Abstract
Biological membranes are composed of isotropic and anisotropic curved nanodomains. Anisotropic membrane components, such as Bin/Amphiphysin/Rvs (BAR) superfamily protein domains, could trigger/facilitate the growth of membrane tubular protrusions, while isotropic curved nanodomains may induce undulated (necklace-like) membrane protrusions. We review the role of isotropic and anisotropic membrane nanodomains in stability of tubular and undulated membrane structures generated or stabilized by cyto- or membrane-skeleton. We also describe the theory of spontaneous self-assembly of isotropic curved membrane nanodomains and derive the critical concentration above which the spontaneous necklace-like membrane protrusion growth is favorable. We show that the actin cytoskeleton growth inside the vesicle or cell can change its equilibrium shape, induce higher degree of segregation of membrane nanodomains or even alter the average orientation angle of anisotropic nanodomains such as BAR domains. These effects may indicate whether the actin cytoskeleton role is only to stabilize membrane protrusions or to generate them by stretching the vesicle membrane. Furthermore, we demonstrate that by taking into account the in-plane orientational ordering of anisotropic membrane nanodomains, direct interactions between them and the extrinsic (deviatoric) curvature elasticity, it is possible to explain the experimentally observed stability of oblate (discocyte) shapes of red blood cells in a broad interval of cell reduced volume. Finally, we present results of numerical calculations and Monte-Carlo simulations which indicate that the active forces of membrane skeleton and cytoskeleton applied to plasma membrane may considerably influence cell shape and membrane budding.
Highlights
The lipid bilayer, embedded with inclusions like proteins and lipids, is the main element of biological membranes [1,2]
Taking into account nematic in-plane ordering with direct interactions between anisotropic membrane components could explain the experimentally observed wide stability window of the reduced volume values v for stable oblate shapes of red blood cells (RBC)
We described the impact of different membrane curved nanodomains and passive and active forces of cytoskeleton and membrane skeleton on closed membrane shapes and membrane budding
Summary
The lipid bilayer, embedded with inclusions like proteins and lipids, is the main element of biological membranes [1,2]. Biological membranes can be viewed as a complex multicomponent system [2,8], composed of lipid molecules, proteins, carbohydrates and many other biologically active components [9] These components/inclusions may promote local membrane curvature changes, sometimes resulting in a global adjustment of the cell shape [10,11,12,13,14,15,16,17,18,19,20,21]. Typical example of anisotropic proteins are for example Bin/Amphiphysin/Rvs (BAR) superfamily protein domains [103] (see Figure 3) These can change the local and global membrane curvature resulting in formation of the membrane tubular structures [103]. Within a statistical-mechanical approach, it has been indicated that in some membrane regions, the average orientation of lipid molecules cannot be neglected in spite of their rotational movement [81]
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