The Structure of Biological Membranes Thursday 8282018 Mike

The Structure of Biological Membranes Thursday 8/28/2018 Mike Mueckler mmueckler@wustl. edu

Functions of Cellular Membranes 1. Plasma membrane acts as a selectively permeable barrier to the environment • Uptake of nutrients • Waste disposal • Maintains intracellular ionic milieu 2. Plasma membrane facilitates communication • With the environment • With other cells 3. Intracellular membranes allow compartmentalization and separation of different chemical reaction pathways • Increased efficiency through proximity • Prevent futile cycling through separation • Protein secretion

Composition of Animal Cell Membranes • Hydrated, proteinaceous lipid bilayers • By weight: 20% water, 80% solids • Solids: Lipid ~90% Protein Carbohydrate (~10%) • Phospholipids responsible for basic membrane bilayer structure and physical properties • Membranes are 2 -dimensional fluids into which proteins are dissolved or embedded

The Most Common Class of Phospholipid is Formed from a Gycerol-3 -P Backbone

Saturated Fatty Acid • Palmitate and stearate most common • 14 -26 carbons • Even # of carbons

Unsaturated Fatty Acid

Structure of Phosphoglycerides All Membrane Lipids are Amphipathic Figure 10 -2 Molecular Biology of the Cell (© Garland Science 2008)

Phosphoglycerides are Classified by their Head Groups Phosphatidylethanolamine Phosphatidylcholine Phosphatidylserine Ether Bond at C 1 Phosphatidylinositol PS and PI bear a net negative charge at neutral p. H

Sphingolipids are the Second Major Class of Phospholipid in Animal Cells Sphingosine Ceramides contain sugar moities in ether linkage to sphingosine

Glycolipids are Abundant in Brain Cells Figure 10 -18 Molecular Biology of the Cell (© Garland Science 2008)

Membranes are formed by the tail to tail association of two lipid leaflets Exoplasmic Leaflet Cytoplasmic Leaflet

Bilayers are Thermodynamically Stable Structures Formed from Amphathic Lipids (Hydrophobic) (Hydrophilic) Figure 10 -11 a Molecular Biology of the Cell (© Garland Science 2008)

Lipid Bilayer Formation is Driven by the Hydrophobic Effect • HE causes hydrophobic surfaces such as fatty acyl chains to aggregate in water • Water molecules squeeze hydrophobic molecules into as compact a surface area as possible in order to the minimize the free energy state (G) of the system by maximizing the entropy (S) or degree of disorder of the water molecules • DG = DH – TDS

Lipid Bilayer Formation is a Spontaneous Process Gently mix PL and water Vigorous mixing Figure 1 -12 Molecular Biology of the Cell, Fifth Edition (© Garland Science 2008)

The Formation of Cell-Like Spherical Water-Filled Bilayers is Energetically Favorable Figure 10 -8 Molecular Biology of the Cell (© Garland Science 2008)

Phospholipids are Mesomorphic Figure 10 -7 Molecular Biology of the Cell (© Garland Science 2008)

Phospho. Lipid Movements within Bilayers (µM/sec) (1012 -1013/sec) (108 -109/sec) Figure 10 -11 b Molecular Biology of the Cell (© Garland Science 2008)

Pure Phospholipid Bilayers Undergo Phase Transition Gauche Isomer Formation All-Trans Below Lipid Phase Transition Temp Above Lipid Phase Transition Temp

Phosphoglyceride Biosynthesis Occurs at the Cytoplasmic Face of the ER Figure 12 -57 Molecular Biology of the Cell (© Garland Science 2008)

A “Scramblase” Enzyme Catalyzes Symmetric Growth of Both Leaflets in the ER Figure 12 -58 Molecular Biology of the Cell (© Garland Science 2008)

The Two Plasma Membrane Leaflets Possess Different Lipid Compositions Enriched in PC, SM, Glycolipids Enriched in PE, PS, PI Figure 10 -16 Molecular Biology of the Cell (© Garland Science 2008)

A “Flippase” Enzyme promotes Lipid Asymmetry in the Plasma Membrane Figure 12 -58 Molecular Biology of the Cell (© Garland Science 2008)

Membrane Proteins May Selectively Interact with Specific Lipids (Lipid Annulus or Halo) Lipid Asymmetry Also Exists in the Plane of the Bilayer

Phospholipids are Involved in Signal Transduction 1. Activation of Lipid Kinases PI-4, 5 P Figure 10 -17 a Molecular Biology of the Cell (© Garland Science 2008) PI-3, 4, 5 P

2. Phospholipases Produce Signaling Molecules via the Degradation of Phospholipids

Phospholipase C Activation Produces Two Intracellular Signaling Molecules Diacylglycerol PI-4, 5 P I-1, 4, 5 P Figure 10 -17 b Molecular Biology of the Cell (© Garland Science 2008)

Amphipathic Lipids Containing Rigid Planar Rings Are Important Components of Biological Membranes Flexible Hydrocarbon Tail Hydrophilic End

How Cholesterol Integrates into a Phospholipid Bilayer C 12 Figure 10 -5 Molecular Biology of the Cell (© Garland Science 2008)

Cholesterol Biosynthesis Occurs in the Cytosol and at the ER Membrane Through Isoprenoid Intermediates (Rate-Limiting Step in ER)

Lipid Rafts are Microdomains Enriched in Cholesterol and SM that may be Involved in Cell Signaling Processes Insoluble in Triton X-100 Figure 10 -14 b Molecular Biology of the Cell (© Garland Science 2008)

Electron Force Microscopic Visualization of Lipid Rafts in Artificial Bilayers Yellow spikes are GPI-anchored protein Figure 10 -14 a Molecular Biology of the Cell (© Garland Science 2008)

3 Ways in which Lipids May be Transferred Between Different Intracellular Compartments Vesicle Fusion Direct Protein-Mediated Soluble Lipid Transfer Binding Proteins

The 3 Basic Categories of Membrane Protein GPI Anchor Single-Pass Multi-pass b-Strands Transmembrane Helix Linker Domain Fatty acyl anchor Integral Figure 10 -19 Molecular Biology of the Cell (© Garland Science 2008) Lipid. Anchored Peripheral (can also interact via PL headgroups)

3 Types of Lipid Anchors Figure 10 -20 Molecular Biology of the Cell (© Garland Science 2008)

Membrane Domains are “Inside-Out” Right-Side Out Soluble Protein Figure 3 -5 Molecular Biology of the Cell (© Garland Science 2008)

Functional Characterization of Integral Membrane Proteins Requires Solubilization and Subsequent Reconstitution into a Lipid Bilayer Figure 10 -31 Molecular Biology of the Cell (© Garland Science 2008)

Detergents are Critical for the Study of Integral Membrane Proteins (Used to denature proteins) (Used to purify Integral Membrane Proteins)

Detergents Exist in Two Different States in Solution (Critical Micelle Concentration) Figure 10 -29 b Molecular Biology of the Cell (© Garland Science 2008)

Detergent Solubilization of Membrane Proteins Desirable for Purification of Integral Membrane Proteins

Beta Sheet Secondary Structure Polypeptide backbone is maximally extended (rise of 3. 3 Å/residue) H-Bond Anti-parallel Strands Side chains alternately extend into opposite sides of the sheet

b-Barrel Structure of the Omp. X Porin Protein in the Outer Membrane of E. coli Aromatic Side Chain anchors Alternating Aliphatic Side Chains

Transmembrane a-Helices H-Bond 1. Right-handed 2. Stability in bilayer results from maximum hydrogen bonding of peptide backbone 3. Usually > 20 residues in length (rise of 1. 5 Å/residue) 4. Exhibit various degrees of tilt with respect to the membrane and can bend due to helix breaking residues 5. Mostly hydrophobic side-chains in single -pass proteins 6. Multi-pass proteins can possess hydrophobic, polar, or amphipathic transmembrane helices

Transmembrane Domains Can Often be Accurately Identified by Hydrophobicity Analysis Figure 10 -22 b Molecular Biology of the Cell (© Garland Science 2008)

Bacteriorhodopsin of Halobacterium Figure 10 -32 Molecular Biology of the Cell (© Garland Science 2008)

Structure of Bacteriorhodopsin Figure 10 -33 Molecular Biology of the Cell (© Garland Science 2008)

Structure of the Glycophorin A Homodimer Coiled-Coil Domains in Van Der Waals Contact Arg and Lys Side Chain Anchors 23 residue transmembrane helices

Structure of the Photosynthetic Reaction Center of Rhodopseudomonas Viridis Figure 10 -34 Molecular Biology of the Cell (© Garland Science 2008)

4 Ways that Protein Mobility is Restricted in Biological Membranes Intramembrane Protein Interactions Interaction with the cytoskeleton Interaction with the extracellular maxtrix Intercellular Protein Interactions Figure 10 -39 Molecular Biology of the Cell (© Garland Science 2008)