Post-translational Processing of Proteins

Question 1

What are some examples of how a protein can be processed after translation?

Answer 1

Proteolytic cleavage and chemical modification

Question 2

What is proteolytic cleavage?

Answer 2

Breaking peptide bonds to remove part of a protein

Question 3

Where are proteins destined for the cytosol or post-translational import into organelles synthesised?

Answer 3

Free ribosomes

Question 4

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

Answer 4

Ribosomes on the rough ER

Question 5

What is required for protein sorting?

Answer 5

An intrinsic signal, a receptor that recognises the signal, a translocation machinery and energy

Question 6

What is the intrinsic signal in proteins destined for peroxisomes?

Answer 6

Serine-lysine-leucine (SKL)

Usually present on the C terminus of the protein

Question 7

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

Answer 7

PTS receptor Pex5

Binds to cargo protein in the cytoplasm

Question 8

What is the translocation machinery in protein targeting to peroxisomes?

Answer 8

13 Pex proteins make up a transport channel across the peroxisomal membrane which binds to the Pex5-cargo complex

Question 9

What is required to allow recycling of the PTS receptor?

Answer 9

ATP hydrolysis

Question 10

What is dissociation often due to?

Answer 10

Change in pH

Question 11

What are some examples of peroxisome biogensis disorders?

Answer 11

Zellweger syndrome and Rhizomelic Chondrodysplasia Punctata

Question 12

What is constitutive secretion?

Answer 12

Occurs all the time - constant flow of extracellular proteins out of the cell. Eg collagen from fibroblasts

Question 13

What is regulated secretion?

Answer 13

Occurs at specific times - only secreted in response to signals. Eg endocrine cells secreting hormones

Question 14

What do secretory cells have an abundance of?

Answer 14

Rough ER

Question 15

What is a signal sequence?

Answer 15

N-terminal amino acid sequence, 5-30 amino acid’s in length, central region rich in hydrophobic resides, able to form alpha helix which allows it to cross membrane easily

Question 16

What does the ‘pre’ part of preproalbumin define?

Answer 16

The signal sequence which is removed during processing

Question 17

What is the signal recognition particle?

Answer 17

Receptor needed to bind the signal peptide on proteins destined for the ER, composed of 6 proteins and a short piece of RNA, recognises the signal peptide and the ribosome

Question 18

What does the stop transfer sequence do?

Answer 18

Halts transfer of the peptide across the ER membrane, acts as an anchor to hold protein in position - type I

Question 19

What are the functions of the Endoplasmic Reticulum?

Answer 19

Insertion of proteins into membranes, specific proteolytic cleavage, glycosylation, formation of S-S bonds, proper folding of proteins, assembly of multisubunit proteins, hydroxylation of selected Lys and Pro residues

Question 20

What is N-linked glycosylation?

Answer 20

Sugars are added on an asparagine side chain - occurs in ER

Question 21

Why is glycosylation of proteins important?

Answer 21

Correct protein folding, protein stability, facilitates interactions with other molecules

Question 22

Between which residues are disulphide bonds formed?

Answer 22

Cystine

Question 23

What is the role of protein disulphide isomerase (PDI)?

Answer 23

Ensure correct disulphide bonds form

Question 24

Why may there be protein folding problems?

Answer 24

Proteins may be trapped in mis-folded conformation, protein contains mutation resulting in mis-folding, protein may be incorrectly associated with other subunits

Question 25

What is the role of ER chaperone proteins in folding problems?

Answer 25

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

Question 26

What are examples of ER chaperone proteins?

Answer 26

BiP: ‘Binding Immunoglobulin Protein’

Calnexin and Calreticulin

Question 27

What happens if mis-folding can not be corrected?

Answer 27

Protein may be returned to cytosol for degradation or accumulate to toxic levels in the ER resulting in disease

Question 28

What is O-linked glycosylation?

Answer 28

Attachment of sugar to OH group

Question 29

Where does O-linked glycosylation occur?

Answer 29

Golgi

Question 30

What is O-linked glycosylation important in?

Answer 30

Proteoglycans - component of ECM and mucus secretions

Question 31

What post-translational modifications does preproinsulin undergo?

Answer 31

The signal peptide at the N terminus of preproinsulin is cut, directing it to enter the endoplasmic reticulum. Proinsulin results and folds into the correct conformation with the formation of disulphide bonds. The B chain is removed and there is trimming of the caboxyl termini. It then enters the golgi as insulin

Question 32

Where is the signal sequence located in proteins targeted for the ER?

Answer 32

N terminus

Question 33

Where is the signal sequence located for proteins targeted for the nucleus?

Answer 33

Various positions, must be on surface of folded protein

Question 34

Where is the signal sequence located for proteins targeted for mitochondria?

Answer 34

N terminus

Question 35

Where is the signal sequence located for proteins targeted for the lysosomes?

Answer 35

Signal patch distinguishes lysosomal proteins from other mannose-labelled proteins