Of The Structure of Biological Membranes Thursday 8/28 2014 Mike Mueckler mmueckler@wustl.edu

The Structure of Biological Membranes

Thursday 8/28 2014Mike Mueckler

mmueckler@wustl.edu

Functions of Cellular Membranes1. Plasma membrane acts as a selectively permeable barrier to the

environment• Uptake of nutrients• Waste disposal• Maintains intracellular ionic milieu2. Plasma membrane facilitates communication• With the environment• With other cells3. 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 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

~90%

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

Figure 10-2 Molecular Biology of the Cell (© Garland Science 2008)

Structure of Phosphoglycerides

All Membrane Lipids are Amphipathic

Phosphoglycerides are Classified by their Head Groups

Phosphatidylethanolamine

Phosphatidylcholine

Phosphatidylserine

PhosphatidylinositolEther Bond at C1

PS and PI bear a net negative chargeat neutral pH

Sphingolipids are the Second Major Class of Phospholipid in Animal Cells

Sphingosine

Ceramides contain sugar moities in ether linkage to sphingosine

Figure 10-18 Molecular Biology of the Cell (© Garland Science 2008)

Glycolipids are Abundant in Brain Cells

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

Cytoplasmic Leaflet

Exoplasmic Leaflet

Figure 10-11a Molecular Biology of the Cell (© Garland Science 2008)

(Hydrophobic)

(Hydrophilic)

Bilayers are Thermodynamically Stable Structures Formed from Amphathic Lipids

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

Figure 1-12 Molecular Biology of the Cell, Fifth Edition (© Garland Science 2008)

Lipid Bilayer Formation is a Spontaneous Process

Gently mix PL and water

Vigorous mixing

Figure 10-8 Molecular Biology of the Cell (© Garland Science 2008)

The Formation of Cell-Like Spherical Water-Filled Bilayers is Energetically Favorable

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

Phospholipids are Mesomorphic

Figure 10-11b Molecular Biology of the Cell (© Garland Science 2008)

PhosphoLipid Movements within Bilayers(µM/sec)

(1012-1013/sec) (108-109/sec)

Pure Phospholipid Bilayers Undergo Phase Transition

Below Lipid Phase Transition Temp Above Lipid Phase Transition Temp

Gauche Isomer Formation

All-Trans

Figure 12-57 Molecular Biology of the Cell (© Garland Science 2008)

Phosphoglyceride Biosynthesis Occurs at the Cytoplasmic Face of the ER

Figure 12-58 Molecular Biology of the Cell (© Garland Science 2008)

A “Scramblase” Enzyme Catalyzes Symmetric

Growth of Both Leaflets in the ER

Figure 10-16 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 12-58 Molecular Biology of the Cell (© Garland Science 2008)

A “Flippase” Enzyme promotes Lipid

Asymmetry in the Plasma Membrane

Membrane Proteins May Selectively Interact with Specific Lipids (Lipid Annulus or Halo)

Lipid Asymmetry Also Exists in the Plane of the Bilayer

Figure 10-17a Molecular Biology of the Cell (© Garland Science 2008)

Phospholipids are Involved in Signal Transduction

PI-4,5P PI-3,4,5P

1. Activation of Lipid Kinases

2. Phospholipases Produce Signaling Molecules via the Degradation of

Phospholipids

Figure 10-17b Molecular Biology of the Cell (© Garland Science 2008)

Phospholipase C Activation Produces Two Intracellular Signaling Molecules

PI-3,4,5P

I-3,4,5P

Diacylglycerol

Hydrophilic End

Flexible Hydrocarbon Tail

Amphipathic Lipids Containing Rigid Planar Rings Are Important Components of Biological Membranes

Figure 10-5 Molecular Biology of the Cell (© Garland Science 2008)

C12

How Cholesterol Integrates into a Phospholipid Bilayer

Cholesterol Biosynthesis Occurs in the Cytosol and at the ER Membrane Through Isoprenoid Intermediates

(Rate-Limiting Step in ER)

Figure 10-14b Molecular Biology of the Cell (© Garland Science 2008)

Lipid Rafts are Microdomains Enriched in Cholesterol and SM that may be Involved in Cell

Signaling Processes

Insoluble in Triton X-100

Figure 10-14a Molecular Biology of the Cell (© Garland Science 2008)

Electron Force Microscopic Visualization of Lipid Rafts in Artificial Bilayers

Yellow spikes are GPI-anchored protein

3 Ways in which Lipids May be Transferred Between Different Intracellular Compartments

Vesicle FusionDirect Protein-Mediated

TransferSoluble Lipid

Binding Proteins

Figure 10-19 Molecular Biology of the Cell (© Garland Science 2008)

The 3 Basic Categories of Membrane Protein

Integral Lipid-Anchored

Peripheral(can also interact via PL headgroups)

Single-Pass Multi-pass

Fatty acyl anchor

GPI Anchor

Transmembrane Helix

Linker Domain

b-Strands

Figure 10-20 Molecular Biology of the Cell (© Garland Science 2008)

3 Types of Lipid Anchors

Figure 3-5 Molecular Biology of the Cell (© Garland Science 2008)

Membrane Domains are “Inside-Out”

Right-Side Out Soluble Protein

Figure 10-31 Molecular Biology of the Cell (© Garland Science 2008)

Functional Characterization of Integral Membrane Proteins Requires Solubilization and Subsequent Reconstitution into a Lipid Bilayer

(Used to denature proteins)

(Used to purify Integral Membrane Proteins)

Detergents are Critical for the Study of Integral Membrane Proteins

Figure 10-29b Molecular Biology of the Cell (© Garland Science 2008)

Detergents Exist in Two Different States in Solution

(Critical Micelle Concentration)

Detergent Solubilization of Membrane Proteins

Desirable for Purification of Integral Membrane Proteins

Beta Sheet Secondary Structure

Anti-parallel Strands

Side chains alternately extend into opposite sides of the sheet

H-Bond

Polypeptide backbone is maximally extended (rise of 3.3 Å/residue)

b-Barrel Structure of the OmpX Porin Protein in the Outer Membrane of E. coli

Aromatic Side Chain anchors

Alternating Aliphatic Side Chains

Transmembrane a-Helices1. Right-handed2. 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

H-Bond

Figure 10-22b Molecular Biology of the Cell (© Garland Science 2008)

Transmembrane Domains Can Often be Accurately Identified by Hydrophobicity Analysis

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

Bacteriorhodopsin of Halobacterium

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

Structure of Bacteriorhodopsin

Structure of the Glycophorin A Homodimer

Arg and Lys Side Chain Anchors

23 residue transmembrane helices

Coiled-Coil Domains in Van Der WaalsContact

Figure 10-34 Molecular Biology of the Cell (© Garland Science 2008)

Structure of the Photosynthetic Reaction Center of Rhodopseudomonas Viridis

Figure 10-39 Molecular Biology of the Cell (© Garland Science 2008)

4 Ways that Protein Mobility is Restricted in Biological Membranes

Intramembrane Protein-Protein Interactions

Interaction with the cytoskeleton

Interaction with the extracellular maxtrix

Intercellular Protein-Protein Interactions