Unfolded Protein Degradation

Question 1

How are unfolded proteins recognised?

Answer 1

Exposed hydrophobic residues

Question 2

Role of ‘folding police’

Answer 2

Looks for hydrophobic residues

Ubiquitin ligase adds a chain of ubiquitins to protein, directing it to the proteasome

Protein digested into 10-15 aa pieces

Question 3

Proteasome

Answer 3

Big hollow cylinder that recognises the polyubiquitin tag

Has a motor that unwinds the protein, feeding it into the top

Question 4

Fate of digested protein

Answer 4

Pieces can be reused by cell OR displayed on cell surface by MHC protein for immune recognition

Question 5

N-end rule

Answer 5

Average lifetime of a protein is determined by the aa at the N-terminal

100 hrs - valine
less than an hr - glutamine

Question 6

Proteins expressed in the Endoplasmic Reticulum (ER)

oi robbie don’t change my cards bitch

Answer 6

Proteins that will end up in membrane, secreted or require covalent modification

1/3 of all proteins made

Question 7

Proteins made in cytoplasm

Answer 7

Proteins that are destined for mitochondria are prevented from folding by chaperones. Otherwise, if not entering organelles, folding occurs simultaneously to synthesis.

Question 8

Why do mt-proteins need chaperones to prevent folding?

Answer 8

Folded proteins cannot pass into mitochondria as it would require a big pore
- need to prevent influx of unwanted molecules

Question 9

Other role of ER (additional to protein synthesis)

Answer 9

Lipid synthesis

Has an export system: ER –> Golgi –> Cell membrane

Question 10

Protein covalent modification in ER

Answer 10

Glycosylation

Disulphide Bridge

Question 11

Glycosylation

Answer 11

Important to protein function
Helps them fold up correctly in the membrane and be inserted the right way around

Incorrectly glycosylated proteins get tagged and exported to cytoplasm for disposal

Question 12

Disulphide bridging

Answer 12

Proteins have several chances to fold right
If they can’t they get sent into cytoplasm for degradation

Question 13

ER

Answer 13

Protein quality control area
- does NOT have its own system for digesting proteins
- has to be excreted into cytoplasm first

Question 14

Proteostasis

Answer 14

body tries to keep protein levels fairly constant all the time

dynamic equilibrium in which protein synthesis and folding is balanced with degradation

Question 15

Unfolded Protein Response (UPR)

Answer 15

Occurs in ER when there is an abundance of unfolded protein

Question 16

Three main proteins involved in UPR

Answer 16

PERK, IRE1-alpha, ATF6
Sit in ER membrane and have a sensor domain inside the ER
- sense level of unfolded protein and signals to cytoplasm

Question 17

Short-term UPR

Answer 17

Short-term response used to reduce protein load on ER

Non-specifically digests mRNA (IRE1-alpha)

Slows down translation (PERK) but shutting off eIF-2
- Eukaryotic translation initiation factor
- Translation cannot start

Question 18

Medium-term UPR

Answer 18

More varied and extensive response

Autophagy

ERAD (ER-associated protein degradation)

Inflammation

Chaperones

Question 19

Autophagy

Answer 19

Cell starts digesting damaged components using membrane encapsulation and fusing with lysosome

Question 20

ERAD

Answer 20

Misfolded proteins are exported from ER into cytoplasm, tagged with with Ub and digested by proteasome

Question 21

Long-term UPR

Answer 21

Apoptosis

Question 22

PERK, IRE1-alpha, ATF6

Answer 22

PERK and IRE1-alpha both lead to short and medium term responses

ATF6 only leads to medium term changes AND needs to move to Golgi for processing before re-entering cytoplasm

Question 23

Inhibiton of PERK

Answer 23

Reduces the level of phosphorylated tau and so reduces neurodegeneration but causes diabetes due to pancreatic beta-cell failure