Protein Targeting

Question 1

What is a signal sequence?

Answer 1

A short sequence of amino acids usually located on the N terminus on newly synthesised proteins that are destined for the secretory pathway

Question 2

What is the Signal Recognition Particle?

Answer 2

Protein that recognises and targets specific proteins to the endoplasmic reticulum

Question 3

What do chaperone proteins in the lumen of the ER do?

Answer 3

Help the polypeptide to fold correctly and protect the nascent polypeptide from interacting with other proteins

Question 4

On which terminus is the signal sequence of proteins targeted to the ER located?

Answer 4

N terminus

Question 5

Where is the signal sequence of proteins targeted to the nucleus located?

Answer 5

Various positions, must be on surface of folded protein

Question 6

On which terminus is the signal sequence of proteins targeted to mitochondria located?

Answer 6

N terminus

Question 7

On which terminus is the signal sequence of proteins destined for retention the ER located?

Answer 7

C terminus

Question 8

Are proteins targeted to the ER folded or unfolded during transfer?

Answer 8

Unfolded - synthesised through membrane

Question 9

Is the signal sequence in proteins destined for the ER cleaved or retained?

Answer 9

Cleaved by signal peptidase

Question 10

Is energy required in protein targeting to the ER?

Answer 10

Yes - hydrolysis of GTP by SRP

Question 11

What is an example of a protein retained in the ER?

Answer 11

PDI (protein disulphide isomerase)

Question 12

What is the ER retention signal?

Answer 12

KDEL (Lysine-Aspartate-Glutamate-Leucine)

Question 13

Are proteins retained in the ER folded or unfolded during transfer?

Answer 13

Folded - delivered via vesicle

Question 14

Are proteins targeted to lysosomes folded or unfolded during transfer?

Answer 14

Folded - delivered via vesicle

Question 15

Are proteins targeted to the nucleus folded or unfolded during transfer?

Answer 15

Folded - large pores in double membrane of nucleus

Question 16

Are proteins targeted to the mitochondria folded or unfolded during transfer?

Answer 16

Held partially unfolded by chaperones (eg MSF)

Question 17

Is the signal sequence in proteins destined for the nucleus cleaved or retained?

Answer 17

Retained - facilitates re-importing of proteins when nucleus reforms after cell division

Question 18

Is the signal sequence in proteins destined for the mitochondria cleaved or retained?

Answer 18

Cleaved

Question 19

Is the signal sequence in proteins destined for the lysosomes cleaved or retained?

Answer 19

Phosphate removed by phosphatase

Question 20

Is the signal sequence in proteins retained in the ER cleaved or retained?

Answer 20

Retained

Question 21

Is energy required in protein targeting to the nucleus?

Answer 21

Yes - hydrolysis of GTP

Question 22

Is energy required in protein targeting to the mitochondria?

Answer 22

Yes - ATP hydrolysis by mHsp70 drives translocation; MSF uses ATP to keep some precursors unfolded

Question 23

Is energy required in protein targeting to the lysosomes?

Answer 23

Yes - phosphotransferase ultimately requires ATP

Question 24

Is energy required in protein targeting to remain in the ER?

Answer 24

No - involves binding and release dependent on pH

Question 25

Briefly describe how proteins are targeted to the ER

Answer 25

• Signal sequence translated on N terminus of protein
• SRP binds to signal sequence
• Binding of SRP halts translation
• SRP receptors present on the membrane of ER
• SRP alongside signal sequence, polypeptide, mRNA and ribosome bind to SRP receptor on ER membrane
• SRP unbinds from signal sequence which binds to translocon
• Translation resumes
• Growing polypeptide ‘droops’ into lumen, where chaperone proteins associate with it and signal sequence is cleaved by signal peptidase

Question 26

What specialist proteins are involved in protein targeting for proteins retained in ER?

Answer 26

KDEL receptor in cis-golgi

Question 27

What specialist proteins are involved in protein targeting to ER?

Answer 27

SRP, SRP receptor

Question 28

What specialist proteins are involved in protein targeting to nucleus?

Answer 28

Importin recognises NLS and mediates transport

RanGTP displaces it in nucleus and drives out export cargo

Question 29

What specialist proteins are involved in protein targeting to mitochondria?

Answer 29

Mitochondrial-import stimulating factor (MSF)

Tom and Tim channel complex

Question 30

What specialist proteins are involved in protein targeting to lysosomes?

Answer 30

M-6-P Receptor in trans golgi