Exam 3- The Secretory Pathway

Question 1

Which are the cellular organelles involved in the secretory pathway?

Answer 1

ER and Golgi Apparatus

Question 2

What is the Pathway for the sec pathway?

Answer 2

Pathway is: ER → Golgi → Secretory Vesicles → Cell Exterior

• proteins that end up in PM use same pathway, except for cell exterior
• secretory pathway happens co-translationally. Very few proteins go through secretory pathway post-translationally

Question 3

What are the potential cellular destinations followed by a protein synthesized in free cytosolic ribosomes?

Answer 3

cytoplasm, nucleus, mitochondria, chloroplast

Question 4

What are the potential cellular destinations followed by a protein synthesized in ER membrane-bound ribosomes?

Answer 4

PM, secretory vesicles, lysosomes, endosomes, golgi and ER

Question 5

-What’s the difference between membrane-bound and free ribosomes?

Answer 5

Nothing, they are identical in terms of shape and function.

The only difference is where the proteins end up due to where the ribosome is bound.

Question 6

What determines whether a ribosome remains free in the cytoplasm until it finishes synthesizing a protein or becomes membrane bound?

Answer 6

The ribosome gets targeted to the membrane of the ER if a ERsignal sequence is on the transcribed protein. ERSS. This sequence is more than 12 hydrophobic amino acids in a row near the amino terminus.

Question 7

What’s the function of the ER Signal Sequence?

Answer 7

Recruits SRP where it binds to the SRP receptor to allow protein to bind to the translocon.
-signal recognition particle
.Recruitment of SRP stops translation until the ribosome is bounded to the ER.

Question 8

What’s the Signal Recognition Particle? What is it made of? What’s its function?

Answer 8

Recognizes the ERSS and escorts the complex of mRNA, protein and ribosome to the membrane of the ER where it binds the SRP receptor.

SRP consists of RNA, 6 polypeptides associated to the RNA and a signal sequence

Question 9

What does the SRP bind to in the surface of the ER?

Answer 9

Signal recognition receptor, bringing the ribosome into close proximity to the ER.

Question 10

What’s a transmembrane domain?

Can transmembrane domains be predicted using biocomputing methods (i.e. computer algorithms)?

Answer 10

It is a sequence of a minimum of 20 hydrophobic amino acids in a row that targets the protein to the membrane of the ER.
Form stable alpha helix in the membrane

Question 11

Proteins destined for the plasma membrane are first inserted in where?

Answer 11

ER membrane

Question 12

Does the topology of a membrane protein change during the different steps involved in the secretory pathway?

Answer 12

Topology of a membrane protein is dictated by amino acid sequence, maintained throughout secretory pathway, even as it reaches the PM.

Question 13

Which ones are the most frequent post-translational modifications enacted upon proteins during their traffic in the secretory pathway?

Answer 13

IN ER:
C- cleave Signal Sequence
S- formation of disulfide bonds 
F- folding of proteins and subunit assembly 
N- glycosylation 
G- addition of GPI anchor

IN GOLGI:
Modify N-glyco
O-Link glyco