Membrane proteins

Question 1

Describe the process of fluorescence recovery after photobleaching (FRAP)

Answer 1

-cells labelled with a fluorescent reagent that binds to specific membrane lipid/protein
-laser light focused on a small area of surface
-irreversibly bleaches the bound reagent, reducing fluorescence
-fluorescence of bleached patch increases as unbleached fluorescent surface molecules diffuse in + bleached ones diffuse out

Question 2

What can FRAP be used to see?

Answer 2

Rapid lateral movement of proteins

Question 3

Why is the asymmetry of proteins in the membrane preserved?

Answer 3

Proteins don’t rotate from 1 side to the other of the membrane

Question 4

What are the different types of membrane proteins?

Answer 4

-integral membrane proteins (intrinsic)
-peripheral or membrane-associated proteins (extrinsic)
-lipid-anchored membrane proteins

Question 5

Describe integral membrane proteins

Answer 5

-all or part fully embedded in the membrane
-often transmembrane - right the way across
-residues interact with hydrophobic interior
-often extra domains in aqueous phase

Question 5

How do integral membrane get right the way across the membrane?

Answer 5

-use alpha helices with high percentage of hydrophobic residues
-can also use special beta sheet arrangements that wrap around to form beta barrels

Question 6

How are integral membrane proteins released from the membrane?

Answer 6

Using detergent

Question 7

In what phase do lipid-anchored membrane proteins remain in?

Answer 7

Protein polypeptide remains in aqueous phase

Question 7

Describe peripheral proteins

Answer 7

-interact with membrane via lipid polar head groups or integral membrane proteins
-readily dissociated from membranes

Question 8

What can lipid-anchored membrane proteins be anchored to?

Answer 8

-fatty acid anchored
-isoprenoid anchored
-glycosylphosphatidyl-inositol (GPI) anchored

Question 9

What do large, polar or charged molecules require to move across the membrane?

Answer 9

Integral membrane proteins

Question 10

What are 2 main classes of membrane transfer proteins?

Answer 10

-channels/pores
-transporters

Question 11

Describe channels/pores

Answer 11

-central passage for ions or molecules (solutes)
-diffusion occurs both ways - depending on conc. grad

Question 12

What are the 2 types of transporters?

Answer 12

-active transporters
-passive transporters

Question 13

Describe passive transporters

Answer 13

-specifically bind solutes
-facilitate diffusion faster than would normally occur
-transport in direction of conc. grad

Question 14

Describe active transporters

Answer 14

-transport against conc. grad
-requires an energy source

Question 15

What does uniport, symport and antiport transport mean?

Answer 15

-uniport - a single solute
-symport - 2 solutes in the same direction
-antiport - 2 solutes in opposite directions

Question 16

What is primary active transport?

Answer 16

Transport is directly coupled to an energy source

Question 17

What is secondary active transport?

Answer 17

Transport is coupled to an ion concentration gradient