S8) Post-translational Modifications

Question 1

Describe the basis for post-translational modification

Answer 1

• All proteins adopt a unique 3D structure to become active
• Some proteins need additional processing after translation to adopt this structure

Question 2

Identify and describe 2 forms of post-translational modifications that can occur

Answer 2

• Proteolytic cleavage – breaking peptide bonds to remove part of the protein
• Chemical modification – addition of functional groups to amino acid residues

Question 3

What are the 2 different post-translational destinations for proteins?

Answer 3

• Proteins destined for the cytosol or posttranslational import into organelles
• Proteins destined for a membrane or secretory pathway via co-translational insertion

Question 4

Where are proteins destined for the cytosol /posttranslational import into organelles synthesised?

Answer 4

Synthesised on free ribosomes

Question 5

Where are proteins destined for a membrane / secretory pathway via co-translational insertion synthesised?

Answer 5

Synthesised on ribosomes bound to rER

Question 6

What are the 4 requirements for protein sorting?

Answer 6

• A signal (address), intrinsic to the protein
• A receptor that recognises the signal and which directs it to the correct membrane
• A translocation machinery
• Energy to transfer the protein to its new place

Question 7

Identify 3 organelles which are targeted by proteins

Answer 7

• Peroxisomes
• Mitochondria
• Nucleus

Question 8

Identify 3 types of proteins which are targeted for secretion

Answer 8

• Extracellular proteins
• Membrane proteins
• Vesicular proteins (lysosomes, endosomes etc)

Question 9

Identify the 2 types of secretion from cells

Answer 9

Question 10

What is consitutive secretion?

Answer 10

Constitutive secretion is the constant flow of extracellular proteins out of the cell e.g. collagen is a secreted and modified protein

Question 11

What are the 3 forms of regulated secretion?

Answer 11

• Endocrine – secreting hormones
• Exocrine – secreting digestive juices
• Neurocrine – secreting neurotransmitters

Question 12

In 7 steps, explain how proteins are targeted to the ER/secretory pathway (co-translational transport)

Answer 12

⇒ Protein synthesis on bound ribosomes

⇒ Co-translational transport of proteins into or across ER membrane

⇒ Budding and fusion of ER-to-Golgi vesicles to form cis-Golgi

⇒ Cisternal progression

⇒ Consitutive secretion / Regulated secretion

⇒ Sorting into Lysosomes

⇒ Endocytosis

Question 13

Identify 6 functions of the ER

Answer 13

• Insertion of proteins into membranes
• Specific proteolytic cleavage
• Glycosylation
• Formation of S-S bonds
• Proper folding of proteins
• Assembly of multi-subunit proteins

Question 14

What is N-linked glycosylation and where does it occur?

Answer 14

• N-linked glycosyation is a reaction involving an amino group where sugars are added on an asparagine side chain
• Occurs in ER

Question 15

What does glycosylation of proteins do?

Answer 15

• Folds protein correctly
• Stabilises protein
• Facilitates interactions with other molecules

Question 16

What can deficiencies in N-linked glycosylation lead to?

Answer 16

Severe inherited human diseases - congenital disorders of glycosylation

Question 17

What is a disulphide bond?

Answer 17

A disulphide bond is a linkage derived by the coupling of two thiol groups

Question 18

What happens if there are protein folding problems?

Answer 18

• Protein may be trapped in mis-folded conformation
• Protein contains mutation resulting in mis-folding
• Protein may be incorrectly associated with other sub-units

Question 19

Identify 3 ER chaperone proteins that attempt to correct misfolding

Answer 19

• BiP: “Binding Immunoglobulin Protein”
• Calnexin
• Calreticulin

Question 20

How do ER chaperone proteins attempt to correct protein misfolding?

Answer 20

• Retain unfolded proteins in the ER
• Act as sensors to “monitor” extent of protein mis-folding
• Mediate increased transcription of chaperones
• Mediate reduction in translation

Question 21

What happens if protein misfolding cannot be corrected?

Answer 21

• Protein may be returned to cytosol for degradation
• Protein may accumulate to toxic levels in the ER causing disease

Question 22

What is O-linked glycolisation and where does it occur?

Answer 22

• O-linked glycosylation is a process important in proteoglycans, involving the attachment of sugar to -OH group of serine/threonine
• Occurs in Golgi apparatus

Question 23

Insulin is a protein that undergoes post-translation modifications.

Describe the processing of preproinsulin

Answer 23

• Signal peptide removed on entry into ER
• Disulphide bonds formed in ER
• Removal of C chain in Golgi

Question 24

Proteins may be delivered to the plasma membrane via the regulated or secretory constitutive pathway.

Identify 4 proteins delivered via the constitutive secretory pathway

Answer 24

• Immunoglobulins
• Albumin
• Prohormones
• Collagen

Question 25

Proteins may be delivered to the plasma membrane via the constitutive or regulated secretory pathway. Identify 3 proteins delivered via the regulated secretory pathway

Answer 25

- Insulin - Glucagon - Trypsin