Biological Membranes 1MCB 110 - Spring 2008 - NogalesMCB 110: Biological MembranesEva Nogales708C Stanley [email protected] Hours:3-4 PM, Thursdays and FridaysBiological Membranes 2MCB 110 - Spring 2008 - Nogales1 MEMBRANE LIPIDS AND LIPID BILAYERI Intro to Biological MembranesII Membrane LipidsIII Lipid Self-AssemblyIV Membrane Fluidity2 MEMBRANE PROTEINSI Introduction to Membrane ProteinsII Integral Membrane ProteinsIII Peripheral Membrane ProteinsIV Lipid-Anchored Membrane ProteinsV Diffusion of Membrane Proteins3 TRANSPORT ACROSS MEMBRANESI Intro: Permeability of the Cell MembranesII DiffusionIII Facilitated DiffusionIV Active TransportV Membrane Potential and Nerve Impulses4 MEMBRANE TRAFFICKINGI Introduction: Secretory PathwayII Endoplasmic ReticulumIII GolgiIV Vesicle TransportV LisosomesVI Endocytosis5 CELL SIGNALINGI IntroductionII G Protein-coupled ReceptorsIII Receptor Tyrosine KinasesIV Integrin SignalingV Cell Signaling and ApoptosisVI Relationships betweenMCB 110BIOLOGICAL MEMBRANESBiological Membranes 3MCB 110 - Spring 2008 - NogalesSuggested Reading: Lodish, Chapter 5 - 5.1 (Chapter 10.1 in new edition!);Alberts, Chapter 10Biological Membranes 4MCB 110 - Spring 2008 - NogalesBiological Membranes 5MCB 110 - Spring 2008 - NogalesBiological Membranes 6MCB 110 - Spring 2008 - NogalesBiological Membranes 7MCB 110 - Spring 2008 - NogalesBiological Membranes 8MCB 110 - Spring 2008 - NogalesBiological Membranes 9MCB 110 - Spring 2008 - NogalesBAError in original table!!Biological Membranes 10MCB 110 - Spring 2008 - NogalesBiological Membranes 11MCB 110 - Spring 2008 - NogalesBiological Membranes 12MCB 110 - Spring 2008 - NogalesIV Membrane FluidityIV A – Definition and Function•Fluidity is defined as “easy of flow” and is opposed to viscosity (resistance to flow).Lipids bilayer are fluid in the liquid crystal state, but under the melting temperature Tmbecome rigidified: liquid crystal to gel transition.Fluidity of a biological membrane isrequired for movement of membraneproteins, and for processes requiringmembrane fusion such as cell divisionor exocytosisIV B – Effect of Lipid Composition•Very sensitive to the presence of unsaturated fatty chains (one or more double bonds) that make packing difficult and thusdecrease the Tm. In contrast saturated lipids have very highmelting temperatures.•The length of the fatty acid chain also influences Tm.The longer the chain the larger the energy of packing. Thus,shorter chains result in lower melting temperatures.Bacteria control the fluidity of their membrane in changingenvironments by regulating the synthesis of lipids (with shorter of longer chains) and by desaturation.•Exp. – Doubling time larger at the restricted temperature forbacteria with mutant desaturases.Biological Membranes 13MCB 110 - Spring 2008 - NogalesBiological Membranes 14MCB 110 - Spring 2008 - Nogales• Cholesterol h as an important effect inmembrane fluidity. Its ring structurerigidifies the membrane, but it also makestransition to gel phase more difficult byhindering packing of phospholipids. The neteffect is a broadening of the transition.• Eukaryotes control fluidity bychanges the amount of cholesterolpresent in the membrane.Biological Membranes 15MCB 110 - Spring 2008 - NogalesIV D – Lipid Distribution in the leaflets• The lipid composition of the bilayer in the twoleaflets is unequal at the cell membrane, giving thetwo sides very specific properties.Biological Membranes 16MCB 110 - Spring 2008 - NogalesMembrane CurvatureMembrane ThicknessBiological Membranes 17MCB 110 - Spring 2008 - NogalesCholesterol and sphingolipids form rigid microdomains or rafts (~ 70 nm in diameter).These rafts concentrate certain membrane proteins and are important in cell signaling.Biological Membranes 18MCB 110 - Spring 2008 - NogalesIV C - Lipid Mobility: lateral movement and flip-flop Lipids are free to move laterally in a lipid bilayer Flip-flop movement is very energeticallyunfavorable and in the cell is catalyzed by flippases.Biological Membranes 19MCB 110 - Spring 2008 - NogalesMEMBRANE PROTEINSI Introduction• Models of biological membranes• Protein/lipids ratios• Asymmetric orientation in membranes• Types of membrane proteinsII Integral Membrane Proteins• Amphipatic character• Visualization by freeze-fracture• Purification• Structural characterizationIII Peripheral Membrane ProteinsA. Non-covalent association. Extraction methodsB. FunctionsIV Lipid-anchored ProteinsA. Outside leaflet. GPIB. Inside leafletV Diffusion of Membrane ProteinsA. Cell fusion experimentsB. FRAPC. SPTSuggested Reading: Lodish, Chapter 5 - 5.2 (Chapter 10.2 in new edition!)Alberts, Chapter 10Biological Membranes 20MCB 110 - Spring 2008 - NogalesIINTRODUCTIONII A – MODELS OF BIOLOGICALMEMBRANESInitial models of biological membranesincluded only lipids.In the 20’s and 30’s hints fromsolubility and surface tensionexperiments indicated that there wereother components in addition to lipids.An initial model of a b iologicalmembrane that included proteins hadthe proteins lining the lipid bilayer withproteins.In 1972 Singer and Nicholson proposed the fluid mosaic model of the membrane thatwe used today. In this model membranes both link to lipids or across the bilayer movein a sea of fluid lipid.While this model is overall correct is has its limitations.For example, we now know that certain components ofthe cell membrane can be fixed in space by attachmentto the underlying cytoskeleton.proteinsBiological Membranes 21MCB 110 - Spring 2008 - NogalesIC – Types of Membrane ProteinsProteins associated with membranes can be classified into three different types depending onthe type of interaction they have with the bilayer.1 – Integral Membrane Proteins pass through the lipid bilayer2 – Peripheral Proteins associate with the bilayer by non-covalent interactions3 – Lipid-anchored ProteinsBiological Membranes 22MCB 110 - Spring 2008 - NogalesII Integral Membrane ProteinsII A – Proteins that traverse the lipid bilayer have a surface with amphipathic character,in contrast with the hydrophilic nature of the surface of soluble proteins.Hydrophilic residues are present also on the inside of the protein, making hydrogenbonds with ligands or with one another, or forming aqueous channels.Biological Membranes 23MCB 110 - Spring 2008 - NogalesII B – Integral membrane proteins can be directly visualized by