MAE 545: Lecture 14 (11/10) Mechanics of cell membranes

Transcription

1 MAE 545: ecture 14 (11/10) Mechanics of cell membranes

2 Cell membranes Eukaryotic cells E. Coli FIBROBAST 10 mm E. COI nuclear pore complex 1 mm inner membrane plasma membrane secretory complex ribosome cell wall outer membrane lipopolysaccharide rough endoplasmic reticulum nuclear envelope Figure 11.2: Key examples of membranes in biological systems. Eukaryotic cells, such as this fibroblast, are rife with many specialized membranes. The plasma membrane is a single phospholipid bilayer riddled with membrane proteins. The rough endoplasmic reticulum, also a single bilayer, is the site of synthesis of membrane-bound and secreted proteins. The ribosomes synthesizing these proteins are intimately associated with a transport apparatus in the endoplasmic Biology reticulumof membrane. the Cell The 2 nuclear envelope consists of two phospholipid bilayers with a thin space between them. This nuclear envelope is perforated by R. Phillips et al., Physical

3 Cell membrane lipids 5nm 3

4 ipid membrane behaves like fluid membrane lipid membrane protein membrane protein ipid molecules and proteins can move around! Flipping of lipid molecules between the layer is unlikely. 4

5 Membrane attached spectrin network provides solid-like behavior Spectrin network provides structural stability for cells red blood cell capillary Alberts et al., Molecular Biology of the Cell 5 a few microns in diameter 7.5µm

6 ipid membrane In water solution lipid molecules spontaneously aggregate to prevent undesirable interactions between water and hydrophobic tails. 6

7 Flat lipid bilayers vs lipid vesicles flat bilayer vesicle 2R energy cost on the edge between lipid tails and water molecules E / bending energy cost E / const arge vesicles have lower energy cost then flat bilayers! 7

8 Shape of lipid molecules can induce spontaneous curvature of structures bilayer micelle H -phase bilayer micelle H-II phase R. Phillips et al., Physical Biology of the Cell 8 inverted micelle

9 Membrane proteins can induce REVIEWS spontaneous curvature a binding of rigid curved proteins ers. In a key mbrane-carted to occur reas that are e area. Even the inserted ect would be so it would otal area difl examples chanism is function of roteins 59. It re known to ns and buds ture (FIG. 3b). mplexes and ide scaffolds and BAR (Bin, eins (includand provide mechanism membranes ng proteins. e absence of ans that it is pe of a split nfirm this; urthermore, forms cylinructure and helix 36, b c J. Zimmerberg and M.M. Kozlov, Figure 3 Mechanisms by which proteins can generate membrane curvature. a The scaffold mechanism. A rigid protein, or protein domain (for example, the BAR (Bin, amphiphysin, Rvs) domain), that has an intrinsic curvature Nat. Rev. Mol. Cel. Biol. 7, 9 (2006) 9 interactions between coat proteins bend the membrane insertions of protein parts between lipid molecules on one side of the layer

10 Membrane deformations stretch bend shear thickness change 10 R. Phillips et al., Physical Biology of the Cell

11 Energy cost for stretching and shearing undeformed square patch isotropic deformation shear deformation patch area A = 2 anisotropic stretching + (1 + 2 ) E A = B 2 + A A 2 B E A = µ 2 2 (1 + 1 ) E A B 2 ( ) 2 + µ 2 ( 1 2) 2 bulk modulus B 0.2N/m (lipid bilayer) shear modulus µ 10 5 N/m (spectrin network) 1, 2 1 (shearing can be interpreted as anisotropic stretching) 11