Protein targeting to the Nucleus

Question 1

What is attached to the outer nuclear membrane?

Answer 1

Ribosomes, connected to the rough ER in some places

Question 2

What is attached to the inner nuclear membrane?

Answer 2

The nuclear lamina, chromatin

Question 3

What is the purpose and structure of the nuclear lamina?

Answer 3

It gives structural support to otherwise squishy membrane. It is made of 3 proteins: Lamin A, B, and C that create a meshwork

Question 4

What is the structure of the nuclear pore complex?

Answer 4

Octagonal symmetry. 8 joined filaments extend into the nucleoplasm and form the nuclear basket. 8 filaments also extend into the cytoplasm. The channel in the middle is blocked by FG-nucleoporins that form a mesh like structure.

Question 5

How do FG-nucleoporins form the mesh-like structure that blocks the openings in nuclear pores?

Answer 5

They are mostly hydrophilic polypeptide chains that aren’t folded very much. There are small sections of phenylalanine and glycine that interact and form the mesh-like structure

Question 6

What can get through the FG-nucleoporins without any help?

Answer 6

Water, ions, small proteins, metabolites

Question 7

What are the characteristics of the nuclear localization sequence (NLS)?

Answer 7

8-30 amino acids, with lots of arginine and lysine (positively charged)

Question 8

How was it determined that nuclear transport proteins were in the cytoplasm and not the nuclear pores?

Answer 8

When the cytoplasm was removed no nuclear transport occurred. Only when the cytoplasm was put back in did proteins go to nucleus

Question 9

What are the transport proteins responsible for bringing proteins into the nucleus?

Answer 9

Importins

Question 10

What are the transport proteins responsible for bringing proteins out of the nucleus?

Answer 10

Exportins

Question 11

What is the energy source for nuclear transport?

Answer 11

A concentration gradient of a G-protein: Ran

Question 12

What establishes the concentration gradient of Ran used for energy for nuclear transport?

Answer 12

Precise placement of GEFs and GAPs. Any Ran in the cytoplasm immediately becomes RanGDP. Any Ran in the nucleus immediately becomes RanGTP

Question 13

What are the steps for nuclear import?

Answer 13

1. Importin binds to the NLS on the cargo protein
2. The importin translocates through the nuclear pore with the cargo
3. Importin binds to RanGTP in the nucleus
4. Cargo protein is released
5. RanGTP diffuses out of the nucleus and brings importin with it
6. RanGTP gets turned into RanGDP by a GAP
7. Affinity between Ran and importin is reduced
8. Importin dissociates
9. RanGDP diffuses back into the nucleus and becomes RanGTP because of a nearby GEF

Question 14

What are the steps for nuclear export?

Answer 14

1. Exportin binds to RanGTP
2. Exportin binds to the NES on the cargo
3. The complex moves through the nuclear pore into the cytoplasm
4. RanGTP gets hydrolyzed into RanGDP by a nearby GAP
5. The exportin dissociates
6. The cargo detaches
7. Exportin and RanGDP go back into the nucleus down their concentration gradients