Lecture 3: Mitochondria and ER nuclear export

Question 1

What is the matrix?

Answer 1

Contains enzymes needed for oxidation and citric acid cycle

Question 2

What is the inner membrane of mitochondria?

Answer 2

Folded up into cristae
Contains proteins that do oxidative phosphorylation
Contains transport proteins that transport in and out of the matrix Impermeable

Question 3

What is the outer membrane of the mitochondria?

Answer 3

Contains porins
Permeable to molecules of 5000 daltons or less

Question 4

What is the intermembrane space?

Answer 4

Contains enzymes that use ATP from matrix to phosphorylate other nucleotides
Contains proteins that are released during apoptosis

Question 5

Where are mitochondria located in the cell?

Answer 5

They move around the cell and can divide and fuse
Near sites that use a lot of ATP

Question 6

What are mitochondrial targeting signal sequences?

Answer 6

Have high content of Arg and SER
At the N terminus
20-80 amino acids long
Cleaved after import into matrix

Question 7

What is the structure of mitochondrial targeting sequences?

Answer 7

They form an amphiphilic alpha helix with two different faces
One side is positive residues One side is non polar

Question 8

What structure must the protein be in before mitochondrial import?

Answer 8

Unfolded or loosely folded

Question 9

How are mitochondrial proteins imported?

Answer 9

1. MTS is recognised by receptor in outer membrane
2. Translocator channel moves protein into inter-membrane space
3. Signal binds to second translocator in inner membrane
4. Chaperone proteins (hsp70) pull protein into the matrix and refold it. Driven by ATP hydrolysis

Question 10

What is the structure of the ER?

Answer 10

Extensive membrane system
Network of sacs and tubules

Question 11

What are the two types of ER?

Answer 11

Smooth - does lipid synthesis Rough - does protein synthesis

Question 12

Which cells have a large amount of rough ER?

Answer 12

Pancreatic cells

Question 13

Which type of cells have a large amount of smooth ER?

Answer 13

Hepatocytes

Question 14

Why do different cells have different amounts of smooth and rough ER?

Answer 14

Depends on how much they need protein and lipid synthesis

Question 15

What are the two types of proteins that enter the ER?

Answer 15

Soluble proteins
Integral membrane proteins

Question 16

What are soluble proteins?

Answer 16

Water soluble
Secretory
In the lumen of organelles of the endomembrane system

Question 17

What are integral membrane proteins?

Answer 17

Hydrophobic sequences
Form membrane spanning domains
Embedded in the lipid bilayer of membranes of endomembrane system and plasma membrane

Question 18

If proteins are destined for different organelles what do they have to enter first?

Answer 18

The ER

Question 19

ER signal sequence features

Answer 19

8+ hydrophobic amino acid residues usually near N-terminus

Question 20

What happens to ribosomes synthesising proteins without a signal sequence?

Answer 20

They remain free in the cytosol

Question 21

What happens to the ribosome synthesising proteins with an ER signal sequence?

Answer 21

They are directed to the ER while the protein is still being translated

Question 22

What is co-translational translocation?

Answer 22

When translation happens at the same time as translocation

Question 23

What is the difference between the translocation of the ER proteins and the mitochondrial and nuclear proteins?

Answer 23

ER does co-translational translocation
Mitochondrial and nuclear proteins do post-translational translocation

Question 24

What is the role of SRP (signal recognition particle) in targeting to the ER?

Answer 24

1. SRP in cytosol binds to ERSS when it leaves ribosome and translation is paused
2. SRP binds to SRP receptor in ER membrane
3. SRP is displaced and the Ribosome passes to the translocator and translation is resumed

Question 25

What is the Sec61 channel?

Answer 25

A multi-subunit complex of proteins forming a channel in the ER membrane

Question 25

What is the function of the Sec61 channel?

Answer 25

It allows the protein chain to be translocated across the ER membrane through a central pore
The pore is closed when there is no translating polypeptide

Question 26

What is the process of the ERSS binding to the Sec61 channel?

Answer 26

1. ERSS binds to Sec61 translocator and the channel opens
2. The polypeptide containing the ERSS enters the channel as a loop, as ERSS is held in Sec61, while translation happens
3. ERSS cleaved by signal peptidase and lets only the protein through the membrane