1. Membranes Structure of Membrane Proteins

2. The Manifold Roles of Membranes The border of the cell/organelle –Barrier to toxic molecules –Helps accumulate & retain nutrients –Carries out energy transduction –Modulate signal transduction –Mediate cell-cell interactions –export of proteins Facilitates cell motion Assists in reproduction

3. Like snowflakes, lipids usually travel in packs Hydrophobic interactions are the key Polar Head Group Hydrophobic Tail air water Monolayers arrange their hydrophobic tails in the air Spontaneously formed lipid structures

4. Micelles bury the nonpolar tails in the center of a spherical structure water Micelles reverse in nonpolar solvents Spontaneously formed lipid structures con’t nonpolar solvent

5. Still more spontaneously formed lipid structures unilamellar vesicles (liposomes) Lipid bilayers: The basis for biological membranes multilamellar vesicles

6. The Fluid Mosaic Model of Singer & Nicholson The phospholipid bilayer is a fluid matrix The bilayer is a two-dimensional solvent Lipids and proteins can undergo rotational and lateral movement Two classes of proteins: –peripheral proteins (extrinsic proteins) –integral proteins (intrinsic proteins)

7. Motion in the bilayer Lipid chains can bend, tilt and rotate Lipids and proteins can migrate ("diffuse") in the bilayer Frye and Edidin proved this (for proteins), using fluorescent-labelled antibodies against membrane proteins (figure  ) Lipid diffusion has been demonstrated as well

8. Membranes are Asymmetric Lateral Asymmetry of Proteins: –Proteins can associate and cluster in the plane of the membrane - they are not uniformly distributed in many cases eg neurons Lateral Asymmetry of Lipids: –Lipids can cluster in the plane of the membrane - they are not uniformly distributed –certain types may cluster around particular proteins in the membrane (a lipid entourage) –Can also induce asymmetry eg with calcium ion treatment 

9. Functions of membrane proteins depends on their orientation within the membrane  Membrane proteins are not tossed into the membrane randomly but have a specific topology eg Glycophorin  Outside Inside Transverse Asymmetry of Membranes

10. phosphatidylcholine phosphatidyethanolamine phosphatidylserine sphingomyelin In most cell membranes, the lipid composition of the outer monolayer differs from that of the inner monolayer: total percentage of phospholipid Transverse asymmetry of membrane lipids

11. Lipids can be moved from one monolayer to the other by flippase proteins Some flippases operate passively and do not require an energy source Other flippases appear to operate actively and require the energy of hydrolysis of ATP How to establish & maintain lipid asymmetry? The role of flippases

12. Membrane Phase Transitions: The "melting" of membrane lipids Below a certain transition temperature, membrane lipids are rigid & tightly packed (Gel Phase) Above the transition temperature, lipids are more flexible and mobile (Liquid crystal phase) The transition temperature is characteristic of the lipids in the membrane Only pure lipid systems give sharp, well-defined transition temperatures Gel Liquid crystal

13. Observing Membrane Phase Transitions by Calorimetry Anti conformation Gauche conformations Gel Liquid crystal Temperature Heat absorbed Transition Temperature

14. Structure of Membrane Proteins Lipid-anchored proteins Integral (intrinsic) proteins Peripheral (extrinsic) proteins

15. Peripheral Membrane Proteins Not strongly bound to the membrane Can be dissociated with mild detergent treatment or with high salt concentrations What holds them there in the first place?

16. Integral Membrane Proteins (IMPs) Strongly imbedded in the bilayer Can only be removed by disrupting the membrane (eg detergents) Often span the membrane (transmembrane) “Inside out” compared to other globular proteins..meaning?

17. Identification of cell type (‘face’) Structural eg adhesion proteins Signalling: eg mediate cell growth & differentiation Pumps & Channels: import & export control Roles of IMPs (will expand on next term & beyond)

18. Bacteriorhodopsin: a classic example of a serpentine IMP Function: a light-driven proton pump Consists of 7 transmembrane helical segments with short loops that interconnent the helices Binds a light-senstive cofactor (retinal) in the hydrophobic core Found in purple patches of Halobacterium halobium

19. Porins: Classic example of a “  -basket” IMP Function: act as selective pores for various small molecules Structure: ~ membrane- spanning 18-strand  barrel forming a hollow cylinder Interior of cylinder lined with hydrophilic residues Found in outer membrane of Gm - bacteria & mitochondria eg maltoporin

20. Lipid-Anchored Proteins Relatively new class of membrane proteins Four types have been found: –Amide-linked myristoyl anchors –Thioester-linked fatty acyl anchors –Thioether-linked prenyl anchors –Glycosyl phosphatidylinositol anchors Critical for proper protein function: location, location, location! Often found on proteins involved in signal transduction In Out

21. Amide-Linked Myristoyl Anchors Anchor is myristic acid (what is the abbreviated name?) Myristic acid forms an amide linkage with the protein at its amino terminus N-terminal residue is always glycine Examples:  subunits of G proteins endothelial nitric oxide synthase signal transduction

22. Ester-linked Acyl Anchors Broad specificity for lipids - myristate, palmitate, stearate, oleate all found Broad specificity for amino acid links - Cys, Ser, Thr all found e.g G-protein-coupled receptors, Transferrin receptor

23. N-myristoylation S-palmitoylation !

24. Thioether-linked Prenyl Anchors Prenylation refers to linking of "isoprene"-based groups Consensus sequence CAAX (C=Cys, A=Aliphatic, X= any) Isoprene groups include farnesyl (15-C) & geranylgeranyl (20 C) groups e.g yeast mating factors, intracellular signalling proteins

25. What’s the Point? Lipid Anchors are Signaling Devices Anchors are transient Reversible anchoring and de- anchoring can control signalling pathways (Similar to phosphorylation/ dephosphorylation, substrate binding/ dissociation, proteolytic cleavage triggers and signals) eg farnesylation

26. Glycosyl Phosphatidylinositol Anchors (GPI anchors) Anchors protein lying outside the cell Always attached to a C-terminal residue Ethanolamine linked to a phosphate linked to an oligosaccharide linked in turn to inositol of phosphatidyl inositol (embedded in the membrane) Examples: surface antigens, adhesion molecules, cell surface hydrolases Protein C CO HNCH 2 CH 2 OPO - O O (Man) 3 Man = mannose GN = glucosamine PI = phosphatidyl inositol GN PI