Protein Targetting

Question 1

When ribosomes are attached to the ER, what is the destination of their secretion?

Answer 1

Either secretion or for the membrane.

Question 2

What are the four components necessary for protein sorting?

Answer 2

A signal, a receptor, translocation machinery and energy to transfer the protein.

Question 3

What is the signal found on a protein destined for the ER?

Answer 3

6-12 hydrophobic amino acids at the N terminus of the protein. This signal is cleaved after utilisation.

Question 4

A protein with an amphipathic helix, and hydrophobic side chains found at the N terminus of the protein is destined for….

Answer 4

Mitochondrial matrix.

Question 5

What is the signal for a protein targeted to the nucleus?

Answer 5

5basic amino acids or 2 small clusters 10 base pairs apart, internal on the protein. This is not cleaved.

Question 6

How do proteins destined for the mitochondrial matrix enter this organelle?

Answer 6

They are kept unfolded by chaperones. They pass through a channel in the outer membrane and then an adjacent channel in the inner membrane. Here a protease cleaves off the signal.

Question 7

In terms of protein targeting, explain Pyruvate dehydrogenase deficiency.

Answer 7

A point mutation on the gene means that there is a proline which breaks the helix meaning there is no signal. This means it is not taken up into the mitochondria, and so there is therefore deficiency. This leads to a build up of lactic acid which can lead to neurological problems.

Question 8

What is the name of the carrier for proteins into the nucleus?

Answer 8

Importin

Question 9

In transport of proteins into the nucleus, what displaces the NLS cargo from importin?

Answer 9

Ran-GTP. This is then hydrolysed in the cytoplasm so that importin can pick up more NLS containing cargo.

Question 10

What is SRP?

Answer 10

SRP is a multidimensional riboprotein which mediates association between receptor, ribosome and signal peptide. This is present in the ER.

Question 11

What are the two methods of secretion from the ER?

Answer 11

Constitutive pathway, regulated secretion.

Question 12

Describe the process of proteins being targeted to the ER.

Answer 12

Protein synthesis begins in the cytoplasm, and then the SRP receptor recognises signal and stops synthesis. The unit is activated and GTP is hydrolysed which opens translocon. The signal is cleaved and the protein is delivered through the channel, before it folds in the ER.

Question 13

Name the enzyme which cleaves the signal in protein targeting to the ER.

Answer 13

Signal peptidase.

Question 14

What happens in membrane synthesis?

Answer 14

A stop transfer anchor sequence anchors the protein in the membrane before the rest of the protein is synthesised in the cytoplasm.

Question 15

What is the importance of glycosylation of proteins? Where does this take place?

Answer 15

This leads to correct protein folding, protein stability, and facilitates interaction with other molecules. This takes place in the ER

Question 16

What is peptidyl-prolyl isomers see and why are they necessary?

Answer 16

They accelerate conversion from trans to cis isomer of proline and therefore are necessary for the correct folding of proteins.

Question 17

What is KDEL?

Answer 17

It is a molecule which moves proteins to be retained in the ER back to the ER after they have moved to the Golgi.

Question 18

Explain how KDEL accepts and released proteins.

Answer 18

At the Golgi, pH is low and so the protein is taken up with high affinity. At the ER, pH is high and so affinity is lower and protein is released.

Question 19

What are the three causes of misfolding?

Answer 19

Protein trapped in mis folded shape, protein contains a mutation which leads to misfolding, protein is incorrectly associated with other subunits.

Question 20

What is the purpose of chaperones in the ER?

Answer 20

These try to rectify misfolding, and also ensure that the protein is retained in the ER and a mis folded protein is not secreted.

Question 21

When lysosomal enzymes pass through the Golgi, what is added as the signal?

Answer 21

Phosphate group is added to the hydroxyl group of the mannose sugar.

Question 22

What happens if there is deficiency in the enzyme which gives lysosomal enzymes their signal?

Answer 22

This is fatal because undegraded material causes bloated lysosomes.

Question 23

Where does O linked glycosylation occur?

Answer 23

In the Golgi. Sugar attaches to the hydroxyl group of Serine/Threonine and this is important in proteoglycans.

Question 24

By what secretory pathway is collagen secreted?

Answer 24

Constitutive

Question 25

What is the basic unit of collagen?

Answer 25

Tropocollagen.

Question 26

Describe the structure of collagen.

Answer 26

It is made up of 3 alpha chains which each have XYGly as their amino acid sequence. This is stabilised by hydrogen bonds.

Question 27

Where does assembly of collagen fibres occur?

Answer 27

Outside of the cell. Otherwise this would be a destructive process.

Question 28

What modification occurs to collagen in the ER?

Answer 28

Signal peptide is cleaved, some proline and lysine are hydroxylated, addition of N linked oligosaccharides and addition of galactose to hydroxylysine residues.

Question 29

What is the function of prolyl hydroxylase in collagen synthesis?

Answer 29

It allows hydrogen bonds to form which stabilise the triple helix.

Question 30

How are collagen molecules structured?

Answer 30

They connect to form fibrils and these aggregate to form fibres.

Question 31

In collagen synthesis, what is the action of Lysyl oxidase?

Answer 31

It oxidises NH3 groups so that covalent cross linkages can form.

Question 32

Explain how insulin is synthesised.

Answer 32

Insulin is synthesised in the ER and it folds with two disulphide bonds forming. Post Golgi this connecting peptide is cleaved (peptideC). This is used as a biochemical marker as to how much insulin our bodies are producing.

Question 33

Where is a ribosome located if the protein it synthesised is destined for the cytosol?

Answer 33

In the cytoplasm.

Question 34

What is the secretory pathway for insulin?

Answer 34

Regulated.