Secretory Pathway and Intracellular Traffic

Question 1

What are the 3 mechanisms of protein transport?

Answer 1

Nuclear transport (gated transport between cytosol and nucleus)

Transmembrane transport (across a membrane from the cytosol into an organelle through translocators - i.e. protein synthesis and mitochondrial import

Vesicular transport (membrane bound transport intermediates move proteins and lipids from one compartment to another)

Question 2

What are the major organelles of exocytosis?

Answer 2

sER, rER
golgi complex
plasma membrane
vesicles and tubules

Question 3

What are the 6 major functions of the ER?

Answer 3

1. Synthesis of lipids (phospholipid, ceramide, cholesterol) - sER
2. Control of cholesterol homoeostasis (sensor and synthesis)
3. Storage of Ca+ (uptake and release)
4. Synthesis of proteins on membrane bound ribosomes - rER
5. Co-translational folding of proteins and early posttranslational modifications
6. Quality control

Question 4

What is the function of BiP?

Answer 4

Binds proteins as they enter the ER and help them to fold properly => interact with protein disulfide isomerase and create disulfide bonds

Question 5

What is SRP?

Answer 5

a Signal Recognition Particle

- 6 protein complex bound to 1 RNA molecule

Question 6

What is the function of SRPs and how do they work?

Answer 6

Functon:
recognizes ER signal sequence and directs newly formed proteins to ER

How:

• binding pocket recognizes signal sequence
• SRP binding induces pause in translation until SRP receptor bound
• ribosome attaches to translocon
• SRP releases

Question 7

What is a translocon?

Answer 7

protein channel allowing polypeptide chain to enter the ER

Question 8

How are proteins with transmembrane domains (TMDs) synthesized

Answer 8

1. Single TMD “Type I membrane protein”
   - mRNA contians stop transfer signal
   - released by translocon- remainder synthesized on cytosolic face (either c terminal or amine terminal out)
2. multiple TMDs
   - same
   - just internal stop and start to go back and forth across
   - synthesized on correct side => made into membrane

Question 9

Where does N-linked glycosylation occur?

Answer 9

in ER lumen when asparagine carbohydrate complex is added to membrane protein

Question 10

What is the function of an asparagine carbohydrate group?

Answer 10

prevent protein aggregation (at hydrophobic domains)

tags to monitor unfolded proteins

Question 11

What are vesicle coats made of?

Answer 11

soluble cytoplasmic proteins and lipids

Question 12

How does COPII travel?

Answer 12

ER to golgi (forward)

Question 13

How does COPI travel?

Answer 13

Golgi to ER (backward)

Question 14

How does clathrin travel?

Answer 14

Golgi to plasma membrane

Question 15

Name a protein that is recycled?

Answer 15

SCAP that bind SERBP

Question 16

What is the function of SCAP & SCRBP?

Answer 16

regulate cholesterol synthesis and export from ER

Question 17

What are the 4 major functions of the golgi aparatus?

Answer 17

1. Synthesis of comples sphingolipids from ceramide backbone
2. Additional post-translational modifications of proteins and lipids
3. Proteolytic processing
4. Sorting of proteins and lipids fro post-golgi compartments

Question 18

What are the compartments of the golgi in order of increasing distance from the nucleus?

Answer 18

cis
medial
trans
TGN - trans golgi network

Question 19

Where does sulfation occur?

Answer 19

trans and TGN

Question 20

Where does most glycosylation occur?

Answer 20

Golgi

Question 21

What does constitutive secretion mean?

Answer 21

Contents will be secreted immediately

Question 22

What does regulated secretion mean?

Answer 22

Contents will only be release once the proper signal has been received