Lecture 9 - The Endoplasmic Reticulum and secretory pathways

Question 1

Fun facts (3)

Answer 1

Endoplasmic reticulum is continuous with nuclear membrane/ envelope.
“Reticulum” = little net in latin.
Continuous network of tubules forming a ’net’.

Question 2

Nuclear lamina proteins (2)

Answer 2

Network of proteins that lines the inside of the nuclear envelope, has a role in maintaining the structure of nucleus. Involved in gene regulation and transcription.

Question 3

Functions of ER (6)

Answer 3

Protein synthesis.
Glycosylation.
Ca2+ sequestration.
Detoxification by cytochrome P450 enzymes.
Folding and assembly and multi-protein complexes.
Lipid synthesis (cholesterol, phospholipids).

Question 4

Insulin (7)

Answer 4

Helps lower blood sugar.
Made in beta cells in the islets of Langerhans.
Signal sequence is cleaved to form proinsulin. Disulphide bridges (bonds) form between cysteine amino acids between chains of amino acids, here the protein is forming its final 3D structure.
Protein modification only takes place in the lumen of the ER. Cytoplasmic proteins do not have disulphide bonds, only the enzymes that catalyse their formation only exist in ER.
Proteolysis occurs, and the C peptide is removed to form a mature peptide.

Question 5

DSPP - Dentin sialophosphoprotein

Protein

Answer 5

A precursor protein for other proteins found in the teeth.

Changing tyrosine at position 6 to aspartic acid.

Question 6

Process of DSPP protein mutation (5)

Answer 6

1. The DSPP gene is not transcribed.
2. The DSPP mRNA is not translated.
3. The DSPP protein accumulates in the cytoplasm.
4. The DSPP protein accumulates in the ER.
5. The DSPP protein is selected but not functional.

Question 7

DSPP mutation position 6 (4)

Answer 7

Step 3) as it is a position six mutation it has occurred in the signal sequence.
The purpose of this is to guide the polypeptide into the ER. If there is a fault with the signalling sequence this means a SRP complex will not occur, so the protein will remain in the cytoplasm.

Question 8

Protein modifications in the ER (5)

Answer 8

Proteolysis (signal peptide).
Disulphide bond formation.
Glycosylation.
Deglycosylation.
Protein folding and assembly (tertiary and quaternary structure).

Question 9

Protein export (5)

Answer 9

Proteins in vesicles bind with target bilayer and fuse and release protein via exocytosis.
Signal recognition particle (SRP) is found in the cystol.
The SRP binds to the ribosome and ER signal sequence.
Binding the SRP pauses protein synthesis.
Proteins that fail any one quality check (as listed above in protein modifications) will not be exported from the ER and are degraded instead.

Question 10

Example of protein export problem (2)

Answer 10

Cystic fibrous caused by mutation in the plasma membrane protein made in ER. The ribo protein is made in ER and transported via golgi apparatus to plasma membrane, but because of a mutation the protein can’t fold properly and is never inserted into an export vesicle and never reaches golgi apparatus instead it is degraded.

Question 11

Roles of Golgi apparatus

Answer 11

Protein modification.
Lipid synthesis.
Protein & Lipid sorting.

Question 12

GA - Protein Modification (4)

Answer 12

Glycosidases, glycosyltransferases.
O-linked glycosylation.
Sulfatases.
Proteases.

Question 13

GA - Lipid synthesis (2)

Answer 13

Sphingomyelin.

Glucosylceramide.

Question 14

GA - Protein & Lipid sorting (5)

Answer 14

Secretory granules.
Plasma membrane.
Basolateral versus apical membrane.
Endosomes.
Lysosomes.