Understanding Cell Membrane Functions and Structure

1. BCOR 011 Lecture 9: Cell membrane structure Sept 19, 2005

2. Cell membranes 1. What are the functions of cell membranes? 2. What is the current model of membrane structure? 3. Evidence supporting the fluid mosaic model 4. How appropriate fluidity is maintained

3. Membrane: organized arrangement of lipids and proteins that encloses and separates the cell from its surroundings prokaryote eukaryote Membranes define spaces with distinctive character and function

4. Membrane Functions 1. boundaries 6. Cell-cell adhesion 2. Localize specific functions 5. Cell-cell communication 3. transport 4. Signal detection

5. (a) Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy ssource to actively pump substances across the membrane. ATP (b) Enzymes Enzymatic activity. A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. (c) Signal transduction. A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell. Signal Receptor Figure 7.9 3. transport major functions of membrane proteins 2. Localize specific functions 4. Signal detection

6. (d) Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells. Glyco- protein (e) Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions (see Figure 6.31). (f) Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes (see Figure 6.29). Figure 7.9 5. Cell-cell communication 6. Cell-cell adhesion 1. boundaries

7. Transport – Lect 10 materials across membranes Cell Signaling – Lect 11 external signals trigger internal events Biochemical functions – Lects 16-19 Oxidative Phosphor, Photosynthesis Importance of Membranes in biochemical Rxns

8. Current Understanding of Membrane Structue: Fluid Mosaic Model 1972 Singer & Nicholson Proteinsembedded and floating in a sea of phospholipids Hydrophobic region of protein Familiar features? Problems ? Phospholipid bilayer Figure 7.3 Hydrophobic region of protein

9. N-terminus C-terminus CYTOPLASMIC SIDE a Helix Figure 7.8 Span the phospholipid bilayer – usually a-helices Why do proteins cross membranes as -helices? • Integral Membrane proteins EXTRACELLULAR SIDE Must present hydrophobic surface

10. hydrophobic A B hydrophilic

11. Sugars commonly found on glycoproteins Sialic acid (SIA) - charge -D-mannose -D-galactose N-acetyl--D-glucosamine c. Carbohydrates – small amts often linked to proteins or lipids

12. Glycocalyx: “sugar coat” Outside cell Inside cell

13. 1 Transmembrane glycoproteins Secretory protein Glycolipid 2 Golgi apparatus Vesicle 3 Plasma membrane: Cytoplasmic face 4 Extracellular face Transmembrane glycoprotein Secreted protein Membrane glycolipid Figure 7.10 • Membrane proteins and lipids • Are synthesized in the ER and Golgi apparatus ER

14. Membrane proteins • Integral • Peripheral • Lipid-anchored

15. Roles of membrane proteins? • Transport – channels and pumps • Links to structural proteins • Receptors - doorbells • Enzymes – localized biochemical rxns • Energy Generation – utilize gradient

16. Fluid Mosaic Model Proteinsembedded and floating in a sea of phospholipids Hydrophobic region of protein Hydrophobic region of protein Evidence?

17. Evidence for Phospholipid (b) (a) Gorter & Grendel – Langmuir trough Red blood cells had enough lipid to twice cover their surface Conclude lipid is a bilayer – hydrophilic heads faced aqueous environment

18. Evidence for integral membrane proteins: Freeze-Fracture Electron Microscopy Extracellular layer A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers. Proteins Knife Plasma membrane Cytoplasmic layer Illustrates: asymmetry of membrane componets Cytoplasmic Leaflet External Leaflet Figure 7.4 Extracellular layer Cytoplasmic layer

19. Fluid Mosaic Model predicts: A. Membranes are fluid: lipids & proteins move in the plane of the bilayer B. Proteins and lipids are asymmetrically distributed in the bilayers

20. Evidence for protein asymmetry I125 I125 LP adds I to protein LP can’t pass thru intact membrane intact Lactoperoxidase (LP) + I125 permeable

21. Evidence for lipid asymmetry? Cut off head groups off of exposed lipids Intact red blood cells SM, PC PE, PS SM & PC Broken red blood cells Digested them with phospholipase Results:isolated different types of phospholipids suggesting lipids were distributed differently in the inner and out parts of the bilayer SM, sphingomyelin; PC, phosphatidylcholine; PE, phosphatidylcholine; PS phosphatidylserine

22. Mosaic: Lipids are asymmetrically distributed Extracellular space phosphatidylcholine sphingomyelin glycolipid cholesterol phosphatidylinositol Cytosol phosphatidylserine phosphatidylethanolamine

23. Fluid Mosaic Model predicts: A. Membranes are fluid: lipids & proteins move in the plane of the bilayer B. Proteins and lipids are asymmetrically distributed in the bilayers

24. Lateral movement (~107 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids Figure 7.5 A The Fluidity of Membranes • Phospholipids can move laterally within the bilayer

25. Movement of membrane phospholipids 1. Rotation about long axis 2. Lateral exchanges 1x107 times/sec. moves several m/sec 3. Flip-flop – rare <1 time/wk to 1 time/few hrs “flippases”

26. Evidence for lipid fluidity: Photobleaching

27. Evidence for membrane protein fluidity? Cell fusion: 1970 D. Frye & M. Edidin mouse human Figure 7.6

28. Lipids: critical role in maintaining membrane fluidity • Saturated fatty acids stack nicely stiffer • Unsaturated fatty acids –more fluid; double bond causes kinks • Stacks poorly More fluid Shorter chains – stack poorly; More movement Length & saturation of hydrocarbon tails affect packing & membrane fluidity

29. Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails (b) Membrane fluidity Figure 7.5 B

30. Sterols – affect membrane fluidity OH- Cholesterol (animal) Hopanoid (prokaryotes)

31. Cholesterol Figure 7.5 (c) Cholesterol within the animal cell membrane cholesterol • At high temperature has a loosening effect • At low temperature has a stiffening effect

32. Cholesterol is common in animalcells • Paradox: • fluidity at high temp. • fluidity at low temp.

33. Most organisms regulate membrane fluidity “Homeoviscous adaptation” Fish, plantsMammals, palm trees 0-20ºC 30-37ºC Polyunsaturated F.A. Saturated F.A. Shorter chains Longer chains Cholesterol cholesterol

34. Restricting movement of membrane proteins -> Membrane Domains B A • Cell cortex • Extracellular matrix • Cell/cell junctions C

35. Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Microfilaments of cytoskeleton Peripheral protein Cholesterol Integral protein CYTOPLASMIC SIDE OF MEMBRANE Figure 7.7 Fibers of extracellular matrix (ECM) Tethering of membrane proteins to the Extracellular Matrix or The Cytoskeleton

36. Summary: Membranes • 1.Fluid Mosaic Model: fluid nature & asymmetric distribution of components • 2. Components: • Lipids– phospholipids, sterols, glycolipids • Fluidity • Proteins – integral, peripheral, lipid-linked • transport, receptors, enzymes, structural support, • electron transport, specialized functional domains • Carbohydrates – as glycolipids & glycoproteins • external glycocalyx