Membrane Dynamics:Properties of biological membrane (plasma membrane)
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Membrane Dynamics:Properties of biological membrane (plasma membrane)
- 2. Biological Membrane Biological membranes are fluid dynamic closed structures that separate the inside from the outside of cells and compartments within cells.
- 3. Plasma Membrane Membrane is composed of: A. Lipids Phospholipids Sterols B. Proteins Integral Peripheral C. Carbohydrates Glycolipids Glycoproteins
- 4. Membrane Dynamics • One remarkable feature of all biological membranes is their flexibility-their ability to change shape without Iosing their integrity and becoming leaky. Our focus: Membrane Dynamics • the motions that occur in membrane and the transient structures allowed by these motions
- 5. Membrane Dynamics • Membrane Fluidity • Transbilayer movement • Lateral movement • Membrane Rafts and Caveolins • Cell-Cell interaction and Adhesion • Membrane Fusion
- 6. Membrane Fluidity • Although the lipid bilayer structure is quite stable, its individual phospholipid and sterol molecules have much freedom of motion which is the main reason of fluidity property of bilayer. • Membrane Fluidity is influenced by: - Temperature - Fatty Acid Composition (Length & Unsaturation) - Sterol Content
- 7. Membrane Fluidity • As temperatures lowers, membranes switch from a fluid state to a solid state as the phospholipids are more closely packed. • Membranes rich in unsaturated fatty acids are more fluid than those dominated by saturated fatty acids, because the kinks in the unsaturated fatty acid tails prevent tight packing.
- 8. Membrane Fluidity • Below normal physiological temperatures -The lipids in a bilayer form a semisolid gel phase, paracrystalline state. • Above physiological temperatures, -The interior of the bilayer is more fluid than solid and the bilayer is like a sea of constantly moving lipid i.e liquid-disordered state, or fluid state • At intermediate (physiological) temperatures -The lipids exist in a intermediate condition of above mentioned extremes i.e liquid-ordered state.
- 9. Membrane Fluidity • At its transition temperature (TM), the bilayer goes from an ordered crystalline state to an a ordered fluid one which when further heated converted to disordered fluid..
- 10. Cholesterol: A “Fluidity Buffer” • Below Tm - cholesterol disrupts close packing of acyl chains increases fluidity • Above Tm - cholesterol constrains motion of acyl chains decreases fluidity • Broadens/abolishes phase transitions From P.R. Cullis & M.J. Hope, In: D.E. Vance & J.E. Vance (1985) Biochemistry of Lipids and Membranes
- 11. Membrane Dynamics • Membrane Fluidity • Transbilayer movement • Lateral movement • Membrane Rafts and Caveolins • Cell-Cell interaction and Adhesion • Membrane Fusion
- 12. Uncatalyzed Transbilayer movement Phospholipids in a bilayer have free lateral diffusion.
- 13. Uncatalyzed Transbilayer movement Phospholipids in a bilayer have restricted movement between the two faces.
- 14. Catalyzed Transbilayer movement FLIPPASES, FLOPPASES, and SCRAMBLASES catalyze movement between the two faces.
- 15. Membrane Dynamics • Membrane Fluidity • Transbilayer movement • Lateral movement • Membrane Rafts and Caveolins • Cell-Cell interaction and Adhesion • Membrane Fusion
- 16. 16 Lipids and Proteins Diffuse Laterally in the Bilayer Measurement of lateral diffusion rates of lipids by fluorescence recovery after photobleaching (FRAP)
- 17. Membrane Proteins •Integral proteins (includes lipid-linked): need detergents to remove •Peripheral proteins: removed by salt, pH changes •Amphitropic proteins: sometimes attached, sometimes not
- 18. •Some membrane proteins have restricted movement. •May be anchored to internal structures (e.g., glycophorin is tethered to spectrin). Membrane Protein Dynamics
- 19. Membrane Dynamics • Membrane Fluidity • Transbilayer movement • Lateral movement • Membrane Rafts and Caveolins • Cell-Cell interaction and Adhesion • Membrane Fusion
- 20. •Lipid rafts are membrane microdomains enriched in sphingolipids, cholesterol, and certain lipid-linked proteins. •Thicker and less fluid than neighboring domains. •The raft is the mainly responsible for signaling and communication Membrane Raft
- 21. Caveolin (caveolae “little caves”)… •Integral membrane protein. •Two globular domains connected by hairpin like hydrophobic structures. •Bound inward to cytoplasmic leaflet. •Three palmitoylgroups attached to carboxyl-terminal globular domain attach to the membrane. •Functions Membrane trafficking Signal transduction Caveolins
- 22. Membrane Dynamics • Membrane Fluidity • Transbilayer movement • Lateral movement • Membrane Rafts and Caveolins • Cell-Cell interaction and Adhesion • Membrane Fusion
- 23. Certain Integral Proteins Mediate Cell- Cell Interactions and Adhesion Four examples of integral protein types that function in cell-cell interactions. Integral proteins play role as… •Transporters, Ion-channels, receptors, cell-cell recognition, endocytosis,etc
- 24. •Functions of Integrins Attaches to cell surface, is transmembrane protein, directs outside Provides binding site for several extracellular proteins Serves as receptors and signal transducers Cadherins. Interacts with cadherins of other cells. •Immunoglobulin-like proteins. Shows homophilic or heterophilic interaction with integrins of neighbouring cells. •Selectins. In presence of Ca++ binds to specific polysachharides on the adjacent cell. Play important role in Blood clotting.
- 25. Membrane Dynamics • Membrane Fluidity • Transbilayer movement • Lateral movement • Membrane Rafts and Caveolins • Cell-Cell interaction and Adhesion • Membrane Fusion
- 26. 26 Membrane Fusion Is Central to Many Biological Processes Specific fusion of two membranes requires that: (1) they recognize each other (2) their surfaces become closely apposed (3) their bilayer structures become locally disrupted (4) their bilayer fused to form a single continuous bilayer (5) triggered at the appropriate time or by a specific signal
- 27. 27 Fusion induced by the hemagglutinin (HA) protein during viral infection
- 28. Summary of Membrane Dynamics • Lipids in a biological membrane can exist in liquid-ordered or liquid-disordered states; in the latter state, thermal motion of acyl chains makes the interior of the bilayer fluid. Fluidity is affected by temperature, fatty acid composition, and sterol content. • Flip-flop diffusion of lipids between the inner and outer leaflets of a membrane is very slow except when speciflcally catalyzed by flippases, floppases, or scramblases. • Lipids and proteins can diffuse laterally within the plane of the membrane, but this mobility is limited by interactions of membrane proteins with internal cytoskeletal structures and interactions of lipidswith lipid rafts.
- 29. Summary contd… • Caveolinis an integral membrane protein that associates with the inner leaflet of the plasma membrane, forcing it to curve inward to form caveolae, probably involved in membrane transport and signaling. • Speci-fic proteins cause local membrane curvature and mediate the fusion of two membranes, which accompanies processes such as endocytosis, exocytosis and viral invasion. • Integrins are transmembrane proteins of the plasma membrane that act both to attach cells to each other and to carry messages betweent he extracellular matrix and the cytoplasm.
- 30. Thank you! Any queries please write to me on comment below or email me at manjuchhetri@gmail.com
Editor's Notes
- The basis for this property : noncovalent interactions among lipids in the bilayer and the mobility allowed to individual lipids because they are not covalently anchored to one another.
- If it have more unsaturated fatty acids, its more flexible because it melts quicker than saturated one. The presence of sterols reduces the fluidity in the core of the bilayer, thus favoring the liquid-ordered phase, and increases the thickness of the lipid leaflet
- Below normal physiological temperatures, the lipids in a bilayer form a semisolid gel phase, in which all types of motion of individual lipid molecules are strongly constrained; the bilayer is paracrystalline . Above physiological temperatures, individual hydrocarbon chains of fatty acids are in constant motion produced by rotation about the carbon-carbon bonds of the long acyl side chains. In this liquid-disordered state, or fluid state , the interior of the bilayer is more fluid than solid and the bilayer is like a sea of constantly moving lipid. At intermediate (physiological) temperatures, the lipids exist in a liquid-ordered state; there is less thermal motion in the acyl chains of the lipid bilayer, but lateral movement in the plane of the bilayer still takes place.These differences in bilayer state are easily observed in liposomes composed of a single lipid, but biological membranes contain many lipids with a variety of fatty acyl chains and thus do not show sharp phase changes with temperature
- Two extreme states of bilayer lipids.( a) In the paracrystalline state or gel phase polar head groups are uniformly arrayed at the surface and the acyl chains are nearly motionless and packed with regular geometry ( b) In the liquid-disordered state or fluid state, acyl chains under go much thermal motion and have no regular organization. Intermediate between these extremes is the liquid-ordered state, in which individual phospholipid molecules can diffuse laterally but the acyl groups remain extended and more or less ordered.