Lipid trafficking

Question 1

Where can ribosomes in cells be found?

Answer 1

Free in cytosol
OR
Bound to ER membrane

Question 2

What is the sequence which proteins destined for other organelles use?

Answer 2

First imported into the ER——> Golgi—-> Other organelles

Question 3

What are some post-translational modifications and quality control?

Answer 3

• Folding
• Formation of disulphide bonds
• initial glycosylation ( addition of sugars)
• specific proteolytic cleavages
• assembly of multimeric proteins

Question 4

Give an example of a disease that results due to misfolding of protein.

Answer 4

Cystic fibrosis. The CFTR gene affect functioning of the chloride channels in the membrane, leading to CF.
AND
SMED-SL:
causes- short limbs and abnormal calcifications.
Affects the DDR2 is a cell surface protein kinase receptor for collagen, this results in ER retention ( retained in the ER)

Question 5

What are the three parts of the Golgi Apparatus?

Answer 5

Cis, medial and trans

Cis is closest to the ER

Question 6

Describe the Vesicular transport complexity via the exocytosis and endocytosis?

Answer 6

Exocytosis:
-Endoplasmic reticulum to cis Golgi network to medial Golgi network to trans Golgi network to cell surface via different type of secretory pathway.

Endocytosis:
-material recognised at plasma membrane.
-early endosome
(involved in recycling) can keep going round and round
-or be sorted into late endosome, and if it is destined for destruction it will be taken to lysosome.

Question 7

Where does post translational modification take place?

Answer 7

Golgi apparatus

Question 8

What are the two types of secretory pathways and what is the difference between them?

Answer 8

Constitutive – it is unregulated and acts as a shuttle vesicle to the membrane Regulatory – it is regulated. Vesicles in this pathway must receive a signal (e.g. a hormone or neurotransmitter) before the material in the vesicles can be exocytosed. Found in excitatory cells.

Question 9

What are clathrin-coated vesicles?

Answer 9

Have an outer coat made up of protein clathrin.

Question 10

What protein is responsible for pinching off?

Answer 10

Dynamin

The molecule dynamin is involved in “pinching off” the vesicle from the membrane in a GTP-dependent process.

Question 11

How far do membranes need to be or them to fuse?

Answer 11

1.5nm

Question 12

What are the forms of endocytosis?

Answer 12

1. Receptor-mediated endocytosis
2. Pinocytosis ( fluid phase)
3. phagocytosis (fluid/particles e.g. microbes)

Question 13

Give an example of a disease of endocytosis

Answer 13

Familial hypercholesterolaemia.
-caused by mutation in LDL- receptor
therefore body is unable to remove LDL so high levels of LDL in the blood.

Question 14

What are the functions of glycosylation?

Answer 14

• Folding
• protection
• receptors
• recognition

Question 15

Explain the regulated secretory pathway

Answer 15

These are sorted into a specific secretory vesicle and the vesicle is stored in the cytoplasm.
Once triggered by a signal, it will bind to cell surface receptor and trigger an intracellular signalling pathways that causes release of the stored secretory proteins. This only happens in specialised cells.

Question 16

The formation of disulphide bonds in a newly synthesized protein occurs in which cellular compartment?

Answer 16

ER

Question 17

What is the function of the trans-Golgi network (TGN)?

Answer 17

a sorting station for newly synthesized proteins

Question 18

What is the principle molecule responsible for the formation of the distinct basket-like cages?

Answer 18

Clathrin

Question 19

What allows ribosomes to attach to the ER?

Answer 19

A common pool of ribosomes is used to synthesize both the proteins that stay in the cytosol and those that are transported into the ER (i.e. secreted and transmembrane). It is the ER signal sequence (or signal peptide) at the N-terminus of a newly formed polypeptide chain that directs the engaged ribosome to the ER membrane.

Question 20

How are LDL taken into the cells?

Answer 20

LDL is endocyosed following binding to specific LDL receptors, which subsequently cluster in clathrin-coated pits which are then internalised.

Question 21

What are the three fates of endocytosed material?

Answer 21

Degradation, transcytosis and recycling

Question 22

Describe the passage of lysosomal enzymes in the secretory pathway.

Answer 22

Lysosomal hydrolase precursors move from the ER to the cis Golgi
The mannose on the lysosomal hydrolase is phosphorylated by phosphotransferase
The phosphorylated sugar acts as a tag
It moves through the Golgi apparatus and binds to mannose-6-phosphate receptors in the trans Golgi
Once bound, the vesicles move to the late endosome
The late endosome has a proton pump which pumps protons into the late endosome thus making it acidic
The acidity allows the dissociation of the M6P receptor
Phosphohydrolases remove the phosphate from the lysosomal hydrolase –meaning it can’t return to the Golgi
Lysosomal hydrolases accumulate in the late endosome and it matures into a lysosome