Chapt. 11, Membrane Structure Functions of cell membrane 1 Chapt. 11, Membrane Structure Functions of cell membrane As a container/ barrier to movement of small molecules. Figure 11 2 Chapt. 11, Membrane Structure Functions of cell membrane, cont. Import and export of molecules. Information transducer. Movement. Figure 12. 3 1
The lipid bilayer is composed (in animals) of phospholipids, cholesterol and glycolipids. All are amphipathic. Structure of lipids results in selfassembly into a bilayer. 4 Phospholipid structure. 5 Fig 11.6 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 6 of the fatty tails. 2
Phospholipids selfassemble into a 2 dimensional lipid bilayer. The importance of the hydrophobic interactions 7 Review of the hydrophobic interaction. Panel 22 8 Review of the hydrophobic interaction. (Fig 119 modified) 9 20 highenergy water molecules 13 highenergy water molecules 3
Phospholipids selfassemble into a 2 dimensional lipid bilayer. Review of the hydrophobic interaction. The importance of van der Waals forces. 10 Phospholipids selfassemble into a 2 dimensional lipid bilayer. Review of the hydrophobic interaction. Fig 119 The importance of van der Waals forces. Result: Open or closed membrane sheets (Figs. 1112, 1113, 1114) 11 Figs. 1112, 1113 12 4
Figs. 1112, 1114 13 Lipid bilayers are said to be fluid. What do we mean by fluid? The potential types of phospholipid mobility. Fig 1115. 14 Membrane phospholipid structure and degree of fluidity. What is freezing or crystallization? Effect of temperature on fluidity. Effect on length of hydrocarbon tails on fluidity. Effect of unsaturation on fluidity. 15 5
Structure of Cholesterol. Fig 117b, 1116. 16 The effect of cholesterol on membrane fluidity. (Fig. 1116) 17 Glycolipid structure. Fig 117c 18 6
Terminology. Leaflet 19 Terminology. Leaflet Problems with inside and outside Suggested terminology cytosolic and noncytosolic 20 Fig. 1119 The bilayer is asymmetrical. Phospholipids some phospholipids are mainly in the cytosolic leaflet, others in the noncytosolic one. Fig. 1117 21 7
The bilayer is asymmetrical. Glycolipids found (almost) exclusively on the noncytosolic leaflet. Fig. 1117 22 The bilayer is asymmetrical.. Cholesterol found on both leaflets as it easily flips. 23 Amounts of proteins in membranes. Some functions of proteins. (Table 111) 24 8
Types of membrane proteins; transmembrane Fig 1121 25 Types of membrane proteins; lipid linked Fig 1121. 26 Types of membrane proteins; peripheral (protein attached) Fig 1121. 27 9
A more detailed consideration of transmembrane proteins. Proteins that have buried hydrophobic parts go all the way through the bilayer; there are no halftransmembrane proteins. The portion of the protein that is in the hydrophobic interior always adopts into a alpha helix or beta sheet. Why? (Fig. 1124 and Fig 1015 in Big Alberts.) 28 Fig. 1124 and Fig 1015 in Big Alberts. 29 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 1126, 1127 30 10
Fig. 1126 31 Fig 1126 32 You can isolate proteins, Now what? Sequence. Use to raise antibodies (Panel 46). SDS gel electrophoresis (Panel 45). 33 11
12 34 SDS PAGE (preparing the sample) SDS 35 SDS PAGE (loading the gel) ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 36 SDS PAGE (after 2 hrs.) ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
SDS PAGE (after 2 hrs.) 112,000 daltons 90,000 daltons 66,000 daltons 34,000 daltons ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 15,000 daltons 37 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Membrane proteins are polar and asymmetric. NH 2 COOH 38 How do we know membrane proteins are polar and asymmetric? Vectorial labeling Reagent cannot penetrate membranes Reagent can label proteins. 39 13
Vectorial labeling SDS gel 40 Vectorial labeling 41 Vectorial labeling SDS gel stained for total protein SDS gel vectorial label 42 14
The complete structure is known for relatively few proteins. Difficulties in determining structure. Examples of membrane proteins where the structure is known. Bacteriorhodopsin (Fig. 1128). 43 Fig. 1128 Bacteriorhodopsin 44 The complete structure is known for relatively few proteins. Difficulties in determining structure. Examples of membrane proteins where the structure is known. Bacteriorhodopsin (Fig. 1128). Photosynthetic reaction center of Rhodopseudomonas. (Fig. 1129) 45 15
Fig. 1129 Bacterial photochemical reaction center 46 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 1131. 47 Fig. 1132 48 16
49 How common are RBC proteins? RBC proteins and related proteins in human health. Dystrophin Spherocytosis Muscular Dystrophy 50 http://www.diseasedir.org.uk/genetic/genex01.htm Glycoproteins as well as glycolipids have sugars on the noncytosolic side. Especially true for plasma membranes (where they are present on the outside of the cell), but also on luminal side (=inside = noncytosolic side) of internal compartments. Fig 1132 51 17
Fig. 1132 52 Possible functions of sugars on the plasma membrane. Membrane protection and lubrication Electrical insulation Cell recognition An example: snagging neutrophils to infection sites. Fig. 1133 53 54 18
Another example: the embryo makes selectins that snag carbohydrates on the uterine wall. 55 Fazlebas & Kim, 2003, Science 299:355356 Chapt. 11, Protein mobility Many proteins can diffuse within the lipid bilayer. Why does this make sense? What kind of mobility is possible? No flipflop. Rapid spinning Lateral diffusion 56 Chapt. 11, Protein mobility How can one show lateral mobility? Fig 1134 57 19
Chapt. 11, Protein mobility Fluorescence Recovery after Photobleaching (FRAP): a way to measure rates of lateral diffusion. 58 Chapt. 11, Protein mobility Restrictions of lateral mobility. (Fig. 1135) 59 Chapt. 11, Protein mobility An example where this is very important: tight junctions in the intestinal epithelium. (Fig. 1136) 60 20