ICPP 3 - Membrane Proteins

Question 1

What is the functional and biochemical evidence that membrane proteins exist?

Answer 1

Functional = The fact that membranes can allow facilitated diffusion to occur and ion gradients to become established.

Biochemical = SDS-page of a membrane reveals banding patterns of multiple easily identify-able proteins.

• Alternatively can use freeze fracture, where cell is frozen within a crystal and fractured with a knife.

Question 2

Which modes of mobility are shown by proteins in lipid bilayers?
Which mode of mobility cannot occur?

Answer 2

• Conformational change, rotation, lateral diffusion.

- No flip-flop, due to the amount of hydrophilic structure exposed to water (too large to flip across membrane).

Question 3

What 3 things restrict the mobility of membrane proteins?

Answer 3

1) Aggregation - proteins can form aggregates which restrict them
2) Tethering - proteins can adhere to molecules in substratum also restricting mobility
3) Interaction - proteins can interact with other cells to form tissue, again restricting mobility

Question 4

What 2 categories can membrane proteins be split into? Describe the features of each category

Answer 4

1) Peripheral

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

2) Integral

• Interact with hydrophobic domains of lipid bilayer
• Cannot be removed by changing pH or ionic strength
• Removed by agents competing for non-polar interactions, e.g.: organic solvents + detergents.

Question 5

What other factors restrict membrane protein mobility

Answer 5

• Membrane protein associations
• Lipid mediated effects
• Proteins tend to separate out into the fluid phase or cholesterol poor regions.

Question 6

How do membrane proteins contribute to the cytoskeleton?

Answer 6

• The erythrocyte cytoskeleton is composed of spectrin diamonds joined at the ends to form the lattice structure of the cytoskeleton.
• Transmembrane proteins anchors spectrin lattice structure of the cytoskeleton through attachment proteins, which hold the lattice against the membrane.

Question 7

Describe how the hereditary spherocytosis and hereditary elliptocytosis forms of haemolytic anaemias arise.

Answer 7

1) Hereditary spherocytosis = Spectrin depleted by 40-50% by spectrin allele mutations. Lattice structure doesn’t form and erythrocytes become spherical. Leads to lysis and decrease in oxygen carrying capability.
2) Hereditary elliptocytosis = Defect in spectrin molecule leads to fragile elliptoid shape cells - effect is the same, cells lyse and Hb released, leading to aneamia.