Lecture 13 - Protein Sorting: Endoplasmic Reticulum

Question 1

How do new proteins go from a ribosome to the organelle where they function?

Answer 1

by using sorting signals in its amino acid sequence

Question 2

What are the main steps of the secretory pathway?

Answer 2

1. Translate protein from mRNA in the ribosomes in the cytoplasm
2. Enters the ER lumen
3. Goes from ER to Golgi in a vesicle
4. Transits the Golgi
5. Leaves the Golgi in a vesicle
6. Vesicle fuses to cell membrane
7. Protein is outside

Question 3

Mutant categories (of vesicles??)

Answer 3

depend on where the mutated category was required
1. fail ER import
2. fail to produce ER vesicles
3. vesicles don’t fuse to Golgi
4. fail to leave the Golgi, Golgi vesicles do not form
5. vesicles don’t fuse with cell membrane

Question 4

Where were the main proteins involved in protein trafficking discovered?

Answer 4

in yeasts; including Sec proteins

Question 5

ER organization

Answer 5

organized into a netlike labyrinth of branching tubules and flattened sacs that extends throughout cytosol
- ER has a single internal space = ER lumen

Question 6

What is a signal peptide?

Answer 6

a short peptide present at the N-terminus of newly synthesized proteins that are destined toward the secretory pathway

• tells proteins where to go

Question 7

What is cotranslational import?

Answer 7

Import and translation happens together
- easier due to protein size => if folded first, protein will not fit through pore in ER

Question 8

What two things guide the ER signal sequence to the ER membrane?

Answer 8

1. a signal-recognition particle (SRP): binds to signal sequence
2. SRP receptor on ER membrane

Question 9

What happens to SRP when a signal sequence binds?

Answer 9

it exposes a binding site for an SRP receptor

Question 10

What is an SRP receptor?

Answer 10

a transmembrane protein complex in the rough ER membrane

Question 11

What proteins do membrane-bound ER ribosomes make? What about free ribosomes?

Answer 11

membrane-bound ER ribosomes: proteins that are co-translated across the ER membrane

free ribosomes: all other proteins

Question 12

What does the polypeptide chain pass through to get into the ER lumen? What about multipass transmembrane proteins?

Answer 12

it passes through a signal sequence-gated aqueous channel in the translocator (/translocon)

polypeptide chain of a multipass transmembrane protein passes back and forth repeatedly across the lipid bilayer

Question 13

What does protein folding in the ER involve/ need?

Answer 13

• chaperone proteins
• disulfide bonds
• glycosylation

Question 14

Translocated polypeptide chains fold and assemble in the ____________

Answer 14

lumen of the rough ER

Question 15

What are disulfide bonds?

Answer 15

post-translational modification that occur in the ER
- force proteins into a certain shape

Question 16

Protein glycosylation in the rough ER

Answer 16

proteins synthesized in the rough ER are glyscosylated by the addition of a common N-linked oligosaccharide

Question 17

What are oligosaccharides used for?

Answer 17

they’re like tags used to mark the state of protein folding
- if protein is folded properly, a glucosidase removes the last glucose
- if protein is not folded properly, a glucosyl transferase adds a glucose

calnexin binds to this glucose to anchor the protein to the membrane to allow time for proper folding

Question 18

Calnexin

Answer 18

A chaperone protein that binds to monoglucosylated on incompletely folded proteins and retains them in the ER
- it recruits an oxidoreductase to add more disulfide bonds
- if folding is good, GlsII removes the final glucose residue

Question 19

What do ER chaperones do?

Answer 19

they prevent protein misfolding and aggregation
- protect peptides from interacting with other misfolded proteins
- create a folding envrionment
- unfold misfolded proteins to give them a 2nd chance at folding properly

Question 20

How are misfolded proteins recognized?

Answer 20

they have exposed hydrophobic residues
- these residues should not be outside