Session 9-Post-translational Processing Of Proteins

Question 1

What is post-translational modification?

Answer 1

Some proteins need additional processing after translation as some proteins are first non-functional and then become active in the right location

Question 2

What are the two methods of post-translational modification?

Answer 2

Proteolytic cleavage

Chemical/covalent modification

Question 3

What is proteolytic cleavage?

Answer 3

Breaking peptide bonds to remove part of a protein

Question 4

What is chemical/covalent modification?

Answer 4

Addition of functional groups to AA residues

Question 5

Where are proteins destined for the cytosol synthesised?

Answer 5

On free ribosomes

Question 6

Where are proteins destined for the membrane or secretory pathway synthesised?

Answer 6

Ribosomes on RER

Question 7

What is required for protein sorting? (4)

Answer 7

1. Signal, intrinsic to protein
2. Receptor that recognises signal and which directs it to the correct membrane
3. Translocation machinery-to transport protein across membrane
4. Energy to transfer protein to new place

Question 8

What is the signal in protein targeting to peroxisomes and where is it present on the protein?

Answer 8

-serine-lysine-leucine = SKL
This SKL sequence is present in proteins going to the peroxisome and is called the peroxisome targeting sequence (PTS).
It is present on the C terminus of the protein

Question 9

What is the receptor that recognises the signal in protein targeting to peroxisomes?

Answer 9

PTS receptor Pex5 which recognises SKL and binds to the protein with this sequence.

Question 10

What is the translocation machinery involved in protein targeting to peroxisomes?

Answer 10

13 Pex proteins make up the transport channel across the peroxisomal membrane - pore is produced which allows the protein to pass through the membrane. These proteins bind to Pex5 to form a cargo complex

Question 11

Where does the energy to transfer the protein in protein targeting to peroxisomes come from?

Answer 11

ATP hydrolysis

Question 12

Which disorder is a result of protein targeting to peroxisomes going wrong and give two examples of this disorder?

Answer 12

Peroxisome biogenesis disorder:

1. Zellweger syndrome
2. Rhizomelic Chondrodysplasia Punctata

Question 13

What causes Zellweger syndrome?

Answer 13

Mutation in several different Pex proteins-cannot break down fatty acids so they accumulate and cause impairment in neuronal function

Question 14

What happens in Rhizomelic Chondrodysplasia Punctata?

Answer 14

Mutation in Pex7 protein which leads to long bone shortening

Question 15

What are the two types of secretion from cells?

Answer 15

Constitutive secretion

Regulated secretion

Question 16

What is constitutive secretion?

Answer 16

Continuous secretion of extracellular proteins (eg collagen, immunoglobulins, albumin)

Question 17

What is regulated secretion?

Answer 17

Controlled secretion-proteins secreted at specific times

Question 18

What do endocrine cells secrete?

Answer 18

Hormones

Question 19

What do exocrine cells secrete?

Answer 19

Digestive juices

Question 20

What do neurocrine cells secrete?

Answer 20

Neurotransmitters

Question 21

What does the pre- part of preproalbumin define?

Answer 21

Signal sequence which is removed during processing

Question 22

What does the -pro- part of preproalbumin mean?

Answer 22

Partially mature protein

Question 23

What is a signal sequence?

Answer 23

N-terminal AA sequence, 5-39 AA in length. There is a central region, rich in hydrophobic residues that are able to form an alpha helix, allowing the protein to cross the lipid bilayer

Question 24

What is a signal recognition particle (SRP)?

Answer 24

Composed of 6 proteins and a short piece of RNA, recognises signal peptide and ribosome

Question 25

Describe the synthesis of secretory proteins and their translocation across the ER membrane (5)

Answer 25

1. Starts as free ribosomes
2. Signal sequence recognised by SRP
3. Binds to ribosome
4. Pauses translation
5. SRP falls off and translation continues

Question 26

What are the functions of the ER? (7)

Answer 26

1. Insertion of proteins into membranes
2. Specific proteolytic cleavage
3. Formation of S-S bonds
4. Proper folding of proteins
5. Assembly of multisubunit proteins
6. Hydroxylation of selected Lys and Pro resides for collagen processing
7. Glycosylation

Question 27

What are the two types of glycosylation?

Answer 27

N-linked

O-linked

Question 28

Why is glycosylation of proteins important? (4)

Answer 28

1. Correct protein folding
2. Protein stability
3. Facilitates interactions with other molecules
4. Deficiencies in N-linked glycosylation can lead to severe inherited diseases (congenital disorders of glycosylation or CDG)

Question 29

What happens in N-linked glycosylation?

Answer 29

Sugars are added on the asparagine (Asn) side chain, the reaction involves an amino group hence N-linked. This occurs in the ER

Question 30

What happens in O-linked glycosylation?

Answer 30

Attachment of sugar to -OH group and occurs in the golgi. The sugar is attached to the hydroxyl group of serine or threonine.

Question 31

What happens if there are folding problems?

Answer 31

Protein may be trapped in misfolded conformation

Protein may be incorrectly associated with other sub-units

Question 32

What attempts to correct any issues with protein folding?

Answer 32

ER chaperone proteins

Question 33

Give examples of ER chaperone proteins

Answer 33

BiP (binding immunoglobulin protein)

Calnexin and calreticulin

Question 34

What do ER chaperone proteins do?

Answer 34

Retain unfolded proteins in the ER and act as sensors to monitor the extent of protein mis-folding

Question 35

What happens if mis-folding cannot be corrected?

Answer 35

Protein may be returned to cytosol for degradation

Question 36

What happens if there is too much misfolding in proteins?

Answer 36

Protein may accumulate to toxic levels in the ER resulting in disease