Lecture 12: Membrane Targeting Protein

Question 1

What are type 1 proteins ?

Answer 1

Have a single transmembrane span. The N-terminal is in the extracellular domain and the c-terminal is in the intracellular domain. Have a cleavable n-terminal leader sequence

Question 2

What are type 2 proteins ?

Answer 2

Lack a leader sequence. C terminal is now extracellular and the n-terminal is cytoplasmic. Have a single transmembrane span

Question 3

What is the type 3 protein ?

Answer 3

Single transmembrane span. The n -terminal is extracellular and the c-terminal is on the cytoplasmic side. No n-terminal cleavable sequence

Question 4

What is a type 4 protein ?

Answer 4

Multi span, can have terminals in both cytoplasms on both extracellular

Question 5

What are type 5 and 6 protiens ?

Answer 5

Have in addition to or covalent modification with lipid. Type 6 is anchored to membrane by lipid

Question 6

What are the other types of protein ?

Answer 6

1. Beta barrel
2. Proteins that only associate with membrane
3. Proteins that are soluble

Question 7

What do spanning regions look like ?

Answer 7

Consist of 20-25 hydrophobic amino acid residues that form a right hand alpha helix that crosses the bilayer

Question 8

Where are all nuclear encoded proteins synthesised ?

Answer 8

In cytoplasm

Question 9

Where are membrane proteins directed for translocation ?

Answer 9

Endoplasmic reticulum

Question 10

Every protein has its own signal, what is the only common feature ?

Answer 10

A central segment of 6-20 hydrophobic residues

Question 11

Where does translation of mRNA begin ?

Answer 11

Ribosome

Question 12

How does a signal sequence direct a protein to ER for translocation through membrane ?

Answer 12

1. Translation
2. Signal sequence emerges and is recognised by SRP
3. SRP biinds to signal sequence enabling interaction with ER membrane

Question 13

What does docking require ?

Answer 13

GTP binding

Question 14

What do interactions betwen the signal sequence and sec61alpha lead to ?

Answer 14

Stabilising interactions of ribosome and translocon and leads to insertion of the signal sequence with the translocon

Question 15

What are transmembrane domains recognised by ?

Answer 15

Translocon because of their hydrophobic nature

Question 16

How do transmembrane domains exit the translocon ?

Answer 16

By moving laterally through a lipid protein interface and become integrated into the bilaer

Question 17

What is a frequent feature of stop transfer ?

Answer 17

Presence of basic residues immediately proxiaml to the cytoplasmic side of the sequence

Question 18

What are signal sequences used for ?

Answer 18

Targeting to and translocation across the membranes of other organelles

Question 19

lipid membrane bilayers are required by cells to prevent what ?

Answer 19

1. Loss of molecules by simple diffusion that a cell wishes to retain
2. Entry of molecules by simple diffusion that a cell wishes to exclude

Question 20

Why are membrane transport systems required by cells ?

Answer 20

Hydrophobic barrier properties of lipid bilayers prevent the free diffusion of all charged and hydrophilic molecuels that a cell wishes to either to take up or to discard

Question 21

What are the characteristics of carrier mediated transport ?

Answer 21

1. Saturable
2. Specific
3. Competition
4. Temperature
5. Mode

Question 22

What is valinomycin ?

Answer 22

An ionophore produced by bacteria that specifically allows movement of potassium

Question 23

What are the types of active transport ?

Answer 23

1. Coupling via ion gradients
2. ATP coupled
3. Group transfer