Lecture 3: Membrane Proteins, Asymmetry and the Cytoskeleton

Question 1

What is the evidence for membrane proteins?

Answer 1

Functional - facilitated diffusion, ion gradients, specificity of cell responses
Biochemical - gel electrophoresis, freeze fracture

Question 2

How do membrane proteins move?

Answer 2

• Conformational change
• Rotational
• Lateral
• NO FLIP FLOP

Question 3

How can membrane protein movement be restricted?

Answer 3

• Membrane protein associations within the membrane
• Association with extra-membranous proteins like the cytoskeleton
• Lipid mediated effects
• Proteins tend to separate out into the fluid phase or cholesterol poor regions

Question 4

How do membrane proteins associate with the lipid belayer?

Answer 4

Peripheral

• Bound to surface
• Electrostatic and hydrogen bonds interactions
• Removed by changes in pH or ionic strength

Integral

• Interact extensively with hydrophobic domains of the lipid belayer
• Cannot be removed by manipulation of pH and ionic strength
• Removed by agents that compete for non-polar interactions (detergents and organic solvents)

Question 5

How may membrane proteins contribute to the cytoskeleton?

Answer 5

Spectrin dimers (peripheral proteins) joined end to end to form the lattice structure of cytoskeleton 
Lattice held in place by attachment proteins which are then attached to transmembrane (integral) protein anchors

Question 6

How are membrane proteins inserted into membranes?

Answer 6

• N-terminal signal sequence folds into the membrane, positioning positively charged residues on the cytoplasmic side
• signal peptidase cleaves the signal sequence
• N-terminal extrudes into ER lumen until hydrophobic stop transfer sequence synthesised
• Ribosome detaches rom ER and completes protein synthesis in cytoplasm

Question 7

What is the difference between membrane proteins and lipids?

Answer 7

Proteins can adopt discrete conformational changes following specific molecular or bio electric signals which underpins function

Question 8

What is a key structural feature of proteins that enables function?

Answer 8

Flexible and elastic

Question 9

What are lipid anchored proteins?

Answer 9

• On external cell face: GPI linked proteins where the protein is linked via an oligosaccharide chain attached to a phospholipid
• On internal cell face: cytoplasmic proteins linked by single fatty acid within the inner membrane

Question 10

What do lipid rafts have to do with proteins?

Answer 10

Lipid rafts host specific proteins, positioning them to optimize their function as it limits ‘random drift’ of proteins in the bilayer

Question 11

What are the restrictions on protein movement in the membrane exemplified by the integral and peripheral protein interaction with cytoskeleton elements as exemplified by RBCs?

Answer 11

• Ankyrin: anchors spectrum to an anchor point (integral proteins like transporter, ion channels and adhesion proteins) in the membrane
• Band 4.1 (peripheral): anchors at the junctions of spectrin to glycophorin (integral membrane protein)

Question 12

How does the cytoskeleton impart immense structural flexibility to the RBC as an example cell?

Answer 12

• Exponentially increases the range of forces and distributes any localized nanometer point forces throughout the network
• Weaker electrostatic interactions of cytoskeleton proteins allow more flexibility and elasticity for plasma membrane to spring back to normal (spectrin is flexible and its helical nature makes it ‘stretchy’j
• When small forces act upon cell membrane, the local electrostatic bonds between proteins give way, allowing shifting of the cytoskeleton, it will just jiggle about the anchor points
• When large forces are applied, it can spring back to normal due to the positional sum of electrostatic forces acting around the network