The ER and secretory pathway

Question 1

Describe the structure of ER.

Answer 1

The ER is a network of tubules and flattened sacs that serve a variety of functions in plant and animal cells.

Question 2

What are the functions of the ER.

Answer 2

1. Protein synthesis, protein glycosylation, folding and assembly, and multi-protein complexes.
2. Many steps of lipid synthesis are initiated in the ER (cholesterol, phospholipids).
3. Ca2+ sequestration - storage of Ca2+ ions inside the lumen of the ER, it is released when needed.
4. Detoxification by cytochrome P450 enzymes - Cytochrome P450 enzymes are found in the liver and they function to metabolise potentially toxic compounds. They also can transform the transform the inactive form of a frug to the active form.

Question 3

What is the basic functions of the rough ER?

Answer 3

Functions:-

• Synthesis of proteins
• Glycosylation of the made proteins

Glycosylation meaning - the controlled enzymatic modification of an organic molecule, especially a protein, by addition of a sugar molecule.

Question 4

What are resident proteins?

Answer 4

Resident proteins are proteins that are manufactured in the ER and stay in the ER.

Question 5

How do resident proteins stay in the ER?

Answer 5

The resident proteins have a retention signal which makes them stay in the ER.

Question 6

What does BiP (Binding immunoglobin protein) do?

Answer 6

BiP identifies proteins that have been improperly made and keep them from going to their final destinations.

Question 7

Why do secreted proteins differ from resident proteins?

Answer 7

Secretory proteins have a signal sequence at the 5’ end which is rich in hydrophobic amino acids.

Question 8

What is the difference between free and bound ribosomes?

Answer 8

• Free ribosomes - they are found in the cytoplasm and synthesise proteins that are used in the cell.
• Bound ribosomes - they are found attached to the surface of the rough ER and they synthesise proteins that are transported out of the cell.

Question 9

Explain the steps for co-translational protein targeting to the ER.

Answer 9

1. The targeting pathway begins with the synthesis of a protein on a free ribosome (ribosome that is free in the nucleus). AUG is the start codon for which an mRNA is translated. This happens at the 5’end of the mRNA molecule.
2. The proteins that are to be secreted from the cells differ from other proteins. These secretory proteins have an signal sequence (between 10 - 30 amino acids) which is very rich in hydrophobic amino acids.
3. As the signal sequence emerged from the ribosome, it is recognised and bound by a signal recognition particle (SRP).
4. The SRP guides the complex (ribosome and the signal sequence) to bind to a SRP receptor on the ER membrane.
5. GTP molecules bind to the SRP and the SRP receptor.
6. This binding of GTP causes the transfer of the signal sequence from the SRP to a membrane channel called a translocon.
7. The hydrolysis of GTP to GDP leads to the dissociation of SRP from both the receptor and the ribosome - mRNA complex.
8. Elongation of the polypeptide now resumes until the complete protein has been synthesised.
9. The signs sequence is cleaved by signal peptidase and the polypeptide is released into the lumen of the ER.

Question 10

What does the signal sequence determine?

Answer 10

It determines whether the ribosome that is translated is a free or bound ribosome.

Question 11

How does protein get modified?

Answer 11

• When insulin is secreted it is called preproinsulin (it is a 110 amino acid protein) and has a signal sequence guiding it to a translocon.
• The signal sequence is cleaved off by proteolysis and after that it is called proinsulin.

Proteolysis meaning - the breakdown of proteins into smaller polypeptides or amino acids.

Question 12

What happens to the faulty proteins?

Answer 12

They are not released and are degraded instead.

Question 13

What are the 3 different domains of the Golgi?

Answer 13

1. The cis face
2. The medial Golgi - where most of the enzymes that carry out lipid and protein modification are found.
3. The trans face

Question 14

What are the functions of Golgi apparatus?

Answer 14

1. Protein modification
   
   1. Glycosidases, glycosyltransferases
   2. O-linked glycosylation
   3. Sulfatases
   4. Proteases
2. Lipid synthesis
   
   1. Sphingomyelin, glucosylceramide
3. Protein and lipid sorting to:
   
   1. Secretory granules
   2. Plasma membrane
   3. Basolateral versus apical membrane - (Apical is the portion of the cell exposed to the lumen and basolateral is the rest of the cell).
   4. Endosomes
   5. Lysosomes