The endoplasmic reticulum and the secretory pathway

Question 1

Structure of endoplasmic reticulum

Answer 1

1. Endoplasmic reticulum is continuous within the nuclear envelope
2. Endoplasmic reticulum one large organelle that often permeates throughout the entire cell
3. Continuous network of membrane tubules

Question 2

Functions of the endoplasmic reticulum

Answer 2

1. Site of Protein synthesis
2. Glycosylation (attachment of carbohydrates to proteins)
3. Site of folding, assembly of multi-protein complexes which are synthesised at ER
4. First steps of membrane Lipid biosynthesis occur within the ER (cholesterol, phospholipids)
5. Ca2+ sequestration
6. Detoxification by cytochrome P450 family of enzymes

Question 3

What is the name of the protein responsible for pumping Ca2+ into endoplasmic reticulum?

Answer 3

Sarco/endoplasmic reticulum ca2+ ATPase (SERCA)

Question 4

What role do cytochrome p450 enzymes play within the body?

Answer 4

Catalyse a series of reactions which render water-insoluble drugs sufficiently water soluble to allow them to be excreted via urine.
Without these reactions, the water insoluble drugs would accumulate within the cell membranes to toxic levels

Question 5

What is the rough endoplasmic reticulum?

Answer 5

Endoplasmic reticulum with ribosomes on its surface

Question 6

What is the smooth endoplasmic reticulum?

Answer 6

Endoplasmic reticulum without ribosomes on its surface

Question 7

What is the process that allows for newly synthesised proteins to be secreted from the cell?

Answer 7

Co-translational protein targeting

Question 8

How do proteins that need to be secreted from the cell differ to other proteins?

Answer 8

Proteins that need to be secreted contain a signal sequence at the N terminus

Question 9

What is the molecule that interacts with the signal sequence? and what does it do?

Answer 9

Signal recognition particle (SRP) recognises and binds to the signal sequence

Question 10

What occurs once the SRP binds to the signal sequence?

Answer 10

Translation of the protein stops. Then the SRP guides the ribosome to the ER plasma membrane where the ribosome will dock onto the Peptide translocation complex while the SRP binds to the SRP receptor

Question 11

What occurs once the ribosome docks onto the peptide translocation complex?

Answer 11

Translation is re-initiated so that the newly synthesised polypeptide is threaded through the peptide translocation complex into the ER lumen

Question 12

What enzyme cleaves the signal sequence from the polypeptide once translation is completed?

Answer 12

Signal peptidase

Question 13

What happens to the signal sequence once it has been cleaved off the polypeptide?

Answer 13

It is degraded

Question 14

Example of a protein that needs to be targeted to the endoplasmic reticulum?

Answer 14

Insulin

Question 15

Describe the processes that occur during the post-translational modification of insulin

Answer 15

1. Signal sequence of preproinsulin gets cleaved off
2. 3 Disulphide bonds then form between the thiol groups of cysteine amino acids within polypeptide chain (molecule now known as proinsulin)
3. C peptide then cleaved out of the proinsulin molecule to form the mature insulin molecule

Question 16

Features of mature insulin

Answer 16

Contains 2 separate peptides: A chain and B chain.
A and B chains are covalently bonded together by 2 of the 3 disulphide bonds
The third disulphide bond is intramolecular (Bonded only to A chain)

Question 17

Examples of protein modifications that take place in the ER

Answer 17

1. Proteolysis
2. Disulfide bond formation
3. Glycosylation (addition of carbohydrates to proteins)
4. De-glycosylation (removal of carbohydrates from proteins)
5. Protein folding and assembly into tertiary/quaternary structures

Question 18

If proteins are glycosylated in the ER then why are they also de-glycosylated in the ER?

Answer 18

Most carbohydrates that are attached usually taken off quickly because they aren’t needed later on or glycosylation of the protein was part of a reaction cycle

Question 19

Once a protein is modified in the ER where must it it be transported to?

Answer 19

Golgi apparatus (Cis side of Golgi)

Question 20

How is the protein transported to the Golgi?

Answer 20

Vesicular transport - transport vesicles bud off of ER membrane to travel through cytoplasm where they will fuse with target bilayer of Golgi.
As they fuse their contents are secreted into target membrane via exocytosis

Question 21

What is ER quality control?

Answer 21

Process that controls whether protein synthesised at ER will be allowed to enter transport vesicle

Question 22

What are some of the checks involved in ER quality control?

Answer 22

1. Proteolysis
2. Glycosylation
3. Disulfide bond formation
4. Folding
5. Assembly into complex

Question 23

What happens to a protein that fails ER quality control?

Answer 23

Proteins that fail any one of the quality checks will not be exported from the ER and will be degraded instead

Question 24

What is the process of protein degradation within the ER called?

Answer 24

ER associated degradation (ERAD)

Question 25

What are the 3 stages of ER associated degradation?

Answer 25

1. Recognition and targeting of misfolded/unassembled proteins
2. Retro-translocation of protein back to the cytosol (cytoplasm)
3. Degradation (ubiquitin-proteasome dependent degradation)

Question 26

Structure of Golgi apparatus

Answer 26

Contains 3 different domains:
Cis face – where vesicles containing proteins fuse with when target bilayer is within Golgi apparatus

Medial Golgi – Where most enzymes that carry out protein/lipid modification of proteins within the Golgi are located

Trans face- where vesicles leave the Golgi apparatus and transport their contents to other areas of the cell, e.g. lysosomes or plasma membrane

Question 27

Roles of the Golgi Apparatus

Answer 27

1. Protein modification - e.g. O-linked glycosylation
2. Lipid Synthesis

3. Protein and lipid sorting to: 
Secretory granules
Plasma membrane
Endosomes
Lysosomes