MCB Lecture 31 Gated Transport & Transmembrane Transport

Question 0

Describe the structure of a nuclear pore

Answer 0

Octamer: eight proteins line the pore complex

Question 1

How do molecules get in and out of the nucleus?

Answer 1

Gated transport

Question 2

How do larger molecules get across the nuclear envelope?

Answer 2

Need a translocator

1. Nuclear Localisation Sequence found in protein
2. Nuclear Import Receptor recognises the NLS
3. Protein is shuttled through the Nuclear Pore Complex on the Nuclear Import Receptor
4. Ran-GTPase hydrolysis GTP, cargo dissociates in the nucleus
5. GDP dissociates on the cytosol side

Question 3

What is the name of the sequence that is recognised on a protein for its import into the nucleus?

Answer 3

Nuclear localisation sequence

Question 4

What is the name of the receptor that moves proteins into the nucleus?

Answer 4

Nuclear import receptor

Question 5

What are the features of the nuclear localisation sequence?

Answer 5

Lysine rich

15-60 amino acids long

Question 6

Describe the steps involved with exporting a protein from the nucleus

Answer 6

Same as the import, except the protein binds to the Nuclear Import Receptor on the inside, as well as Ran-GTP.
Ran-GTP is hydrolysed in the cytosol, and the cargo is released

Question 7

Describe the function of Ran-GTPase

Answer 7

Hydrolysis GTP in the cytosol

Question 8

Where are nuclear proteins synthesised?

Answer 8

In the cytosol

Question 9

When do nuclear proteins fold?

Answer 9

In the cytosol

Question 10

How is the import of nuclear proteins regulated?

Answer 10

Phosphorylation of a protein causes conformational change, and the Nuclear Localisation Sequence is exposed
Now, the protein can be imported

Question 11

When is transmembrane transport used?

Answer 11

To move protein from the cytosol into the mitochondria or ER

Question 12

Describe how proteins traverse the two mitochondrial membranes

Answer 12

1. Signal sequence on protein recognised by the TOM receptor
2. TOM and TIM line up in the membranes -> translocation channel
3. The protein is fed through into the matrix of the mitochondria
4. Signal sequence cleaved
5. Protein folds

Question 13

Describe the features of the signal sequence for proteins entering the mitochondria

Answer 13

The signal sequence is an amphiphilic sequence.

The receptor I recognises the hydrophobic region

Question 14

Describe the structure of the translocation channels on the mitochondrial membrane

Answer 14

TOM on outer membrane

TIM on inner membrane

Question 15

How do proteins get into the peroxisomes?

Answer 15

Translocators. The mechanism is not well understood

Question 16

What is cotranslational translocation?

Answer 16

This is when a protein is fed into the ER while it is still being translated

Question 17

What is the sequence that is recognised on a protein so that it many enter the ER?

Answer 17

Signal sequence

Question 18

What is the signal recognition particle?

Answer 18

This is a separate protein that binds to the signal sequence on a protein that is being fed into the ER
When is binds, it is then recognised by the Signal Recognition Particle Receptor
The protein is fed through the translocator

Question 19

What is the Signal Recognition Particle Receptor?

Answer 19

This recognises the Signal Recognition Particle bound to the signal sequence of a protein being translated

Question 20

What is Sec61?

Answer 20

This is the translocator complex and the ribosome in the ER membrane

Question 21

Which types of proteins are fed through an ER translocator?

Answer 21

Soluble proteins

Question 22

What is signal peptidase?

Answer 22

Cleaves the signal sequence, so that the protein is free in the lumen of the ER

Question 23

Where do ER proteins fold?

Answer 23

In the lumen of the ER

Question 24

What are the two binding sites on the Nuclear import receptor?

What is special about the binding sites?

Answer 24

1. Cargo binding domain
2. Ran binding domain

The nuclear import receptor can only bind one of these at a time

Question 25

What is Ran?

Answer 25

It is a protein that binds GTP and hydrolysis it.

It is important in the movement on proteins in and out of the nucleus

Question 26

What sort of molecules are shuttled across the nuclear pore by nuclear import and export receptors?

Answer 26

Proteins

Question 27

What is the name of the receptor that exports proteins from the nucleus across the nuclear pore complex?

Answer 27

Nuclear export receptor?

Question 28

How does binding of RAN-GTP differ in nuclear export and import?

Answer 28

It doesn’t

It binds in the nucleus, moves across and hydrolysed and dissociates in the cytosol

Question 29

What is interesting about the NFAT protein that is shuttle across the nuclear pore?

Answer 29

The NLS is cryptic, but is revealed when phosphorylated by a Ca2+ kinase

Thus nuclear import is regulated and calcium dependent

Question 30

How is it ensured that mitochondrial proteins do not fold before they enter the mitochondria?

Answer 30

Chaperones bind to them

Question 31

What is the structure of the signal sequence of a protein that is recognised by the translocator receptor?

Answer 31

It is an alpha helix

Question 32

Which proteins enter the ER?

Answer 32

Secreted
Proteins destined for membrane embedding
Proteins bound for lysosomes, Golgi and ER

Question 33

Describe the locations involved for translation of proteins that are bound for the ER

Answer 33

Initiation: ALWAYS in the cytosol on free ribosomes
Termination: ribosomes bound to the endoplasmic reticulum

Question 34

How does the signal sequence of proteins bound for the ER associate with the ER?

Answer 34

Signal recognition particle (SRP)

SRP receptor protein