major impact on cluster sizes. Clustered PI(4,5)P₂ and PI(3,5)P₂ are observed to present considerably higher affinity for more ordered lipid phases than the monomeric species or than PI(4)P, possibly reflecting a more general tendency of clustered lipids for insertion into ordered domains. These results support a model for the description of the lateral organization of PIPs in cellular membranes, where both divalent cation interaction and membrane order are key modulators defining the lateral organization of these lipids.

Calcium Directly Regulates Phosphatidylinositol 4,5-Bisphosphate Headgroup Conformation and Recognition

The orchestrated recognition of phosphoinositides and concomitant intracellular release of Ca²⁺ is pivotal to almost every aspect of cellular processes, including membrane homeostasis, cell division and growth, vesicle trafficking, as well as secretion. Although Ca²⁺ is known to directly impact phosphoinositide clustering, little is known about the molecular basis for this or its significance in cellular signaling. Here, we study the direct interaction of Ca²⁺ with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂), the main lipid marker of the plasma membrane. Electrokinetic potential measurements of PI(4,5)P₂ containing liposomes reveal that Ca²⁺ as well as Mg²⁺ reduce the zeta potential of liposomes to nearly background levels of pure phosphatidylcholine membranes. Strikingly, lipid recognition by the default PI(4,5)P₂ lipid sensor, phospholipase C delta 1 pleckstrin homology domain (PLC δ1-PH), is completely inhibited in the presence of Ca²⁺, while Mg²⁺ has no effect with 100 nm liposomes and modest effect with giant unilamellar vesicles. Consistent with biochemical data, vibrational sum frequency spectroscopy and atomistic molecular dynamics simulations reveal how Ca²⁺ binding to the PI(4,5)P₂ headgroup and carbonyl regions leads to confined lipid headgroup tilting and conformational rearrangements. We rationalize these findings by the ability of calcium to block a highly specific interaction between PLC δ1-PH and PI(4,5)P₂, encoded within the conformational properties of the lipid itself. Our studies demonstrate the possibility that switchable phosphoinositide conformational states can serve as lipid recognition and controlled cell signaling mechanisms.

Engineering Lipid Structure for Recognition of the Liquid Ordered Membrane Phase

The selective partitioning of lipid components in phase-separated membranes is essential for domain formation involved in cellular processes. Identifying and tracking the movement of lipids in cellular systems would be improved if we understood how to achieve selective affinity between fluorophore-labeled lipids and membrane assemblies. Here, we investigated the structure and chemistry of membrane lipids to evaluate lipid designs that partition to the liquid ordered (L_o) phase. A range of fluorophores at the headgroup position and lengths of PEG spacer between the lipid backbone and fluorophore were examined. On a lipid body with saturated palmityl or palmitoyl tails, we found that although the lipid tails can direct selective partitioning to the L_o phase through favorable packing interactions, headgroup hydrophobicity can override the partitioning behavior and direct the lipid to the disordered membrane phase (L_d). The PEG spacer can serve as a buffer to mute headgroup-membrane interactions and thus improve L_o phase partitioning, but its effect is limited with strongly hydrophobic fluorophore headgroups. We present a series of lipid designs leading to the development of novel fluorescently labeled lipids with selective affinity for the L_o phase.

Cholesterol-Dependent Phase-Demixing in Lipid Bilayers as a Switch for the Activity of the Phosphoinositide-Binding Cytoskeletal Protein Gelsolin

The lateral distribution of phosphatidylinositol 4,5-bisphosphate (PIP2) in lipid bilayers is affected both by divalent cation-mediated attractions and cholesterol-dependent phase demixing. The effects of lateral redistribution of PIP2 within a membrane on PIP2-protein interactions are explored with an N-terminal fragment of gelsolin (NtGSN) that severs actin in a Ca(2+)-insensitive manner. The extent of NtGSN inhibition by PIP2-containing large unilamellar vesicles (LUVs) depends on the lateral organization of the membrane as quantified by an actin-severing assay. At a fixed PIP2 mole fraction, the inhibition is largely enhanced by the segregation of liquid ordered/liquid disordered (Lo/Ld) phases that is induced by altering either cholesterol content or temperature, whereas the presence of Ca(2+) only slightly improves the inhibition. Inhibition of gelsolin induced by demixed LUVs is more effective with decreasing temperature, coincident with increasing membrane order as determined by Laurdan generalized polarization and is reversible as the temperature increases. This result suggests that PIP2-mediated inhibition of gelsolin function depends not only on changes in global concentration but also on lateral distribution of PIP2. These observations imply that gelsolin, and perhaps other PIP2-regulated proteins, can be activated or inactivated by the formation of nanodomains or clusters without changing PIP2 bulk concentration in the cell membrane.

The effect of multivalent cations and Tau on paclitaxel-stabilized microtubule assembly, disassembly, and structure

In this review we describe recent studies directed at understanding the formation of novel nanoscale assemblies in biological materials systems. In particular, we focus on the effects of multivalent cations, and separately, of microtubule-associated protein (MAP) Tau, on microtubule (MT) ordering (bundling), MT disassembly, and MT structure. Counter-ion directed bundling of paclitaxel-stabilized MTs is a model electrostatic system, which parallels efforts to understand MT bundling by intrinsically disordered proteins (typically biological polyampholytes) expressed in neurons. We describe studies, which reveal an unexpected transition from tightly spaced MT bundles to loose bundles consisting of strings of MTs as the valence of the cationic counter-ion decreases from Z=3 to Z=2. This transition is not predicted by any current theories of polyelectrolytes. Notably, studies of a larger series of divalent counter-ions reveal strong ion specific effects. Divalent counter-ions may either bundle or depolymerize paclitaxel-stabilized MTs. The ion concentration required for depolymerization decreases with increasing atomic number. In a more biologically related system we review synchrotron small angle x-ray scattering (SAXS) studies on the effect of the Tau on the structure of paclitaxel-stabilized MTs. The electrostatic binding of MAP Tau isoforms leads to an increase in the average radius of microtubules with increasing Tau coverage (i.e. a re-distribution of protofilament numbers in MTs). Finally, inspired by MTs as model nanotubes, we briefly describe other more robust lipid-based cylindrical nanostructures, which may have technological applications, for example, in drug encapsulation and delivery.

Calcium Promotes the Formation of Syntaxin 1 Mesoscale Domains through Phosphatidylinositol 4,5-Bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) is a minor component of total plasma membrane lipids, but it has a substantial role in the regulation of many cellular functions, including exo- and endocytosis. Recently, it was shown that PI(4,5)P2and syntaxin 1, a SNARE protein that catalyzes regulated exocytosis, form domains in the plasma membrane that constitute recognition sites for vesicle docking. Also, calcium was shown to promote syntaxin 1 clustering in the plasma membrane, but the molecular mechanism was unknown. Here, using a combination of superresolution stimulated emission depletion microscopy, FRET, and atomic force microscopy, we show that Ca(2+)acts as a charge bridge that specifically and reversibly connects multiple syntaxin 1/PI(4,5)P2complexes into larger mesoscale domains. This transient reorganization of the plasma membrane by physiological Ca(2+)concentrations is likely to be important for Ca(2+)-regulated secretion.

Biophysical methods for the characterization of PTEN/lipid bilayer interactions

PTEN, a tumor suppressor protein that dephosphorylates phosphoinositides at the 3-position of the inositol ring, is a cytosolic protein that needs to associate with the plasma membrane or other subcellular membranes to exert its lipid phosphatase function. Upon membrane association PTEN interacts with at least three different lipid entities: An anionic lipid that is present in sufficiently high concentration to create a negative potential that allows PTEN to interact electrostatically with the membrane, phosphatidylinositol-4,5-bisphosphate, which interacts with PTEN's N-terminal end and the substrate, usually phosphatidylinositol-3,4,5-trisphosphate. Many parameters influence PTEN's interaction with the lipid bilayer, for example, the lateral organization of the lipids or the presence of other chemical species like cholesterol or other lipids. To investigate systematically the different steps of PTEN's complex binding mechanism and to explore its dynamic behavior in the membrane bound state, in vitro methods need to be employed that allow for a systematic variation of the experimental conditions. In this review we survey a variety of methods that can be used to assess PTEN lipid binding affinity, the dynamics of its membrane association as well as its dynamic behavior in the membrane bound state.

Sec14-nodulin proteins and the patterning of phosphoinositide landmarks for developmental control of membrane morphogenesis

A Sec14-nodulin protein model is used to identify the nodulin domain as a novel phosphoinositide effector module with a role in controlling lateral organization of phosphoinositide. The domain organization of Sec14-nodulin proteins suggests a versatile principle for the bit mapping of membrane surfaces into high-definition lipid-signaling screens.

Polarized membrane morphogenesis is a fundamental activity of eukaryotic cells. This process is essential for the biology of cells and tissues, and its execution demands exquisite temporal coordination of functionally diverse membrane signaling reactions with high spatial resolution. Moreover, mechanisms must exist to establish and preserve such organization in the face of randomizing forces that would diffuse it. Here we identify the conserved AtSfh1 Sec14-nodulin protein as a novel effector of phosphoinositide signaling in the extreme polarized membrane growth program exhibited by growing Arabidopsis root hairs. The data are consistent with Sec14-nodulin proteins controlling the lateral organization of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂) landmarks for polarized membrane morphogenesis in plants. This patterning activity requires both the PtdIns(4,5)P₂ binding and homo-oligomerization activities of the AtSfh1 nodulin domain and is an essential aspect of the polarity signaling program in root hairs. Finally, the data suggest a general principle for how the phosphoinositide signaling landscape is physically bit mapped so that eukaryotic cells are able to convert a membrane surface into a high-definition lipid-signaling screen.

Competitive cation binding to phosphatidylinositol-4,5-bisphosphate domains revealed by X-ray fluorescence

Calcium ions bind strongly to PIP₂ at physiological concentrations, leading to condensation and decreased effective charge for PIP₂. Calcium displaces the more numerous magnesium and potassium ions, but some potassium ions remain.

Separation of the Stern and diffuse layer in coarse-grained models: the cases of phosphatidyl serine, phosphatidic acid, and PIP2 monolayers

We present here a method to separate the Stern and diffuse layer in general systems into two regions that can be analyzed separately. The Stern layer can be described in terms of Bjerrum pairing and the diffuse layer in terms of Poisson-Boltzmann theory (monovalent) or strong coupling theory plus a slowly decaying tail (divalent). We consider three anionic phospholipids: phosphatidyl serine, phosphatidic acid, and phosphatidylinositol(4,5)bisphosphate (PIP2), which we describe within a minimal coarse-grained model as a function of ionic concentration. The case of mixed lipid systems is also considered, which shows a high level of binding cooperativity as a function of PIP2 localization. Implications for existing experimental systems of lipid heterogeneities are also discussed.

Formation and characterization of supported lipid bilayers containing phosphatidylinositol-4,5-bisphosphate and cholesterol as functional surfaces

Solid-supported lipid bilayers (SLBs) mimicking a biological membrane are commonly used to investigate lipid-lipid or lipid-protein interactions. Simple binary or ternary lipid systems are well established, whereas more complex model membranes containing biologically important signaling lipids such as phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) and cholesterol have not been extensively described yet. Here we report the generation of such bilayers and their relevant biophysical properties and in particular the accessibility of PI(4,5)P2 for protein binding. Ternary mixtures of POPC with 20% cholesterol and either 3 or 5 mol % dioleoyl-phosphatidylinositol-4,5-bisphosphate were probed by employing the quartz crystal microbalance and atomic force microscopy. We show that these mixtures form homogeneous solid-supported bilayers that exhibit no intrinsic phase separation and are characterized by long-term stability (>8 h). Bilayers were formed in a pH-dependent manner and were characterized by the accessibility of PI(4,5)P2 on the SLB surface as shown by the interaction with the PI(4,5)P2 binding domain of the cortical membrane-cytoskeleton linker protein ezrin. A time-dependent reduction of PI(4,5)P2 levels in the upper leaflet of SLBs was observed, which could be effectively inhibited by the incorporation of a negatively charged lipid such as phosphatidylserine. Furthermore, quartz crystal microbalance measurements revealed that cholesterol affects bilayer adsorption to the solid support.

Role of calcium in membrane interactions by PI(4,5)P₂-binding proteins

Ca²⁺ and phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] are key agents in membrane-associated signalling events. Their temporal and spatial regulation is crucial for activation or recruitment of proteins in the plasma membrane. In fact, the interaction of several signalling proteins with PI(4,5)P₂ has been shown to be tightly regulated and dependent on the presence of Ca²⁺, with co-operative binding in some cases. In these proteins, PI(4,5)P₂ and Ca²⁺ binding typically occurs at different binding sites. In addition, several PI(4,5)P₂-binding proteins are known targets of calmodulin (CaM), which, depending on the presence of calcium, can compete with PI(4,5)P₂ for protein interaction, translating Ca²⁺ transient microdomains into variations of PI(4,5)P₂ lateral organization in time and space. The present review highlights different examples of calcium-dependent PI(4,5)P₂-binding proteins and discusses the possible impact of this dual regulation on fine-tuning of protein activity by triggering target membrane binding in the presence of subtle changes in the levels of calcium or PI(4,5)P₂.

Counterion-mediated pattern formation in membranes containing anionic lipids

Most lipid components of cell membranes are either neutral, like cholesterol, or zwitterionic, like phosphatidylcholine and sphingomyelin. Very few lipids, such as sphingosine, are cationic at physiological pH. These generally interact only transiently with the lipid bilayer, and their synthetic analogs are often designed to destabilize the membrane for drug or DNA delivery. However, anionic lipids are common in both eukaryotic and prokaryotic cell membranes. The net charge per anionic phospholipid ranges from -1 for the most abundant anionic lipids such as phosphatidylserine, to near -7 for phosphatidylinositol 3,4,5 trisphosphate, although the effective charge depends on many environmental factors. Anionic phospholipids and other negatively charged lipids such as lipopolysaccharides are not randomly distributed in the lipid bilayer, but are highly restricted to specific leaflets of the bilayer and to regions near transmembrane proteins or other organized structures within the plane of the membrane. This review highlights some recent evidence that counterions, in the form of monovalent or divalent metal ions, polyamines, or cationic protein domains, have a large influence on the lateral distribution of anionic lipids within the membrane, and that lateral demixing of anionic lipids has effects on membrane curvature and protein function that are important for biological control.

Mutations in BIN1 associated with centronuclear myopathy disrupt membrane remodeling by affecting protein density and oligomerization

The regulation of membrane shapes is central to many cellular phenomena. Bin/Amphiphysin/Rvs (BAR) domain-containing proteins are key players for membrane remodeling during endocytosis, cell migration, and endosomal sorting. BIN1, which contains an N-BAR domain, is assumed to be essential for biogenesis of plasma membrane invaginations (T-tubules) in muscle tissues. Three mutations, K35N, D151N and R154Q, have been discovered so far in the BAR domain of BIN1 in patients with centronuclear myopathy (CNM), where impaired organization of T-tubules has been reported. However, molecular mechanisms behind this malfunction have remained elusive. None of the BIN1 disease mutants displayed a significantly compromised curvature sensing ability. However, two mutants showed impaired membrane tubulation both in vivo and in vitro, and displayed characteristically different behaviors. R154Q generated smaller membrane curvature compared to WT N-BAR. Quantification of protein density on membranes revealed a lower membrane-bound density for R154Q compared to WT and the other mutants, which appeared to be the primary reason for the observation of impaired deformation capacity. The D151N mutant was unable to tubulate liposomes under certain experimental conditions. At medium protein concentrations we found 'budding' structures on liposomes that we hypothesized to be intermediates during the tubulation process except for the D151N mutant. Chemical crosslinking assays suggested that the D151N mutation impaired protein oligomerization upon membrane binding. Although we found an insignificant difference between WT and K35N N-BAR in in vitro assays, depolymerizing actin in live cells allowed tubulation of plasma membranes through the K35N mutant. Our results provide insights into the membrane-involved pathophysiological mechanisms leading to human disease.