Protein trafficking 2

Question 1

Where are ER targeting sequences located?

Answer 1

The N-terminus of proteins.

Question 2

What do the targeting sequences for ER contain?

Answer 2

They are a hydrophobic stretch of amino acids (usually 8-15) and normally have a positively charged N-terminus and a cleavage sequence.

Question 3

What is the signal recognition particle (SRP)?

Answer 3

A ribonucleoprotein complex that recognises a signal sequence from a ribosome. It binds to the signal sequence (and amino acid) to halt translation.

Question 4

What happens after the SRP has bound to the ribosome/partially synthesised protein complex?

Answer 4

It binds to the ER membrane which contains an SRP-receptor.

Question 5

What happens when the SRP/ribosome/protein complex binds to the SRP-receptor?

Answer 5

The receptor guides the complex to the translocon (protein conducting channel) and both the SRP and SRP receptor hydrolyse their GTP.

Question 6

What happens upon GTP hydrolysis of SRP and SRP receptor.

Answer 6

There is a conformational change and the SRP is released from the ribosome/protein chain and the receptor can be used again for the same process - it is recycled.

Question 7

How does the polypeptide chain get into the lumen?

Answer 7

The continued translation and extension of the chain pushes it into the lumen.

Question 8

What happens when the protein is being transported through the ER membrane?

Answer 8

The protein is unfolded.

Question 9

What is a nascent chain?

Answer 9

It is a partially synthesised protein - this is bound to the SRP and ribosome during the ER targeting and translocation process.

Question 10

What does lumenally located signal peptidase protein complex do?

Answer 10

It cleaves signal sequences from proteins that are soluble in the lumen of the ER to create a new N-terminus of the protein.

Question 11

What are signal sequences that are non-cleavable called?

Answer 11

Signal-anchor sequences.

Question 12

What are some features of the signal sequences that are non-cleavable?

Answer 12

They have longer hydrophobic stretches of amino acids and no signal peptidase cleavage sequence.

Question 13

Where are proteins that are targeted to the ER synthesised?

Answer 13

The cytosol.

Question 14

What usually happens to proteins after they enter the ER?

Answer 14

They may enter the endomembrane system.

Question 15

What is the endomembrane system?

Answer 15

All of the organelles and membranes of the secretory and endocytic pathways such as the ER, golgi, lysosomes, plasma membrane and endosomes.

Question 16

What are the 3 pathways of protein trafficking?

Answer 16

Anterograde pathways, retrograde pathways and endocytic pathways.

Question 17

What is a general mechanism for how molecules move from one organelle to the next?

Answer 17

Budding and fusion.

Question 18

Describe the process of budding and fusion.

Answer 18

The vesicle buds from the donor compartment, the vesicle pinches off and translocates from the donor to the acceptor compartment. The vesicle then docks with the acceptor compartment and fuses with this to release the contents into the lumen.

Question 19

How is the process of budding helped?

Answer 19

Protein coats assemble onto membranes and surround the bud that is forming. The vesicle is then coated by protein which breaks off to leave the naked transport vesicle.

Question 20

What are clathrin coats?

Answer 20

They are assembled from clathrin proteins and adaptor proteins that surrounding forming buds.

Question 21

What formation do the clathrin coats take?

Answer 21

They form a Trisklelion structure composed of heavy and light chains.

Question 22

What protein is involved in the formation of clathrin coated vesicles?

Answer 22

The final scission event is carried out by dynamin.

Question 23

What happens if dyanamin is not present or is a mutant?

Answer 23

The organism may become paralysed as they cannot endocytose membrane for generation of new synaptic vesicles.

Question 24

What can coats on vesicles be made from other than clathrin?

Answer 24

COPI and COPII coats are involved with forming buds at the golgi and ER respectively.

Question 25

What budding is clathrin involved in?

Answer 25

Budding from the plasma membrane and from the TGN (trans-golgi network).