Plasma Membrane Cytoskeleton Interactions & Membrane Asymmetry Flashcards
1
Q
Functions Of Membrane Proteins
A
- 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
2
Q
Red Blood Cells Are A Good Model For Studying Membranes And Membrane Proteins
A
- 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
3
Q
Purifying Cytoskeleton Of RBCs
A
- 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
4
Q
RBC Cytoskeleton Proteins
A
- Peripheral proteins:
- Spectrin
- Actin (in complex with tropomyosin, band 4.1, adducin)
- Ankyrin
- Transmembrane (integral) proteins
- Glycophorin
- Band 3
5
Q
RBC Cytoskeleton Mesh Structure
A
- 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)
6
Q
Relative Mobilities Of Membrane Components
A
- 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
7
Q
Tight Junctions
A
- tight junctions prevent movement between apical and basolateral membranes
8
Q
Restraints On Movement Of Proteins In Membrane
A
- Physical structures eg tight junctions
- Indirect interactions with cytoskeleton eg band 3 & glycophorin
9
Q
Restraints On Movement Of Lipids In Membrane
A
Phospholipids
- Sequestered by binding to specific proteins
- Segregated into domains - lipid rafts
- Physical barriers eg tight junctions ( only outer leaflet of membrane)
10
Q
Membranes are asymmetric
A
- Proteins always have same orientation in membrane
- Lipid compostion of 2 bilayers is different
11
Q
The glycolax
A
- The cell coat
- The sugar residues associated with the transmembrane and peripheral proteins at the extracellular surface of the plasma membrane
12
Q
Why Are Membranes Asymmetric?
Proteins
A
- 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
13
Q
Why Are Membranes Asymmetric?
Phospholipids
A
- 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
- Glycolipids are in extracellular leaflet - glycoproteins outside
- -vely charged phospholipids (phosphatidylserine (signalling)) mainly in cytosolic leaflet
14
Q
How Is Lipid Asymmetry Achieved/ Maintained?
A
- Eqm causes asymmetry
- Scramblase enzyme: equilibriates lipids (randomly)
- Energy-dependent flippase ensures membrane asymmetry is maintained
15
Q
Transfer Of Phosphatidylserine (signalling) In Membrane
A
- 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
