《医学细胞生物学课件：2 Cell Membrane》由会员分享，可在线阅读，更多相关《医学细胞生物学课件：2 Cell Membrane（112页珍藏版）》请在金锄头文库上搜索。
1、Cell MembraneModel of the cell membraneMembrane structureMembrane skeletonSpecialized structure on cell surfurcecell membrane disease Model of cell membrane 1.A brief history of studies on the structure of the plasmic membraneA. Conception: Plasma membrane(cell membrane), Intracellular membrane, Bio
2、membrane.B. The history of studyOverton(1890s): Lipid nature of PM; J.D.Robertson(1959): The TEM showing:the trilaminar appearance of PM; Unit membrane model; S.J.Singer and G.Nicolson(1972): fluid-mosaic model; K.Simons et al(1997): lipid rafts model; Functional rafts in Cell membranes. Nature 387:
3、569-572 2. Singer and Nicolsons Model of membrane structure: The fluid-mosaic model is the “central dogma” of membrane biology. A.The core lipid bilayer exists in a fluid state, capable of dynamic movement.B.Membrane proteins form a mosaic of particles penetrating the lipid to varying degrees. The F
4、luid Mosaic Model, proposed in 1972 by Singer and Nicolson, had two key features, both implied in its name.Its like a fluidIts like a mosaicIts the Fluid Mosaic Model!fluid mosaic model A membrane is a mosaicA membrane is a mosaicProteins and other molecules are embedded in a Proteins and other mole
5、cules are embedded in a framework of phospholipidsframework of phospholipids A membrane is fluidA membrane is fluidMost protein and phospholipid molecules can move Most protein and phospholipid molecules can move laterallylaterallyThe fluid-mosaic model is The fluid-mosaic model is the “central dogm
6、a” of the “central dogma” of membrane biologymembrane biologyCellular membranes have 4 components:phospholipid bilayertransmembrane proteinsinterior protein networkcell surface markersThe chemical composition of membranesmembrane% protein% lipid% carbohydratemyelin18793human erythrocyte plasma membr
7、ane49438mitochondrial inner membrane79240amoeba plasma membrane54424The comparison of the cell membrane componentsMembrane LipidsMembrane Lipids: The Fluid Part of the Model Phospholipids: Phosphoglyceride and sphingolipids Glycolipids Sterols ( is only found in animals)vMembrane lipids are amphipat
8、hic.vThere are three major classes of lipids.Phospholipid structure consists of-glycerol a 3-carbon polyalcohol acting as a backbone for the phospholipid-2 fatty acids attached to the glycerol-phosphate group attached to the glycerolPhospholipidsThe fatty acids are nonpolar chains of carbon and hydr
9、ogen.-Their nonpolar nature makes them hydrophobic (“water-fearing”).The phosphate group is polar and hydrophilic (“water-loving”).PhospholipidsThe partially hydrophilic, partially hydrophobic phospholipid spontaneously forms a bilayer:-fatty acids are on the inside-phosphate groups are on both surf
10、aces of the bilayerPhospholipidsFigure. Four major phospholipids in mammalian plasma membranes. Note that different head groups are represented by different symbols in this figure and the next. All of the lipid molecules shown are derived from glycerol except for sphingomyelin, which is derived from
11、 serine.phosphoglyceridecardiolipin Phosphatidyl inositol sphingolipidglycolipids are membrane components composed of lipids that are covalently bonded to monosaccharides or polysaccharides. glycolipidFigure. Glycolipid molecules. Galactocerebroside (A) is called a neutral glycolipid because the sug
12、ar that forms its head group is uncharged. A ganglioside (B) always contains one or more negatively charged sialic acid residues (also called N-acetylneuraminic acid, or NANA), whose structure is shown in (C). Whereas in bacteria and plants almost all glycolipids are derived from glycerol, as are mo
13、st phospholipids, in animal cells they are almost always produced from sphingosine, an amino alcohol derived from serine, as is the case for the phospholipid sphingomyelin. Gal = galactose; Glc = glucose, GalNAc = N-acetylgalactos-amine; these three sugars are uncharged. The amount of cholesterol ma
14、y vary with the type of membrane. Plasma membranes have nearly one cholesterol per phospholipid molecule. Other membranes (like those around bacteria) have no cholesterol. They immobilize the first few hydrocarbon groups of the phospholipid molecules. This makes the lipid bilayer less deformable and
15、 decreases its permeability to small water-soluble molecules. Without cholesterol (such as in a bacterium) a cell would need a cell wall. Cholesterol prevents crystallization of hydrocarbons and phase shifts in the membrane. CholesterolFigure. The structure of cholesterol. Cholesterol is represented
16、 by a formula in (A), by a schematic drawing in (B), and as a space-filling model in (C). membranecholesterolphosphotidyl-cholinesphingo-myelinphosphotidyl-serineglycolipidsrat liver plasma membrane3018149-rat liver RER65533-rat liver nuclear membrane105533-rat liver myelin22116712E coli cytoplasmic
17、 membrane00-不同细胞膜磷脂的组成micelles; bilayersA liposome is an artificially-prepared vesicle composed of a lipid bilayer. The liposome can be used as a vehicle for administration of nutrients and pharmaceutical drugs.Liposomes can be prepared by disrupting biological membranes (such as by sonication).Lipo
18、somes are composed of natural phospholipids, and may also contain mixed lipid chains with surfactant properties. A liposome design may employ surface ligands for attaching to unhealthy tissue.liposomeliposome治疗性蛋白，如治疗性蛋白，如TRAILTRAIL，IL-12IL-12脂质体的应用：脂质体的应用：Her2+Her2+Her2-Her2- 激光共聚焦激光共聚焦显微微镜下下观察察P(M
19、DS-co-CES)/ Herceptin和和纳米米颗粒的粒的胞内分布（胞内分布（时间点如点如图所示）：所示）：（A）在）在HER2高表达的高表达的BT474 细胞中分布胞中分布（B）在）在HER2阴性的阴性的HEK293细胞中分布胞中分布 蓝色：胞核色：胞核经DAPI染色染色 红色：色：Alexa Fluor 647-Herceptin 绿色：色：载有有FITC的的 P(MDS-co-CES)Biomaterials, 30(5): 919-927, 2009,Any Questions?membrane proteinStudies Show Proteins In MembraneMem
20、brane Proteins Many FunctionsEach type of protein in a membrane has a special functionAdhesion proteins hold to surface, cellsRecognition proteins recognize “self”Receptor proteins receive messagesEnzymes greatly speed-up reactionsTransport proteins (active and passive) active require energy to tran
21、sport passive no energy required for transportMembrane proteins can be associated with the lipid bilayer in different waysWhy areproteins the perfect molecule to build structures in the cell membrane?Classes of amino acidsWhat do these amino acids have in common?nonpolar & hydrophobicClasses of amin
22、o acidsWhat do these amino acids have in common?polar & hydrophilicI like thepolar onesthe best! Proteins domains anchor moleculeWithin membranenonpolar amino acids hydrophobic anchors protein into membraneOn outer surfaces of membranepolar amino acids hydrophilicextend into extracellular fluid & in
23、to cytosolPolar areasof proteinNonpolar areas of proteinThe polypeptide chains of most transmembrane proteins cross the bilayer in an a-helical conformationA typical transmembrane -helix consists of 20-25 hydrophobic amino acidsFigure. A segment of a transmembrane polypeptide chain crossing the lipi
24、d bilayer as an a helix. Only the a-carbon backbone of the polypeptide chain is shown, with the hydrophobic amino acids in green and yellow. (J. Deisenhofer et al., Nature 318:618-624 and H. Michel et al., EMBO J. 5:1149-1158) Figure. A typical single-pass transmembrane protein. Note that the polype
25、ptide chain traverses the lipid bilayer as a right-handed a helix and that the oligosaccharide chains and disulfide bonds are all on the noncytosolic surface of the membrane. Disulfide bonds do not form between the sulfhydryl groups in the cytoplasmic domain of the protein because the reducing envir
26、onment in the cytosol maintains these groups in their reduced (-SH) form.Glycophorin monomers span the red blood cell membrane with a single transmembrane -helixThe TM a-helices of two glycophorin membrane spanning regions associate as a coiled-coil structure forming a dimerPorins are pore-forming p
27、roteins that span the bilayer as a -barrelRhodobacter porin monomer (a trimer in membrane)16 antiparallel -sheetsHydrophobic side chains exposed to bilayerHydrophilic residues exposed to poreFigure. The three-dimensional structure of a porin trimer of Rhodobacter capsulatus determined by x-ray cryst
28、allography. (A) Each monomer consists of a 16-stranded antiparallel b barrel that forms a transmembrane water-filled channel. (B) The monomers tightly associate to form trimers, which have three separate channels for the diffusion of small solutes through the bacterial outer membrane. A long loop of
29、 polypeptide chain (shown in red), which connects two b strands, protrudes into the lumen of each channel, narrowing it to a cross-section of 0.6 x 1 nm. (Adapted from M.S. Weiss et al., FEBS Lett. 280: 379-382) Intrinsic membrane proteins can pass through the bilayer many timesMammalian glucose sym
30、porterFigure. The three-dimensional structure of a bacteriorhodopsin molecule. The polypeptide chain crosses the lipid bilayer as seven a helices. The location of the chromophore and the probable pathway taken by protons during the light-activated pumping cycle are shown. When activated by a photon,
31、 the chromophore is thought to pass an H+ to the side chain of aspartic acid 85. Subsequently, three other H+ transfers are thought to complete the cyclefrom aspartic acid 85 to the extra-cellular space, from aspartic acid 96 to the chromophore, and from the cytosol to aspartic acid 96. (R. Henderso
32、n et al. J. Mol. Biol.213:899-929) Aquaporin Channel Structure Aquaporin Channel StructureOsmotic swelling of oocytesOther membrane proteins are attached to the bilayer by covalently attached lipidsMyristoylated proteins contain a covalently attached 14-carbon fatty acid at the N-terminus of the pro
33、teinMyristoylation(豆蔻酰化) occurs in initial phases of protein synthesisPrenyl and palmitoyl groups are attached to cysteine residues via a thioether linkagePrenyl groups（异戊二烯基） are unsaturated intermediates of sterol synthesisPalmitic acid(棕榈酸) is a 16 carbon saturated fatty acidThese protein modific
34、ations occur after the protein is synthesizedGlycerophosphatidylinositol（糖基肌醇磷脂） serves as a covalently bound phospholipid anchor for certain cell surface proteinsGPI proteins are found on cell surfaceLipid modification occurs after protein is inserted through ER bilayer糖基化磷脂酰肌醇锚定蛋白质 Other proteins
35、may be anchored to specific sites in a membrane through the cytoskeletonSpectrin tetramerAnkyrin linker蛋白与膜的结合方式蛋白与膜的结合方式 整合蛋白整合蛋白 脂锚定蛋白脂锚定蛋白 外周蛋白外周蛋白Membrane protein purificationSDS; Triton-X100Figure. The use of mild detergents for solubilizing, purifying, and reconstituting functional membrane pr
36、otein systems. In this example functional Na+-K+ ATPase molecules are purified and incorporated into phospholipid vesicles. The Na+-K+ ATPase is an ion pump that is present in the plasma membrane of most animal cells; it uses the energy of ATP hydrolysis to pump Na+ out of the cell and K+ in, as dis
37、cussed in Chapter 11. Any Questions?Characteristics of cell membraneMembranes are fluidityvFluidity of membrane lipid. It give membranes the ability to fuse, form networks, and separate charge;vMotility of membrane protein.Phospholipids can rapidly diffuse along the plane of the membraneNearest neig
38、hbor replacement rate is 10-8/secFlip-flop is a rare process leaflet exchange rate is 6 - 20 h10-8 sec6- 20 hours for flip-flopFlippasesA relatively new discovery! Lipids can be moved from one monolayer to the other by flippase proteins Some flippases operate passively and do not require an energy s
39、ource Other flippases appear to operate actively and require the energy of hydrolysis of ATP Active flippases can generate membrane asymmetriesVan der Waals interactions between fatty acyl chains are the main determinants of acyl chain mobilityBelow the phase transition temperature fatty acyl chains
40、 are in a gel-like (crystalline) stateAbove the phase transition temperature, fatty acyl chains are in rapid motionMembrane Phase TransitionsThe transition temperature is characteristic of the lipids in the membrane.Double bonds reduce the number of potential van der Walls interactions between fatty
41、 acyl chainsCholesterol is an amphipathic steroid that is abundant in plasma membranesCholesterol can pack with phospholipids in a 1:1 ratio The “Fluidity” of a Lipid Bilayer Is Determined by Its CompositionShort chain fatty acyl groups tend to increase lateral mobilityUnsaturated fatty acids tend t
42、o increase fluidityCholesterol and other sterols tend to impede fatty acid mobility (act as a fluidity buffer)Protein fluidityFigure. Experiment demonstrating the mixing of plasma membrane proteins on mouse-human hybrid cells. The mouse and human proteins are initially confined to their own halves o
43、f the newly formed heterocaryon plasma membrane, but they intermix with time. The two antibodies used to visualize the proteins can be distinguished in a fluorescence microscope because fluorescein is green whereas rhodamine is red. (Based on observations of L.D. Frye and M. Edidin, J. Cell Sci. 7:3
44、19-335) Figure. Antibody-induced patching and capping of a cell-surface protein on a white blood cell. The bivalent antibodies cross-link the protein molecules to which they bind. This causes them to cluster into large patches, which are actively swept to the tail end of the cell to form a cap. The
45、centrosome, which governs the head-tail polarity of the cell, is shown in orange. The lateral diffusion of membrane lipids can demonstrated experimentally by a technique called Fluorescence Recovery After Photobleaching (FRAP)（光脱色荧光恢复技术光脱色荧光恢复技术 ）. Figure. Diagram of an epithelial cell showing how a
46、 plasma membrane protein is restricted to a particular domain of the membrane. Protein A (in the apical membrane) and protein B (in the basal and lateral membranes) can diffuse laterally in their own domains but are prevented from entering the other domain, at least partly by the specialized cell ju
47、nction called a tight junction. Lipid molecules in the outer (noncytoplasmic) monolayer of the plasma membrane are likewise unable to diffuse between the two domains; lipids in the inner (cytoplasmic) monolayer, however, are able to do so. Figure. Three domains in the plasma membrane of guinea pig s
48、perm defined with monoclonal antibodies. A guinea pig sperm is shown schematically in (A), while each of the three pairs of micrographs shown in (B), (C), and (D) shows cell-surface immunofluorescence staining with a different monoclonal antibody (on the right) next to a phase-contrast micrograph (o
49、n the left) of the same cell. The antibody shown in (B) labels only the anterior head, that in (C) only the posterior head, whereas that in (D) labels only the tail. (Courtesy of Selena Carroll and Diana Myles.) Figure. Four ways in which the lateral mobility of specific plasma membrane proteins can
50、 be restricted. The proteins can self-assemble into large aggregates (such as bacteriorhodopsin in the purple membrane of Halobacterium) (A); they can be tethered by interactions with assemblies of macromolecules outside (B) or inside (C) the cell; or they can interact with proteins on the surface o
51、f another cell (D). Membranes are AsymmetricLateral Asymmetry of Proteins: Proteins can associate and cluster in the plane of the membrane - they are not uniformly distributed in many cases Lateral Asymmetry of Lipids: Lipids can cluster in the plane of the membrane - they are not uniformly distribu
52、ted Membranes are AsymmetricTransverse asymmetry of proteins Mark Bretscher showed that N-terminus of glycophorin is extracellular whereas C-terminus is intracellular Transverse asymmetry of lipids In most cell membranes, the composition of the outer monolayer is quite different from that of the inn
53、er monolayer Figure. Simplified diagram of the cell coat (glycocalyx). The cell coat is made up of the oligosaccharide side chains of glycolipids and integral membrane glycoproteins and the polysaccharide chains on integral membrane proteoglycans. In addition, adsorbed glycoproteins and adsorbed pro
54、teoglycans (not shown) contribute to the glycocalyx in many cells. Note that all of the carbohydrate is on the noncytoplasmic surface of the membrane. Membrane skeleton膜骨架模式图膜骨架模式图Figure. Spectrin molecules from human red blood cells. The protein is shown schematically in (A) and in electron microgr
55、aphs in (B). Each spectrin heterodimer consists of two antiparallel, loosely intertwined, flexible polypeptide chains called a and b these are attached noncovalently to each other at multiple points, including both ends. The phosphorylated head end, where two dimers associate to form a tetramer, is
56、on the left. Both the a and b chains are composed largely of repeating domains 106 amino acids long. In (B) the spectrin molecules have been shadowed with platinum. (D.W. Speicher and V.T. Marchesi, Nature 311:177-180; B, D.M. Shotton et al., J. Mol. Biol. 131:303-329) Figure. The spectrin-based cyt
57、oskeleton on the cytoplasmic side of the human red blood cell membrane. The structure is shown schematically in (A) and in an electron micrograph in (B). The arrangement shown in (A) has been deduced mainly from studies on the interactions of purified proteins in vitro. Spectrin dimers associate hea
58、d-to-head to form tetramers that are linked together into a netlike meshwork by junctional complexes composed of short actin filaments (containing 13 actin monomers), tropomyosin, which probably determines the length of the actin filaments, band 4.1, and adducin. The cytoskeleton is linked to the me
59、mbrane by the indirect binding of spectrin tetramers to some band 3 proteins via ankyrin molecules, as well as by the binding of band 4.1 proteins to both band 3 and glycophorin (not shown). The electron micrograph in (B) shows the cytoskeleton on the cytoplasmic side of a red blood cell membrane af
60、ter fixation and negative staining. (B, courtesy of T. Byers and D. Branton, PNSA. 82:6153-6157) Specialized structure on cell surfurce糖萼与微绒毛糖萼与微绒毛 Cell membrane disease1.1 Hereditary spherocytosis1.2 cystic fibrosis1.3 LDL receptor deficiency diseaseHereditary spherocytosisHereditary spherocytosisH
61、ereditary spherocytosisHereditary spherocytosiscystic fibrosisProtein Misfolding Diseases(also known as Protein Conformational Diseases)Disorders arising from the failure of a specific peptide or protein to adopt, or remain in, its native structureDiseases in which an impairment in the folding effic
62、iency of a given proteinresults in a reduction of native folded protein.ribosomenascent chainof CFTRribosomefolded,functionalCFTR on thecell membraneend of translation,folding on the ER membrane,translocation to the cell membranenascent chainof CFTRribosomenascent chainof CFTRribosomenascent chainof
63、 CFTRmisfoldedCFTRribosomenascent chainof CFTRmisfoldedCFTRno foldedCFTR on thecell membranecystic fibrosisLDL receptor deficiency diseaseThank you for your attention!Figure. The three-dimensional structure of the photosynthetic reaction center of the bacterium Rhodopseudomonas viridis. The structur
64、e was determined by x-ray diffraction analysis of crystals of this transmembrane protein complex. The complex consists of four subunits, L, M, H, and a cytochrome. The L and M subunits form the core of the reaction center, and each contains five a helices that span the lipid bilayer. The locations of the various electron carrier coenzymes are shown in black. (Adapted from a drawing by J. Richardson based on data from J. Deisenhofer et al., Nature 318:618-624)
