Protein Targeting

Question 1

Where do proteins need to be moved to?

Answer 1

Organelles
Membrane
To be secreted
Cytosol

Question 2

What organelles can proteins be targeted towards?

Answer 2

Endoplasmic reticulum
Mitochondria
Nucleus
Lysosomes
CSM
Secretory vesicles

Question 3

How do proteins get targeted to inside the ER?

Answer 3

A signal sequence on the end of the protein (on the ribosome still), binds to an SRP (signal recognition particle, complex). The SRP then binds to the SRP receptor on the membrane of the ER; the ribosome is then bound to the membrane and the protein is guided through a translocation protein (translocon) into the ER lumen.
Signal sequence then cleaved by signal peptidase and SRP recycled.

Question 4

How do proteins get targeted to the ER membrane

Answer 4

A signal sequence on the end of the protein (on the ribosome still), binds to an SRP (signal recognition particle, complex). The SRP then binds to the SRP receptor on the membrane of the ER; the ribosome is then bound to the membrane and the protein is guided through a translocation protein (translocon) until a stop transfer sequence is meant to pass through; it is hydrophobic so stays in translocation protein.
Then signal peptidase cleaves the signal sequence and then the SRP is recycled.

Question 5

What happens to get proteins from ER to Golgi and what happens there?

Answer 5

Proteins synthesised, folded and modified in ER then vesicles form that move to and merge with the cis-Golgi; in which further protein modification takes place. Vesicles move between cisterna of Golgi via vesicles.

Question 6

How can the position of a protein within a transport vesicle effect its final position?

Answer 6

If the protein is within the membrane of the vesicle then when the vesicle coalesces with the CSM it will end up being a membrane protein in the CSM.
If it is completely within the vesicle then it will be excreted.

Question 7

Targeting proteins to mitochondria

Answer 7

Happens after translation but before complete folding.
Protein is complexed to a chaperone (HSP70)
Signal sequence binds; protein is guided through a translocator, through both membranes.
Signal sequence is cleaved

Question 8

What is the chaperone molecule used to bring a protein to mitochondria?

Answer 8

HSP70

Question 9

Targeting proteins to nucleus

Answer 9

After both translation and folding have been completed.
Nuclear proteins contain the nuclear localisation signal (NLS, PKKKRKV)
NLS binds to importin (needs ATP) and is transported into nuclear pore.
Requires a G-protein (Ran) and hydrolysis of GTP

Question 10

Targeting proteins to lysosome

Answer 10

Protein needs tagged with mannose-6-phosphate