1. Chapter 10: Membrane Structure

2. Membrane Structure Membrane Function
Plasma membrane defines cell and maintains differences btwn cytosol and extracellular environment
Defines individual organelles
Establish ion gradients
Contain proteins act as sensors of external signals allow cell to adapt to changing environment

3. Membrane Structure General Membrane Structure
Thin film of lipid and protein held together by noncovalent interactions
lipid bilayer serves as basic fluid structure; impermeable barrier
Proteins mediate all other functions of membrane

4. The Lipid Bilayer Lipids of Membrane
50% of total membrane mass, 5 x 106/um2
Amphipathic
Self-sealing
Phosphlipids, cholestrol, glycolipids
Phospholipids= most abundant lipid in membrane
Spontaneously aggregate to bury hydrophobic tail

5. The Lipid Bilayer Fluidity of Lipid Bilayer
Rapid lateral diffusion of phospholipids 10-8cm/sec
Change places 107 times/sec
Rarely flip-flop
Fluidity depends on composition (phospholipid and cholesterol) and temperature
Phase transition= the temperature at which there is a change of state from liquid to solid

6. The Lipid Bilayer
Fluidity and length and saturation of FA hydrocarbon chains
Short hydrocarbon chain lengths fluidity
Double bonds fluidity

7. The Lipid Bilayer Fluidity and cholesterol content
Cholesterol fluidity
Provides mechanical stability
1 cholesterol/phospholipid in eucaryotes
No cholesterol in procaryotes; mechanical stabililty imparted by cell wall

8. The Lipid Bilayer
Major Membrane Phospholipids

9. The Lipid Bilayer

10. The Lipid Bilayer Lipid Rafts
Microdomains enriched in sphingolipids, cholesterol and membrane proteins
Long saturated FA chain of sphingolipids = attractive forces that hold adjacent molecules together
Thicker than other parts of bilayer
able to accommodate membrane proteins concentrating for transport or to enable proteins to function together

11. The Lipid Bilayer Membrane asymmetry
Phospholipid distribution: phosphatidylcholine and sphingomyelin confined to outer monolayer
phosphatidylethanolamine and phosphatidylserine are on inner monolayer
Charge
Proteins
Impt to function (ie, apoptosis and translocation of phosphatidylserine)
glycolipids

12. The Lipid Bilayer Glycolipids
Sugar containing lipid molecules w/ most extreme assemmetry in distribution
Found exclusively on noncytoplasmic monolayer
On surface of all plasma membranes
gangliosides= most complex, sialic acid containing oligosaccharides, net negative chg, most abundant in pm of nerve cells
Function- protection, cell recognition, transmission of electrical impulses

13. Membrane Proteins Proteins Carry Out Specific Functions of Membrane
Give ea type of membrane characteristic functional properties
Amts and types of proteins highly variable
By mass proteins represent 50% lipids 50%; lipids small thus 1 protein/ lipid molecules
Associate w/ membrane in various ways depending on function: transmembrane, integral, peripheral

14. Membrane Proteins
Transmembrane Proteins Typically Cross as Alpha Helix
Unique orientation reflection syn and insertion in ER and function
Membrane spanning doamin comprised of nonpolar aa 20-30
Alpha helix is predominate conformation but beta shees form closed beta barrels that can span membrane as well
Can predict membrane spanning regions via hydropathy plot

15. Membrane Proteins Beta Barrels form Transmembrane Channels
Tend to be more rigid
Comprised of 8-22 strands
Some pore forming proteins generating water-filled channels for select hydrophilic solutes
Polar side chains line channel on inside; nonpolar side chains project out interact w/hydrophobic core of lipid bilayer

16. Membrane Proteins Many Membrane Proteins are Glycolslated
Majority of transmembrane proteins in animal cells are glycolsylated
Sugars added in lumen of ER and golgi
Oligosaccharide chains always present on noncytosolic side

17. Membrane Proteins
Membrane Proteins can be Solubilized and Purified in Detergents
Distrupt hydrophobic interactions
Bind to hydrophobic regions of membrane and displace lipid molecules
SDS, triton

18. Membrane Proteins Red Blood Cells Model system for studying membranes
Available in lg numbers
Easy to isolate uncontaminated from other cell types
No nucleus or internal organelles
Can prepare ghosts and in-side-out vesicles

19. Membrane Proteins Ghosts and InSide-Out RBCs Ghosts= empty RBCs
prepared by placing cells in soln of low salt to cause water to move into and lyse cells
can reseal or be studied while leaky
Inside-Out Vesicles

20. Membrane Proteins Use of Sealed and Unsealed RBC ghosts
Demonstrated some proteins extend across lipid bilayer
Enabled sidedness of proteins to be determined
Label intact sealed ghosts and inside-out vesicles w/ water solutble label that cannot penetrate lipid bilayer– perform SDS PAGE
Exposed internal or external surface to proteolytic enzymes that are membrane impremeant (transmembrane protein will be partially digested from both sides
Labeled antibodies

21. Membrane Proteins Studies of RBC Plasma Membrane Proteins by SDS PAGE
15 major protein bands btwn 15, ,000 daltons
3 most prominent comprose more than 65% protein mass; ea arranged differently in membrane
Spectrin
Glycophorin
Band 3

22. Membrane Proteins
Spectrin= Cytoskeletal Protein Assoc. w/ Cytosolic Side of Membrane
Most proteins are peripheral and assoc w/ cytosolic side
Spectrin most abundant
Long, thin, flexible rod 100 nm length, constitutes 25% protein mass
2.5 x 105 copies/cell
Principle component of cytoskeleton underlying RBC, maintains structural integrity and biconcave shape
Heterodimer formed from 2 lg structurally similar subunits that assoc head to head to form 200 nm long tetramer
Tail ends of 4-5 tetramers linked by binidng short actin filaments and other cytoskeletal components forming meshwork
Abnormalities in spectrin results in anemia and spherical shaped RBCs that are fragile
Ankyrin binds spectrin and band 3 to membrane

23. Membrane Proteins
Spectrin

24. Membrane Proteins Glycophorin sm singlepass glycoprotein of 131 aa
Most of mass on external surface of membrane
100 sugar residues on 16 diff side chains which account for 60% protein mass
>90% sialic acid most of negative chg on surface of RBC
1 million molec/cell
Function unknown

25. Membrane Proteins Band 3 Multipass membrane protein
Catalyzes couple transport of anions
930 aa thought to extend across bilayer 12X
Allows HCO3 to cross membrane in exchg for Cl- increasing amt of CO2 delivered to lungs

26. Membrane Proteins Bacteriorhodopsin is a Proton Pump
Halobacterium salinarum “purple membrane”
7 transmembrane helices of 25 aa
Retinal light absorbing chromophore linked to lys side chain
Light excites chromophore causing conf chg which results in transfer of H+ from inside to outside cell
Electrochemical gradient used to syn ATP
Can pump up to 100 H+/sec

27. Membrane Proteins
Membrane Proteins Often Function as Lg Complexes

28. Membrane Proteins Membrane Proteins Diffuse in Plane of
Membrane but Do Not Flip-Flop

29. Membrane Proteins Techniques for Measuring the Lateral
Diffusion of Membrane Proteins

30. Membrane Proteins Confining Proteins and Lipids to Specific
Regions of the Membrane

31. Membrane Proteins Cell surface Coated with Sugar Residues
Glycocalyx= CHO rich region on cell surface
Proteoglycans= long polysaccharide chains linked to protein core
Oligosahharides also linked to proteoglycans
Possbile functions: protection, receptors, electrical conductivity