1. Vesiculation of biological membrane driven by curvature induced frustrations in membrane orientational orderingDalija Povše Jesenek, Šárka Perutková, Wojciech Góźdź, Veronika Kralj-Iglič, Aleš Iglič, Samo Kralj, 2013, izvirni znanstveni članek Opis: 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). Ključne besede: structural transitions, topological defects, membrane microvesicles, membrane curvature, membrane fission, vesiculation Objavljeno v DKUM: 03.08.2017; Ogledov: 979; Prenosov: 386 Celotno besedilo (4,92 MB) Gradivo ima več datotek! Več... |
2. Numerical study of membrane configurationsLuka Mesarec, Miha Fošnarič, Samo Penič, Veronika Kralj-Iglič, Samo Kralj, Wojciech Góźdź, Aleš Iglič, 2014, izvirni znanstveni članek Opis: We studied biological membranes of spherical topology within the framework of the spontaneous curvature model. Both Monte Carlo simulations and the numerical minimization of the curvature energy were used to obtain the shapes of the vesicles. The shapes of the vesicles and their energy were calculated for different values of the reduced volume. The vesicles which exhibit inplane ordering were also studied. Minimal models have been developed in order to study the orientational ordering in colloids coated with a thin sheet of nematic liquid crystal (nematic shells).The topological defects are always present on the surfaces with the topology of a sphere.The location of the topological defects depends strongly on the curvature of the surface. We studied the nematic ordering and the formation of topological defects on vesicles obtained by the minimization of the spontaneous curvature energy. Ključne besede: biological membranes, vesicles, spontaneous curvature model, Monte Carlo simulations, nematic shells, orientational ordering, topological defects Objavljeno v DKUM: 14.06.2017; Ogledov: 895; Prenosov: 339 Celotno besedilo (4,38 MB) Gradivo ima več datotek! Več... |
3. On the role of anisotropy of membrane constituents on the elastic properties of highly curved lipid membranesŠarka Perutkova, 2017, doktorska disertacija Opis: 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. Ključne besede: Biomembranes, Lipid anisotropy, Inverted hexagonal phase, Rotational averaging, Rod-like proteins, Membrane nanotubes, Membrane
protein sorting Objavljeno v DKUM: 25.05.2017; Ogledov: 1656; Prenosov: 95 Celotno besedilo (12,09 MB) |