Types of Membrane Proteins

Question 1

What are the 3 types of membrane proteins?

Answer 1

1. Integran (intrinsic)
2. Peripheral (extrinsic)
3. Lipid-linked

Question 2

How do intrinsic proteins interact with the membrane?

Answer 2

Interact with core of membrane - are tightly bound through interactions with fatty acid chains in the hydrophobic core of the bilayer

Question 3

Can intrinsic proteins be removed?

Answer 3

Not without breaking the membrane. Can only be extracted (solubilised) by using agents that disrupt the membrane structure e.g. organic solvents (chloroform) or detergents

Question 4

How do intrinsic proteins span the membrane? How are they distributed?

Answer 4

Span the membrane with single or multiple transmembrane segments

Are asymmetrically distributed across the bilayer

Question 5

Describe Glycophorin A

Answer 5

• Is a glycoprotein
• Single transmembrane domain
• Sequence passing through membrane is roughly 20 hydrophobic amino acids

Question 6

Describe extracellular domain of Glycophorin A

Answer 6

Extracellular domain is hydrophilic and glycosylated (extra carbohydrates are added)

Question 7

Where is Glycophorin A found and why?

Answer 7

In RBCs - enables the to slip through vasculature easily

Question 8

What type of protein is Glycophorin A?

Answer 8

Intrinsic

Question 9

What type of protein is Bacteriorhodopsin?

Answer 9

Intrinsic

Question 10

Describe Bacteriorhodopsin

Answer 10

• 7 transmembrane helices alpha embedded in hydrophobic interior of bilayer
• Bundle of 7 TM helices creates a pore through the membrane
• 20-30 amino acids in an alpha helix to transverse the membrane

Question 11

How are each of the 7 TM helices linked in Bacteriorhodopsin?

Answer 11

By a short hydrophilic region of the polypeptide chain that is exposed on the extracellular cytosolic side of the membrane

Question 12

What does the bundle of 7 TM helices through membrane create Bacteriorhodopsin?

Answer 12

Creates hydrophilic core channel - allows transportation of hydrogen ions through

Question 13

How do lipid-linked proteins interact with the membrane?

Answer 13

Proteins are covalently linked to a lipid which is inserted in the membrane (e.g. covalently attached to fatty acids, have GPI anchor)

Question 14

How do peripheral proteins interact with the membrane?

Answer 14

Don’t interact with the hydrophobic core of the membrane

Simply interact with hydrophilic heads and sit outside membrane

Question 15

Where can peripheral proteins be found?

Answer 15

On the outer or inner surface of the bilayer

Question 16

Can peripheral proteins be removed from the bilayer?

Answer 16

Yes can be separated without damaging the membrane as they are less tightly bound

Question 17

How do peripheral proteins interact with lipid head groups or with other membrane proteins? How can these interactions be disrupted?

Answer 17

Through ionic interactions

By washing the membrane with high salt solution (high ionic strength). This washes the peripheral membrane proteins off the bilayer in soluble form

Question 18

What class of proteins do cytoskeletal proteins belong to?

Answer 18

Peripheral proteins

Question 19

What are spectrin and actin examples of?

Answer 19

Cytoskeletal proteins

Question 20

What do cytoskeletal proteins form?

Answer 20

Form a scaffold on the cytosolic side of the plasma membrane - this network is attached to the cytosolic domains of the integral membrane proteins

Question 21

What is job of spectrin?

Answer 21

Forms long filaments

Question 22

What is job of actin?

Answer 22

Joins spectrin filaments

Question 23

What is importance of cytoskeleton in RBCs?

Answer 23

Maintaining shape and rigidity of cell and restricting lateral motion of integral membrane proteins

Question 24

What happens if RBC peripheral proteins are removed?

Answer 24

Cytoskeleton affected

• ‘Ghost’ as lose rigid shape
• Membrane proteins become laterally mobile

Question 25

What are 2 hereditary diseases that affected cytoskeleton?

Answer 25

Hereditary spherocytosis and elliptocytosis

Question 26

What is result of these hereditary diseases?

Answer 26

Mutations in genes encoding spectrin or ankyrin which results in abnormally shaped erythrocytes

Erythrocytes are degraded more rapidly by spleen –> ANAEMIA