P. I. Kuzmin

Variation of lipid membrane composition caused by strong bending

Dynamic coupling between the morphology and molecular composition of cellular membranes is crucial for formation of the intracellular organelles and transport vesicles. Most of the membrane proteins and lipids discriminate membrane curvatures. However, it remains unclear whether the curvature alone is sufficient to support heterogeneous distribution of lipids. Here we demonstrate that the curvature-driven redistribution of phospholipids, such as dioleoylphosphatidylethanolamine (DOPE), requires strong membrane bending. We used cylindrical lipid nanotubes (NTs) pulled from planar lipid membranes with lateral tension of ∼1 dyn/cm. Such high tensions forced extreme curvatures of the NT membrane, with luminal radius approaching the thickness of the lipid bilayer, 5nm. When the NT contained lipid species with high spontaneous curvature (SC), such as DOPE, we observed slow reduction of its radius. This reduction indicated the redistribution of DOPE between the inner and outer monolayers of the NT. Accordingly, the SC of DOPE was recovered from the measured changes in the radii: the SC value, calculated under the assumption that the DOPE content is coupled to the monolayer curvature, was ∼0.4 nm−1, consistent with the published data. Thus, redistribution of lipids should be taken into account in calculations of composition and material properties of strongly deformed membrane structures, such as intermediate structures arising in the processes of membrane fusion and fission.

Stabilization of bilayer structure of raft due to elastic deformations of membrane

Line tension of the boundary of specific domains (rafts) rich in sphingomyelin was calculated. The line tension was calculated based on macroscopic theory of elasticity under assumption that the bilayer in raft is thicker than in the surrounding membrane. The calculations took into account the possibility of lateral shift of the domain boundaries located in different monolayers of the membrane. The line tension was associated with the energy of elastic deformations appearing in the vicinity of the boundary in order to compensate for the difference in the thickness of the monolayers. Spatial distribution of deformations and the line tension was calculated by minimization of elastic free energy of the system. Dependence of the line tension on the distance between the domains boundaries located in different monolayers was obtained. It was shown that the line tension is minimal if the distance is about 4 nm. Thus, membrane deformations stabilize the bilayer structure of rafts observed experimentally. The calculated value of line tension is about 0.6 pN for the difference between the monolayer thickness of raft and surrounding membrane of about 0.5 nm, which is in agreement with the experimental data available.

Model of membrane fusion: Continuous transition to fusion pore with regard of hydrophobic and hydration interactions

We consider the process of fusion of lipid membranes from the stage of stalk with minimal radius to the stage of fusion pore. We assume that stalk directly developed into the fusion pore, omitting the stage of hemifusion diaphragm. Energy of intermediate stages is calculated on the basis of the classical elasticity theory of liquid crystals adapted for lipid membranes. The trajectory of transition from stalk to pore is obtained with regard to hydrophobic and hydration interactions. Continuous change of orientation of lipids in distal monolayers occurs along the trajectory. The orientation changes from the direction along rotational axis of the system specific to stalk to the direction corresponding to the fusion pore. Dependence of energy of intermediate stages on the value of spontaneous curvature of distal monolayers of the fusing membranes is obtained. We demonstrate that the energy barrier of the stalk-to-pore transition decreases when distal monolayers have positive spontaneous curvature, which is in accordance with available experimental data.

Membrane fusion. Two possible mechanisms underlying a decrease in the fusion energy barrier in the presence of fusion proteins

Here we explored a contribution of fusion proteins to stalk formation, the first stage of membrane fusion, and considered two likely mechanisms, by which these proteins could influence the membrane transformation. One mechanism represents the induction of spontaneous membrane curvature, while another is membrane disturbance by a force generated by attached proteins. The energy barrier arising due to the deformation of approaching membranes and hydration repulsion between them was calculated. In addition, a dependence of an energy barrier height on certain protein features, such as spontaneous curvature, was analyzed. It was found that if fusion proteins do not produce a force directly applied to fusing membranes, they negligibly affect the barrier height irrespective of a value of spontaneous protein curvature. Thus, the overall results provide evidence that if fusion proteins are unable to exert force, they cannot provide monolayer fusion of the membranes.

Detection of DNA molecules in a lipid nanotube channel in the low ion strength conditions

Investigation of the transport phenomena in the nanoscopic channels/pores with the diameter smaller than 100 nm is of utmost importance for various biological, medical, and technical applications. Presently, the main line of development of nanofluidics is creation of biosensors capable of detecting single molecules and manipulating them. Detection of molecules is based on the measurement of electric current through a channel of appropriate size: when the molecule enters the channel, which diameter is comparable with the molecule size, the ion current reduces. In order to improve transport properties of such channels, their walls are often coated with a lipid bilayer, which behaves as two-dimensional liquid and thus is capable of supporting transport phenomena. In the present work, we utilized this property of lipid membranes for the development of a method for detecting and controlling transport of single-stranded DNA through channels formed by membrane cylinders with the luminal radii of 5–7 nm. We have demonstrated that in the conditions of small ion strength, the appearance of a DNA molecule inside such channel is accompanied by an increase of its ion conductivity and can be controlled by the polarity of the applied voltage. The amplitude of the ion current increase allows evaluating the amount of DNA molecules inside the channels. It was also demonstrated that upon adsorption of DNA molecules on the lipid bilayer surface, the membrane cylinder behaves as a voltage-sensitive selective ion channel.

Membrane shape changes at initial stage of membrane fusion under the action of proteins inducing spontaneous curvature

This paper studies change of membrane shape at the initial stage of the fusion process due to the fusion proteins inducing spontaneous curvature in the membrane. As protein inclusions are embedded into the membrane, a highly curved surface forms in the center of the membrane; it facilitates the formation of short-lived hydrophobic defects and leads to the merger of the contact monolayers of the membranes. Membrane is considered as continuous liquid-crystal medium subject to elastic deformations. One deformational mode of splay is taken into account; energy is calculated in the quadratic approximation on this deformation. In case of positive spontaneous curvature induced by the protein there is no bulge on the top of the membrane despite high deviation of membrane shape from the equilibrium state. In case of negative spontaneous curvature a bulge is formed and its height and curvature increase with the increase of the membrane curvature in the initial state.