Post Translatiomal Processing Of Proteins

Question 1

What is protein sorting ?

Answer 1

The process by which proteins are transported to their appropriate destinations

Question 2

Proteins that are destined for cytosol or into organelles are made …..

Answer 2

On free ribosomes

Question 3

Proteins that are destined for membrane or secretory pathway are synthesised …..

Answer 3

On the ribosomes on the rough ER.

Question 4

What type of proteins are made on ribosomes on the rough endoplasmic recticulum

Answer 4

• extracellular proteins

Membrane proteins

Vesicular proteins ( lysosomes , endosomes)

Question 5

What are the 4 requirements for protein sorting ?

Answer 5

1) a signal which is intrinsic to the protein
2) a receptor that recognises the signal and directs it to the correct membrane
3) translocations machinery
4) energy to transfer the protein to its new place

Question 6

What are the two types of secretion?

Answer 6

1) constitutive secretion

2) regulated secretion

Question 7

What is constitutive secretion

Answer 7

• proteins are secreted from a cell continuously , regardless of external factors or signals.
• it is not regulated.
• an example would be albumin from the liver.

Question 8

What is regulated secretion ?

Answer 8

• where proteins are secreted in large amounts when a signal is detected.
• an example would be endocrine cells eg beta cells secreting insulin when they detect that glucose levels are high.
• another example would be exocrine cells
• another example would be neurocrine cells secreting neurotransmitters

Question 9

What is a signal sequence ?

Answer 9

N-terminal aa sequence

• 5-30 amino acids in length
• central region is rich in hydrophobic residues which form an alpha helical chain,
• the ‘pre’ part on a protein will define the signal sequence.

Question 10

How many proteins make up the signal recognition particle (SRP)?

Answer 10

6 proteins and a short piece of RNA

Question 11

How are secretatory proteins translocated in the ER?

Answer 11

1) secretatory proteins have a signal sequence.
2) signal recognition particles bind to the signal sequence and also binds to the ribosome simultaneously. At this point , translocation is halted.
3) the Signal recongition particle then targets the entire complex to a signal recognition receptors on the membrane of the ER.
4) GTP binds to the signal recognition particle and receptor - this triggers the signal sequence to be transferred to the membrane channel - translocon.
5) GTP is hydrolysed which causes dissosiation of the SRP from the receptor and ribosome-mRNA complex.
6) signal peptidase the breaks the signal peptide
7) translocation continues as polypeptide released into the lumen of the ER.

Question 12

What are the functions of the endoplasmic reticulum ?

Answer 12

1) glycosylation
2) formation of S-S bonds
3) proper folding of proteins

Question 13

What is N linked glycosylation?

Answer 13

• the process by which a sugar is added to the N atom ( in an amide group) of an asparagine residue.

Question 14

Why is glycosylation of proteins important ?

Answer 14

1) ensures correct protein folding
2) protein stability
3) facilitates interactions with other molecules

Question 15

What is the role of the protein disulphide isomerase ?

Answer 15

• forms the disulphides bonds in the ER lumen during protein folding.

Question 16

What happens if Mid folding cannot be corrected ?

Answer 16

• protein may be returned to cytosol for degradation.

2) protein may accumulate to toxic levels in the ER resulting in disease.

Question 17

What are the three potential destinations of proteins from the Golgi apparatus ?

Answer 17

1) plasma membrane
2) lysosomes
3) secretory vesicles

Question 18

Where is the cis network of the Golgi apparatus located ?

Answer 18

• beat th start of the Golgi apparatus- where the protein enters,

Question 19

Where is the trans Golgi located ?

Answer 19

At the end of the Golgi apparatus - from where the protein is secreted,

Question 20

What occurs at the cis Golgi network ?

Answer 20

• phosphorylation of oligossccharides on lysosomal proteins

- sorting also occurs

Question 21

What occurs at the trans Golgi network ?

Answer 21

• sulfstion of tyrosines and carbohydrates

- sorting also occurs

Question 22

What is one example of post-translationally modified secreted protein ?

Answer 22

Collagen

Question 23

Describe the structure of a collagen fibre

Answer 23

• fibrous protein
• rod shaped
• has 3 polypeptide ( alpha chains )
• glycine is in every 3rd position
• triple helix shape
• hydrogen bonds between alpha chains to stabilise the structure
• high tensile strength
• non extensible
• non compressible

Question 24

What is a basic unit of collagen called ?

Answer 24

Tropocollagen

Question 25

Outline the synthesis and modification of collagen in the endoplasmic reticulum

Answer 25

1) entry of the polypeptide chain into the lumen of the ER.
2) cleavage of the signal sequence by signal peptidase.
3) hydroxylation of selected proline and lysine residues ( add -OH).
4) N-linked glycosylation ( adding sugars to the amide groups of asparagine residues)
5) addition of galactose to hydroxylysine residues.
6) chain alignment of 3 alpha collagen molecules. And disulfide bonds form at the c terminus.
7) triple helical procollagen is formed from the N terminus to C terminus,
8) glucose is added and the procollagen molecule is added into a vesicles

Question 26

Why is hydroxylation of the proline / lysine residues important

Answer 26

• allows hydrogen bonds to be formed between the alpha helix chains which stabilises the collagen molecule - can be stable at high temperatures

Question 27

What is scurvy caused by?

Answer 27

Lack of vitaminmC

• this results in weak tropocollagen triple helices as prolyl hydroxylase would not be made ( addition of OH to proline residues ) so hydrogen bonds cannot form between alpha helices,
• thus, weak collagen fibrils results in connective tissue defect.

Question 28

How is procollagen secreted out of the ER?

Answer 28

1) once procollagen enters the vesicles - it leaves by exocytosis
2) the removal of N and C terminals occur and TROPOcollagen is formed.

Question 29

Would you find tropocollagen intracellular or extracellular ?

Answer 29

Extracellular

Question 30

Why are collagen molecules shorter than the polypeptide chain encoded by the COL1A1gene ?

Answer 30

• because their N and C terminals are removed,

Question 31

how. does the formation of a collagen fibre occur ?

Answer 31

• removal of N and C terminal propeptides
  2) lateral association of collagen molecules followed by covalent cross linking which forms a collagen fibril
  3) aggregation of collagen fibrils creates a collagen fibre

Question 32

Why is tropocollagen secreted before final processing occurs ?

Answer 32

• because if you remove the N and C terminal propeptides inside the cell , then the tropocollagens will align themselves laterally to form collagen fibrils ( covalent cross links between them ) then later aggregate to form collagen fibres.
• this would cause the cell to burst,