Lecture 2: Nuclear import

Question 1

How did membrane enclosed organelles arise from the asgard archeon?

Answer 1

1. Internal membranes surround the archael DNA and aerobic bacterium is engulfed
2. Endosymbiont sheds encasing plasma membrane and escapes into cytosol
3. Internal membranes pinch off to form nucleus and endomembrane system

Question 2

How do proteins reach their target area?

Answer 2

They are translated in cytosol and cross organelle membranes to reach their place of function

Question 3

What is the signal hypothesis?

Answer 3

Idea that proteins have signals needed to target proteins to organelles

Question 4

What is the role of cytosol in protein targeting?

Answer 4

Default destination
When there’s no signal the protein stays in the cytosol (70%)

Question 5

What are the 4 rules of protein targeting>

Answer 5

1. Signal targets the protein to the right place
2. Receptors recognise the signal
3. Transport machinery lets proteins cross the membrane
4. Energy ATP is needed

Question 6

What are signal sequences?

Answer 6

Polypeptide sequences made of specific amino acids
Might be removed when protein is delivered

Question 7

What is the nuclear envelope made up of?

Answer 7

Outer nuclear membrane
Inner nuclear membrane
Nuclear lamina

Question 8

What is the outer nuclear membrane?

Answer 8

Continuous with the ER and has similar composition

Question 9

What is the inner nuclear membrane?

Answer 9

Has proteins that bind to chromosomes and lamina

Question 10

What is the nuclear lamina?

Answer 10

Protein network made of lamins

Question 11

What are nuclear pores?

Answer 11

Gateway to nucleus
Small water soluble molecules can diffuse through
Folded proteins can enter
Larger things use active transport

Question 12

What are nuclear targeting signals?

Answer 12

Target proteins to the nucleus
Have lots of positive amino acids
Under 12 aa long
Not removed after protein is targeted

Question 13

How is protein imported into the nucleus?

Answer 13

1. Nuclear import receptor binds to protein with nuclear localization signal in cytosol
2. Fibrils direct receptor to the pore
3. Receptors bind to pore proteins
4. Cargo proteins move into nucleus

Question 14

What is GTP hydrolysis?

Answer 14

Drives nuclear import

Question 15

What are the two types of small GTPase Ran?

Answer 15

GTP bound in nucleus
GDP bound in cytosol

Question 16

What is the process of Ran controlling nuclear import?

Answer 16

1. There is a protein with nuclear localisation sequence which binds to nuclear import receptor
2. This moves protein through pores
3. Receptor and cargo protein imported into nucleus
4. In nucleus Ran-GTP binds to receptor which displaces cargo protein from receptor
5. Receptor bound to Ran-GTP recycled back into cytosol
6. Ran hydrolyses GTP and dissociates from receptor

Question 17

What are cut and paste experiments?

Answer 17

Removing the NLS and seeing its role
Found its necessary for nuclear import