Protein Targeting and Sorting

Question 1

Where does most polypeptide synthesis take place?

Answer 1

in the cytosol with transcripts leaving the nucleus and associating with free ribosomes

Question 2

what is cotranslational import?

Answer 2

transfer of a growing polypeptide chain across the ER membrane as polypeptide synthesis proceeds

Question 3

How does cotranslational import occur?

Answer 3

ribosomes become attached to the ER membranes early in translation, and polypeptide chains are transferred across the ER membrane as synthesis takes place

Question 4

What is postranslational import?

Answer 4

uptake by organelles of completed polypeptide chains after they have been synthesized, mediated by specific targetting signals within the polypeptide

Question 5

Where do the polypeptide chains go during postranslational import?

Answer 5

cytosol, mitochondira, cholorplast, peroxisomes or nuclear interior

Question 6

What did the signal hypothesis model describe?

Answer 6

proposed that some sort of intrinstic molecular signal distinguishes polypeptides from the many polypeptides destined to be released into the cytosol

for polypeptides destined for the ER, the first segment of the polypeptide to by synthesized (n-terminus) contains an ER signal sequence that directs the ribosome mRNA polypeptide complex to the surface of the rough ER

Question 7

What did Milstein et al determine about protein targeting and sorting?

Answer 7

found evidence that a 20 amino acid sequence at the N-terminus of the light chain of immunoglobulin G is a signal sequence

Question 8

What are preproteins?

Answer 8

proteins containing sequences at the N-terminus to target the ER

Question 9

What is the Signal Recognition Particle? (SRP)

Answer 9

a particle that reconizes and binds to the ER signal sequence of the newly forming polypeptide and then binds to the ER membrane

Question 10

What does the SRP contain?

Answer 10

both protein and RNA

Question 11

Cotranslational Import:

Describe the first 3 steps that occur

Answer 11

1. SRP binds to the signal sequence and blocks further translation
2. SRP binds the ribosome to the ER membrane with a translocon protein, the SRP binds SRP receptor with the help of GTP allowing ribosome to bind to ribosome receptor. This transfers the signal sequence to the pore protein
3. The central channels open as the signal sequence is inserted

Question 12

Cotranslational import:

describe the last 3 steps that occur

Answer 12

4. GTP is hydrolyzed and the SRP is released
5. the polypeptide elongates and passes into the ER lumen where signal peptidase cleaves the signal peptide
6. polypeptide is released into the lumen, ribosome detaches from the ER membrane and the subunits dissociate and release the mRNA

Question 13

After Cotranslational Import in the ER lumen:

What are BiP proteins?

Answer 13

member of the Hsp 70 family of chaperones; present in the ER lumen, where it facilitates protein folding by reversibly binding to the hydrophobic regions of polypeptide chains

Question 14

How do BiPs assist with protein folding in the ER lumen?

Answer 14

BiP binds to the hydrophobic regions of polypeptide chains and prevents aggregation of polypeptides with similar regions

releases the polypeptide chains, accompanied by ATP hydrolysis, giving the polypeptides a short opportunity to fold

the hydrophobid region is buried in the interior of the molecule

Question 15

How do disulfide bonds form during folding of proteins?

Answer 15

facilitated by protein disulfide isomerase, enzyme present in the ER lumen that catalyzes the formation and breakage of disulfide bonds between cytesine residues

Question 16

When proteins repeatedly fail to fold what helps control this?

Answer 16

unfolded protein response - use sensor molecules in the ER membrane to detect misfolded proteins, activate pathways that shut down the snthesis of most proteins while enhancing the production of those required for protein folding and degradation

ER-associated degradation (ERAD) - recognizes misfolded or unassembled proteins and exports or “retranslocates” then back across the ER membrane to the cytosol where they are degraded by proteasomes

Question 17

What happens after proteins are folded in the ER lumen?

Answer 17

glycoproteins are sent to the Golgi complex for further glycosylation and processing of carbohydrate side chains

soluble proteins move from the Golgi to secretory vesicles for secretion from the cell

the proteins not destined for secretion have specific side chains that target them to destinations within the endomembrane system

proteins who return to the ER have a KDEL sequence

Question 18

Describe the two main mechanisms postulated for the insertion of integral membrane proteins.

Answer 18

1. polypeptide with an internal stop-transfer sequence and a terminal ER signal sequence - allows an SRP to bind the ribosome-mRNA complex to the ER membrane, elongation of the polypeptide continues until the hydrophobic transmembrane domain is synthesized
2. polypeptide with onle a single, internal stop-transfer sequence - STP binds and targets to the ER membrane, hydrophobic region functions as a membrane anchor