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

3. 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

4. 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%

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

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

8. Unsaturated Fatty Acid

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

10. Phosphoglycerides are Classified by their Head Groups Phosphatidylethanolamine Phosphatidylcholine Phosphatidylserine Phosphatidylinositol Ether Bond at C1 PS and PI bear a net negative charge at neutral pH

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

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

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

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

15. 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  G =  –  S

17. 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

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

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

20. Figure 10-11b Molecular Biology of the Cell (© Garland Science 2008) PhosphoLipid Movements within Bilayers (µM/sec) (10 12 -10 13 /sec) (10 8 -10 9 /sec)

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

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

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

24. 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

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

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

27. 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

28. 2. Phospholipases Produce Signaling Molecules via the Degradation of Phospholipids

29. 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

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

31. Figure 10-5 Molecular Biology of the Cell (© Garland Science 2008) C12 How Cholesterol Integrates into a Phospholipid Bilayer

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

33. 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

34. 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

35. 3 Ways in which Lipids May be Transferred Between Different Intracellular Compartments Vesicle Fusion Direct Protein-Mediated Transfer Soluble Lipid Binding Proteins

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

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

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

40. 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

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

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

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

44. 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)

45.  -Barrel Structure of the OmpX Porin Protein in the Outer Membrane of E. coli Aromatic Side Chain anchors Alternating Aliphatic Side Chains

46. Transmembrane  -Helices 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 H-Bond

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

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

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

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

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

52. 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