A monolayer assay tailored to investigate lipid-protein systems

Binding and Characterization of DNA Origami Nanostructures on Lipid Membranes

DNA origami is an extremely versatile nanoengineering tool with widespread applicability in various fields of research, including membrane physiology and biophysics. The possibility to easily modify DNA strands with lipophilic moieties enabled the recent development of a variety of membrane-active DNA origami devices. Biological membranes, as the core barriers of the cells, display vital structural and functional roles. Therefore, lipid bilayers are widely popular targets of DNA origami nanotechnology for synthetic biology and biomedical applications. In this chapter, we summarize the typical experimental methods used to investigate the interaction of DNA origami with synthetic membrane models. Herein, we present detailed protocols for the production of lipid model membranes and characterization of membrane-targeted DNA origami nanostructures using different microscopy approaches.

Measuring protein insertion areas in lipid monolayers by fluorescence correlation spectroscopy

Membrane insertion of protein domains is an important step in many membrane remodeling processes, for example, in vesicular transport. The membrane area taken up by the protein insertion influences the protein binding affinity as well as the mechanical stress induced in the membrane and thereby its curvature. To our knowledge, this is the first optical measurement of this quantity on a system in equilibrium with direct determination of the number of inserted protein and no further assumptions concerning the binding thermodynamics. Whereas macroscopic total area changes in lipid monolayers are typically measured on a Langmuir film balance, finding the number of inserted proteins without perturbing the system and quantitating any small area changes has posed a challenge. Here, we address both issues by performing two-color fluorescence correlation spectroscopy directly on the monolayer. With a fraction of the protein being fluorescently labeled, the number of inserted proteins is determined in situ without resorting to invasive techniques such as collecting the monolayer by aspiration. The second color channel is exploited to monitor a small fraction of labeled lipids to determine the total area increase. Here, we use this method to determine the insertion area per molecule of Sar1, a protein of the COPII complex, which is involved in transport vesicle formation. Sar1 has an N-terminal amphipathic helix, which is responsible for membrane binding and curvature generation. An insertion area of (3.4 ± 0.8) nm² was obtained for Sar1 in monolayers from a lipid mixture typically used in COPII reconstitution experiments, in good agreement with the expected insertion area of the Sar1 amphipathic helix. By using the two-color approach, determining insertion areas relies only on local fluorescence measurements. No macroscopic area measurements are needed, giving the method the potential to also be applied to laterally heterogeneous monolayers and bilayers.

Fluorescence Correlation Spectroscopy to Examine Protein-Lipid Interactions in Membranes

Fluorescence correlation spectroscopy (FCS) is a versatile technique to study membrane dynamics and protein-lipid interactions. It can provide information about diffusion coefficients, concentrations, and molecular interactions of proteins and lipids in the membrane. These parameters allow for the determination of protein partitioning into different lipid environments, the identification of lipid domains, and the detection of lipid-protein complexes on the membrane. During the last decades, FCS studies were successfully performed on model membrane systems as also on living cells, to characterize protein-lipid interactions. Recent developments of the method described here improved quantitative measurements on membranes and decreased the number of potential artifacts. The aim of this chapter is to provide the reader with the necessary information and some practical guidelines to perform FCS studies on artificial and cellular membranes.

FCS Analysis of Protein Mobility on Lipid Monolayers

In vitro membrane model systems are used to dissect complex biological phenomena under controlled unadulterated conditions. In this context, lipid monolayers are a powerful tool to particularly study the influence of lipid packing on the behavior of membrane proteins. Here, monolayers deposited in miniaturized fixed area-chambers, which require only minute amounts of protein, were used and shown to faithfully reproduce the characteristics of Langmuir monolayers. This assay is ideally suited to be combined with single-molecule sensitive fluorescence correlation spectroscopy (FCS) to characterize diffusion dynamics. Our results confirm the influence of lipid packing on lipid mobility and validate the use of FCS as an alternative to conventional surface pressure measurements for characterizing the monolayer. Furthermore, we demonstrate the effect of lipid density on the diffusional behavior of membrane-bound components. We exploit the sensitivity of FCS to characterize protein interactions with the lipid monolayer in a regime in which the monolayer physical properties are not altered. To demonstrate the potential of our approach, we analyzed the diffusion behavior of objects of different nature, ranging from a small peptide to a large DNA-based nanostructure. Moreover, in this work we quantify the surface viscosity of lipid monolayers. We present a detailed strategy for the conduction of point FCS experiments on lipid monolayers, which is the first step toward extensive studies of protein-monolayer interactions.

Control of lipid domain organization by a biomimetic contractile actomyosin cortex

The cell membrane is a heterogeneously organized composite with lipid-protein micro-domains. The contractile actin cortex may govern the lateral organization of these domains in the cell membrane, yet the underlying mechanisms are not known. We recently reconstituted minimal actin cortices (MACs) (Vogel et al., 2013b) and here advanced our assay to investigate effects of rearranging actin filaments on the lateral membrane organization by introducing various phase-separated lipid mono- and bilayers to the MACs. The addition of actin filaments reorganized membrane domains. We found that the process reached a steady state where line tension and lateral crowding balanced. Moreover, the phase boundary allowed myosin driven actin filament rearrangements to actively move individual lipid domains, often accompanied by their shape change, fusion or splitting. Our findings illustrate how actin cortex remodeling in cells may control dynamic rearrangements of lipids and other molecules inside domains without directly binding to actin filaments.

DOI:http://dx.doi.org/10.7554/eLife.24350.001

Protein Patterns and Oscillations on Lipid Monolayers and in Microdroplets

The Min proteins from E.coli position the bacterial cell-division machinery through pole-to-pole oscillations. In vitro, Min protein self-organization can be reconstituted in the presence of a lipid membrane as a catalytic surface. However, Min dynamics have so far not been reconstituted in fully membrane-enclosed volumes. Microdroplets interfaced by lipid monolayers were employed as a simple 3D mimic of cellular compartments to reconstitute Min protein oscillations. We demonstrate that lipid monolayers are sufficient to fulfil the catalytic role of the membrane and thus represent a facile platform to investigate Min protein regulated dynamics of the cell-division protein FtsZ-mts. In particular, we show that droplet containers reveal distinct Min oscillation modes, and reveal a dependence of FtsZ-mts structures on compartment size. Finally, co-reconstitution of Min proteins and FtsZ-mts in droplets yields antagonistic localization, thus demonstrating that droplets indeed support the analysis of complex bacterial self-organization in confined volumes.

Validity and applicability of membrane model systems for studying interactions of peripheral membrane proteins with lipids

The cell membrane serves, at the same time, both as a barrier that segregates as well as a functional layer that facilitates selective communication. It is characterized as much by the complexity of its components as by the myriad of signaling process that it supports. And, herein lays the problems in its study and understanding of its behavior - it has a complex and dynamic nature that is further entangled by the fact that many events are both temporal and transient in their nature. Model membrane systems that bypass cellular complexity and compositional diversity have tremendously accelerated our understanding of the mechanisms and biological consequences of lipid-lipid and protein-lipid interactions. Concurrently, in some cases, the validity and applicability of model membrane systems are tarnished by inherent methodical limitations as well as undefined quality criteria. In this review we introduce membrane model systems widely used to study protein-lipid interactions in the context of key parameters of the membrane that govern lipid availability for peripheral membrane proteins. This article is part of a Special Issue entitled Tools to study lipid functions.

The design of MACs (minimal actin cortices)

The actin cell cortex in eukaryotic cells is a key player in controlling and maintaining the shape of cells, and in driving major shape changes such as in cytokinesis. It is thereby constantly being remodeled. Cell shape changes require forces acting on membranes that are generated by the interplay of membrane coupled actin filaments and assemblies of myosin motors. Little is known about how their interaction regulates actin cell cortex remodeling and cell shape changes. Because of the vital importance of actin, myosin motors and the cell membrane, selective in vivo experiments and manipulations are often difficult to perform or not feasible. Thus, the intelligent design of minimal in vitro systems for actin-myosin-membrane interactions could pave a way for investigating actin cell cortex mechanics in a detailed and quantitative manner. Here, we present and discuss the design of several bottom-up in vitro systems accomplishing the coupling of actin filaments to artificial membranes, where key parameters such as actin densities and membrane properties can be varied in a controlled manner. Insights gained from these in vitro systems may help to uncover fundamental principles of how exactly actin-myosin-membrane interactions govern actin cortex remodeling and membrane properties for cell shape changes. © 2013 Wiley Periodicals, Inc.