COR 011 Lecture 9: ell membrane structure ept 19, 2005 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 Membrane: organized arrangement of lipids and proteins that encloses and separates the cell from its surroundings Membrane Functions 2. Localize specific functions 1. boundaries 6. Cell-cell adhesion prokaryote eukaryote Membranes define spaces with distinctive character and function 3. transport 5. Cell-cell communication 4. Signal detection
major functions of membrane proteins (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) 3. 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. (b) 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 specific in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. (c) 2. Localize functions 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. 4. Signal detection Enzymes Signal ATP identification tags that are specifically recognized by other cells. 5. Cell-cell communication (e) (f) 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). 6. Cell-cell adhesion 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 1. to the boundaries ECM can coordinate extracellular and intracellular changes (see Figure 6.29). Glycoprotein Figure 7.9 Receptor Figure 7.9 ransport Lect10 materials across membranes ell Signaling Lect11 external signals trigger internal events iochemical functions Lects 16-19 Oxidative Phosphor, Photosynthesis Importance of Membranes in biochemical Rxns Current Understanding of Membrane Structue: Fluid Mosaic Model 1972 Singer & Nicholson Proteins embedded and floating in a sea of phospholipids Familiar features? Problems? Phospholipid bilayer Hydrophobic region of protein
Integral Membrane proteins Span the phospholipid bilayer usually α-helices Why do proteins cross membranes as α-helices? N-terminus ust present ydrophobic urface EXTRACELLULAR SIDE A hydrophobic B C-terminus hydrophilic Figure 7.8 α Helix CYTOPLASMIC SIDE ugars commonly found on glycoproteins Glycocalyx: sugar coat β-d-mannose β-d-galactose N-acetyl-β-D-glucosamine Sialic acid (SIA) -charge Outside cell c. Carbohydrates small amts often linked to proteins or lipids Inside cell
Membrane proteins and lipids Are synthesized in the ER and Golgi apparatus Transmembrane glycoproteins 1 Membrane proteins Integral Peripheral Lipid-anchored Secretory protein ER Glycolipid Golgi apparatus 2 Vesicle 3 Secreted protein 4 Transmembrane glycoprotein Plasma membrane: Cytoplasmic face Extracellular face Figure 7.10 Membrane glycolipid Roles of membrane proteins? Fluid Mosaic Model Proteins embedded and floating in a sea of phospholipids Hydrophobic region of protein Evidence?. Transport channels and pumps. Links to structural proteins. Receptors - doorbells. Enzymes localized biochemical rxns Hydrophobic region of protein
vidence for Phospholipid (a) (b) 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. Knife Proteins orter & Grendel Langmuir trough ed blood cells had enough lipid to twice cover their surface onclude lipid is a bilayer hydrophilic heads faced queous environment Figure 7.4 Extracellular layer Plasma membrane Cytoplasmic layer Cytoplasmic layer Illustrates: asymmetry of membrane componets External Leaflet Cytoplasmic Leaflet Fluid Mosaic Model predicts: A. Membranes are fluid: lipids & proteins move in the plane of the bilayer. Proteins and lipids are asymmetrically istributed in the bilayers Evidence for protein asymmetry intact permeable I 125 I 125 LP adds I to protein LP can t pass thru intact membrane Lactoperoxidase (LP) + I 125
vidence for lipid asymmetry? ut off head groups off of exposed lipids igested them with phospholipase ntact red blood cells SM, PC Mosaic: Lipids are asymmetrically distributed Extracellular space phosphatidylcholine sphingomyelin roken red blood cells PE, PS SM & PC esults:isolated different types of phospholipids uggesting lipids were distributed differently the inner and out parts of the bilayer SM, sphingomyelin; PC, phosphatidylcholine; PE, phosphatidylcholine; PS phosphatidylserine Cytosol glycolipid cholesterol phosphatidylinositol phosphatidylserine phosphatidylethanolamine Fluid Mosaic Model predicts: The Fluidity of Membranes Phospholipids can move laterally within the bilayer A. Membranes are fluid: lipids & proteins move in the plane of the bilayer. Proteins and lipids are asymmetrically istributed in the bilayers Lateral movement (~10 7 times per second) (a) Movement of phospholipids Flip-flop (~ once per month)
Movement of membrane phospholipids 1. Rotation about long axis Evidence for lipid fluidity: Photobleaching 2. Lateral exchanges 1x10 7 times /sec. moves several µm/sec 3. Flip-flop rare <1 time/wk to 1 time/few hrs flippases vidence for membrane protein fluidity? ell fusion: 1970 D. Frye & M. Edidin mouse Lipids: critical role in maintaining membrane fluidity Saturated fatty acids stack nicely Unsaturated fatty acids more fluid; double bond causes kinks Stacks poorly Shorter chains stack poorly; More movement stiffer More fluid Figure 7.6 human Length & saturation of hydrocarbon tails affect packing & membrane fluidity
Sterols affect membrane fluidity Fluid Viscous Unsaturated hydrocarbon tails with kinks (b) Membrane fluidity gure 7.5 B Saturated hydro- Carbon tails Hopanoid (prokaryotes) Cholesterol (animal) OH- cholesterol At high temperature has a loosening effect At low temperature has a stiffening effect Cholesterol is common in animal cells Paradox: a) fluidity at high temp. b) fluidity at low temp. Cholesterol gure 7.5 (c) Cholesterol within the animal cell membrane
Most organisms regulate membrane fluidity Homeoviscous adaptation Restricting movement of membrane proteins -> Membrane Domains B Fish, plants 0-20ºC Polyunsaturated F.A. Shorter chains Cholesterol Mammals, palm trees 30-37ºC Saturated F.A. Longer chains cholesterol A C (A) Cell cortex (B) Extracellular matrix (C) Cell/cell junctions ethering of membrane proteins to the Extracellular Matrix or he Cytoskeleton Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Summary: Membranes 1. Fluid Mosaic Model: fluid nature & asymmetric distribution of components 2. Components: Lipids phospholipids, sterols, glycolipids Fluidity Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Proteins integral, peripheral, lipid-linked transport, receptors, enzymes, structural support, electron transport, specialized functional domains Microfilaments of cytoskeleton Cholesterol Peripheral protein Integral protein CYTOPLASMIC SIDE Carbohydrates as glycolipids & glycoproteins external glycocalyx