Chapt. 11, Membrane Structure - SUNY Geneseolewisj/Cell.04/Lectures/Chapt11...1 1 Chapt. 11, Membrane Structure •Functions of cell membrane 2 Chapt. 11, Membrane Structure •Functions

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Chapt. 11, Membrane Structure

• Functions of cell membrane

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• Functions of cell membrane– As a container/ barrier to movement of

small molecules. Figure 1-1

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• Functions of cell membrane, cont.– Import and export of molecules.– Information transducer.– Movement. Figure 1-2.


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• The lipid bilayer is composed (in animals) ofphospholipids, cholesterol and glycolipids.

• All are amphipathic.

• Structure of lipids results in self-assemblyinto a bilayer.

Chapt. 11, Membrane Structure, Lipids

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• Phospholipid structure.


Fig 11.6

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• Phospholipid structure.– Phospholipids make up the bulk of the membrane

bilayer.– The four major components of a phospholipid.– Hydrophobic and hydrophilic components of a

phospholipid.– There are several types of organic headgroups.– There is usually one or more double bonds in one

of the fatty tails.


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• Phospholipids self-assemble into a 2dimensional lipid bilayer.– The importance of the hydrophobic interactions


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– Review of the hydrophobic interaction. Panel 2-2Chapt. 11, Membrane Structure, Lipids

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– Review of the hydrophobic interaction. (Fig 11-9modified)


20 high-energy watermolecules

13 high-energy watermolecules 10

• Phospholipids self-assemble into a 2dimensional lipid bilayer.– Review of the hydrophobic interaction.– The importance of van der Waals forces.


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• Phospholipids self-assemble into a 2dimensional lipid bilayer.– Review of the hydrophobic interaction. Fig 11-9– The importance of van der Waals forces.– Result: Open or closed membrane sheets (Figs.

11-12, 11-13, 11-14)


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Figs. 11-12, 11-13

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Figs. 11-12, 11-14

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• Lipid bilayers are said to be fluid.– What do we mean by “fluid”?– The potential types of phospholipid mobility. Fig

11-15.


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• Membrane phospholipid structure and degreeof fluidity.– What is “freezing” or “crystallization”?– Effect of temperature on fluidity.– Effect on length of hydrocarbon tails on fluidity.– Effect of unsaturation on fluidity.


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• Structure of Cholesterol. Fig 11-7b, 11-16.


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• The effect of cholesterol on membranefluidity. (Fig. 11-16)


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• Glycolipid structure. Fig 11-7c


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• Terminology.– “Leaflet”


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• Terminology.– “Leaflet”– Problems with

“inside” and “outside”– Suggested

terminology“cytosolic” and “non-cytosolic”


Fig. 11-19

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• The bilayer is asymmetrical.– Phospholipids - some phospholipids are mainly in

the cytosolic leaflet, others in the non-cytosolicone. Fig. 11-17


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• The bilayer is asymmetrical.– Glycolipids - found (almost) exclusively on the

non-cytosolic leaflet. Fig. 11-17


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• The bilayer is asymmetrical..– Cholesterol -- found on both leaflets as it easily

flips.


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• Amounts of proteins in membranes.• Some functions of proteins. (Table 11-1)

Chapt. 11, Membrane Structure, Proteins

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• Types of membrane proteins; transmembraneFig 11-21


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• Types of membrane proteins; lipid linked Fig11-21.


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• Types of membrane proteins; peripheral(protein attached) Fig 11-21.


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• A more detailed consideration oftransmembrane proteins.– Proteins that have buried hydrophobic parts go

all the way through the bilayer; there are nohalf-transmembrane proteins.

– The portion of the protein that is in thehydrophobic interior always adopts into a alphahelix or beta sheet. Why? (Fig. 11-24 and Fig10-15 in Big Alberts.)


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Fig. 11-24 and Fig 10-15 in Big Alberts.

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• Difficulties in studying membrane proteins.– Hard to physically separate protein of interest

from other proteins.– Even if you could, hydrophobic portions of the

proteins would aggregate.

• The solution: detergents. Fig 11-26, 11-27


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Fig. 11-26

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Fig 11-26

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• You can isolate proteins, Now what?– Sequence.– Use to raise antibodies (Panel 4-6).– SDS gel electrophoresis (Panel 4-5).


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SDS PAGE (preparing the sample)

SDS

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SDS PAGE (loading the gel)

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SDS PAGE (after 2 hrs.)

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SDS PAGE (after 2 hrs.)

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112,000 daltons90,000 daltons

66,000 daltons

34,000 daltons

15,000 daltons

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• Membrane proteins are polar andasymmetric.


NH2

COOH

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• How do we know membrane proteins are polarand asymmetric?

• Vectorial labeling– Reagent cannot penetrate membranes– Reagent can label proteins.


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Vectorial labeling

SDS gel

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Vectorial labeling

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Vectorial labeling

SDS gelstained for total protein

SDS gelvectoriallabel

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• The complete structure is known forrelatively few proteins.– Difficulties in determining structure.– Examples of membrane proteins where the

structure is known.• Bacteriorhodopsin (Fig. 11-28).


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Fig. 11-28 Bacteriorhodopsin

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• The complete structure is known forrelatively few proteins.– Difficulties in determining structure.– Examples of membrane proteins where the

structure is known.• Bacteriorhodopsin (Fig. 11-28).• Photosynthetic reaction center of Rhodopseudomonas.

(Fig. 11-29)


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Fig. 11-29

Bacterialphoto-chemicalreactioncenter

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• The red blood cell and the plasma membrane.– Why study the R !BC?– Kinds of proteins present

• A multipass membrane protein (band 3)• A single pass membrane protein (glycophorin)• Numerous peripheral proteins including:

– Spectrin– Actin

• A summary Fig 11-31.


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Fig. 11-32

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• How common are RBC proteins?

• RBC proteins and related proteins in humanhealth.– Spherocytosis– Muscular Dystrophy


http://www.diseasedir.org.uk/genetic/genex01.htm

Dystrophin

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• Glycoproteins as well as glycolipids havesugars on the non-cytosolic side.– Especially true for plasma membranes (where

they are present on the outside of the cell), butalso on luminal side (=inside = non-cytosolic side)of internal compartments.

– Fig 11-32


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Fig. 11-32

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• Possible functions of sugars on the plasmamembrane.– Membrane protection and lubrication– Electrical insulation– Cell recognition

• An example: snagging neutrophils to infection sites.Fig. 11-33


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55Fazlebas & Kim, 2003, Science 299:355-356

• Another example: the embryo makes selectins that snag carbohydrates on the uterine wall.

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• Many proteins can diffuse within the lipidbilayer.– Why does this make sense?– What kind of mobility is possible?

• No flip-flop.• Rapid spinning• Lateral diffusion

Chapt. 11, Protein mobility

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• How can one show lateral mobility? Fig 11-34


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• Fluorescence Recovery after Photobleaching(FRAP): a way to measure rates of lateraldiffusion.


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• Restrictions of lateral mobility. (Fig. 11-35)


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• An example where this is very important:tight junctions in the intestinal epithelium.(Fig. 11-36)


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Chapt. 11, Membrane Structure - SUNY Geneseolewisj/Cell.04/Lectures/Chapt11...1 1 Chapt. 11, Membrane Structure •Functions of cell membrane 2 Chapt. 11, Membrane Structure •Functions