9. The Endoplasmic Reticulum and Secretory Pathway

Question 1

Describe Exocytosis (3pts)

Answer 1

1. The cells from a vesicle around material that needs to be expelled from the cell.
2. The vesicle is transported to the cell membrane
3. The vesicle membrane fuses with the cell membrane and releases the contents from the cell.

Question 2

How is the endoplasmic reticulum visualised ?

Answer 2

Light microscope allows us to visualise large components of cells such as the nucleus however electron microscopes are vital to see small organelles such as ribosomes

Question 3

Describe the endoplasmic reticulum (4pts)

Answer 3

1. Extensive membrane system continuous with the nuclear envelope
2. Single-membrane compartment
3. Consists of the Rough ER which is needed for protein synthesis and the folding of proteins
4. Consists of the Smooth ER which is needed for lipid synthesis and the detoxification of certain drugs and toxins by cytochrome p450 enzymes.

Question 4

Describe the Rough ER (6pts)

Answer 4

1. Has ribosomes
2. Found near the nucleus
3. Originates from nuclear membranes
4. Mainly composed of cisternae
5. Function is the synthesis, folding and transport of proteins
6. Well developed in protein forming and secretory cells.

Question 5

Describe the Smooth ER (6pts)

Answer 5

1. Does not have ribosomes
2. Found closer to the cell membrane
3. Originates from the rough ER by giving off the ribosome
4. Mainly composed of tubules
5. Function= synthesis and transport of lipids
6. Mainly present in lipid forming cells

Question 6

Describe Signal sequences

Answer 6

Signal sequences are amino-acids at the N-terminal end of the protein that are recognised by enzyme systems within the cell that transport the protein to the correct destination

Question 7

Describe the way proteins travel (3pts)

Answer 7

1. Proteins produced in the ribosome
2. Proteins will travel from the rough ER to the smooth ER to the golgi.
3. They will either be secreted outside the cell, accumulated in the lysosomes and digested or secreted to the cell surface plasma membrane.

Question 8

Describe protein synthesis and modification (9pts)

Answer 8

1. A ribosome starts protein synthesis during initiation by assembling at the 5-prime cap.
2. Once it encounters the start codon, it begins elongating the polypeptide chain
3. Proteins that are to be secreted from the cell have a signal sequence at the beginning of the Amino acid sequence.
4. This signal sequence pauses translation
5. The signal sequence leads to the recruitment of the SRP. The SRP interacts with the signal sequence as well as the ribosome and guides the ribosome to the translocon that sits in the membrane of the ER with the SRP also binding to the SRP receptor on the ER membrane.
6. Once the ribosome binds to the complex, translation is resumed.
7. The newly synthesised growing polypeptide chain is threaded through the translocation channel into the lumen of the ER.
8. The signal peptidase enzyme then cleaves off the signal sequence which is then degraded. The SRP is recycled and released from the ribosome complex.
9. Translation resumes

Question 9

How is Insulin modified inside the rough ER?

Answer 9

Insulin is made by beta cells in the pancreas and there are 3 forms:

1. Peproinuslin- has signal peptide in the N terminus which is then cleaved off. The signal peptide is 20-25 amino acids long
2. Proinsulin- Large bits removed such as the C peptide which is not used at all. It is cleaved off to form a mature protein
3. Mature insulin- as a results of all these modifications. Sugars such as glycosylation and degylocsylation are added to proteins as they enter the ER.

Question 10

Describe Proteolysis (1pts)

Answer 10

Proteins being cleaved off

Question 11

Describe the ER quality control check (5pts)

Answer 11

For export to occur in Golgi the protein must have passed the quality control check:

1. Proteolysis- protein must be properly cleaved
2. Must be Glycosylated
3. Disulphide bridges must be formed in the right places.
4. Protein should have been formed/ folded
5. Assembled to larger complexes

Question 12

What happens if proteins fail the ER quality check (1pt)

Answer 12

Proteins that fail any quality check will not be exported from the Er and are degraded by ubiquitination and proteasome in ERAD- Endoplasmic-reticlum-associated protein degradation.

Question 13

Describe the Golgi Apparatus (6pts)

Answer 13

1. Looks like a stack of pancakes
2. Proteins travel from cis to trans face and are packaged and modified along the way.
3. Cis-face= where vesicles would fuse, coming from the ER.
4. Trans-face= where vesicles are pinched off and is transferred out of Golgi.
5. Each pancake has cisternae- the middle stack is called the medial cisternae
6. Golgi stacks form a ribbon structure

Question 14

Describe Vesicle transport proteins (4pts)

Answer 14

1. Vesicle transport proteins move from the Golgi Apparatus to lysosomes, the plasma membrane or the exterior. The destination is determined in the ER when the proteins binds to a specific receptor.
2. Bud formation is facilitated by binding of different coat proteins
3. Once transport vesicle is formed and released coat proteins are removed revealing v-Snare which are integral proteins
4. V- snare binds to t-snare (target) in the target membrane, the transport vesicle fuses to the target membrane and the cargo delivered.