Secretory Pathways

Question 1

What are six major functions of the ER

Answer 1

• synthesis of lipids (primarily in the smooth ER)
• control of cholesterol homeostasis (cholesterol sensor and synthesis)
• storage of Ca2+ (rapid uptake and release)
• synthesis of proteins of membrane bound ribosomes (rough ER)
• co-translational folding of proteins and early post translational modification
• quality control

Question 2

What does the ER signal sequence on a newly formed polypeptide chain do?

Answer 2

• “directs” the engaged ribosome to the ER membrane

- the signal sequence is recognized by a signal recognition particle (SRP), six proteins bound to one RNA molecule

Question 3

What does the signal recognition particle do?

Answer 3

• binds to the ER signal sequence on the polypeptide chain, stopping protein synthesis (binding pocket is flexible and can bind to a variety of signal sequences)
• Binding induces a pause in translation
• SRP detaches once the ribosome is attached to the translocon
• mature protein is cleaved into the ER lumen

Question 4

What is a translocon?

Answer 4

a protein channel allowing the polypeptide chain to enter the ER

Question 5

How is a soluble protein translocated across the ER membrane?

Answer 5

• after binding an ER signal sequence, the translator (or translocon) opens its pore, allowing the transfer of the polypeptide chain across the lipid bilayer as a loop
• hydrophobic signal sequence (previously bound to SRP) is cleaved and diffused into lipid bilayer for degradation

Question 6

What regulates the translocon?

Answer 6

the ribosome at the cytoplasmic face and by BiP, a chaperone protein, at the luminal face

Question 7

What does BiP do?

Answer 7

binds the protein as it’s entering the ER and helps it to fold and to interact with a protein disulfide isomerase to create disulfide bonds

Question 8

How is a protein with a transmembrane domain (TMD) synthesized?

Answer 8

• mRNA contains a sequence recognized bu the translocon as a “stop transfer” signal
• the stop transfer is released by the translocon and the remainder of the protein, the C-terminal end, is synthesized on the cytsolic face
• can have multiple orientations
• positively charged amino acids orient the protein

Question 9

How is a protein with multiple TMDs synthesized?

Answer 9

they have alternating internal stop and start sequences

Question 10

Many membrane proteins have a ______ ________ complex added to an ______ in the ER lumen

Answer 10

carbohydrate complex; asparagine

Question 11

What is the purpose of N-linked glycosylation that occurs in the ER lumen?

Answer 11

• helps keep proteins from aggregating when hydrophobic domains are exposed
• glucose residues act as tags to monitor unfolded proteins

Question 12

How are proteins shuttled from the ER to the Golgi Complex?

Answer 12

budding of vesicles, fusion of some of these vesicles into tubules, and fusion of these vesicles/tubules with the next compartment

Question 13

The movement of cargo via budding of vesicles requires what?

Answer 13

the formation of coated vesicles and adapter proteins that recognize the cargo, the coat, and the membrane proteins/lipids destined to move

Question 14

What is COPII?

Answer 14

a coat protein for ER to Golgi vesicle movement

Question 15

What is COPI?

Answer 15

a coat protein for Golgi to ER vesicle movement

Question 16

What is clathrin?

Answer 16

a coat protein for Golgi to plasma membrane

Question 17

What are the major functions of the Golgi Complex?

Answer 17

• synthesis of complex sphingolipids from the ceramics backbone
• additional post-translational modifications of proteins and lipids (glycosylation and sulfation)
• proteolytic processing
• sorting of proteins and lipids for post-Golgi compartments