Endoplasmic Reticulum

Question 1

Describe endoplasmic reticulum

Answer 1

Network of enclosed tubules, continuous with the nuclear membrane

Question 2

What is the main function of RER and SER?

Answer 2

RER - protein folding and synthesis

SER - lipid synthesis

Question 3

What is the ER a store of?

Answer 3

Ca2+ which is released during certain cellular events

Question 4

By what two pathways are proteins trafficked through the ER to the cell and membrane?

Answer 4

1. Co-translational route

2. Post-translational route

Question 5

What is a signal sequence?

Answer 5

The first part of a protein to be translated, which contains 20 amino acids, a region of hydrophobic amino acids and a cleavage site

Question 6

What is the structure of a signal recognition particle?

Answer 6

Made of 6 polypeptides, an RNA molecule and a translational pause domain

Question 7

How do the signal sequence and the signal recognition particle interact?

Answer 7

• At first cause a pause in translation and sends signal for the ribosome (at this point free in the cytosol) to bind to the ER and become part of the RER
• SRP particle then binds to SRP receptor on the RER, then guides to protein translocator (translocon) which allows protein to pass into RER and translation to start again

Question 8

What happens when translation finished (co-translational route)?

Answer 8

Signal sequence is cleaved off allowing the protein to enter the lumen

Question 9

How common is the co-translational route for mammalian cells?

Answer 9

Used 80% of the time

Question 10

How is the post-translational route different to the co-translation route?

Answer 10

The entire polypeptide chain is synthesized on a free ribosome

Question 11

How does the polypeptide chain from the free ribosome arrive at the ER?

Answer 11

Chaperone proteins maintain protein in an unfolded state and bring it to the ER

Question 12

How does the polypeptide chain then enter the lumen of the ER?

Answer 12

• Signal sequence interacts with the sec62/63 and the rest of the chain attaches to the translocon
• BiP ratches polypeptide through the translocon
• Signal sequence is cleaved

Question 13

When is the post-translational route mostly used?

Answer 13

Mitochondria, chloroplasts and peroxisomes

Question 14

How is a single-pass protein inserted into the cell membrane?

Answer 14

• Hydrophobic start sequence binds to translocon, and protein is transferred into the lumen until hydrophobic stop sequence is reached
• Signal peptidase then cleaves off start sequence and stop sequence changes shape to pass through the lumen, protein is then released by the translocon

Question 15

How are multiple-pass proteins achieved?

Answer 15

Multiple start and stop sequences which are cleaved off

Question 16

When are proteins folded?

Answer 16

Almost immediately after entering the lumen of the ER, due to oxidizing environment forming disulphide bridges

Question 17

What is N-linked glycolysation and where does is occur?

Answer 17

The addition of complex sugar molecules to the polypeptide chain, usually occurs on specific asparagine residues.

Question 18

How does N-linked glycolysation occur?

Answer 18

• Oligosaccharide is attached to a lipid carrier and transferred to the polypeptide via the enzyme olisaccharyl transferase
• 3*glucose is usually cleaved off

Question 19

How is a GPI anchor attached to a protein?

Answer 19

• GPI anchor composed of glycosylphosphatidyl-inositol (2 fatty acids, an oligosaccharide and a head group such as ethanol amine)
• Transferred to acetyl end of a protein when the signal sequence is cleaved

Question 20

How are phospholipids synthesized?

Answer 20

Glycerol 3-phosphate combines with 2 Fatty acid CoA to produce phosphatic acid which can go on to produce diglycerol which can then produce phosphatidylcholine/phosphatidylserine/phosphatidylethanolamine

Question 21

How are phospholipids ordered in the in the membrane?

Answer 21

• When asymmetrically added to cytosolic side enzyme scramblase catalyses flipping
• When 2 layers delivered via vesicles then flipase catalyses the flipping of specific phospholipids to form cytoplasmic monolayer