Endoplasmic Reticulum

Question 0

What is the central role of the ER?

Answer 0

Transmembrane lipid and protein synthesis. Also acts as a calcium store for cell signalling.

Question 1

How is the ER organised?

Answer 1

In a network labyrinth of branching tubules and flattened sacks that extend throughout the cytosol, with one internal space called the ER lumen.

Question 2

Describe import of proteins into the ER?

Answer 2

Co-translational: the import begins before the complete synthesis of the polypeptide chain. The ribosome that has synthesised the polypeptide is attached directly to the ER and allows one end to be translocated and the other to be still synthesised.

Question 3

What has smooth ER got and what is it’s function?

Answer 3

No ribosomes. Has transitional ER which is used to bud vesicles from for newly synthesised proteins and lipids.

Question 4

When is smooth ER abundant?

Answer 4

When cells requires a lot of lipid biosynthesis.

Question 5

How are transmembrane and water-soluble proteins translocated differently?

Answer 5

Transmembrane proteins are translocated partially whereas water-soluble proteins are fully translocated.

Question 6

How are proteins directed to the ER?

Answer 6

Using N-terminal, short hydrophobic signal sequences with two components the signal recognition particle (SRP) and the SRP receptor.

Question 7

Describe the SRP complex?

Answer 7

Complex with six different polypeptide chains bound to a single small RNA molecule, all have an eight (or more) non-polar sequence of amino acids at the centre. The binding site is a large hydrophobic pocket that is lined with methionines.

Question 8

What does SRP do?

Answer 8

Recognises and targets specific proteins to the ER in eukaryotes and the plasma membrane in prokaryotes.

Question 9

What occurs in the ER lumen?

Answer 9

Translocated polypeptide chains fold and assemble there. ER resident proteins reside there detected by their ER retention signal which is a c-terminal, 4 amino acids long sequence.

Question 10

Why are disulphides bonds favoured in the ER lumen?

Answer 10

As it’s an oxidising environment.

Question 11

Why are disulphides bonds important?

Answer 11

As the correct cysteine residues to form a disulphides bonds is key to correct folding of the protein.

Question 12

How are proteins glycosylated?

Answer 12

Addition of a N-linked oligosaccharide, 14 sugars long containing glucosamine, mannose and glucose, by oligosaccharyl transferase.

Question 13

How do ER chaperone proteins assist in the correct folding of proteins?

Answer 13

Bind to the single terminal glucose on the oligosaccharide and prevent incompletely folded proteins from aggregating and leaving. Only once the protein is correctly folded the single terminal glucose is cleaved and the protein is free to leave.

Question 14

How is improper folding recognised in the ER?

Answer 14

ERAD: ER-associated degradation system. Recognises when a mannose residue is trimmed in an improperly folded protein and sends it for degradation via retro-translocated using SEC61 translocon. The de-glycosylated polypeptide is ubiquitylated and then fed to the proteasomes.

Question 15

What are the two retrieval motifs?

Answer 15

1. C-terminal KKXX motif, for ER membrane proteins, which is exposed to the cytosol and retrieved by coatamer complex (COP1).
2. C-terminal KDEL motif, for luminal ER proteins, which is recognised by the KDEL receptor and also using COP1 vesicles.

Question 16

What does COP1 do?

Answer 16

Forms transport vesicles to carry the proteins from the cis-Golgi to the ER.

Question 17

What are the diseases of the ER commonly associated with?

Answer 17

Proteins misfolding.

Question 18

What is the mutation that causes CF?

Answer 18

> 1000 mutations in gene that encodes CFTR (cystic fibrosis transmembrane conductance regulator).

Question 19

What are the symptoms of CF?

Answer 19

Thick mucus secretions in the lungs and intestines which cause inflammation and infection

Question 20

What is the CF transmembrane conductance regulator?

Answer 20

And ATP-binding cassette transported which couples ATP hydrolysis to transport of ions and molecules across membranes.

Question 21

What is the mutation that causes oestogeneis imperfecta?

Answer 21

Mutation in collagen 1 that prevents the ER from folding can result in degradation of procollagen 1 by the ERAD.

Question 22

What is the result of oestogeneis imperfecta?

Answer 22

Severe bone formation defects.

Question 23

What is the mutation that causes alpha-1-antitrypsin deficiency?

Answer 23

A mutation in chromosome 14.

Question 24

What does cholera toxin promote?

Answer 24

Electrolyte and water movement into the intestinal lumen resulting in severe diarrhoea and further spread of bacteria.

Question 25

How does cholera toxin act?

Answer 25

Cholera toxin has two subunits: the B subunit binds to GM1 Glycolipids on the surface of cells that line the gut. The toxin is endocytosed and trafficked retrogradely through the secretory pathway to the ER. The A subunit has a retrieval KDEL motif which is used. In the ER the A subunit dissociates into two parts, one is unfolded and retrotranslocated to the cytosol and binds adenylate cyclase increasing cAMP activating protein kinase A. Protein kinase A phosphorylates CFTR promoting calcium secretion.

Question 26

What is ricin?

Answer 26

A plant toxin present in castor beans and used as a poison.

Question 27

What are the symptoms of ricin poisoning?

Answer 27

Nausea, diarrhoea, seizures and hypertension.

Question 28

What is the action of the ricin toxin?

Answer 28

Ricin is a two subunit protein, the A-chain is enzymatically active and the B-chain binds to glycolipids and glycoproteins on the surface of cells that line. Ricin is endocytosed and retrogradely trafficked through the secretory pathway to the ER. It splits into its A and B chain in the ER, and the A chain goes back to the cytosol where it targets the 28S ribosomal RNA and cleaves blocking the binding of the elongation factor.