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Vesiculation of biological membrane driven by curvature induced frustrations in membrane orientational ordering
Dalija Povše Jesenek, Šárka Perutková, Wojciech Góźdź, Veronika Kralj-Iglič, Aleš Iglič, Samo Kralj, 2013, original scientific article

Abstract: Membrane budding often leads to the formation and release of microvesicles. The latter might play an important role in long distance cell-to-cell communication, owing to their ability to move with body fluids. Several mechanisms exist which might trigger the pinching off of globular buds from the parent membrane (vesiculation). In this paper, we consider the theoretical impacts of topological defects (frustrations) on this process in the membranes that exhibit global in-plane orientational order. A Landau–de Gennes theoretical approach is used in terms of tensor orientational order parameters. The impact of membrane shapes on position and the number of defects is analyzed. In studied cases, only defects with winding numbers m = ±1/2 appear, where we refer to the number of defects with m = 1/2 as defects, and with m = –1/2 as anti-defects. It is demonstrated that defects are attracted to regions with maximal positive Gaussian curvature, K. On the contrary, anti-defects are attracted to membrane regions exhibiting minimal negative values of K. We show on membrane structures exhibiting spherical topology that the coexistence of regions with K > 0 and K < 0 might trigger formation of defect–anti-defect pairs for strong enough local membrane curvatures. Critical conditions for triggering pairs are determined in several demonstrative cases. Then the additionally appeared anti-defects are assembled at the membrane neck, where K < 0. Consequent strong local fluctuations of membrane constituent anisotropic molecules might trigger membrane fission neck rupture, enabling a membrane fission process and the release of membrane daughter microvesicles (ie, vesiculation).
Keywords: structural transitions, topological defects, membrane microvesicles, membrane curvature, membrane fission, vesiculation
Published in DKUM: 03.08.2017; Views: 869; Downloads: 377
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On the role of anisotropy of membrane constituents on the elastic properties of highly curved lipid membranes
Šarka Perutkova, 2017, doctoral dissertation

Abstract: The primary objective of the thesis covers in the theoretical study the role of anisotropic membrane components in the elasticity of highly curved biological membranes. To show the importance of anisotropy, we focused on one type of non-lamellar membrane self-assembly - the inverted hexagonal phase and the membrane tubular protrusions with attached proteins. These two structures represent excellent examples of highly curved structures in which the anisotropy of molecules or small domains plays an important role. In the first part of the thesis, we developed a theoretical model describing the stability of the inverted hexagonal phase, which considers lipid anisotropy and deviations from the circularity of the pivotal plane cross-section. We applied a wedge-like model of phospholipid molecules, in which the phospholipid molecule is described as a wedge, with the angle of the wedge increasing with temperature. However, we also took into account the average orientation of lipids by including the deviatoric bending energy contribution derived from statistical physics. Theoretical predictions of our model showed that a crosssection of the inverted hexagonal phase is an intermediate between a circle and a hexagon, and that it has lower energy than the circular cross-section. The results were in agreement with observations gathered by the small angle X-ray scattering. By comparing our results with the experiments, we predicted some values of the mean intrinsic curvature and the phospholipid chain stiffness. In the second part of the thesis, we developed a theoretical model, which describes the stabilisation of membrane nanotubes containing attached anisotropic flexible rod-like proteins. We derived the free energy of a vesicle with nanotube taking into account the rotational averaging of the anisotropic attached proteins. We also added the entropy contribution due to the non-homogeneous lateral distribution of proteins. Our theoretical results showed that rod-like attached proteins and membrane domains can stabilise the membrane tubular protrusions if we consider the protein/ domain anisotropy. Our results were also in agreement with experimental results in which isotropic membrane constituents were found on the tips of the nanotube or on the mother vesicle; however, the anisotropic membrane constituents were detected along the nanotubes. Our results showed that rod-like attached proteins and membrane domains can stabilise the membrane tubular protrusions if we consider the protein/domain anisotropy. The anisotropy of membrane constituents can lower the membrane free energy in regions of high curvature. The main aim of the thesis was to show that the anisotropy of membrane constituents can lower the membrane free energy in regions of high curvature and that the rotational averaging of anisotropic membrane components should be considered in the evaluation of the membrane free energy at highly curved membrane structures.
Keywords: Biomembranes, Lipid anisotropy, Inverted hexagonal phase, Rotational averaging, Rod-like proteins, Membrane nanotubes, Membrane protein sorting
Published in DKUM: 25.05.2017; Views: 1495; Downloads: 86
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