Endoplasmic reticulum and secretory pathway

Question 1

Define a signal peptide

Answer 1

A short peptide (16-30 amino acids long) present at the N-terminus of the majority of newly synthesised proteins that are destined towards the secretory pathway

Question 2

Describe the ER as a single-membrane compartment

Answer 2

Continuous network of tubular and flat vesicular structures in the cytoplasm (“little net”).

Question 3

What is inside the compartment of the ER

Answer 3

A space which is connected with the space between the two membrane surfaces of the nuclear membrane (Continuous with the nuclear envelope)

Question 4

Describe the two parts of the ER and briefly their functions

Answer 4

Granular/Rough ER (ribosomes attached (hence rough) for the translation and folding of new proteins)

Agranular/Smooth ER - Synthesis of lipids and detoxification of certain drugs and toxins by cytochrome p450 enzymes

Question 5

What do the RER and the SER look like on an electron micrograph

Answer 5

RER - Long tubular lines covered with black spots on them representing the ribosomes

SER - Quite smooth and flat tubular structure, with no black spots (no ribosome)

Question 6

State the functions of the RER

Answer 6

Protein synthesis, glycosylation, folding and assembly and multi-protein complexes

Question 7

State the functions of the SER

Answer 7

Lipid synthesis (cholesterol, phospholipids) important for cell membrane for example. Doesn’t synthesise proteins

Ca2+ sequestration - Important in contraction of muscle cells

Detoxification by cytochrome P450 enzymes - Catalyse reactions that make drugs lipid soluble and waste water soluble to expel from the body

Question 8

Name some differences between the RER and SER

Answer 8

Ribosomes v No ribosomes
Found near the nucleus v Found closer to the cell membrane
Originates from nuclear membranes v Originates from rough ER by giving off the ribosomes
Mainly composed of cisternae vs Mainly composed of tubules
Involved in synthesis, folding and transport of proteins v Synthesis and transport of lipids
Well developed in protein forming and secretory cells v Mainly present in lipid forming cells

Question 9

What happens in the first step of SRP

Answer 9

Recognition of the signal peptide - Ribosome

Question 10

What happens in the second step of SRP

Answer 10

When the SRP particle binds the ribosome-nascent-chain complex, for this one GTP molecule is needed to provide energy for the process.

Question 11

What happens in the third step of SRP

Answer 11

Binding of the complex composed of ribosome-nascent protein + the SRP particle + GTP to the SRP receptor. Two other GTP molecules are needed.

Question 12

What is the initial synthesised form of insulin?

Answer 12

Preproinsulin

Question 13

What happens when the insulin peptide is inside the RER

Answer 13

The first modification occurs, where the preproinsulin is brought to proinsulin and consists of a proteolysis, where the signal sequence is removed

Question 14

What would the second modification be

Answer 14

Cleavage of the proinsulin into the three different peptides, the A, B and C chains

Question 15

What happens to the A, B and C chains in the third modification

Answer 15

Disulfide bonds between A and B chains, C chain is removed to form a a biologically active insulin that comprises 51 amino acids.

Question 16

What happens in the fourth step

Answer 16

The insulin goes through more modifications, the additions and removals of sugar/protein molecules, glycosylation, deglycosylation

Question 17

Where does glycosylation/deglycosylation occur

Answer 17

Golgi

Question 18

Why is glycolysation/deglycosylation important

Answer 18

Sorting secreted proteins (E.g. lysosomal proteins to lysosomes)

Question 19

Where does protein folding and assembly occur?

Answer 19

Golgi

Question 20

What happens to proteins that fail a quality check during protein synthesis?

Answer 20

They will not be exported from the ER. They will be degraded by ubiquitination and the proteasome.

ERAD - Endoplasmic-reticulum-associated-protein degradation

Question 21

Describe the golgi structure

Answer 21

Single-membrane compartment consisting of a 4 to 8 stacked layers of thin, flat, enclosed vesicles (cisternae) lying near one side of the nucleus

Question 22

Name and describe the 3 networks that make up the golgi

Answer 22

1. cis (First cisternae structure, closer to nucleus)
2. Medial and trans compartments (Final structure closer to cell membrane)

Question 23

How does the golgi aid in protein modification

Answer 23

1. Glycosidases, glycosyltransferases
2. O-linked glycosylation
3. Sulfatases
4. Proteases

Question 24

How does golgi aid in lipid Synthesis

Answer 24

SPhingomyelin
Glucosylceramide

Question 25

How does the golgi aid in protein and lipid sortng

Answer 25

Helps sort to secretory granules, plasma membrane, basolateral versus apical membrane, endosomes, lysosomes

Question 26

What are the 3 stages of vesicles from the RER fusing to the golgi to drop their cargo inside the complex

Answer 26

Budding
Movement
Fusion

Question 27

What is the direction of vesicles transporting proteins from one cisternae to another

Answer 27

Cis to trans

Question 28

How does bud formation occur

Answer 28

Binding of different Coat proteins (E.g. COPI and COPII)