Redistribution Of Membrane Components Research Articles

From reversible to irreversible: When the membrane nanotube pearling is coupled with phase separation

Membrane nanotubes, which are ubiquitous in biology and act as channels maintaining transport between different cells and organelles, readily undergo pearling in response to external stimuli. Membrane nanotube pearling involves generation of heterogeneous curvature coupled with redistribution of membrane components that may interfere with the shape recovery of pearled nanotubes. However, the mechanism underlying such delicate process remains unclear and difficult to study at the molecular scale in vivo. By means of molecular dynamics simulation, here we investigate pearling of multi-component membrane nanotubes and reversibility through manipulating system temperature and osmotic pressure. With the equilibrium shape of membrane nanotubes controlled by the osmotic pressure, our results demonstrate that the process of membrane nanotube pearling can be reversible or irreversible, depending on the phase segregation state. For the pearled nanotube releasing high surface energy, different lipid components redistribute along the tube axial direction. Lipids with unsaturated tails prefer gathering at the high-curvature shrinking region, whereas the swelling region is constituted by saturated lipids forming the liquid-ordered phase of a higher bending rigidity. Such curvature sensitive phase segregation minimizes the system free energy by reducing both the membrane bending energy and line tension at the phase boundary. As such, the pearled nanotube fails to recover its shape upon retracting stimuli, suggesting irreversibility of the membrane nanotube pearling coupled with phase separation. Given importance of membrane nanotube pearling in various cellular activities, these results provide a new mechanism of controlling equilibrium shapes of membrane nanotubes in complex cellular environment.

Structure-function relationship of the influenza fusion peptide

Influenza is an enveloped virus which enters the host cell through en-docytosis-mediated mechanism. To enable the release genetic material, a process of fusion between viral and host cell membranes occurs, which is mediated by influenza hemagglutinin (HA) protein. The N-terminal fragment of hemagglutinin HA2 subunit, directly interacting with the membrane, is named a fusion peptide (HAfp), since it is able to promote fusion also as a synthetic fragment. Its C-terminal part contains three residues (W21-Y22-G23), which are highly conserved among various serotypes of Influenza A. It has been shown that the peptide length has an influence on its structure: HAfp1-20 forms a boomerangin contrast to a tight helical hairpin formed by HAfp1-23. To gain more insight into the role of the conserved residues, we studied the effect of peptide length on fusion properties, its structural dynamics, and par-titioning to the phospholipid bilayer. By means of molecular dynamics simulations and spectroscopic measurements, we showed that the presence of three C-terminal residues in HAfp1-23 promotes the for-mation of hairpin structure. In contrast to less structured HAfp1-20,it orients perpendicularly to the membrane plane and induces more disorder in the surrounding lipids. Using a novel fusion visualization assay based on FLIM microscopy on giant unilamellar vesicles (GUV), we observed that HAfp1-23 promotes fusion to a higher extent than HAfp1-20. Moreover, we report cholesterolenriched domain formation exclusively by the longer fusion peptide. This redistribution of membrane components in fluid phases is likely to play a role during membrane fusion.

Lateral redistribution of transmembrane proteins and liquid-ordered domains in lipid membranes with inhomogeneous curvature

A number of processes in living cells are accompanied by significant changes of the geometric curvature of lipid membranes. In turn, heterogeneity of the lateral curvature can lead to spatial redistribution of membrane components, most important of which are transmembrane proteins and liquid-ordered lipid-protein domains. These components have a so-called hydrophobic mismatch: the length of the transmembrane domain of the protein, or the thickness of the bilayer of the domain differ from the thickness of the surrounding membrane. In this work we consider redistribution of membrane components with hydrophobic mismatch in membranes with non-uniform geometric curvature. Dependence of the components’ energy on the curvature is calculated in terms of theory of elasticity of liquid crystals adapted to lipid membranes. According to the calculations, transmembrane proteins prefer regions of the membrane with zero curvature. Liquid-ordered domains having a size of a few nm distribute mainly into regions of the membrane with small negative curvature appearing in the cell plasma membrane in the process of endocytosis. The distribution of domains of a large radius is determined by a decrease of their perimeter upon bending; these domains distribute into membrane regions with relatively large curvature.

Radio-induced structural membrane modifications: a potential bioindicator of ionizing radiation exposure?

Purpose: This study focused on radio-induced membrane alterations in order to assess some related parameters as potential biological indicators of ionizing radiation effects in cases of accidental overexposure. Materials and methods: Radio-induced membrane alterations were assessed after gamma-irradiation of human blood. Biophysical techniques based on fluorescent probe incorporation into isolated living lymphocytes and erythrocytes membranes were applied. Results: Using the technique of fluorescence polarization, the lipophilic phase of the membrane was shown to be more fluid whereas the lipid-protein interface of the membrane was shown to be more rigid after gamma-irradiation. Fluorescent anisotropy modifications showed dose-time effect relationships after radiation exposure. Ionizing radiation induced a decrease in steadystate anisotropy values but did not affect the probe's lifetime as assessed by fluorescence lifetime distribution technique. These data suggest that the anisotropy variations are representative of the local properties of the fluorescent probe's micro-environment. However, the distribution width showed a decrease pointing towards radiation-induced changes of membrane domain organization, probably due not only to membrane water penetration related to lipoperoxidation, but also to compositional changes and redistribution of membrane components. In contrast, the lack of radiation effect observed using the lateral diffusion index technique may be related to the integrated overview of the radio-induced modifications of the membrane provided by this technique, which pointed out radio-induced damage to the membrane in micro-domains. Conclusion: These findings suggest the utility of structural membrane modification measurements as an early bio-indicator of ionizing radiation exposure.

Increased membrane heterogeneity in stimulated human granulocytes

TMA-DPH fluorescence decay in human PMN before and after stimulation with FMLP was studied using frequency domain fluorometry. Membrane heterogeneity was assessed by the width of the continuous distributions of lifetime values of Lorentzian shape used to describe the fluorescence decay. In non-stimulated granulocytes TMA-DPH fluorescence decay is characterized by two distributions of lifetime values centered at 6.5 and 1.0 ns and full width at half maximum of 0.3 and 1.2 ns, respectively. Within 15 min after stimulation, the center values of the two distribution components were 5.1 and 0.8 ns and the distribution width was 0.8 and 0.6 ns, respectively. These results indicate changes of membrane domain organization which can be ascribed to compositional changes redistribution of membrane components.

Studies of factors regulating the ageing of human erythrocytes—V. The role of vesiculation in the loss of membrane-bound sialic acid

1. 1. Erythrocytes aged in vitro in protein-free media with and without leukocytes, release glycopeptides and vesicles. 2. 2. Addition of soybean trypsin inhibitor (SbTI) blocks almost completely the release of glycopeptides but not the vesiculation. 3. 3. Vesicles appear as a heterogenous population with a diameter of 125 ± 20 nm and with a lower content of N-acetylneuraminic acid (NAN) than ghosts of the original erythrocytes. 4. 4. The NAN:phosphorus and NAN:cholesterol ratios in the vesicles are lower than in intact membranes. 5. 5. The results indicate a redistribution of membrane components during in vitro erythrocyte incubation and the major importance of sialopeptides release in comparison with vesiculation.