Plasma Membrane Cytoskeleton Interactions & Membrane Asymmetry

Question 1

Functions Of Membrane Proteins

Answer 1

• Integral proteins
  
  • Enzymes (lipase, protease)
  • Carriers
  • Channels
  • Receptors (GPCR)
  • Cell-cell recognition
• Peripheral proteins
  
  • Cell shape determination (eg cytoskelton)
  • Communication with internal and external environment (eg glycoproteins and phosphatidylinositol linked proteins) (Ras signalling protein)
  • Intracellular transport

Question 2

Red Blood Cells Are A Good Model For Studying Membranes And Membrane Proteins

Answer 2

• RBCs are predominantly plasma membrane
• RBC membranes have high tensile strength - need to withstand high forces to maintain ionic environment

Hypotonic (RBC ghost) (low salt-lyse) - Isotonic - Hypertonic (high salt-shrivel)

• RBC plasma membrane is anchored to the cytoskeleton

Question 3

Purifying Cytoskeleton Of RBCs

Answer 3

• Take RBC ghosts & solubilise in detergent (SDS)
• Proteins can then be analysed byb SDS polyacrylamide gel electrophoresis
  
  • allows proteins to be seperated based on size
• Few proteins in RBC membrane

Question 4

RBC Cytoskeleton Proteins

Answer 4

• Peripheral proteins:
  
  • Spectrin
  • Actin (in complex with tropomyosin, band 4.1, adducin)
  • Ankyrin
• Transmembrane (integral) proteins
  
  • Glycophorin
  • Band 3

Question 5

RBC Cytoskeleton Mesh Structure

Answer 5

• Biconcave-disc shape of RBCs is maintained by a mesh of proteins lining inner surface of membrane
• Spectrin (dimers) attached to glycophorin by junctional complex (incl. actin)

Question 6

Relative Mobilities Of Membrane Components

Answer 6

• Phospholipids ~2x10^-4 cm/sec
• Proteins ~10^-4 - 10^-6 cm/sec
• Mobility of band 3 and glycophorin are restrained by cytoskelton of RBCs - tethered by mesh

Question 7

Tight Junctions

Answer 7

• tight junctions prevent movement between apical and basolateral membranes

Question 8

Restraints On Movement Of Proteins In Membrane

Answer 8

• Physical structures eg tight junctions
• Indirect interactions with cytoskeleton eg band 3 & glycophorin

Question 9

Restraints On Movement Of Lipids In Membrane

Answer 9

Phospholipids

• Sequestered by binding to specific proteins
• Segregated into domains - lipid rafts
• Physical barriers eg tight junctions ( only outer leaflet of membrane)

Question 10

Membranes are asymmetric

Answer 10

• Proteins always have same orientation in membrane
• Lipid compostion of 2 bilayers is different

Question 11

The glycolax

Answer 11

• The cell coat
• The sugar residues associated with the transmembrane and peripheral proteins at the extracellular surface of the plasma membrane

Question 12

Why Are Membranes Asymmetric?

Proteins

Answer 12

• Membrane bound enzymes are orientated to take up substrates from one side of the membrane & deliver products to other side
• Transport molecules only work in one direction
• Receptors are orientated so they can bind extracellular ligands

Question 13

Why Are Membranes Asymmetric?

Phospholipids

Answer 13

• Asymmetry results in different charge distribution across membrane - usually inner surface -ve ( important for maintaining electrochemical grdt)
• Can result in 2 leaflets of bilayer having different fluidity
• Certain proteins involved in signalling specifically recognise lipid head groups found in cytosolic monolayer of lipid bilayer
• Regulated transfer of some phospholipids from one leaflet to the other can have important functional effects

1. Glycolipids are in extracellular leaflet - glycoproteins outside
2. -vely charged phospholipids (phosphatidylserine (signalling)) mainly in cytosolic leaflet

Question 14

How Is Lipid Asymmetry Achieved/ Maintained?

Answer 14

• Eqm causes asymmetry

• Scramblase enzyme: equilibriates lipids (randomly)
• Energy-dependent flippase ensures membrane asymmetry is maintained

Question 15

Transfer Of Phosphatidylserine (signalling) In Membrane

Answer 15

• The aminophospholipid translocase transfers phosphtidylerine (&phosphotidylethanolamine) from outer to inner leaflet of plasma membrane (ATP dependent)
• Scramblase abolishes asymmetry
• Net eqm favours translocase under normal conditions

Question 16

Importance Of Phospholipid Asymmetry

Answer 16

• Coagulation (Clot formation)
  
  • Phosphotidylserine on platelets & other cell membranes provides the nucleation site for the coagulation cascade
  • In response to tissue damage translocase activity is inhibited resulting in phophatidylserine in extracellular leaflet
  • Scotts Disease (bleeding disorder)- due to deficiency of scrambalase, phosphatidylserine isn’t moved sufficientlyt quickly to extracellular leaflet - results in inhibition of coagulation
• Cell Recognition & Apoptosis
  
  • Macrophages remove dead cells by phagocytosis
  • Macrophage plasma membrane contains receptors, which recognise aminophospholipids ( phosphatidylserine/ phosphatidylethanolamine)
  • Old RBCs, platelets & apoptotic cells all express aminophospholipids in outer leaflet of plasma membrane
  • Aminophospholipids act as signal for phagocytosis