MCGB - Protein Processing And Targeting - done to slide 21

Question 1

If proteases were activated while they are still inside the cell they were synthesised in, they would destroy the cell. How is this issue overcome?

Answer 1

They are activated outside the cell in “post-translational modification.”

Question 2

What is proteolytic cleavage?

Answer 2

Breaking peptide bonds to remove part of the protein.

Question 3

What is chemical modification?

Answer 3

The addition of functional groups to amino acid residues.

Question 4

If a protein is destined for the cytosol or for posttranslational import into organelles, where is it synthesised?

Answer 4

Free ribosomes

Question 5

If a protein is destined for the membrane, lysosomes, secretory vesicles or other organelles, where is it synthesised?

Answer 5

Ribosomes on rough ER.

Question 6

What is required for protein sorting?

Answer 6

A SIGNAL intrinsic to the protein, a RECEPTOR which can recognise the signal and direct it to the correct membrane, a TRANSLOCATION machinery, ENERGY to transfer protein to new place.

Question 7

Which type of protein targeting is this? Three amino acids termed the “PTS” are present on the C-terminus of the protein. PTS is recognised by PTS receptor Pex5 which binds to “cargo protein”, 13 Pex proteins make up a transport channel across membrane of organelle, binds to Pex5-cargo complex. ATP hydrolysis required for recycling of receptor.

Answer 7

Protein targeting to peroxisomes

Question 8

What is the difference between constitutive and regulated secretion?

Answer 8

Constitutive - going on all the time, constant flow of extracellular proteins out of the cell, eg. Fibroblasts

Regulated - occurs at specific times from cell, eg. Neurocrine cells

Question 9

Give some characteristics of a signal sequence of a secreted protein.

Answer 9

N-terminal amino acid sequence, 5-30 amino acids in length, central region rich in hydrophobic residues, able to form alpha helix.

Question 10

What is post translational modification?

Answer 10

The changes made to proteins after they are formed.

Question 11

Give some examples of proteins that would be targeted for secretion.

Answer 11

Extracellular proteins, membrane proteins, vesicular proteins eg lysosomes.

Question 12

Ina cell that secretes proteins there is a lot of one particular organelle. Which one?

Answer 12

RER

Question 13

Why is it important that secreted proteins are able to form an alpha helix?

Answer 13

It allows them to quickly cross the lipid bilayer.

Question 14

What is a signal recognition particle?

Answer 14

A molecule composed of six proteins and a short piece of RNA which recognises the signal peptide and the ribosome.

Question 15

Give some functions of the endoplasmic reticulum.

Answer 15

Insertion of proteins into membranes, specific proteolytic cleavage, glycosylation, formation of disulphide bonds, proper folding/assembly of proteins, hydroxylation of residues.

Question 16

What is N-linked glycosylation?

Answer 16

Adding a sugar group to an amino acid through a nitrogen group in the ER.

Question 17

Why is glycosylation of proteins important?

Answer 17

Provides stability, facilitates interaction with other molecules, allows correct protein folding.

Question 18

What is the role of protein disulphide isomerase?

Answer 18

It ensures the correct disulphide bond forms and replaces them if incorrect.

Question 19

Why is it less of an issue if protein misfolding occurs in the ER?

Answer 19

ER chaperone proteins can help the protein to refold correctly.

Question 20

What happens to a mis-folded protein which cannot be corrected?

Answer 20

It is returned to the cytosol for degradation.

Question 21

What is O-linked glycosylation?

Answer 21

Attachment of a sugar to an OH group, which occurs in the Golgi complex.

Question 22

Collagen is not very abundant in the body - true or false?

Answer 22

False, it’s the most abundant.

Question 23

What is a collagen fibre made up of?

Answer 23

Molecules called TROPOCOLLAGEN (rod shaped protein, made of 3 polypeptides, repeated glycine structure, triple helix)

Question 24

Give some characteristics of the triple helix structure of collagen.

Answer 24

Non-extensible, non-compressible, high tensile strength.

Question 25

Why is glycine present in every third position of the alpha helix of collagen?

Answer 25

Glycine is the only amino acid small enough to fit into the middle of the helix.

Question 26

What sort of bonds form between alpha chains to stabilise the structure?

Answer 26

Hydrogen bonds.

Question 27

Which organelle do fibroblasts have an uncommonly large amount of?

Answer 27

RER

Question 28

The signal peptide is cleaved off as the prepro alpha chains of collagen enter the ER. What are they then called?

Answer 28

Pro alpha chains

Question 29

Why are collagen molecules weaker if sufficient vitamin C is not present?

Answer 29

The amino acids cannot be hydroxylated, so they cannot form hydrogen bonds and the molecule is weaker (scurvy).

Question 30

Outline the stages of tropocollagen formation and modification.

Answer 30

1) synthesis and entry to lumen of RER
2) cleavage of signal peptide
3) hydroxylation of Pro and Lys
4) N-glycosylation and galactose added
5) chain alignment, S-S bonds
6) precollagen forms
7) glucose added, enters transport vesicle
8) exocytosis, removal of N and C terminal propeptides

Question 31

What is removed when procollagen is transformed into tropocollagen?

Answer 31

N and C terminal peptides

Question 32

How does tropocollagen form collagen?

Answer 32

N and C terminal propeptides removed, collagen molecules laterally associated and covalently cross-linked, aggregation of fibrils.

Question 33

What does lysyl oxidise cause tropocollagens to do?

Answer 33

Form cross-links

Question 34

Why must tropocollagen be secreted outside the cell?

Answer 34

It is extremely large when cross-linked and would destroy the cell if it formed inside.

Question 35

Give some examples of disorders that can occur if targeting to peroxisomes goes wrong.

Answer 35

Zellweger syndrome, Rhizomelic Chondrodysplasia Punctata (both are peroxisome biogenesis disorders)

Question 36

What is a signal sequence?

Answer 36

N-terminal sequence of 5-30 amino acids