Protein sorting and trafficking lecture 6

Question 1

How many proteins does a eukaryotic cell contain?

Answer 1

10 billion

Question 2

What are the routes for proteins to take after they have been synthesised in the cytosol?

Answer 2

• stay put
• nucleus
• ER (trafficking)
• mitochondrion

OR in plants

• Peroxisomes
• Chloroplasts/Plastids

Question 3

What are sorting signals?

Answer 3

specific stretches of amino acids in proteins that can determine where the cell is destined to go

Question 4

How are proteins modified for nuclear targeting

Answer 4

postally charged amino acids are arranged in long stretches the protein. These form a patch of on the folded protein . This positive patch is recognised by the nuclear protein receptor

Question 5

What is significant about the nuclear membrane and the Endoplasmic reticulum?

Answer 5

nuclear membrane is continuous with the ER

Question 6

What are some key components of the nuclear pores?

Answer 6

• fibrils
• protein subunits
• spanning the cytoplasm

Question 7

How do molecules get into the nuclear pore?

Answer 7

• can only pass through the nuclear pores
• small molecules can diffuse in and out of the nucleus via the pores
• Large molecules require active transport through the nuclear pore

Question 8

What do Nuclear import receptors bind to?

Answer 8

NLS’ ( Nuclear localisation signals)

Question 9

How do NLS bind to the nuclear import receptors?

Answer 9

Proteins (the cargo) containing nuclear localisation sequences (NLS’s) are recognised by nuclear import receptors. The complex of receptor and cargo binds to the fibrils on the cytoplasmic side.

Question 10

What are the steps for the cargo to be transported through the nuclear pore?

Answer 10

• import is of a full folded protein
  Mechanism
• NLS binds to the cytosolic nuclear import receptors
• Transported to pre
• Energy dependent, the energy is provided by GTP hyrolysis

Question 11

How does the nuclear Import Receptor release the cargo?

Answer 11

GTP has a high affinity for the nuclear import receptor. After entering, the RAN- GTP binds to the nucleus

Question 12

How does the Ran - Gtp dissociate from the nuclear import receptor in the cytoplasm?

Answer 12

Turns into Ran - GDP and therefore its affinity for nuclear import receptor is significantly reduced and it dissociates

Question 13

How are proteins adapted to recognise the mitochondria?

Answer 13

The molecules are amphipathic. This means that they have different charges on different sides of the protein.

Question 14

3 key facts about mitochondrial targeting?

Answer 14

• always at the N - terminus
• Varies in size from the from 20 - 30 amino acids
• multiple positively charged amino acids make an amphipathic alpha - helix

Question 15

What are the names of the receptors on the inside and the outside of the mitochondrial membrane?

Answer 15

TOM (translocator of the outer membrane). TIM (translocator of the inner membrane).

Question 16

How are proteins transported into the mitochondria?

Answer 16

they have to be unfolded

Question 17

How do proteins not refold when they are in the cytosol?

Answer 17

• There are chaperone proteins which bind to the proteins and prevents them from re folding

Question 18

How do the proteins of interest unbind from the chaperones?

Answer 18

Unbind via ATP hydrolysis

Question 19

What Is another function of the chaperone proteins?

Answer 19

Prevents the protein from leaving

Question 20

Why can you not have large pores in the mitochondrial membrane?

Answer 20

The proton gradient would not be maintained

Question 21

What is the mitocochondrial import sequence attracted to?

Answer 21

The negative charge (it is positively charged)

Question 22

Why is import into the mitochondria more complex?

Answer 22

• Mitochondrial proteins can be in outer membrane, inner membrane, intermembrane space or matrix.
• mitochondria also have their own genome and protein synthesis machinery

Question 23

Where should you look to learn about nuclear important and RAN gaP and all that crap

Answer 23

In you notes, you have written some good notes on the nuclear import mechanism